• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ concentrations. Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 16 and 25 km above sea level (NASEM 2021). The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ concentrations. Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 16 and 25 km above sea level (NASEM 2021). The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 16 and 25 km above sea level (NASEM 2021). The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 16 and 25 km above sea level (NASEM 2021). The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 16 and 25 km above sea level (NASEM 2021). The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation

• • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions. (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions. (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions. (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions. (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ concentrations. Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

• Stratospheric Aerosol Injection (SAI) is a strategy for increasing the number of small reflective particles (aerosols) in the stratosphere to increase the reflection and scattering of incoming sunlight (NASEM 2021). The stratosphere is relatively stable, and aerosols can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation (NASEM 2021). Potential aerosols for SAI include sulfur dioxide (SO₂) or hydrogen sulfide gas (H₂S), sulfate particles, or solid particles including alumina, calcite, or rutile (TiO₂). Sulfate is analogous to material expelled in volcanic eruptions. (NASEM 2021). SAI is the most studied and best understood SRM approach to date (NASEM 2021). Modeling evidence suggests that SAI could offset at least half of the radiative forcing caused by a doubling of CO₂ concentrations. Evidence shows that SAI may be able to produce uniform radiative forcing (Kravitz et al. 2017) and reduce key climate hazards substantially (Irvine and Keith 2020).

• Expected physical changes (global)
  • Reduce incoming solar radiation
• Spatial extent (size)
  • Injection would have global impacts. Particles are uniformly distributed zonally (east-west) after injection over a period of weeks. Meridional (north-south) mixing happens more slowly (months-years), moving from the equator toward the poles. Therefore, injection in one hemisphere would be concentrated in that hemisphere.

• Atmosphere / Stratosphere
  • SAI would deploy aerosols in the stratosphere between 7 and 25 km above sea level. The stratosphere is a relatively stable zone in the atmosphere at between 10 and 50 km altitude where there is less vertical than horizontal mixing (Labitzke and Van Loon 2012), meaning an aerosol particle could remain in the stratosphere, reflecting solar radiation for a period measured in years (NASEM 2021).
  • Altitude can impact effect. Higher altitude deployments increase the lifetime of the aerosols, however, there are technical limitations to higher altitude deployments (Lee et al. 2023a).

• Once aerosols are added to the stratosphere they spread uniformly in longitude and are transported poleward in latitude and lead to global effects (NASEM 2021). Injection scenarios tested in models include injection in tropics, subtropics, subpolar, or polar regions, or uniformly across all latitudes (Duffey et al. 2023).
• Focusing aerosol injection at higher latitudes could provide greater cooling in the Arctic than other regions, but effects cannot be isolated to a particular region (NASEM 2021, Duffey et al. 2023). Any scenario focused on Arctic cooling would need to be balanced by southern hemisphere cooling to avoid differences in temperature between hemispheres and changes in precipitation patterns resulting from unbalanced deployment (NASEM 2021, Duffey et al. 2023).

• Effective when there is incoming solar radiation - during daytime, with largest impact during summer. This technique would not have a radiative forcing effect during the winter in the Arctic (Duffey et al. 2023), however, it would still have a climate impact on nights/winters due to ocean heat capacity and atmospheric heat transport.

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer.

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021, Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable.

• Direct or indirect impact on sea ice?
  • Indirect via temperature change.
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general, there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect.

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Due to atmospheric circulation patterns, aerosols have a shorter lifetime at higher latitudes compared to lower latitudes (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary (UNEP 2023).

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023).
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by a single nation or actor.
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012).

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer.

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021, Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect.

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Due to atmospheric circulation patterns, aerosols have a shorter lifetime at higher latitudes compared to lower latitudes (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary (UNEP 2023).

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012).

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Due to atmospheric circulation patterns, aerosols have a shorter lifetime at higher latitudes compared to lower latitudes (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Due to atmospheric circulation patterns, aerosols have a shorter lifetime at higher latitudes compared to lower latitudes (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Due to atmospheric circulation patterns, aerosols have a shorter lifetime at higher latitudes compared to lower latitudes (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment, could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5°C or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024). This study reports a temperature decrease of 1°C in 2050 compared to a scenario without SAI.
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection with cooling around 2°C in the Arctic region by 2050), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5 or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024).
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein).
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years
  • Technology could be developed in about 10 yrs, although estimates vary, UNEP 2023.

• >10 years for technology development
  • Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief).
  • Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
  • Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
  • Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
  • Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year per 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5 or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024).
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
    • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
    • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
    • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein)
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years (technology could be developed in about 10 yrs, although estimates vary, UNEP 2023)

• >10 years for technology development
• Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief)
• Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
• Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
• Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
• Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

Impact on

• Unknown
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Decrease of 1.5 or greater
    • Temperature decrease from SAI scales with the specifics of the deployment. For example, how much material is injected. Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • There is substantial evidence from modeling studies as well as from studies of volcanic eruptions that SAI can decrease temperature globally (NASEM 2021; Duffey et al. 2023, Visioni et al. 2023a).
    • The most efficient deployment locations for global cooling are in the subtropical regions (Zhang et al. 2024).
    • Many SAI simulations result in over-cooling of tropics and under-cooling of polar regions. This effect is greatest for injection at tropics and uniformly across latitude, but injection outside the equator diminishes this effect (reviewed in Duffey et al. 2023). Deployment of SAI at the poles not as efficient for global cooling by a factor of around 2 (Duffey et al. 2023).
• Arctic region
  • Up to 2°C
    • Most studies have a temperature goal, then discuss the implications for achieving that temperature goal.
    • Subpolar deployment could cool average surface temperature in Arctic by 2°C (Smith et al. 2022b).
    • Deployment of SAI at the poles may give greater cooling in the Arctic (Lee et al. 2023b reports 50% more per unit mass injection), but there is disagreement amongst studies (reviewed in Duffey et al. 2023).
    • Modeling studies that keep forcing constant at 2020 levels still see warming in Arctic over time of 0.01-0.03°C/yr (compared to rate of 0.04°C/yr without SAI; Berdahl et al 2014). Adding more aerosols at high latitudes may achieve more cooling in the Arctic.

• Global
  • Decrease of 1 W/m² or greater
    • Most studies have a temperature or radiative forcing goal, then discuss the implications for achieving that goal (e.g., Moore et al. 2010).
    • Volcanic eruptions show demonstration - possible to reduce solar shortwave heating of the planet by at least 1 W/m² with upper bound likely much larger (NASEM 2021 and reviewed in NRC 2015).
• Arctic region
  • Estimate unavailable

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both

• Varies: slow down of loss of Arctic sea ice up to restoration of sea ice
  • There is variation among different climate models that look at how SAI impacts sea ice, and very little consistency in models when looking at regional changes (reviewed in Duffey et al. 2023, cites Moore et al. 2014).
  • Looking across multiple models, Berdahl et al. 2014 finds that September sea ice extent still decreases into the future with SAI, but not as quickly as without SAI. Recent work shows restoration of sea ice extent with high latitude injections (Zhang et al. 2024).
  • Some single model studies show stabilization or restoration of Arctic sea ice extent (reviewed in Duffey et al. 2023, Jones et al. 2018, Jiang et al. 2019, Kravitz et al. 2019). In general there is greater restoration of summer sea ice minimum than winter maximum.

Scalability

• Spatially scalable since SAI has a global effect

• Release of particles at 25 km would be more efficient than at 20 km, but deployment infrastructure doesn’t exist and therefore would be substantially more expensive to do at 25 km than at 20 km (Smith et al. 2022a).
• The tropopause is lower in polar regions, so SAI injection could potentially happen at lower latitudes in the poles with fewer planes and flights required (Duffey et al. 2023 and references therein)
• Aerosol lifetime decreases with latitude up to factor of 4 (Duffey et al. 2023 and references therein). If doing Arctic deployment could deploy strategically during specific times of year to make up for this (Duffey et al. 2023 and references therein).

• >10 years (technology could be developed in about 10 yrs, although estimates vary, UNEP 2023)

• >10 years for technology development
• Once deployed could see planetary cooling within a year (Keith 2013 in C2G 2021 Evidence Brief)
• Note that gradual SAI deployments may take longer for detectable effects than an abrupt deployment where the impact could be detected within a few years (UNEP 2023).

• >10 years for new technology development, although it is possible that timeline could be escalated via governmental support.
• Subscale deployments using existing technology could happen sooner (10 years or less; Keith and Smith 2024). A subscale deployment is defined by Keith and Smith (2024) as “a deployment large enough to substantially increase the amount of aerosol in the stratosphere while being well below the level that is required to delay warming by a decade”.
• Once implemented, SRM methods would have an impact within months (Russell et al. 2012 and references therein).
• Note that gradual SAI deployments may take a decade for regional effects to be detectable compared to an abrupt deployment where impacts could be detected within years (UNEP 2023).

Cost

• Wide range of cost estimates from few billions to $100 billion a year 1°C cooling (C2G 2023, NASEM 2021, Duffey et al. 2023, UNEP 2023)
  • Few billions of dollars a year (slow ramp up) to $15 billion a year for 1°C cooling (NASEM 2021).
  • Low tens of billions per year (Duffey et al. 2023, citing Moriyama et al. 2017 and Smith 2020).
  • About $20 billion per year per 1°C cooling (Smith 2020, UNEP 2023)
  • Development of aircraft estimated to cost around $3.5 billion (Smith and Wagner 2018).
  • Might be inexpensive enough that could be undertaken by single nation or actor
  • Note that these cost estimates do not include indirect costs that could arise, such as policy and governance development and costs for damages that might accrue, which could be substantial (Smith 2020).

• Unknown
  • Energy required for aerosol delivery (Russell et al. 2012)

• 3 – Research into SAI is occurring internationally, primarily through modeling studies. Small-scale field studies have been proposed, and one has been conducted to date. Studies of engineering and deployment needs, cost, governance, and social acceptability have been conducted.
• Summary of existing literature and studies
  • Many modeling studies, including overall reviews (Irvine et al. 2016 and others) and attention given specifically to how SAI could cool the Arctic (Berdahl et al. 2014, Duffey et al. 2023, Lee et al. 2023b).
  • Studies discussing the engineering needs for how to do deployment (see Smith et al. 2022a,2022b, Keith and Smith 2024).
  • There has been some development of aircraft for research purposes (e.g., OSTP 2023).
  • There are studies and reports of governance and social acceptability (NASEM 2021, OSTP 2023, Mettiäinen et al. 2022).
  • Some planned field studies were cancelled
    • Stratospheric Controlled Pertubration Experiment (ScoPEx) was scheduled to take place in 2021 in Sweden and was cancelled in response to opposition from Saami Council and environmental groups. This experiment planned for release of 100g – 2kg of calcium carbonate.
  • One field study to date (results unpublished, described in Temple 2023).
  • Private companies have been raising money for research.

• Feasible
  • On a small scale, with limited aircraft deployment, this technique is technically feasible within 10 yrs (Keith and Smith 2024). A global deployment with temperature impact would take longer and is dependent on the development of additional aircraft.

• 3 – Research into SAI is occurring internationally, primarily through modeling studies. Small-scale field studies have been proposed, and one has been conducted to date. Studies of engineering and deployment needs, cost, governance, and social acceptability have been conducted.
• Summary of existing literature and studies
  • Many modeling studies, including overall reviews (Irvine et al. 2016 and others) and attention given specifically to how SAI could cool the Arctic (Berdahl et al. 2014, Duffey et al. 2023, Lee et al. 2023b).
  • Studies discussing the engineering needs for how to do deployment (see Smith et al. 2022a,2022b, Keith and Smith 2024).
  • There has been some development of aircraft for research purposes (e.g., OSTP 2023).
  • There are studies and reports of governance and social acceptability (NASEM 2021, OSTP 2023, Mettiäinen et al. 2022).
  • Some planned field studies were cancelled
    • Stratospheric Controlled Pertubration Experiment (ScoPEx) was scheduled to take place in 2021 in Sweden and was cancelled in response to opposition from Saami Council and environmental groups. This experiment planned for release of 100g – 2kg of calcium carbonate.
  • One field study to date (results unpublished, described in Temple 2023).
  • Private companies have been raising money for research.

• On a small scale, with limited aircraft deployment, this technique is technically feasible within 10 yrs (Keith and Smith 2024). A global deployment with temperature impact would take longer and is dependent on the development of additional aircraft.

• • 3 – Research into SAI is occurring internationally, primarily through modeling studies. Small-scale field studies have been proposed, and one has been conducted to date. Studies of engineering and deployment needs, cost, governance, and social acceptability have been conducted.
  • Summary of existing literature and studies
    • Many modeling studies, including overall reviews (Irvine et al. 2016 and others) and attention given specifically to how SAI could cool the Arctic (Berdahl et al. 2014, Duffey et al. 2023, Lee et al. 2023b).
    • Studies discussing the engineering needs for how to do deployment (see Smith et al. 2022a,2022b, Keith and Smith 2024).
    • There has been some development of aircraft for research purposes (e.g., OSTP 2023).
    • There are studies and reports of governance and social acceptability (NASEM 2021, OSTP 2023, Mettiäinen et al. 2022).
    • Some planned field studies were cancelled
      • Stratospheric Controlled Pertubration Experiment (ScoPEx) was scheduled to take place in 2021 in Sweden and was cancelled in response to opposition from Saami Council and environmental groups. This experiment planned for release of 100g – 2kg of calcium carbonate.
    • One field study to date (results unpublished, described in Temple 2023).
    • Private companies have been raising money for research.

• • On a small scale, with limited aircraft deployment, this technique is technically feasible within 10 yrs (Keith and Smith 2024). A global deployment with temperature impact would take longer and is dependent on the development of additional aircraft.

• TRL
  • 3 – Research into SAI is occurring internationally, primarily through modeling studies. Small-scale field studies have been proposed, and one has been conducted to date. Studies of engineering and deployment needs, cost, governance, and social acceptability have been conducted.
  • Summary of existing literature and studies
    • Many modeling studies, including overall reviews (Irvine et al. 2016 and others) and attention given specifically to how SAI could cool the Arctic (Berdahl et al. 2014, Duffey et al. 2023, Lee et al. 2023b).
    • Studies discussing the engineering needs for how to do deployment (see Smith et al. 2022a,2022b, Keith and Smith 2024).
    • There has been some development of aircraft for research purposes (e.g., OSTP 2023).
    • There are studies and reports of governance and social acceptability (NASEM 2021, OSTP 2023, Mettiäinen et al. 2022).
    • Some planned field studies were cancelled
      • Stratospheric Controlled Pertubration Experiment (ScoPEx) was scheduled to take place in 2021 in Sweden and was cancelled in response to opposition from Saami Council and environmental groups. This experiment planned for release of 100g – 2kg of calcium carbonate.
    • One field study to date (results unpublished, described in Temple 2023).
    • Private companies have been raising money for research.
• Technical feasibility within 10 yrs
  • On a small scale, with limited aircraft deployment, this technique is technically feasible within 10 yrs (Keith and Smith 2024). A global deployment with temperature impact would take longer and is dependent on the development of additional aircraft.

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Increased aerosols from SAI can impact atmospheric chemistry (e.g., changes in halogens and ozone) that would interact with climate; there is large uncertainty around these impacts and interactions (NASEM 2021).
  • Stratospheric heating and subsequent impacts
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon; NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
  • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016). From Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
  • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
  • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities, and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Increased aerosols from SAI can impact atmospheric chemistry (e.g., changes in halogens and ozone) that would interact with climate; there is large uncertainty around these impacts and interactions (NASEM 2021).
  • Stratospheric heating and subsequent impacts
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Increased aerosols from SAI can impact atmospheric chemistry (e.g., changes in halogens and ozone) that would interact with climate; there is large uncertainty around these impacts and interactions (NASEM 2021).
  • Stratospheric heating and subsequent impacts
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Increased aerosols from SAI can impact atmospheric chemistry (e.g., changes in halogens and ozone) that would interact with climate; there is large uncertainty around these impacts and interactions (NASEM 2021).
  • Stratospheric heating and subsequent impacts
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Increased aerosols from SAI can impact atmospheric chemistry (e.g., changes in halogens and ozone) that would interact with climate; there is large uncertainty around these impacts and interactions (NASEM 2021).
  • Stratospheric heating and subsequent impacts.
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Can impact atmospheric chemistry (NASEM 2021).
  • Stratospheric heating and subsequent impacts.
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In the northern hemisphere this means cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, could reduce concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Can impact atmospheric chemistry (NASEM 2021).
  • Stratospheric heating and subsequent impacts.
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In northern hemisphere get cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, reducing concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• Medium
  • The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
  • Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• High
  • Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
  • Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
  • To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

Physical and chemical changes

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Can impact atmospheric chemistry (NASEM 2021).
  • Stratospheric heating and subsequent impacts.
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In northern hemisphere get cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

Impacts on species

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

Impacts on ecosystems

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

Impacts on society

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, reducing concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

Ease of reversibility

• The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
• Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

Risk of termination shock

• Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
• Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
• To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023).
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023).
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016).
• Risks
  • Can impact atmospheric chemistry (NASEM 2021).
  • Stratospheric heating and subsequent impacts.
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023).
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012).
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In northern hemisphere get cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016).
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth.
  • Changes in hydrological cycles can impact species.
    • Species can be impacted by changes in precipitation.
  • Species can be impacted by acid precipitation.

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012).
• Risks
  • Changes in light regimes may impact ecosystems.
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein).
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems.
    • Ecosystems can be impacted by changes in precipitation.
  • Ecosystems can be impacted by acid precipitation.
  • Risks to ocean ecosystems.
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, reducing concentrating solar power output by about 6% (NASEM 2021).
  • Impeded astronomical observations (Russell et al. 2012).
  • Hazy appearance to sky (Russell et al. 2012).
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023).
    • Unilateral deployment (UNEP 2023).
    • Security issues may arise (C2G 2021 Policy Brief).
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief).

• The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
• Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

• Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018).
• Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
• To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

• Co-benefits
  • Mitigation of climate change impacts
    • Modelling studies consistently show that climate change impacts (e.g., temperature and hydrology) decrease in most regions with a carefully designed deployment compared to continued climate change without an SRM deployment (UNEP 2023).
    • Cooler temperatures may increase water availability (Russell et al. 2012).
    • Some SAI modeling scenarios show reduced climate-induced changes in the tracks of extra-tropical cyclones in the Southern Hemisphere (Gertler et al. 2020).
  • Benefits to the cryosphere
    • Localized SAI deployment in the poles may restore glacier surface mass balance (reviewed in Duffey et al. 2023)
    • Localized SAI deployment in the poles may increase precipitation and snow volume on sea ice (reviewed in Duffey et al. 2023)
    • SAI deployment could reduce permafrost thaw and carbon release (GeoMIP G4 SAI model by Chen et al. 2020 reports 50% less permafrost carbon released as CO₂ and 40% less permafrost carbon released as CH₄ with SAI compared to RCP4.5).
    • SAI deployment could slow the rate of ice sheet loss in Antarctica (Moore et al. 2024).
    • Reductions in ozone loss in the middle and upper stratosphere (NASEM 2021). Stratospheric warming from SAI would suppress the NOx cycle which destroys ozone (reviewed in Irvine et al. 2016)
• Risks
  • Can impact atmospheric chemistry (NASEM 2021)
  • Stratospheric heating and subsequent impacts
    • Stratospheric heating is a consequence of SAI and depends on aerosol size, microphysics, and injection location (NASEM 2021, Bednarz et al. 2023). Stratospheric heating impacts circulation, ozone transport, water vapor, and tropopause temperature (NASEM 2021). Stratospheric heating can cause wintertime warming in high latitudes (reviewed in Duffey et al. 2023)
  • Ozone loss in the lower stratosphere, particularly in polar regions where colder temperatures interact with sulfate (NASEM 2021). Ozone loss can lead to increased UV radiation. Difficult to predict because depends on complex atmospheric processes (NASEM 2021). A decrease in ozone was seen after the 1991 eruption of Mt. Pinatubo, so this is a possibility not just seen in models (Irvine et al. 2016). Although this is a possibility, though, it is estimated that the effect would be small and would not pose substantial risks except in regions already impacted by ozone loss (Irvine et al. 2016).
  • Changes to light regimes
    • Increase in ratio of diffuse to direct light (NASEM 2021).
    • Decrease in photosynthetically active radiation (PAR; Russell et al. 2012)
  • Alteration of temperature seasonality, especially at high latitudes (NASEM 2021, Duffey et al. 2023). In northern hemisphere get cooler summers but warmer winters (than present, not compared to no-SRM baseline). This can impact sea ice and snow in the Arctic. The effect is not uniform over the Arctic (Duffey et al. 2023).
  • Risks to the cryosphere
    • Undercooling in the Arctic winter and reduced seasonal cycle can lead to increased Arctic precipitation, reduced Arctic sea ice extent in winter, increased loss of permafrost in winter, reduced Arctic cloudiness, increased glacier surface melt in winter (Duffey et al. 2023, Zarnetske et al. 2021).
  • Climate alterations
    • Reduced equator to pole temperature gradient (Greater incoming solar radiation at tropics than at poles; McCormack et al. 2016). However, there is emerging evidence that this can be avoided with injections away from the equator (Wells et al. in review).
    • Changes in global and regional hydrological cycles (droughts, flooding, changes to monsoon) (NASEM 2021, Zarnetske et al. 2021). Decreases in globally averaged precipitation, shown in climate models (NASEM 2021). Precipitation distribution, intensity, and seasonality could be altered (Zarnetske et al. 2021 and references therein).
    • Changes in regional climate compared to current climate (NASEM 2021), however these changes are small when compared to climate predictions with ongoing climate change (NASEM 2021).
    • Risk of acid precipitation (Zarnetske et al. 2021), thought to be a small contribution, though (Russell et al. 2012, Kravitz et al. 2009, Irvine et al. 2016) from Kravitz et al. 2009 – if injection of SO₂ is 5 Tg/yr, which is 25% amount from 1991 Mt. Pinatubo eruption, that is an order of magnitude less than global industrial sulfur pollution.
  • Risks to the ocean
    • Changes in ocean temperatures will lag behind changes in atmospheric temperatures due to high specific heat capacity of water.
    • Changes in ocean circulation patterns due to changes in energy flux with reduced atmospheric temperature (McCormack et al. 2016)
    • Note that risks are higher with a large SAI deployment at once versus a ramped up or phased in approach (UNEP 2023).
    • Stratospheric Aerosol Injection could inadvertently seed cirrus clouds due to 1) sedimenting aerosols influencing cirrus formation (Cziczo et al. 2019) and 2) heating of the upper troposphere / lower stratosphere region, decreasing cirrus coverage (e.g., Kuebbeler et al. 2012, Visioni et al. 2018).

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to species.
• Risks
  • Changes in light regimes may impact species
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein)
    • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein); UV causes photo-inhibition, can damage organisms, reducing survival and growth
  • Changes in hydrological cycles can impact species
    • Species can be impacted by changes in precipitation
  • Species can be impacted by acid precipitation

• Co-benefits
  • Mitigation of climate change impacts described above may provide benefits to ecosystems.
  • Increase in diffuse light may increase photosynthetic efficiency and carbon fixation for some species (Russell et al. 2012)
• Risks
  • Changes in light regimes may impact ecosystems
    • Increases in ratio of diffuse to direct light can affect photosynthesis (NASEM 2021, Zarnetske et al. 2021 and references therein)
    • Decreases in PAR may change the depth of the deep chlorophyll maximum with unknown consequences for marine ecosystems (Russell et al. 2012). However, changes in PAR would need to be significant and over long durations of time to disrupt widespread primary productivity in pelagic ecosystems.
  • Increased UV radiation from ozone loss could impact species (Zarnetske et al. 2021 and references therein). However, there would also be direct UV scattering from the sulfate aerosols, potentially mitigating this risk.
    • UV radiation causes photo-inhibition, can decrease productivity, changes in diffuse light as well as UV could lead to changes in phenology, functional traits, geographic ranges of communities and impact carbon uptake. These immediate effects would impact terrestrial plants and well as marine phytoplankton communities but would have cascading impacts up food webs.
  • Changes in hydrological cycles can impact ecosystems
    • Ecosystems can be impacted by changes in precipitation
  • Ecosystems can be impacted by acid precipitation
  • Risks to ocean ecosystems
    • Ocean heating, deoxygenation, and sea level rise in the neartime will continue to happen even with SAI (Zarnetske et al. 2021).
    • Unknown changes to ocean biogeochemistry (Zarnetske et al. 2021). Cooling from SAI could alter nutrient cycling and primary production which may increase CO₂ accumulation in the oceans and atmosphere (Zarnetske 2021).

• Co-benefits
  • SAI deployment could reduce permafrost thaw and carbon release which would avoid economic losses around $8.4 trillion by 2070 and conserve indigenous habits and lifestyles (Chen et al. 2020).
• Risks
  • Increase in ratio of diffuse to direct light can impact solar energy production, reducing concentrating solar power output by about 6% (NASEM 2021)
  • Impeded astronomical observations (Russell et al. 2012)
  • Hazy appearance to sky (Russell et al. 2012)
  • Potential health impacts from pollution, but estimate highly uncertain (reviewed in Irvine et al. 2016).
  • Geopolitical risks
    • Potential for international conflicts due to transboundary effects (UNEP 2023)
    • Unilateral deployment (UNEP 2023)
    • Security issues may arise (C2G 2021 Policy Brief)
    • Plans to deploy SAI will likely strain international relationships (C2G 2021 Policy Brief)

• The stratosphere is relatively stable and aerosols can remain for one to a few years before being transported to the troposphere and eventually removed by sedimentation and precipitation (Irvine et al. 2016, NASEM 2021).
• Climate state estimated to return to without SAI within a decade (Berdahl et al. 2014). However, this is difficult to model, especially considering tipping points and non-linearities in the earth system.

• Increases in temperature would happen within months of termination (Russell et al. 2012, Trisos et al. 2018)
• Climate state returns to without SAI within a decade (Berdahl et al. 2014); any sea ice retention from SAI would go away within this time.
• To avoid termination shock would need to phase out SAI gradually over decades, but unplanned interruption could be disastrous and any implementation would need to have sufficient safeguards (reviewed in Irvine et al. 2016, UNEP 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building.
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize procedural justice (Morrow 2019) and Indigenous self-determination (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible.
    • Include social scientists with engagement expertise on research teams during the research design process.
    • Don’t presuppose what communities will be concerned about.
    • Develop a plan to be responsive to community concern.
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize procedural justice (Morrow 2019) and Indigenous self-determination (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible.
    • Include social scientists with engagement expertise on research teams during the research design process.
    • Don’t presuppose what communities will be concerned about.
    • Develop a plan to be responsive to community concern.
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize procedural justice (Morrow 2019) and Indigenous self-determination (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible.
    • Include social scientists with engagement expertise on research teams during the research design process.
    • Don’t presuppose what communities will be concerned about.
    • Develop a plan to be responsive to community concern.
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize procedural justice (Morrow 2019) and Indigenous self-determination (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize procedural justice (Morrow 2019) and Indigenous self-determination (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• See DSG (2023), A justice-based analysis of solar geoengineering and capacity building
• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).
• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
• Procedural justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
    • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
    • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
    • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
  • Specific to Stratospheric Aerosol Injection:
    • Grasso (2022) provides two criteria of procedural justice of SAI:
      • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
      • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
    • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
• Restorative justice
  • Applicable to all approaches within Solar Radiation Modification:
    • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
    • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
  • Specific to Stratospheric Aerosol Injection:
    • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).

• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
  • Procedural justice
    • Applicable to all approaches within Solar Radiation Modification:
      • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
      • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
      • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
      • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
    • Specific to Stratospheric Aerosol Injection:
      • Grasso (2022) provides two criteria of procedural justice of SAI:
        • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
        • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
      • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
    • Restorative justice
      • Applicable to all approaches within Solar Radiation Modification:
        • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
        • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
      • Specific to Stratospheric Aerosol Injection:
        • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability – Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership, and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).

• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
  • Procedural justice
    • Applicable to all approaches within Solar Radiation Modification:
      • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
      • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
      • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
      • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
    • Specific to Stratospheric Aerosol Injection:
      • Grasso (2022) provides two criteria of procedural justice of SAI:
        • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
        • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
      • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
    • Restorative justice
      • Applicable to all approaches within Solar Radiation Modification:
        • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
        • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
      • Specific to Stratospheric Aerosol Injection:
        • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior, and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability - Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).

• Applicable to all approaches within Solar Radiation Modification:
  • International regulations would likely need to be considered for all Solar Radiation Modification approaches as transboundary effects are likely, especially for larger field experiments and deployment, dependent on scale and area of application. Some research activities may fall under national jurisdiction.
• Specific to Stratospheric Aerosol Injection:
  • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There is no formal governance framework for this approach (UNEP 2023). Governance efforts to date have been scattered and ad hoc (NASEM 2021). Governance is needed for at least two different levels: research and deployment (DSG).
    • The National Academy of Sciences’ (2021) report “Reflecting Sunlight: Recommendations for solar geoengineering research and research governance landscape” provides an overview of laws and international treaties that might apply to SRM. These include:
      • Domestic Law
        • US National Environmental Policy Act and state analogs
        • US Weather Modification Reporting Act and state analogs
        • Regulatory statutes
        • Tort Liability
        • Intellectual property law
      • International Environmental Law
        • • Treaty Law
            • UN Convention on Biological Diversity
            • London Convention/London Protocol
            • UN Framework Convention on Climate Change
            • Vienna Convention and Montreal Protocol
            • Convention on Long-Range Transboundary Air Pollution (CLRTAP)
            • Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques (ENMOD)
            • UN Convention on the Law of the Sea
          • Customary International Law and Principles
            • Prevention of transboundary harm principle
            • Principle of intergenerational equity
            • The precautionary principle
            • Sustainable development goals
  • It will be important to distinguish small-scale perturbation experiments without climate relevance versus larger-scale testing that may be indistinguishable from deployment.
  • NASEM (2021) provides a proposed framework and approach for SRM research and governance, which emphasis engagement, input, and assessment. This includes exit ramps – “criteria and protocols for terminating research programs or areas” (NASEM 2021).
  • A report by the Climate Overshoot Commission (2023) calls for research on SRM and governance discussions as well as moratorium on SRM deployment and large-scale outdoor experiments.
  • UNESCO World Commission on the Ethics of Scientific Knowledge and Technology’s (COMEST) 2023 Report on the ethics of climate engineering has a slate of recommendations related to SRM covering governance, participation and inclusion, role of scientific knowledge and research strengthening capacity, and education, awareness, and advocacy.
  • In the absence of a governance framework there have been calls for governments to prohibit the development and deployment of SRM (Gupta et al. 2024). There is a need for governments to discuss coordination of research governance (Jinnah et al. 2024b).
  • An independent advisory committee for Harvard University’s Stratospheric Controlled Perturbation Experiment (SCoPEX) applied a research governance framework to the SCoPEx proposal detailed in the advisory committee’s final report (Jinnah et al. 2024a), which may inform future governance of outdoor experiments; this framework could potentially be applied to other atmospheric SRM approaches.
• Specific to Stratospheric Aerosol Injection:
  • The UN could potentially lead a global conversation on SAI leading to governance considerations and scientific assessment (UNEP 2023), however, a resolution on SRM at UNEA6 was withdrawn and did not have support for further movement (Biermann and Gupta 2024).
  • One proposal is to reach global consensus on a temperature target (through an unknown mechanism), then for states to agree how much SAI would be deployed (Ricke et al. 2013).
  • Outdoor research could be governed by frameworks established specifically for SAI, or by frameworks that regulate input of materials into atmosphere or stratosphere (UNEP 2023).
    • In order to detect a global climate response to SAI for research purposes, the forcing needed would likely be similar to deployment (>0.1 W/m² Duffey et al. 2023 and references therein). Framing governance in terms of the potential impacts of research (i.e., whether or not the research may meaningfully impact the environment), rather than the intent of the activities, may be path forward (SilverLining 2019).
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.
  • SilverLining (2023) provides a roadmap for research for SRM activities in general with a focus on SAI and MCB in the United States.

• Here we define justice related to approaches to slow the loss of Arctic sea ice through distributive justice, procedural justice, and restorative justice. Following COMEST (2023), we consider questions of ethics through a justice lens. Note that this is not an exhaustive list of justice dimensions and as the field advances, so will the related considerations and dimensions.
• General comment on justice: “A well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination” (Morrow 2019).

• Distributive justice
  • Applicable to all approaches within Solar Radiation Modification:
    • Impacts from solar geoengineering have potential to cause disproportionate harm to those least responsible for climate change (DSG 2023). It is also possible that communities most exposed or vulnerable to climate hazards receive the most benefit, depending on the deployment. There is concern from vulnerable populations that research will overlook local needs and worsen global inequities (C2G 2021 Evidence Brief). There is an urgent need for justice-based recommendations (DSG 2023).
  • Specific to Stratospheric Aerosol Injection:
    • No additional information
  • Procedural justice
    • Applicable to all approaches within Solar Radiation Modification:
      • If procedural justice is considered, people affected by research would have an opportunity to participate and have a say in how the approach will be researched, deployed, and governed. Because these approaches have global ramifications, procedural justice will be challenging (Preston 2013).
      • Efforts to support procedural justice for SRM in general to date have been inadequate (DSG 2023). Because of uncertainty in outcomes, variable interests, and the potential for wide-ranging effects, procedural justice is critical (DSG 2023).
      • Diversity within the SRM research community has generally been lacking (NASEM 2021). To address this, some organizations have supported participation in research for the Global South, but there has been a lack of attention on support for participation in governance (DSG 2023). Therefore, organizations and people may understand the science of an approach but not know how to translate their interests around the issue into policy, which is a significant justice gap (DSG 2023).
      • Bennett et al. (2022) suggests an inclusive governance approach that incorporates stakeholder concerns in the design and deployment of approaches and effectively communicates risk. Within the development of such a framework there is an opportunity to prioritize Indigenous self-determination and procedural justice (Chuffart et al. 2023). Note, however, that stakeholders may also include non-local people.
    • Specific to Stratospheric Aerosol Injection:
      • Grasso (2022) provides two criteria of procedural justice of SAI:
        • impartiality – “the involvement in SAI of agents, all of whom having parity of participation”
        • equality of opportunity – “all agents must have the same opportunity to fully understand the issues at stake in SAI”
      • It’s also recommended to continuously check, recalibrate, and contextualize procedural justice and standards (Grasso 2022).
    • Restorative justice
      • Applicable to all approaches within Solar Radiation Modification:
        • If restorative justice is considered, plans would be developed for those who could be harmed by the approach to be compensated, rehabilitated, or restored (Preston 2013).
        • Horton and Keith (2019) proposed an international climate risk insurance pool where states supporting SRM deployment support the pool and opposing states would be insured against SRM risks.
      • Specific to Stratospheric Aerosol Injection:
        • No additional information

• Applicable to all approaches within Solar Radiation Modification:
  • There have been a series of open letters from academics and others that reject (Call for Non-Use Agreement) or support (Importance of Research on SRM, Call for Balance) solar radiation modification research, showing the current varying opinions that are shaping public perception.
  • The SCoPEx independent advisory committee offered four core principles for societal engagement related to solar radiation modification:
    • Start engagement efforts as early as possible
    • Include social scientists with engagement expertise on research teams during the research design process
    • Don’t presuppose what communities will be concerned about
    • Develop a plan to be responsive to community concern
  • A recent study on public perceptions found that people surveyed in the Global South were generally more supportive of research and development into SRM technologies compared to those from the Global North (Baum et al. 2024). Those from the Global South also expressed concern about unequal distribution of risks between rich and poor countries (Baum et al. 2024).
• Specific to Stratospheric Aerosol Injection:
  • There have been a number of public opinion research papers, including some with deliberative elements (e.g., Bolsen et al. 2023, Rosenthal et al. 2023). There have also been many educational/outreach events not resulting in any kind of formal publication. While this approach is receiving increasingly more attention, people are generally unfamiliar with SAI and are susceptible to framing (Raimi et al. 2021, Bolsen et al. 2023). A low level of public engagement with SAI leads to a strong influence of media articles, with more negative arguments about SAI leading to negative public opinions (Bolsen et al. 2023).
    • One Arctic-focused study by Mettiäinen et al. (2022) was conducted in Finland engaging public in SAI research. Findings showed Arctic residents were concerned with impacts of SAI and the decision-making process, and focus group participants prioritized mitigation over SAI (Mettiäinen 2022).
  • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).
  • Exposure to information about climate change increased public support for research and deployment of SAI in Singapore and the United States (Rosenthal et al. 2023).

• Applicable to all approaches within Solar Radiation Modification:
  • The principle of free, prior and informed consent (FPIC) in the United Nations Declaration on the Rights of Indigenous Peoples (UNDRIP) is the foundation for engagement with Indigenous Peoples.
  • Particular to any potential Arctic research or deployment, The Inuit Circumpolar Council (2022) has published Circumpolar Inuit Protocols for Equitable and Ethical Engagement, which include eight protocols:
    • ‘Nothing About Us Without Us’ – Always Engage with Inuit
    • Recognize Indigenous Knowledge in its Own Right
    • Practice Good Governance
    • Communication with Intent
    • Exercising Accountability - Building Trust
    • Building Meaningful Partnerships
    • Information, Data Sharing, Ownership and Permissions
    • Equitably Fund Inuit Representation and Knowledge
  • Any meaningful engagement with Indigenous peoples needs to consider context. Whyte (2018) states, “Indigenous voices should be involved in scientific and policy discussions of different types of geoengineering. But, context matters. Geoengineering discourses cannot just be associated with geoengineering to the exclusion of topics and solutions that Indigenous peoples value.”
• Specific to Stratospheric Aerosol Injection:
  • Despite being widely mentioned as a group affected by climate-altering techniques, indigenous people have not been visible in debates about the future of SAI (C2G 2021 Evidence Brief).
  • Lack of consultation and concern over the impacts of SAI led the Saami Council to actively oppose SAI research. The Saami Council has taken an open stance against stratospheric aerosol injection (SAI) (solar radiation management) calling to halt all SAI experiments, research and development of the technology globally (Saami Council 2021a,b,c,d, Oksanen 2023, Risse 2023).  In March 2021, the Saami Council, wrote an open letter to the Stratospheric Controlled Perturbation Experiment (SCoPEx) Advisory Board. SCoPEX was a project by Harvard University that was to test stratospheric aerosol injection technology in Sweden. In the letter the Saami Council (2021a) calls for cancellation of the planned test flight in Kiruna, Sweden, and states that SAI research and technology development “must not be advanced in the absence of full, global consensus on its acceptability”. The root cause of the position of the Saami Council and environmental civil society groups was “the manner in which the SCoPEx project's test flight was planned on Sámi domicile area, without any consultations” (Oksanen 2023).
    • Key arguments in the letter are:
      • Artificial manipulation of the environment by SAI technology.
      • Contradicting life in harmony with Mother Nature.
      • Failure of SAI to address the core issues of climate change and instead offers a “quick fix”.
      • The de facto moratorium on climate related geoengineering under the Convention of Biological Diversity showing that global conversation is required before approvals for testing of the technology.
      • The Intergovernmental Panel on Climate Change (IPCC) identifies large risks and uncertainties, including the danger of termination shock (de Coninck et al. 2018).
      • Failure to address power imbalances underpinning injustices and the expert-elite technocratic setting in the Global North.
      • Risk of weaponization of SAI technology.
      • Offering a false solution that distracts from addressing root cause to climate change.
      • Unknown risks of solar geoengineering technology risking the future.
    • Sweden’s space agency subsequently cancelled the field tests (Bennett et al. 2022).