• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect is estimated to be warming of 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation.
  • By seeding cirrus clouds, the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017).
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in the Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect is estimated to be warming of 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017).
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect is estimated to be warming of 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect is estimated to be warming of 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect (warming + cooling) is estimated to be 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect (warming + cooling) is estimated to be 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of solar insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect (warming + cooling) is estimated to be 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

• Cirrus Cloud Thinning (CCT) is a strategy for modifying the properties of high-altitude clouds (between 5 and 18 km altitude; Gasparini et al. 2020) in the upper troposphere, increasing the atmosphere’s transparency to outgoing thermal radiation (NASEM 2021). Cirrus clouds have a warming effect from absorption of outgoing longwave radiation as well as a cooling effect by reflecting shortwave radiation. The net effect (warming + cooling) is estimated to be 2-5 W/m² based on estimates from observations and climate models (Gasparini and Lohmann 2016, Hong et al. 2016, L’Ecuyer et al. 2019, Matus & L’ecuyer 2017). CCT focuses on increasing outgoing longwave radiation to decrease the warming effect of cirrus clouds. This can be achieved either by reducing their coverage or optical thickness (Gasparini and Lohmann 2016). Thinning happens by seeding clouds with ice nuclei that facilitate the growth of fewer and larger ice crystals, and CCT is sometimes referred to as Cirrus Cloud Seeding. There has been some exploration into potential substances for CCT including Bismuth triiodide (BiI₃), which is inexpensive and nontoxic, as well as naturally occurring mineral dust (Lohmann and Gasparini 2017, cites Mitchell and Finnegan 2009). CCT and mixed-phase cloud thinning (MPCT) are the only proposed technologies that would result in direct local cooling of the Arctic during winter, which is when Arctic amplification is strongest (Cohen et al. 2014, reviewed in Duffey et al. 2023)

• Expected physical changes (global)
  • Increase outgoing longwave thermal radiation
  • By seeding cirrus clouds the goal is to form heterogeneous cirrus clouds instead of homogenous cirrus clouds. That shift in formation mechanisms leads to a cooling effect with four components:
    • When cirrus clouds form at lower altitudes with CCT their warming effect decreases (Lohmann and Gasparini 2017)
    • Clouds formed with CCT have fewer larger ice crystals and sediment more readily which reduces optical thickness and lifetime of cirrus clouds.
    • Removes water vapor, a potent greenhouse gas, from the upper troposphere more effectively (Lohmann and Gasparini 2017).
    • Heterogeneously formed/seeded cirrus clouds are more transparent for solar and thermal radiation. The last effect dominates on average, decreasing their overall climate warming effect.

• Unknown
  • Targeting areas of large frequency of homogeneous cirrus clouds such as mountain regions (Gryspeerdt et al. 2018) and/or the Southern Ocean.
    • Suggestions for application around the size of Chile, in mountain regions where homogeneous cirrus clouds dominate, the Southern Ocean, Greenland and the air downstream of Greenland, mountains in Scandinavia, Arctic and Antarctic regions (B. Gasparini pers. comm., Froyd et al. 2022, Mitchell et al. 2018).

• In the atmosphere in the upper troposphere (5-18 km), where temperatures are colder than 35°C.

• Potentially areas of cirrus clouds in the Arctic (or Antarctica, i.e. high latitudes)
  • Arctic deployment could be effective because of low concentrations of existing natural ice nuclei (Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), but further research is needed to determine if seeding in the Arctic would be effective.
• If the right conditions existed, this technique could be scaled globally or applied regionally, similar to marine cloud brightening.

• Winter
  • CCT would be most effective during winter in Arctic when clouds have only a warming effect due to the lack of insolation (Gasparini and Lohmann 2016, Storelvmo and Herger 2014 reviewed in Duffey et al. 2023), assuming conditions are appropriate for seeding.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024).
  • Potential for increased albedo due to sea ice lasting longer.

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and, how many particles, are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • A modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat-trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • A modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • A modeling study of Northern Hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • An Arctic-specific modeling study reports cooling but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to the reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change.
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2,000,000 km² in sea ice area (Storelvmo et al. 2014).

Scalability

• Unknown
  • Depends on the current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024).
  • Potential for increased albedo due to sea ice lasting longer.

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2,000,000 km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2,000,000 km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10^⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km²  in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km²  in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km²  in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km²  in sea ice area (Storelvmo et al. 2014)
  • 10⁶km

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2×10⁶ km²  in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2x10^6 km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2x10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2x10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).

• Direct or indirect impact on sea ice?
  • Indirect via temperature change
• New or old ice?
  • Both
• Impact on sea ice
  • Increases of 2x10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

Impact on

• Unknown
  • Possibility of reduced albedo from CCT (Shi et al. 2024)
  • Potential for increased albedo due to sea ice lasting longer

• Global
  • Temperature decrease of 0-2.0°C, with potential for warming
    • Modeling results are inconsistent, and some have been irreproducible. Differences exist between models in terms of how cirrus clouds are represented. A recent modeling study found no effect of CCT and a potential for warming due to a propensity for overseeding.
      • When testing maximum response of CCT in Gasparini et al. 2020 (seeding all cirrus clouds with uniform concentration) results varied between global temperature decreases of 0.6°C -1.4°C depending on which model was used. The two models represent cirrus clouds differently.
      • More updated models (Tully et al. 2022, 2023) found no effect of seeding and also warming from overseeding. Discrepancies have to do with size of particles and how many particles, and are based on optimizing conditions for nucleation.
• Arctic region
  • Varies; contrasting results report increases and decreases in temperature.
    • Modeling study reported decreased temperatures throughout the Arctic region > 2°C (Storelvmo et al. 2014). This study seeded the upper troposphere (areas with temperatures below -35°C) in areas with either the 15% or 45% largest solar zenith angle at noon. This type of deployment avoids low latitude areas where the reflective contribution of cirrus clouds might outweigh their heat trapping contribution. This study reported mean temperature decrease and did not distinguish between summer and winter.
    • Modeling study of globally uniform seeding in areas with temperatures colder than -35°C in the winter reported temperature decrease in the Arctic during winter of 2-4°C (Gasparini et al. 2017). Summer seeding yielded slight warming to cooling <1°C (Gasparini et al. 2017).
    • Modeling study of Northern hemisphere deployment during winter reported temperature increase of 1°C (Tully et al. 2023).
    • Arctic-specific modeling study reports cooling, but does not report specific temperature change in °C (Gruber et al. 2019).

• Global
  • Decrease of 3 to increase of 2.1 W/m²
    • A realistic upper limit on potential decreases in radiative forcing from CCT is 2-3 W/m² (Lohmann and Gasparini 2017, Liu and Shi 2021).
    • CCT can also lead to increases in radiative forcing of +2.1 W/m² because of its overseeding potential (Lohmann and Gasparini 2017).
    • A climate modeling study that used two different models reported decreases in global radiative forcing of 0.8 W/m² and 1.8W/m² (Gasparini et al. 2020). The models represented cirrus clouds differently.
    • A more recent model (Tully et al. 2023) found increased radiative forcing from overseeding.
• Arctic region
  • Varies, net negative or net positive change
    • Modeled Northern Hemisphere deployment reports both positive and negative net changes to the radiative balance depending on seeding particle radius and seeding concentrations (Tully et al. 2023). The negative changes were not significant.
    • Arctic-specific study with high resolution reports increases in longwave radiation with an overall cooling effect (Gruber et al. 2019). Additional cooling is found due to reduction of mixed-phase clouds (Gruber et al. 2019).
• Sea ice
  • Direct or indirect impact on sea ice?
    • Indirect via temperature change
  • New or old ice?
    • Both
  • Impact on sea ice
    • Increases of 2x10⁶ km² in sea ice area (Storelvmo et al. 2014)

Scalability

• Unknown
  • Depends on current and future distribution of where/when homogenous versus heterogeneous nucleation occurs.

• Unknown

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

• >20 yrs
  • Modeling studies show inconsistent predictions (Duffey et al. 2023).

Cost

• Unknown
  • Cost will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• Unknown
  • CO₂ footprint will depend on the size of the emitted aerosol, which determines the mass of material needed to loft. This will also depend on where deployment happens and if it’s in a region that may be effective.

• 2 – Most research has been on implementing CCT in climate models.
• Summary of existing literature and studies:
  • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high-altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
  • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).

• Not feasible within 10 yrs.

• 2 – most research has been on implementing CCT in climate models
• Summary of existing literature and studies:
  • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
  • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).

• Not feasible within 10 yrs

• • 2 – most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).

• • Not feasible within 10 yrs

• TRL
  • 2 – most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).
• Technical feasibility within 10 yrs
  • Not feasible within 10 yrs

• TRL
  • 2 – most Most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).
• Technical feasibility within 10 yrs
  • Not feasible within 10 yrs

• TRL
  • 2 – most Most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).
• Technical feasibility within 10 yrs
  • Not feasible within 10 yrs

• TRL
  • 2 – most Most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).
• Technical feasibility within 10 yrs
  • Not feasible within 10 yrs

• TRL -- 2
  • Most research has been on implementing CCT in climate models
  • Summary of existing literature and studies:
    • Cloud seeding in the lower atmosphere is already done for weather modification but has not been applied to high altitude cirrus clouds (Villanueva et al. 2022). See NASEM (2003) for an assessment of weather modification and a more recent review by the World Meteorological Organization’s Expert Team on Weather Modification that reviewed precipitation enhancement research (Flossmann et al. 2019).
    • Modeling studies have been conducted with inconsistent results (reviewed in Duffey et al. 2023).
• Technical feasibility within 10 yrs
  • Not feasible within 10 yrs

Physical and chemical changes

• Co-benefits
  • Removes water vapor, a potent greenhouse gas, from the upper troposphere (Lohmann and Gasparini 2017). See also an approach proposed by Schwarz et al. (2024) for intentional stratospheric dehydration.
  • Potential additional cooling due to reduction of mixed-phase clouds if done at night (Gruber et al. 2019).
• Risks
  • Warming could happen if clouds are overseeded (Gasparini and Lohmann 2016 and others cited in Duffey et al. 2023).
  • If seed in area where no cirrus clouds form could lead to a warming effect, similar to contrails (Lohmann and Gasparini 2017).
  • If applied globally could intensify tropical convection (Lohmann and Gasparini 2017).
  • Potential shift in Intertropical Convergence Zone (ITCZ) if seeding preferentially cools one hemisphere; the ITCZ would shift toward the warmer hemisphere (Gasparini et al. 2020).
  • Potential for extreme regional precipitation (Kristjánsson et al. 2015, Gasparini et al. 2020).
  • 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).
  • Potential for indirect effects on low clouds (Liu and Shi 2021).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • Unknown

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • Unknown

Impacts on society

• Co-benefits
  • Research into CCT will help us understand cirrus cloud formation (Lohmann and Gasparini 2017).
• Risks
  • Unknown

Ease of reversibility

• Easy
  • Tropospheric aerosols have a lifetime of days (lower troposphere; Parker and Irvine 2018) to weeks (upper troposphere). Lifetimes of tropospheric aerosols are shorter than those in the stratosphere.

Risk of termination shock

• High
  • Temperatures would return to that without intervention in weeks (Parker and Irvine 2018).

Physical and chemical changes

• Co-benefits
  • Removes water vapor, a potent greenhouse gas, from the upper troposphere (Lohmann and Gasparini 2017). See also an approach proposed by Schwarz et al. (2024) for intentional stratospheric dehydration.
  • Potential additional cooling due to reduction of mixed-phase clouds if done at night (Gruber et al. 2019).
• Risks
  • Warming could happen if clouds are overseeded (Gasparini and Lohmann 2016 and others cited in Duffey et al. 2023).
  • If seed in area where no cirrus clouds form could lead to a warming effect, similar to contrails (Lohmann and Gasparini 2017).
  • If applied globally could intensify tropical convection (Lohmann and Gasparini 2017).
  • Potential shift in Intertropical Convergence Zone (ITCZ) if seeding preferentially cools one hemisphere; the ITCZ would shift toward the warmer hemisphere (Gasparini et al. 2020).
  • Potential for extreme regional precipitation (Kristjánsson et al. 2015, Gasparini et al. 2020).
  • 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).
  • Potential for indirect effects on low clouds (Liu and Shi 2021).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • Unknown

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • Unknown

Impacts on society

• Co-benefits
  • Research into CCT will help us understand cirrus cloud formation (Lohmann and Gasparini 2017).
• Risks
  • Unknown

Ease of reversibility

• Tropospheric aerosols have a lifetime of days (lower troposphere; Parker and Irvine 2018) to weeks (upper troposphere). Lifetimes of tropospheric aerosols are shorter than those in the stratosphere.

Risk of termination shock

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018).

• Co-benefits
  • Removes water vapor, a potent greenhouse gas, from the upper troposphere (Lohmann and Gasparini 2017). See also an approach proposed by Schwarz et al. (2024) for intentional stratospheric dehydration.
  • Potential additional cooling due to reduction of mixed-phase clouds if done at night (Gruber et al. 2019).
• Risks
  • Warming could happen if clouds are overseeded (Gasparini and Lohmann 2016 and others cited in Duffey et al. 2023).
  • If seed in area where no cirrus clouds form could lead to a warming effect, similar to contrails (Lohmann and Gasparini 2017).
  • If applied globally could intensify tropical convection (Lohmann and Gasparini 2017).
  • Potential shift in Intertropical Convergence Zone (ITCZ) if seeding preferentially cools one hemisphere; the ITCZ would shift toward the warmer hemisphere (Gasparini et al. 2020).
  • Potential for extreme regional precipitation (Kristjánsson et al. 2015, Gasparini et al. 2020).
  • 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).
  • Potential for indirect effects on low clouds (Liu and Shi 2021).

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Research into CCT will help us understand cirrus cloud formation (Lohmann and Gasparini 2017).
• Risks
  • Unknown

• Tropospheric aerosols have a lifetime of days (lower troposphere; Parker and Irvine 2018) to weeks (upper troposphere). Lifetimes of tropospheric aerosols are shorter than those in the stratosphere.

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018).

• Co-benefits
  • Removes water vapor, a potent greenhouse gas, from the upper troposphere (Lohmann and Gasparini 2017). See also an approach proposed by Schwarz et al. (2024) for intentional stratospheric dehydration.
  • Potential additional cooling due to reduction of mixed-phase clouds if done at night (Gruber et al. 2019).
• Risks
  • Warming could happen if clouds are overseeded (Gasparini and Lohmann 2016 and others cited in Duffey et al. 2023)
  • If seed in area where no cirrus clouds form could lead to a warming effect, similar to contrails (Lohmann and Gasparini 2017)
  • If applied globally could intensify tropical convection (Lohmann and Gasparini 2017).
  • Potential shift in Intertropical Convergence Zone (ITCZ) if seeding preferentially cools one hemisphere; the ITCZ would shift toward the warmer hemisphere (Gasparini et al. 2020).
  • Potential for extreme regional precipitation (Kristjánsson et al. 2015, Gasparini et al. 2020).
  • 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).
  • Potential for indirect effects on low clouds (Liu and Shi 2021).

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Research into CCT will help us understand cirrus cloud formation (Lohmann and Gasparini 2017).
• Risks
  • Unknown

• Tropospheric aerosols have a lifetime of days (lower troposphere; Parker and Irvine 2018) to weeks (upper troposphere). Lifetimes of tropospheric aerosols are shorter than those in the stratosphere.

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018).

• Co-benefits
  • Removes water vapor, a potent greenhouse gas, from the upper troposphere (Lohmann and Gasparini 2017). See also an approach proposed by Schwarz et al. (2024) for intentional stratospheric dehydration.
  • Potential additional cooling due to reduction of mixed-phase clouds if done at night (Gruber et al. 2019).
• Risks
  • Warming could happen if clouds are overseeded (Gasparini and Lohmann 2016 and others cited in Duffey et al. 2023)
  • If seed in area where no cirrus clouds form could lead to a warming effect, similar to contrails (Lohmann and Gasparini 2017)
  • If applied globally could intensify tropical convection (Lohmann and Gasparini 2017).
  • Potential shift in Intertropical Convergence Zone (ITCZ) if seeding preferentially cools one hemisphere; the ITCZ would shift toward the warmer hemisphere (Gasparini et al. 2020).
  • Potential for extreme regional precipitation (Kristjánsson et al. 2015, Gasparini et al. 2020).
  • 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).
  • Potential for indirect effects on low clouds (Liu and Shi 2021).

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Unknown
• Risks
  • Unknown

• Co-benefits
  • Research into CCT will help us understand cirrus cloud formation (Lohmann and Gasparini 2017)
• Risks
  • Unknown

• Tropospheric aerosols have a lifetime of days (lower troposphere; Parker and Irvine 2018) to weeks (upper troposphere). Lifetimes of tropospheric aerosols are shorter than those in the stratosphere.

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018).

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information.
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement.
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
  • OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
    • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
• OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
  • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
• OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
  • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown

• 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 Cirrus Cloud Thinning, Mixed-Phase Cloud Thinning, and Marine Cloud Brightening (Global and Arctic):
  • For activities occurring over/in the ocean:
    • If occurring within Exclusive Economic Zone would be governed by State (C2G 2021 Evidence Brief).
    • In International waters customary international law applies (C2G 2021 Evidence Brief).

• 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 Cirrus Cloud Thinning:
  • The Arctic Council has been called upon as a venue for providing oversight on regional approaches to slow the loss of Arctic sea ice, or to establish working groups to provide guidance (Bodansky and Hunt 2020, Bennett et al. 2022). However, the current geopolitical landscape and lack of participation from Russia makes consensus difficult.
• OSTP (2023) provides a research governance framework and research plan for United States SRM activities, focusing on SAI, CCT, and MCB.

• 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 Cirrus Cloud Thinning:
    • Unknown
      • CCT deployment may impact a smaller area than SAI deployment and impacts, costs, and benefits will vary depending on region and context.
• 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 Cirrus Cloud Thinning:
    • No additional information
• 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 Cirrus Cloud Thinning:
  • 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 Cirrus Cloud Thinning:
  • Very little or no engagement
    • Substate actors could play an important role in public engagement and integration of outputs from engagement into research and governance (Jinnah et al. 2018).

• 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 Cirrus Cloud Thinning:
  • Unknown