• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic.

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter.
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km².

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022).
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022).

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic.

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter.
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km².

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022).
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022).

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic.

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter.
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km².

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022).
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022).

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter.
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km².

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022).
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022).

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter.
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km².

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022).
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022).

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km²

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022)
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022)

• High altitude clouds, which are ice clouds, warm the Earth by trapping heat (longwave radiation) instead of it escaping to space. Low altitude clouds are usually liquid and cool the Earth by reflecting solar radiation (shortwave radiation). There are also low to mid altitude clouds where cloud supercooled water and ice co-exist - mixed-phase clouds (MPCs) where both heat trapping and reflection can occur. During polar night or bright ice, the longwave radiative effects are most important. In mixed-phase cloud thinning (MPCT), a similar approach to cirrus cloud thinning (CCT) would be used on MPCs during winter using ice-nucleating particles to seed clouds to reduce their heat trapping effect. MPCs occur most frequently at high latitudes (Zhang et al. 2018 cited in Villanueva et al. 2022). To be effective, this technique can only be done at the poles during winter because there is no, or very little, solar radiation. In other seasons and locations, MPCs produce a close to neutral or net cooling effect and thus decreasing their coverage would lead to a warming impact on climate. This technique is focused on shallow MPCs, usually in the lower 3 km on the atmosphere, as opposed to deep (or convective) MPCs such as thunderstorms that are abundant in tropical regions. The seeding done with MPCT enhances the seeding that already occurs naturally with natural dust (Villanueva et al. 2022) and in some high latitude ship tracks (Christensen et al. 2014). Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022). This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947, Flossmann et al. 2019).

• Increase outgoing thermal (longwave) radiation. It will also increase incoming solar radiation (shortwave) if applied outside of polar night.
• Expected physical changes (Arctic region)
  • Temperature decline and increase in sea ice extent when applied in Arctic

• Ocean area in high northern and southern latitudes
  • Villanueva et al. 2022 seeded area between 60° and 90°N and 30°W-90°E in winter
  • Zapalac (2023) looks at cooling entire Arctic north of 70°N, area of 15,000,000 km²

• Atmosphere / Troposphere (intermediate) at approximately 2-5 km

• Arctic Ocean area
  • Targeted Arctic deployment; MPCs are found most frequently at high latitudes (Zhang et al. 2018 in Villanueva et al. 2022)
  • MPCs have both a heat-trapping effect as well as a cooling effect (from their reflectivity). High latitude marine clouds with a net heat-trapping effect to do MPCT to produce a net cooling effect in winter (Villanueva et al. 2022).
• Southern Ocean area
  • This technique could have great cooling potential in Antarctica because there is a low concentration of naturally occurring ice nucleating particles (J. Kok pers. comm.). Modeling study by Villanueva et al. (2022) shows greater cooling in the Antarctic than in the Arctic.

• Winter
  • MPCT would be effective during winter (November to February) in the Arctic when MPCs have net positive radiative effect (Villanueva et al. 2022)

Impact on

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

• Global
  • Unknown
    • Potential for an effect based on result from modeling study in the Arctic (Villanueva et al. 2022) and connection of the Arctic to general circulation of the ocean and atmosphere.
• Arctic region
  • Annual mean temperature decrease of 1.03° ± 57°C
    • Modeling study seeded area between 60° and 90°N and 30°W-90°E in winter and found annual mean surface temperature over the Arctic Ocean decreased by 1.03° ± 57°C (Villanueva et al. 2022).
    • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).

• Global
  • Unknown
    • Likely would have some effect by reducing outgoing longwave radiation at the poles.
• Arctic region
  • Unknown

• Direct or indirect impact on sea ice?
  • Indirect via cooling / temperature decrease
• New or old ice?
  • Both
• Impact on sea ice
  • Some potential, but from a single study with an idealized seeding scenario; more research needed. 8% increase in annual Arctic sea ice surface area; 32% increase during winter.
    • Modeling study reported 8% increase in annual Arctic sea ice surface area (Villanueva et al. 2022); increases by 32% during winter and decreasing by 39% during summer.
    • In 2013 there was a 48% increase in September Arctic sea ice compared to 2012 which was attributed to a 20% reduction in cloud coverage in Jan and Feb 2013 (Lui and Key 2014 cited in Zapalac 2023).
    • Estimated that 1 W/m² of reduced forcing for 1 month would thicken sea ice by 0.85 cm (Zapalac 2023 not peer reviewed).

Scalability

• Can only be scaled to size of Arctic and Southern Ocean, will not work in lower latitudes.
  • In simulations, assuming an ice nucleating particle (INP) of 0.1 um diameter, would need injected mass of 0.15 Tg per year (Villanueva et al. 2022). Would need to get to about 2-5 km altitude, which is in reach of currently available unmanned aircraft (Axisa and DeFelice 2016 cited in Villanueva et al. 2022). This is a much lower height than for CCT or SAI.
  • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown
  • Zapalac (2023) estimates would need to be reseeded every 4 days. Seeding entire region over Arctic north of 70°N every 4 days would require at least 890 airships (Zapalac 2023). There may be other means of applying this approach.
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

Cost

• Unknown

• Unknown
  • Flights running every 4 days over Arctic for winter months.

Impact on

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

• Global
  • Unknown
    • Potential for an effect based on result from modeling study in the Arctic (Villanueva et al. 2022) and connection of the Arctic to general circulation of the ocean and atmosphere.
• Arctic region
  • Annual mean temperature decrease of 1.03° ± 57°C.
    • Modeling study seeded area between 60° and 90°N and 30°W-90°E in winter and found annual mean surface temperature over the Arctic Ocean decreased by 1.03° ±57°C (Villanueva et al. 2022).
    • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).

• Global
  • Unknown
    • Likely would have some effect by reducing outgoing longwave radiation at the poles.
• Arctic region
  • Unknown

• Direct or indirect impact on sea ice?
  • Indirect via cooling / temperature decrease
• New or old ice?
  • Both
• Impact on sea ice
  • Some potential, but from a single study with an idealized seeding scenario; more research needed. 8% increase in annual Arctic sea ice surface area; 32% increase during winter.
    • Modeling study reported 8% increase in annual Arctic sea ice surface area (Villanueva et al. 2022); increases by 32% during winter and decreasing by 39% during summer.
    • In 2013 there was a 48% increase in September Arctic sea ice compared to 2012 which was attributed to a 20% reduction in cloud coverage in Jan and Feb 2013 (Lui and Key 2014 cited in Zapalac 2023).
    • Estimated that 1 W/m² of reduced forcing for 1 month would thicken sea ice by 0.85 cm (Zapalac 2023).

Scalability

• Can only be scaled to size of Arctic and Southern Ocean, will not work in lower latitudes.
  • In simulations, assuming an ice nucleating particle (INP) of 0.1 um diameter, would need injected mass of 0.15 Tg per year (Villanueva et al. 2022). Would need to get to about 2-5 km altitude, which is in reach of currently available unmanned aircraft (Axisa and DeFelice 2016 cited in Villanueva et al. 2022). This is a much lower height than for CCT or SAI.
  • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown
  • Zapalac (2023) estimates would need to be reseeded every 4 days. Seeding entire region over Arctic north of 70°N every 4 days would require at least 890 airships (Zapalac 2023). There may be other means of applying this approach.
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

Cost

• Unknown

• Unknown
  • Flights running every 4 d over Arctic for winter months.

Impact on

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

• Global
  • Unknown
    • Potential for an effect based on result from modeling study in the Arctic (Villanueva et al. 2022) and connection of the Arctic to general circulation of the ocean and atmosphere.
• Arctic region
  • Annual mean temperature decrease of 1.03° ± 57°C
    • Modeling study seeded area between 60° and 90°N and 30°W-90°E in winter and found annual mean surface temperature over the Arctic Ocean decreased by 1.03° ±57°C (Villanueva et al. 2022).
    • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).

• Global
  • Unknown
    • Likely would have some effect by reducing outgoing longwave radiation at the poles.
• Arctic region
  • Unknown

• Direct or indirect impact on sea ice?
  • Indirect via cooling / temperature decrease
• New or old ice?
  • Both
• Impact on sea ice
  • Some potential, but from a single study with an idealized seeding scenario; more research needed. 8% increase in annual Arctic sea ice surface area; 32% increase during winter.
    • Modeling study reported 8% increase in annual Arctic sea ice surface area (Villanueva et al. 2022); increases by 32% during winter and decreasing by 39% during summer.
    • In 2013 there was a 48% increase in September Arctic sea ice compared to 2012 which was attributed to a 20% reduction in cloud coverage in Jan and Feb 2013 (Lui and Key 2014 cited in Zapalac 2023).
    • Estimated that 1 W/m² of reduced forcing for 1 month would thicken sea ice by 0.85 cm (Zapalac 2023).

Scalability

• Can only be scaled to size of Arctic and Southern Ocean, will not work in lower latitudes.
  • In simulations, assuming an ice nucleating particle (INP) of 0.1 um diameter, would need injected mass of 0.15 Tg per year (Villanueva et al. 2022). Would need to get to about 2-5 km altitude, which is in reach of currently available unmanned aircraft (Axisa and DeFelice 2016 cited in Villanueva et al. 2022). This is a much lower height than for CCT or SAI.
  • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown
  • Zapalac (2023) estimates would need to be reseeded every 4 days. Seeding entire region over Arctic north of 70°N every 4 days would require at least 890 airships (Zapalac 2023). There may be other means of applying this approach.
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

Cost

• Unknown

• Unknown
  • Flights running every 4 d over Arctic for winter months

Impact on

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

• Global
  • Unknown
    • Potential for an effect based on result from modeling study in the Arctic (Villanueva et al. 2022) and connection of the Arctic to general circulation of the ocean and atmosphere.
• Arctic region
  • Annual mean temperature decrease of 1.03° ± 57°C
    • Modeling study seeded area between 60° and 90°N and 30°W-90°E in winter and found annual mean surface temperature over the Arctic Ocean decreased by 1.03° ±57°C (Villanueva et al. 2022).
    • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022).

• Global
  • Unknown
    • Likely would have some effect by reducing outgoing longwave radiation at the poles.
• Arctic region
  • Unknown

• Direct or indirect impact on sea ice?
  • Indirect via cooling / temperature decrease
• New or old ice?
  • Both
• Impact on sea ice
  • Some potential, but from a single study with an idealized seeding scenario; more research needed. 8% increase in annual Arctic sea ice surface area; 32% increase during winter.
    • Modeling study reported 8% increase in annual Arctic sea ice surface area (Villanueva et al. 2022); increases by 32% during winter and decreasing by 39% during summer.
    • In 2013 there was a 48% increase in September Arctic sea ice compared to 2012 which was attributed to a 20% reduction in cloud coverage in Jan and Feb 2013 (Lui and Key 2014 cited in Zapalac 2023).
    • Estimated that 1 W/m² of reduced forcing for 1 month would thicken sea ice by 0.85 cm (Zapalac 2023).

Scalability

• Can only be scaled to size of Arctic and Southern Ocean, will not work in lower latitudes.
  • In simulations, assuming an ice nucleating particle (INP) of 0.1 um diameter, would need injected mass of 0.15 Tg per year (Villanueva et al. 2022). Would need to get to about 2-5 km altitude, which is in reach of currently available unmanned aircraft (Axisa and DeFelice 2016 cited in Villanueva et al. 2022). This is a much lower height than for CCT or SAI.
  • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022)
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown
  • Zapalac (2023) estimates would need to be reseeded every 4 days. Seeding entire region over Arctic north of 70°N every 4 days would require at least 890 airships (Zapalac 2023). There may be other means of applying this approach.
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

Cost

• Unknown

• Unknown
  • Flights running every 4 d over Arctic for winter months

Impact on

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

• Global
  • Unknown
    • Potential for an effect based on result from modeling study in the Arctic (Villanueva et al. 2022) and connection of the Arctic to general circulation of the ocean and atmosphere.
• Arctic region
  • Annual mean temperature decrease of 1.03° ± 57°C
    • Modeling study seeded area between 60° and 90°N and 30°W-90°E in winter and found annual mean surface temperature over the Arctic Ocean decreased by 1.03° ±57°C (Villanueva et al. 2022).
    • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022)

• Global
  • Unknown
    • Likely would have some effect by reducing outgoing longwave radiation at the poles.
• Arctic region
  • Unknown

• Direct or indirect impact on sea ice?
  • Indirect via cooling / temperature decrease
• New or old ice?
  • Both
• Impact on sea ice
  • Some potential, but from a single study with an idealized seeding scenario; more research needed. 8% increase in annual Arctic sea ice surface area; 32% increase during winter.
    • Modeling study reported 8% increase in annual Arctic sea ice surface area (Villanueva et al. 2022); increases by 32% during winter and decreasing by 39% during summer
    • In 2013 there was a 48% increase in September Arctic sea ice compared to 2012 which was attributed to a 20% reduction in cloud coverage in Jan and Feb 2013 (Lui and Key 2014 cited in Zapalac 2023)
    • Estimated that 1 W/m² of reduced forcing for 1 month would thicken sea ice by 0.85 cm (Zapalac 2023

Scalability

• Can only be scaled to size of Arctic and Southern Ocean, will not work in lower latitudes.
  • In simulations, assuming an ice nucleating particle (INP) of 0.1 um diameter, would need injected mass of 0.15 Tg per year (Villanueva et al. 2022). Would need to get to about 2-5 km altitude, which is in reach of currently available unmanned aircraft (Axisa and DeFelice 2016 cited in Villanueva et al. 2022). This is a much lower height than for CCT or SAI.
  • Unlike CCT, there is no possibility of overseeding of MCPT leading to opposite effect (Villanueva et al. 2022)
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown
  • Zapalac (2023) estimates would need to be reseeded every 4 days. Seeding entire region over Arctic north of 70°N every 4 days would require at least 890 airships (Zapalac 2023). There may be other means of applying this approach.
  • This technique is similar to precipitation enhancement which is already done around the world (e.g., Vonnegut 1947).

• Unknown

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

• Possible within 20 years
  • This technique is similar to precipitation enhancement which is already done around the world.

Cost

• Unknown

• Unknown
  • Flights running every 4 d over Arctic for winter months

• 5 – modeling studies, similar systems for deployment exist for lower altitude application, some outdoor experiments in process. From a technological perspective, this technique could be done. Uncertainty remains as to whether or not it would substantially cool the Arctic.
• Summary of existing literature and studies:
  • Studies indirectly looked at MPCT by studying ice sedimenting from CCT and through studies of shipping emissions (citations in Villanueva et al. 2022).
  • Some studies on cloud seeding with ice nucleating particles to enhance glaciation as well as triggering precipitation events or preventing hail (weather modification) (e.g., Vonnegut 1947, citations in Villanueva et al. 2022).
  • Some modeling studies (Villanueva et al. 2022 uses cloud resolving model simulations as well as a climate model).
  • One simulated technique is to have aircraft fly above clouds dropping 1 kg km^-¹ of dry ice pellets 3 mm in diameter in parallel tracks by 1.2 km creating a curtain of ice (Zapalac 2023).
  • Observational studies of MPCs beginning using existing lidar and radar data to assess cooling potential of aerosol and MPC interaction (L. Russell pers. comm.).
  • Some outdoor experiments underway, with others planned.

• Feasible within 10 years
  • Using technology that already exists; weather modification already happens via cloud seeding (e.g., Vonnegut 1947, Villanueva et al. 2022). The pellets sublimate and freeze surround water vapor creating a trail of ice crystals. 1 kg pellets can generate approximately 10¹⁵ ice crystals.
  • Aircraft already exist.

• 5 – modeling studies, similar systems for deployment exist for lower altitude application, some outdoor experiments in process. From a technological perspective, this technique could be done. Uncertainty remains as to whether or not it would substantially cool the Arctic.
• Summary of existing literature and studies:
  • Studies indirectly looked at MCT by studying looking into ice sedimenting from CCT and through studies of shipping emissions (citations in Villanueva et al. 2022).
  • Some studies on cloud seeding with ice nucleating particles to enhance glaciation as well as triggering precipitation events or preventing hail (weather modification) (e.g., Vonnegut 1947, citations in Villanueva et al. 2022).
  • Some modeling studies (Villanueva et al. 2022 uses cloud resolving model simulations as well as a climate model).
  • One simulated technique is to have aircraft fly above clouds dropping 1 kg km^-¹ of dry ice pellets 3mm in diameter in parallel tracks by 1.2 km creating a curtain of ice (Zapalac 2023).
  • Observational studies of MPCs beginning using existing lidar and radar data to assess cooling potential of aerosol and MPC interaction (L. Russell pers. comm.).
  • Some outdoor experiments underway, with others planned.

• Feasible within 10 years
  • Using technology that already exists; weather modification already happens via cloud seeding (e.g., Vonnegut 1947, Villanueva et al. 2022). The pellets sublimate and freeze surround water vapor creating a trail of ice crystals. 1 kg pellets can generate approximately 10¹⁵ ice crystals.
  • Aircraft already exist.

• • 5 – modeling studies, similar systems for deployment exist for lower altitude application, some outdoor experiments in process. From a technological perspective, this technique could be done. Uncertainty remains as to whether or not it would substantially cool the Arctic.
  • Summary of existing literature and studies:
    • Studies indirectly looked at MCT by studying looking into ice sedimenting from CCT and through studies of shipping emissions (citations in Villanueva et al. 2022).
    • Some studies on cloud seeding with ice nucleating particles to enhance glaciation as well as triggering precipitation events or preventing hail (weather modification) (e.g., Vonnegut 1947, citations in Villanueva et al. 2022).
    • Some modeling studies (Villanueva et al. 2022 uses cloud resolving model simulations as well as a climate model).
    • One simulated technique is to have aircraft fly above clouds dropping 1 kg km^-1 of dry ice pellets 3mm in diameter in parallel tracks by 1.2 km creating a curtain of ice (Zapalac 2023).
    • Observational studies of MPCs beginning using existing lidar and radar data to assess cooling potential of aerosol and MPC interaction (L. Russell pers. comm.).
    • Some outdoor experiments underway, with others planned.

• • Feasible within 10 years
    • Using technology that already exists; weather modification already happens via cloud seeding (e.g., Vonnegut 1947, Villanueva et al. 2022). The pellets sublimate and freeze surround water vapor creating a trail of ice crystals. 1 kg pellets can generate approximately 10¹⁵ ice crystals.
    • Aircraft already exist.

• TRL
  • 5 – modeling studies, similar systems for deployment exist for lower altitude application, some outdoor experiments in process. From a technological perspective, this technique could be done. Uncertainty remains as to whether or not it would substantially cool the Arctic.
  • Summary of existing literature and studies:
    • Studies indirectly looked at MCT by studying looking into ice sedimenting from CCT and through studies of shipping emissions (citations in Villanueva et al. 2022).
    • Some studies on cloud seeding with ice nucleating particles to enhance glaciation as well as triggering precipitation events or preventing hail (weather modification) (e.g., Vonnegut 1947, citations in Villanueva et al. 2022).
    • Some modeling studies (Villanueva et al. 2022 uses cloud resolving model simulations as well as a climate model).
    • One simulated technique is to have aircraft fly above clouds dropping 1 kg km^-1 of dry ice pellets 3mm in diameter in parallel tracks by 1.2 km creating a curtain of ice (Zapalac 2023).
    • Observational studies of MPCs beginning using existing lidar and radar data to assess cooling potential of aerosol and MPC interaction (L. Russell pers. comm.).
    • Some outdoor experiments underway, with others planned.
• Technical feasibility within 10 yrs
  • Feasible within 10 years
    • Using technology that already exists; weather modification already happens via cloud seeding (e.g., Vonnegut 1947, Villanueva et al. 2022). The pellets sublimate and freeze surround water vapor creating a trail of ice crystals. 1 kg pellets can generate approximately 10¹⁵ ice crystals.
    • Aircraft already exist.

• TRL
  • 5 – modeling studies, similar systems for deployment exist for lower altitude application, some outdoor experiments in process. From a technological perspective, this technique could be done. Uncertainty remains as to whether or not it would substantially cool the Arctic.
  • Summary of existing literature and studies:
    • Studies indirectly looked at MCT by studying looking into ice sedimenting from CCT and through studies of shipping emissions (citations in Villanueva et al. 2022)
    • Some studies on cloud seeding with ice nucleating particles to enhance glaciation as well as triggering precipitation events or preventing hail (weather modification) (e.g., Vonnegut 1947, citations in Villanueva et al. 2022)
    • Some modeling studies (Villanueva et al. 2022 uses cloud resolving model simulations as well as a climate model)
    • One simulated technique is to have aircraft fly above clouds dropping 1 kg km^-1 of dry ice pellets 3mm in diameter in parallel tracks by 1.2 km creating a curtain of ice (Zapalac 2023).
    • Observational studies of MPCs beginning using existing lidar and radar data to assess cooling potential of aerosol and MPC interaction (L. Russell pers. comm.).
    • Some outdoor experiments underway, with others planned.
• Technical feasibility within 10 yrs
  • Feasible within 10 years
    • Using technology that already exists; weather modification already happens via cloud seeding (e.g., Vonnegut 1947, Villanueva et al. 2022). The pellets sublimate and freeze surround water vapor creating a trail of ice crystals. 1 kg pellets can generate approximately 10¹⁵ ice crystals
    • Aircraft already exist

Physical and chemical changes

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022).
  • This technique only works in winter, therefore there is a risk of sea ice continuing to melt during summer (Villanueva et al. 2022).
  • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

Impacts on society

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

Ease of reversibility

• Easy
  • INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

Risk of termination shock

• High
  • Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

Physical and chemical changes

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022).
    • This technique only works in winter, therefore there is a risk of sea ice continuing to melt during summer (Villanueva et al. 2022).
    • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

Impacts on society

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

Ease of reversibility

• Easy
  • INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

Risk of termination shock

• High
  • Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

Physical and chemical changes

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022).
    • There is a rebound effect of sea ice melting during summer, especially as this technique only works in winter (Villanueva et al. 2022).
    • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

Impacts on society

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

Ease of reversibility

• Easy
  • INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

Risk of termination shock

• High
  • Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

Physical and chemical changes

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022).
    • There is a rebound effect of sea ice melting during summer, especially as this technique only works in winter (Villanueva et al. 2022).
    • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

Impacts on species

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

Impacts on ecosystems

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

Impacts on society

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

Ease of reversibility

• INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

Risk of termination shock

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022).
    • There is a rebound effect of sea ice melting during summer, especially as this technique only works in winter (Villanueva et al. 2022).
    • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

• INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

• Co-benefits
  • Unknown
• Risks
  • Decrease in precipitation (Villanueva et al. 2022)
    • There is a rebound effect of sea ice melting during summer, especially as this technique only works in winter (Villanueva et al. 2022).
    • If using silver iodide particles, as is used in weather modification, iodine can react with ozone to form iodic acid which can increase liquid droplet concentrations in cloud and increase warming (Baccarini et al. 2020 in Zapalac 2023). However, other ice nucleating particles are being considered for this approach (Villanueva et al. 2022, Zapalac 2023).

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for species, but this is unknown.

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for ecosystems, but this is unknown.

• Co-benefits
  • Unknown
• Risks
  • If silver iodide is used for seeding, scaling up application may have impacts for society, but this is unknown.

• INPs for MPCT have a lower lifetime than stratospheric aerosols, so there’s a quicker shut down if needed for MPCT compared to SAI (Villanueva et al. 2022).

• Temperatures would return to that without intervention in weeks (Parker and Irvine 2018). While the forcing generated by seeding would disappear in weeks, the temperature could take years to return because of the thermal inertia of the oceans (J. Kok pers. comm.).

• 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
• 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
    • 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
    • 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase 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 Mixed-phase Cloud Thinning:
  • In MPCT cooling is much more regionally confined, such that governance challenges and risk of geopolitical instability might be substantially less than with SAI or MCB (J. Kok pers. comm.).
  • 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.
  • For an Arctic-specific application, the 2017 Agreement on Enhancing Arctic Scientific Cooperation is relevant. This is a legally binding agreement signed in 2017 by all Arctic States negotiated in the Arctic Council. It promotes international cooperation and favorable conditions for conducting scientific research, facilitates access to research areas, infrastructure, and facilities, and promotes education and training of scientists in Arctic issues. The agreement also encourages participants to utilize traditional and local knowledge as appropriate as well as encourages communication between traditional and local knowledge holders and participants. This may provide a framework for consultation with stakeholders including Indigenous peoples in intervention research, planning, and testing (Chuffart et al. 2023).

• 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 Mixed-phase Cloud Thinning:
    • Unknown
      • MPCT research and deployment may impact a smaller area relative to other SRM approaches. If it was successful at prolonging the loss of Arctic sea ice it may have benefits that extend beyond the area of research and deployment.
    • 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 Mixed-phase 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 Mixed-phase 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 Mixed-phase 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).
  • This approach is similar to precipitation modification, which is done seasonally already.

• 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 Mixed-phase Cloud Thinning:
  • Unknown