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Recent studies have suggested that stratospheric aerosol injection (SAI) of solid particles for climate intervention could reduce stratospheric warming compared to injection of SO2 ${\text{SO}}_{2}$. However, interactions of microphysical processes, such as settling and coagulation of solid particles, with stratospheric dynamics have not been considered. Using a global chemistry‐climate model with interactive solid particle microphysics, we show that agglomeration significantly reduces the backscatter efficiency per unit of injected material compared to mono‐disperse particles, partly due to faster settling of the agglomerates, but mainly due to increased forward‐ over backscattering with increasing agglomerate size. Despite these effects, some materials substantially reduce required injection rates as well as perturbation of stratospheric winds, age of air and stratospheric warming compared to injection of SO2 ${\text{SO}}_{2}$, with the most promising results being shown by 150 nm diamond particles. Uncertainties remain as to whether stratospheric dispersion of solid particles is feasible without formation of agglomerates. Plain Language Summary: [Stratospheric warming is an undesired side effect of climate intervention via SAI. Recent studies have shown that stratospheric warming could be reduced when injecting solid particles instead of gaseous SO2 ${\text{SO}}_{2}$. However, most of these studies looked at the stratospheric particle mass required for a given radiative forcing (RF), without accounting for gravitational settling of particles or the effect of particles sticking together after mutual collision. We show that accounting for these effects significantly reduces the amount of backward reflected radiation per unit of stratospheric particle mass decreasing the radiative efficiency of the injected material. This is due to the combined effect of faster gravitational settling and the larger fraction of forward reflected radiation over backward reflected radiation with increasing agglomerate size. However, we show that injection of diamond particles at a radius of 150 nm instead of SO2 ${\text{SO}}_{2}$ significantly reduces required stratospheric injection rates as well as perturbation of stratospheric winds, age of stratospheric air and stratospheric water vapor concentrations due to small stratospheric warming per unit of RF. However, large uncertainties remain as to whether it will be feasible to inject solid particles into the stratosphere at concentrations low enough to prohibit that the particles stick together.] Key Points: We explore stratospheric injections of six solid particles and gaseous SO2 within a climate model with comprehensive aerosol microphysicsAccounting for settling and agglomeration of solid particles can substantially reduce the radiative forcing (RF) per unit of injected materialInjection of diamond particles (r = 150 nm) instead of SO2 largely reduces stratospheric temperature, circulation and water vapor anomalies [ABSTRACT FROM AUTHOR]

Solar geoengineering, also called sunlight reflection or solar radiation modification (SRM), is a potential climate response that would cool the Earth's surface and reduce many other climate changes by scattering on order 1% of incoming sunlight back to space. SRM can only imperfectly correct for elevated greenhouse gases, but it might complement other climate responses to reduce risks, while also bringing new risks and new challenges to global governance. As climate alarm and calls for effective near-term action mount, SRM is attracting sharply increased attention and controversy, with many calls for expanded research and governance consultations along with ongoing concerns about risks, misuse, or overreliance. We review SRM's history, methods, potential uses and impacts, and governance needs, prioritizing the approach that is most prominent and promising, stratospheric aerosol injection. We identify several policy-relevant characteristics of SRM interventions and identify four narratives that capture current arguments over how SRM might be developed or used in sociopolitical context to either beneficial or destructive effect, with implications for near-term research, assessment, and governance activity. [ABSTRACT FROM AUTHOR]

Wind renewable energy (WRE) is an essential component of the global sustainable energy portfolio. Recently, there has been increasing discussion on the potential supplementation of this conventional mitigation portfolio with Solar Radiation Modification (SRM). However, the impact of SRM on conventional mitigation measures has received limited attention to date. In this study, we explore one part of this impact, the potential effect of one type of SRM, Stratospheric Aerosol Injections (SAI), on WRE. Using hourly output from the Earth System Model CNRM‐ESM2‐1, we compare WRE potential under a medium emission scenario (SSP245) and a high emission scenario (SSP585) with an SRM scenario that has SSP585 baseline conditions and uses SAI to offset warming to approximately SSP245 global warming levels. Our results suggest that SAI may affect surface wind resources by modifying large‐scale circulation patterns, such as a significant poleward jet‐shift in the Southern Hemisphere. The modeled total global WRE potential is negligibly reduced under SAI compared to the SSP‐scenarios. However, regional trends are highly variable, with large increases and decreases in WRE potential frequently reaching 12% across the globe with SAI. This study highlights potential downstream effects of SRM on climatic elements, such as wind patterns, and offers perspectives on its implications for our mitigation efforts. Plain Language Summary: Wind renewable energy (WRE) is a pivotal element of the global transition toward sustainable energy. Recently, there has been a surge of interest in Solar Radiation Modification (SRM) as a potential means of managing climate impacts, but its effects on existing strategies like WRE have not been much addressed in the current literature. This research examines the potential impact of one type of SRM, Stratospheric Aerosol Injections (SAI), on WRE potential. An Earth System Model is used to compare WRE potential under three climate scenarios: one with moderate emissions, one with high emissions and one where SAI is used to reduce warming in the high‐emission scenario to match the moderate‐emission global warming level. The findings suggest that SAI may result in a minimal reduction in long‐term global mean WRE potential. However, the effects vary considerably by region and season, with areas experiencing changes in WRE potential that frequently reach approximately 12%. This research highlights the potential for SAI to exert complex effects on wind patterns, which could influence the future of renewable energy strategies. Key Points: Stratospheric Aerosol Injections do not fully compensate for atmospheric circulation changes from climate change but create new dynamicsTotal global wind energy potential is negligibly reduced under Stratospheric Aerosol Injections but regional trends tend to vary substantiallyResults are based on hourly output from CNRM‐ESM2‐1. Changes are likely to be larger in other GeoMIP models in the northern hemisphere [ABSTRACT FROM AUTHOR]

Solar radiation modification (SRM) has been proposed to temporarily reduce anthropogenic warming. This study presents an assessment of the regional impacts of SRM via solar dimming and stratospheric aerosol injection (SAI) on temperature and precipitation over 0°–30° N and 90° E–110° E, covering Mainland Southeast Asia and adjacent oceans. Using data from the Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6), we examine regional impacts of SRM using three SRM experiments: (1) G6Sulfur, which reduces radiative forcing from the high-emission SSP5–8.5 scenario to the moderate-emission SSP2–4.5 scenario by injecting sulfate aerosols; (2) G6Solar, which similarly reduces radiative forcing from the high-emission to moderate-emission scenarios but by uniformly reducing the solar constant; and (3) G1ext, which reduces radiative forcing from a quadrupled carbon dioxide concentration to pre-industrial levels by uniform solar constant reduction. Our findings show that higher greenhouse gas emissions increase overall precipitation, along with tendencies to have extreme rainfall events and more dry episodes in between. While SRM can partially cool down the surface temperature warming caused by increased greenhouse gas emissions, its effects on precipitation are complex: Solar dimming in G6Solar and G1ext tends to reduce overall precipitation, and tropical sulfate injection in G6Sulfur could lead to further drying in the tropics because of the stratospheric warming associated with the injected aerosols. Different SRM strategies might result in different responses on precipitation. [ABSTRACT FROM AUTHOR]

Solar climate intervention refers to a group of methods for reducing climate risks associated with anthropogenic warming by reflecting sunlight. Marine cloud brightening (MCB), one such approach, proposes to inject sea‐salt aerosol into one or more regional marine boundary layer to increase marine cloud reflectivity. Here, we assess the potential influence of various MCB experiments on Africa's climate using simulations from the Community Earth System Model (CESM2) with the Community Atmosphere Model (CAM6) as its atmospheric component. We analyzed four idealized MCB experiments under a medium‐range background forcing scenario (SSP2‐4.5), which brighten clouds over three subtropical ocean regions: (a) Northeast Pacific (MCBNEP); (b) Southeast Pacific (MCBSEP); (c) Southeast Atlantic (MCBSEA); and (d) these three regions simultaneously (MCBALL). Our results suggest that the climate impacts of MCB in Africa are highly sensitive to the deployment region. MCBSEP would produce the strongest global cooling effect and thus could be the most effective in decreasing temperatures, increasing precipitation, and reducing the intensity and frequency of temperature and precipitation extremes across most parts of Africa, especially West Africa, in the future (2035–2054) compared to the historical climate (1995–2014). MCB in other regions produces less cooling and wetting despite similar radiative forcings. While the projected changes under MCBALL are similar to those of MCBSEP, MCBNEP and MCBSEA could see more residual warming and induce a warmer future than under SSP2‐4.5 in some regions across Africa. All MCB experiments are more effective in cooling maximum temperature and related extremes than minimum temperature and related extremes. Plain Language Summary: We investigate the potential impact of artificially increasing marine cloud reflectivity on Africa's climate. Human influence on the climate is projected to increase the risk of damaging extreme events across the globe. In Africa, one of the most vulnerable regions to climate change, these impacts are already being felt, especially among communities least able to adapt. It is possible to increase the reflectivity of marine clouds by spraying them with sea salt particulates. This approach was proposed as one possible way of reducing warming by reducing the amount of sunlight reaching the earth's surface. The potential implications of such initiatives for the climate system remain uncertain, especially in Africa. In this study, we analyzed a climate model, that simulates increased reflectivity of clouds to assess the potential impacts of artificially brightening clouds over different subtropical marine regions on the African climate, focusing on mean and extreme precipitation and temperature events. Our results suggest that the impact of artificially increasing marine cloud reflectivity depends on the marine region of intervention. Key Points: The climate impacts of Marine Cloud Brightening (MCB) in Africa are highly sensitive to the deployment regionMCB in the Southeast Pacific could effectively cool Africa and reduces temperature and precipitation extremesIn this study, MCB is more effective in cooling maximum temperature and related extremes than minimum temperature and related extremes [ABSTRACT FROM AUTHOR]

Abstract Solar radiation modification (SRM) has been proposed to temporarily reduce anthropogenic warming. This study presents an assessment of the regional impacts of SRM via solar dimming and stratospheric aerosol injection (SAI) on temperature and precipitation over 0°–30° N and 90° E–110° E, covering Mainland Southeast Asia and adjacent oceans. Using data from the Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6), we examine regional impacts of SRM using three SRM experiments: (1) G6Sulfur, which reduces radiative forcing from the high-emission SSP5–8.5 scenario to the moderate-emission SSP2–4.5 scenario by injecting sulfate aerosols; (2) G6Solar, which similarly reduces radiative forcing from the high-emission to moderate-emission scenarios but by uniformly reducing the solar constant; and (3) G1ext, which reduces radiative forcing from a quadrupled carbon dioxide concentration to pre-industrial levels by uniform solar constant reduction. Our findings show that higher greenhouse gas emissions increase overall precipitation, along with tendencies to have extreme rainfall events and more dry episodes in between. While SRM can partially cool down the surface temperature warming caused by increased greenhouse gas emissions, its effects on precipitation are complex: Solar dimming in G6Solar and G1ext tends to reduce overall precipitation, and tropical sulfate injection in G6Sulfur could lead to further drying in the tropics because of the stratospheric warming associated with the injected aerosols. Different SRM strategies might result in different responses on precipitation.

In recent years, some scientists have called for research into and potential development of 'solar geoengineering' technologies as an option to counter global warming. Solar geoengineering refers to a set of speculative techniques to reflect some incoming sunlight back into space, for example, by continuously spraying reflective sulphur aerosols into the stratosphere over several generations. Because of the significant ecological, social, and political risks posed by such technologies, many scholars and civil society organizations have urged governments to take action to prohibit the development and deployment of solar geoengineering techniques. In this article we take such calls for a prohibitory or a non-use regime on solar geoengineering as a starting point to examine existing international law and governance precedents that could guide the development of such a regime. The precedents we examine include international prohibitory and restrictive regimes that impose bans or restrictions on chemical weapons, biological weapons, weather modification technologies, anti-personnel landmines, substances that deplete the ozone layer, trade in hazardous wastes, deep seabed mining, and mining in Antarctica. We also assess emerging norms and soft law in anticipatory governance of novel technologies, such as human cloning and gene editing. While there is no blueprint for a solar geoengineering non-use regime in international law, our analysis points to numerous specific elements on which governments could draw to constrain or impose an outright prohibition on the development of technologies for solar geoengineering, should they opt to do so. [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection (SAI) has been proposed as a potential supplement to mitigate some climate impacts of anthropogenic warming. Using Community Earth System Model ensemble simulation results, we analyze the response of temperature and precipitation extremes to two different SAI strategies: one injects SO2 at the equator to stabilize global mean temperature and the other injects SO2 at multiple locations to stabilize global mean temperature as well as the interhemispheric and equator‐to‐pole temperature gradients. Our analysis shows that in the late 21st century, compared with the present‐day climate, both equatorial and multi‐location injection lead to reduced hot extremes in the tropics, corresponding to overcooling of the mean climate state. In mid‐to‐high latitude regions, in comparison to the present‐day climate, substantial decreases in cold extremes are observed under both equatorial and multi‐location injection, corresponding to residual winter warming of the mean climate state. Both equatorial and multi‐location injection reduce precipitation extremes in the tropics below the present‐day level, associated with the decrease in mean precipitation. Overall, for most regions, temperature and precipitation extremes show reduced change in response to multi‐location injection than to equatorial injection, corresponding to reduced mean climate change for multi‐location injection. In comparison with equatorial injection, in response to multi‐location injection, most land regions experience fewer years with significant change in cold extremes from the present‐day level, and most tropical regions experience fewer years with significant change in hot extremes. The design of SAI strategies to mitigate anthropogenic climate extremes merits further study. Plain Language Summary: Injecting SO2 into the stratosphere to deflect incoming sunlight back to space has been proposed as a possible means to counteract anthropogenic warming. Previous studies focused on the response of the mean climate change to SO2 injection. Here we analyze how SO2 injection would affect climate extremes using two sets of climate model simulations under a high‐CO2 scenario. In one set of simulations, SO2 is injected into the stratosphere at the equator to stabilize global mean warming at the present‐day level; in another set of simulations, SO2 is injected into the stratosphere at four different latitudes to stabilize global mean temperature, interhemispheric temperature gradient, and equator‐to‐pole temperature gradient at the same time. Our analysis shows that both equatorial injection and multi‐location injection can help reduce temperature and precipitation extreme events induced by global warming, but their effectiveness varies across different regions. In general, compared to equatorial injection, multi‐location injection can be more effective in stabilizing climate extremes at the present‐day level over most regions. This is consistent with the fact that multi‐location injection exhibits smaller changes in mean climate state than that of the equatorial injection relative to the present‐day state. Key Points: We examine the climate extreme response to two stratospheric aerosol injection (SAI) strategies: equatorial injection and multi‐location injectionThe response of climate extremes to SAI shows similar characteristics to mean climate responseMulti‐location injection is more effective in stabilizing climate extremes at the present‐day level [ABSTRACT FROM AUTHOR]

Abstract Wind renewable energy (WRE) is an essential component of the global sustainable energy portfolio. Recently, there has been increasing discussion on the potential supplementation of this conventional mitigation portfolio with Solar Radiation Modification (SRM). However, the impact of SRM on conventional mitigation measures has received limited attention to date. In this study, we explore one part of this impact, the potential effect of one type of SRM, Stratospheric Aerosol Injections (SAI), on WRE. Using hourly output from the Earth System Model CNRM‐ESM2‐1, we compare WRE potential under a medium emission scenario (SSP245) and a high emission scenario (SSP585) with an SRM scenario that has SSP585 baseline conditions and uses SAI to offset warming to approximately SSP245 global warming levels. Our results suggest that SAI may affect surface wind resources by modifying large‐scale circulation patterns, such as a significant poleward jet‐shift in the Southern Hemisphere. The modeled total global WRE potential is negligibly reduced under SAI compared to the SSP‐scenarios. However, regional trends are highly variable, with large increases and decreases in WRE potential frequently reaching 12% across the globe with SAI. This study highlights potential downstream effects of SRM on climatic elements, such as wind patterns, and offers perspectives on its implications for our mitigation efforts.

Abstract Recent studies have suggested that stratospheric aerosol injection (SAI) of solid particles for climate intervention could reduce stratospheric warming compared to injection of SO2. However, interactions of microphysical processes, such as settling and coagulation of solid particles, with stratospheric dynamics have not been considered. Using a global chemistry‐climate model with interactive solid particle microphysics, we show that agglomeration significantly reduces the backscatter efficiency per unit of injected material compared to mono‐disperse particles, partly due to faster settling of the agglomerates, but mainly due to increased forward‐ over backscattering with increasing agglomerate size. Despite these effects, some materials substantially reduce required injection rates as well as perturbation of stratospheric winds, age of air and stratospheric warming compared to injection of SO2, with the most promising results being shown by 150 nm diamond particles. Uncertainties remain as to whether stratospheric dispersion of solid particles is feasible without formation of agglomerates.

Abstract Stratospheric aerosol injection (SAI) has been proposed as a potential supplement to mitigate some climate impacts of anthropogenic warming. Using Community Earth System Model ensemble simulation results, we analyze the response of temperature and precipitation extremes to two different SAI strategies: one injects SO2 at the equator to stabilize global mean temperature and the other injects SO2 at multiple locations to stabilize global mean temperature as well as the interhemispheric and equator‐to‐pole temperature gradients. Our analysis shows that in the late 21st century, compared with the present‐day climate, both equatorial and multi‐location injection lead to reduced hot extremes in the tropics, corresponding to overcooling of the mean climate state. In mid‐to‐high latitude regions, in comparison to the present‐day climate, substantial decreases in cold extremes are observed under both equatorial and multi‐location injection, corresponding to residual winter warming of the mean climate state. Both equatorial and multi‐location injection reduce precipitation extremes in the tropics below the present‐day level, associated with the decrease in mean precipitation. Overall, for most regions, temperature and precipitation extremes show reduced change in response to multi‐location injection than to equatorial injection, corresponding to reduced mean climate change for multi‐location injection. In comparison with equatorial injection, in response to multi‐location injection, most land regions experience fewer years with significant change in cold extremes from the present‐day level, and most tropical regions experience fewer years with significant change in hot extremes. The design of SAI strategies to mitigate anthropogenic climate extremes merits further study.

The illustrative development pathways developed by the Intergovernmental Panel on Climate Change (IPCC) for the Synthesis Report of its Sixth Assessment Report (AR6 (1)) consider mainly different levels of mitigation and adaptation. This article expands the range of development pathways. One assumption is the crossing of tipping points of the climate system with positive feedback. A second assumption is the deployment of solar radiation modification triggered by significantly increase of loss and damage. A prerequisite for such deployment is additional scientific knowledge to inform decisions at the policy level. [ABSTRACT FROM AUTHOR]

As disruptions from climate change increase, so will the urgency to find shorter-term approaches to ameliorating its harms. This may include calls to implement solar geoengineering, an approach to cooling the planet by reflecting incoming sunlight back to space. While the exact effects of solar geoengineering are still highly uncertain, physical science to date suggests that it may be effective at reducing many aspects of climate change in the short term. One of the biggest concerns about solar geoengineering is the extent to which it may interfere with crucial emissions reductions policies, i.e. mitigation. There are multiple channels by which geoengineering could alter mitigation pathways, both financial and behavioural. Here we define three such linkages and present the evidence available to constrain their potential magnitudes. Because solar geoengineering is not a substitute for mitigation, policies to develop or implement technologies that could be used to carry it out should be designed to accentuate its complementary nature to mitigation and deter the possibility it is used to delay decarbonizing the economy. [ABSTRACT FROM AUTHOR]

It has been proposed that increasing greenhouse gas (GHG) driven climate tipping point risks may prompt consideration of solar radiation modification (SRM) climate intervention to reduce those risks. Here, we study marine cloud brightening (MCB) SRM interventions in three subtropical oceanic regions using Community Earth System Model 2 experiments. We assess the MCB impact on tipping element‐related metrics to estimate the extent to which such interventions might reduce tipping element risks. Both the pattern and magnitude of the MCB cooling depend strongly on location of the MCB intervention. We find the MCB cooling effect reduces most tipping element impacts; though differences in MCB versus GHG climate response patterns mean MCB is an imperfect remedy. However, MCB applied in certain regions may exacerbate certain GHG tipping element impacts. Thus, it is crucial to carefully consider the pattern of MCB interventions and their teleconnected responses to avoid unintended climate effects. Plain Language Summary: Marine cloud brightening (MCB) is a proposal to spray sea salt particles into clouds over oceans to increase the reflection of sunlight by the clouds, thus cooling the surface. If greenhouse gas warming continues, technologies like MCB might be considered to avoid climate change impacts such as climate system tipping points. Here, we use state‐of‐the‐art climate model experiments to analyze the MCB impact on elements of the climate system that may have tipping points. In this model, MCB reduces risks for most tipping elements considered here, such as by reducing coral reef heat stress and increasing Atlantic overturning circulation. However, the impact of MCB depends on where it is applied and in some cases adds to GHG impacts, meaning the location of MCB deployments must be carefully considered to avoid unintended regional climate effects. Key Points: The magnitude and pattern of marine cloud brightening (MCB) climate impacts depend strongly on the location of the interventionWe find MCB impacts that have qualitative similarities to prior work, but there are discrepancies that suggest key inter‐model uncertaintiesMCB simulations generally show reduced tipping element risk overall, but certain MCB patterns may exacerbate some tipping element changes [ABSTRACT FROM AUTHOR]

As the effects of anthropogenic climate change become more severe, several approaches for deliberate climate intervention to reduce or stabilize Earth's surface temperature have been proposed. Solar radiation modification (SRM) is one potential approach to partially counteract anthropogenic warming by reflecting a small proportion of the incoming solar radiation to increase Earth's albedo. While climate science research has focused on the predicted climate effects of SRM, almost no studies have investigated the impacts that SRM would have on ecological systems. The impacts and risks posed by SRM would vary by implementation scenario, anthropogenic climate effects, geographic region, and by ecosystem, community, population, and organism. Complex interactions among Earth's climate system and living systems would further affect SRM impacts and risks. We focus here on stratospheric aerosol intervention (SAI), a well-studied and relatively feasible SRM scheme that is likely to have a large impact on Earth's surface temperature. We outline current gaps in knowledge about both helpful and harmful predicted effects of SAI on ecological systems. Desired ecological outcomes might also inform development of future SAI implementation scenarios. In addition to filling these knowledge gaps, increased collaboration between ecologists and climate scientists would identify a common set of SAI research goals and improve the communication about potential SAI impacts and risks with the public. Without this collaboration, forecasts of SAI impacts will overlook potential effects on biodiversity and ecosystem services for humanity.

Carbon dioxide removal and solar radiation modification (SRM) are two classes of proposed climate intervention methods. A thorough understanding of climate system response to these methods calls for a good understanding of the carbon cycle response. In this study, we used an Earth system model to examine the response of global climate and carbon cycle to artificial ocean alkalinization (AOA), a method of CO2 removal, and reduction in solar irradiance that represents the overall effect of solar radiation modification. In our simulations, AOA is applied uniformly over the global ice-free ocean under the RCP8.5 scenario to bring down atmospheric CO2 to the level of RCP4.5, and SRM is applied uniformly over the globe under the RCP8.5 scenario to bring down global mean surface temperature to the level of RCP4.5. Our simulations show that with the same goal of temperature stabilization, AOA and SRM cause fundamentally different perturbations of the ocean and land carbon cycle. By the end of the 21st century, relative to the simulation of RCP8.5, AOA-induced changes in ocean carbonate chemistry enhances global oceanic CO2 uptake by 983 PgC and increases global mean surface ocean pH by 0.42. Meanwhile, AOA reduces land CO2 uptake by 79 PgC and reduces atmospheric CO2 concentration by 426 × 10−6. By contrast, relative to the simulation of RCP8.5, SRM has a minor effect on the oceanic CO2 uptake and ocean acidification. SRM-induced cooling enhances land CO2 uptake by 140 PgC and reduces atmospheric CO2 concentration by 63 × 10−6. A sudden termination of SRM causes a rate of temperature change that is much larger than that of RCP8.5. A sudden termination of AOA causes a rate of temperature change that is comparable to that of RCP8.5 and a rate of ocean acidification that is much larger than that of RCP8.5.

Abstract It has been proposed that increasing greenhouse gas (GHG) driven climate tipping point risks may prompt consideration of solar radiation modification (SRM) climate intervention to reduce those risks. Here, we study marine cloud brightening (MCB) SRM interventions in three subtropical oceanic regions using Community Earth System Model 2 experiments. We assess the MCB impact on tipping element‐related metrics to estimate the extent to which such interventions might reduce tipping element risks. Both the pattern and magnitude of the MCB cooling depend strongly on location of the MCB intervention. We find the MCB cooling effect reduces most tipping element impacts; though differences in MCB versus GHG climate response patterns mean MCB is an imperfect remedy. However, MCB applied in certain regions may exacerbate certain GHG tipping element impacts. Thus, it is crucial to carefully consider the pattern of MCB interventions and their teleconnected responses to avoid unintended climate effects.

As the world struggles to limit warming to 1.5 or 2 °C below pre-industrial temperatures, research into solar climate interventions that could temporarily offset some amount of greenhouse gas-driven global warming by reflecting more sunlight back out to space has gained prominence. These solar climate intervention techniques would aim to cool the Earth by injecting aerosols (tiny liquid or solid particles suspended in the atmosphere) into the upper atmosphere or into low-altitude marine clouds. In a new development, “cooling credits” are now being marketed that claim to offset a certain amount of greenhouse gas warming with aerosol-based cooling. The science of solar climate intervention is currently too uncertain and the quantification of effects insufficient for any such claims to be credible in the near term. More fundamentally, however, the environmental impacts of greenhouse gases and aerosols are too different for such credits to be an appropriate instrument for reducing climate risk even if scientific uncertainties were narrowed and robust monitoring systems put in place. While some form of commercial mechanism for solar climate intervention implementation, in the event it is used, is likely, “cooling credits” are unlikely to be a viable climate solution, either now or in the future. [ABSTRACT FROM AUTHOR]

Solar geoengineering refers to proposals, including stratospheric aerosol injection (SAI), to slow or reverse climate change by reflecting away incoming sunlight. The rapid changes ongoing in the Arctic and Antarctic, and the risk of exceeding tipping points in the cryosphere within decades, make limiting such changes a plausible objective of solar geoengineering. Here, we review the impacts of SAI on polar climate and cryosphere, including the dependence of these impacts on the latitude(s) of injection, and make recommendations for future research directions. SAI would cool the polar regions and reduce many changes in polar climate under future warming scenarios. Some under‐cooling of the polar regions relative to the global mean is expected under SAI without high latitude injection, due to latitudinal variation in insolation and CO2 forcing, the forcing dependence of the polar lapse rate feedback, and altered atmospheric dynamics. There are also potential limitations in the effectiveness of SAI to arrest changes in winter‐time polar climate and to prevent sea‐level rise from the Antarctic ice sheet. Finally, we also review the prospects for three other solar geoengineering proposals targeting the poles: marine cloud brightening, cirrus cloud thinning, and sea‐ice albedo modification. Sea‐ice albedo modification appears unlikely to be viable on pan‐Arctic or Antarctic scales. Whether marine cloud brightening or cirrus cloud thinning would be effective in the polar regions remains uncertain. Solar geoengineering is an increasingly prominent proposal and a robust understanding of its consequences in the polar regions is needed to inform climate policy in the coming decades. Plain Language Summary: It may be possible to temporarily limit the impacts of climate change by reflecting away part of the incoming sunlight reaching the Earth. Ideas for how to do this are termed solar geoengineering. The Arctic and Antarctic are warming rapidly and change in these regions contributes to serious problems caused by climate change, such as global sea level rise. Additionally, we may exceed warming thresholds within several decades beyond which changes in the polar climate system become self‐perpetuating. As a result the Arctic and Antarctic are potential regional targets of solar geoengineering. In this study we review for the first time the evidence on how various forms of solar geoengineering would impact the Arctic and Antarctic. We find that while solar geoengineering is less effective in the polar regions, which receive less sunlight to reflect away than the lower latitudes, global deployment could meaningfully reduce all the projected changes due to climate change which we assess. The potential effectiveness of local polar schemes, which would aim to change the energy balance only over the Arctic or Antarctic, remains very uncertain. The limited evidence we have suggests such options would be insufficient to prevent regional change under high emissions scenarios. Key Points: Global solar geoengineering, if technically feasible, would likely reduce the rapid climate change ongoing in the Arctic and AntarcticTargeted solar geoengineering strategies would be needed to avoid residual warming in the polar regionsNo truly localized polar geoengineering proposal has been shown viable for restoring climate on a pan‐Arctic or Antarctic scale [ABSTRACT FROM AUTHOR]

As atmospheric carbon dioxide concentrations rise and climate change becomes more destructive, geoengineering has become a subject of serious consideration. By reflecting a fraction of incoming sunlight, solar geoengineering could cool the planet quickly, but with uncertain effects on regional climatology, particularly hydrological patterns. Here, we review recent work on projected hydrologic outcomes of solar geoengineering, in the context of a robust literature on hydrological responses to climate change. While most approaches to solar geoengineering are expected to weaken the global hydrologic cycle, regional effects will vary based on implementation method and strategy. The literature on the hydrologic outcomes and impacts of geoengineering demonstrates that its implications for human welfare will depend on assumptions about underlying social conditions and objectives of intervention as well as the social lens through which projected effects are interpreted. We conclude with suggestions to reduce decision-relevant uncertainties in this novel field of Earth science inquiry. The expected hydrological effects of reducing insolation are among the most uncertain and consequential impacts of solar geoengineering (SG). Theoretical frameworks from broader climate science can help explain SG's effects on global precipitation, relative humidity, and other aspects of hydroclimate. The state of the knowledge on hydrological impacts of SG is unevenly concentrated among regions. Projected hydrological impacts from SG are scenario dependent and difficult to characterize as either harmful or beneficial. [ABSTRACT FROM AUTHOR]

Abstract Solar geoengineering refers to proposals, including stratospheric aerosol injection (SAI), to slow or reverse climate change by reflecting away incoming sunlight. The rapid changes ongoing in the Arctic and Antarctic, and the risk of exceeding tipping points in the cryosphere within decades, make limiting such changes a plausible objective of solar geoengineering. Here, we review the impacts of SAI on polar climate and cryosphere, including the dependence of these impacts on the latitude(s) of injection, and make recommendations for future research directions. SAI would cool the polar regions and reduce many changes in polar climate under future warming scenarios. Some under‐cooling of the polar regions relative to the global mean is expected under SAI without high latitude injection, due to latitudinal variation in insolation and CO2 forcing, the forcing dependence of the polar lapse rate feedback, and altered atmospheric dynamics. There are also potential limitations in the effectiveness of SAI to arrest changes in winter‐time polar climate and to prevent sea‐level rise from the Antarctic ice sheet. Finally, we also review the prospects for three other solar geoengineering proposals targeting the poles: marine cloud brightening, cirrus cloud thinning, and sea‐ice albedo modification. Sea‐ice albedo modification appears unlikely to be viable on pan‐Arctic or Antarctic scales. Whether marine cloud brightening or cirrus cloud thinning would be effective in the polar regions remains uncertain. Solar geoengineering is an increasingly prominent proposal and a robust understanding of its consequences in the polar regions is needed to inform climate policy in the coming decades.

Climate change is a prevalent threat, and it is unlikely that current mitigation efforts will be enough to avoid unwanted impacts. One potential option to reduce climate change impacts is the use of stratospheric aerosol injection (SAI). Even if SAI is ultimately deployed, it might be initiated only after some temperature target is exceeded. The consequences of such a delay are assessed herein. This study compares two cases, with the same target global mean temperature of ∼1.5° C above preindustrial, but start dates of 2035 or a ‘delayed’ start in 2045. We make use of simulations in the Community Earth System Model version 2 with the Whole Atmosphere Coupled Chemistry Model version 6 (CESM2-WACCM6), using SAI under the SSP2-4.5 emissions pathway. We find that delaying the start of deployment (relative to the target temperature) necessitates lower net radiative forcing (−30%) and thus larger sulfur dioxide injection rates (+20%), even after surface temperatures converge, to compensate for the extra energy absorbed by the Earth system. Southern hemisphere ozone is higher from 2035 to 2050 in the delayed start scenario, but converges to the same value later in the century. However, many of the surface climate differences between the 2035 and 2045 start simulations appear to be small during the 10–25 years following the delayed SAI start, although longer simulations would be needed to assess any longer-term impacts in this model. In addition, irreversibilities and tipping points that might be triggered during the period of increased warming may not be adequately represented in the model but could change this conclusion in the real world.

Little is known about how climate intervention through stratospheric aerosol injection (SAI) may affect the climatology of the Southern Hemisphere extratropical cyclones under warming scenarios. To address this knowledge gap, we tracked extratropical cyclones from 2015 to 2099 in a set of projections of three international projects: the Assessing Responses and Impacts of Solar Climate Intervention on the Earth System with Stratospheric Aerosol Injection (ARISE), the Stratospheric Aerosol Geoengineering Large Ensemble (GLENS), and the Geoengineering Model Intercomparison Project (GeoMIP/G6sulfur). Comparisons were performed between no-SAI and SAI scenarios as well as between different timeslices and their reference period (2015–2024). Among the findings, both no-SAI and SAI project a decrease in cyclone frequency towards the end of the century although weaker under SAI scenarios. On the other hand, cyclones tend to be stronger under no-SAI scenarios while keeping their intensity more similar to the reference period under SAI scenarios. This means that under SAI scenarios the climatology of cyclones is less affected by global warming than under no-SAI. Other features of these systems, such as travelling distance, lifetime, and mean velocity show small differences between no-SAI and SAI scenarios and between reference and future periods.

Solar radiation modification (SRM) is a proposed method to cool the Earth by intentionally perturbing the Earth's energy balance. One concern about the effect of SRM is disparities in the regional climate response. In this study, we use the Community Earth System Model (CESM1.2) to analyze the regional response of land hydrology and terrestrial carbon uptake to different amounts of SRM. The SRM is implemented by a uniform increase in volcanic‐size sulfate aerosols in the stratosphere under a doubling of atmospheric CO2. Our results show that different amounts of SRM could either moderate or exacerbate CO2‐induced changes in land hydrology including precipitation, precipitation minus evapotranspiration, and soil moisture (SM), but the effect varies widely across regions and specific variables. An "optimal" amount of SRM that moderates land hydrology changes for one region might exacerbate changes for other regions (or vice versa). Also, our study shows that for quite a few regions, partial SRM moderates CO2‐induced change in precipitation minus evaporation but exacerbates changes in CO2‐induced SM. The response of terrestrial Net primary productivity (NPP) to different amounts of SRM shows large regional disparities, depending on whether temperature or water availability constrains NPP more. Our study also shows that the effect of CO2 physiological forcing plays a key role in regulating land hydrology response to SRM, especially at the regional scale. Plain Language Summary: To reduce anthropogenic global warming, different solar radiation modification (SRM) approaches have been proposed. One main concern about the climate effect of SRM is the uneven regional climate response. We use a climate model to analyze how stratospheric aerosol increase would affect regional climate. Our focus is on regional land hydrology (including precipitation, precipitation minus evapotranspiration, and soil moisture [SM]) and carbon cycle response to different intensities of stratospheric aerosol increase. Our results demonstrate that different intensities of aerosol increase would induce strong regional inequalities in reducing the CO2‐induced anomaly of specific quantities. A certain intensity of SRM that reduces the CO2‐induced anomaly of land hydrology for one region might increase the anomaly for other regions. In some regions, SRM reduces the CO2‐induced anomaly in precipitation minus evapotranspiration but increases the CO2‐induced anomaly in SM. In addition, changes in vegetation productivity in response to different intensities of SRM show large regional disparities, depending on whether temperature or water availability is the major limiting factor for vegetation growth. Key Points: We use the Community Earth System Model model to analyze how different amounts of solar radiation modification (SRM) affect regional climateLand hydrology response depends on the amount of SRM, the region, and the specific variables consideredThe response of land productivity to different amounts of SRM is regionally dependent [ABSTRACT FROM AUTHOR]

Current impacts and escalating risks of climate change require strong and decisive action to reduce greenhouse gas (GHG) emissions. They also highlight the urgency of research to enhance safety for human and natural systems, especially for those most vulnerable. This is reflected in two recent US National Academies of Science, Engineering, and Medicine studies that recommended a national focus on advancing our understanding of how to manage urgent current and future climate risks, and the study of approaches for increasing the reflection of sunlight from the atmosphere to reduce global warming, a process referred to as sunlight reflection modification (SRM). Here, we build on these recommendations by proposing a roadmap approach for the planning, coordination, and delivery of research to support a robust scientific assessment of SRM to reduce near-term climate risks in a defined timeframe. This approach is designed to support the evaluation of SRM as a possible rapid, temporary, additive measure to reduce catastrophic impacts from anthropogenic climate change, not as a substitute for aggressive GHG mitigation. Assessing SRM is proposed to be undertaken in the context of climate hazard risks through 2050, weighing the impacts associated with likely climate change trajectories against scenarios of possible SRM implementations. Provided that research is undertaken openly and that scientific resources are made widely available, the transparency of the process and the evidence generated would contribute to the democratization of information, participation by diverse stakeholders, more informed decision-making, and better opportunities for all people to weigh SRM options against climate change risks. [ABSTRACT FROM AUTHOR]

Abstract Solar radiation modification (SRM) is a proposed method to cool the Earth by intentionally perturbing the Earth's energy balance. One concern about the effect of SRM is disparities in the regional climate response. In this study, we use the Community Earth System Model (CESM1.2) to analyze the regional response of land hydrology and terrestrial carbon uptake to different amounts of SRM. The SRM is implemented by a uniform increase in volcanic‐size sulfate aerosols in the stratosphere under a doubling of atmospheric CO2. Our results show that different amounts of SRM could either moderate or exacerbate CO2‐induced changes in land hydrology including precipitation, precipitation minus evapotranspiration, and soil moisture (SM), but the effect varies widely across regions and specific variables. An “optimal” amount of SRM that moderates land hydrology changes for one region might exacerbate changes for other regions (or vice versa). Also, our study shows that for quite a few regions, partial SRM moderates CO2‐induced change in precipitation minus evaporation but exacerbates changes in CO2‐induced SM. The response of terrestrial Net primary productivity (NPP) to different amounts of SRM shows large regional disparities, depending on whether temperature or water availability constrains NPP more. Our study also shows that the effect of CO2 physiological forcing plays a key role in regulating land hydrology response to SRM, especially at the regional scale.

Making informed future decisions about solar radiation modification (SRM; also known as solar geoengineering)--approaches such as stratospheric aerosol injection (SAI) that would cool the climate by reflecting sunlight--requires projections of the climate response and associated human and ecosystem impacts. These projections, in turn, will rely on simulations with global climate models. As with climate-change projections, these simulations need to adequately span a range of possible futures, describing different choices, such as start date and temperature target, as well as risks, such as termination or interruptions. SRMmodeling simulations to date typically consider only a single scenario, often with some unrealistic or arbitrarily chosen elements (such as starting deployment in 2020), and have often been chosen based on scientific rather than policy-relevant considerations (e.g., choosing quite substantial cooling specifically to achieve a bigger response). This limits the ability to compare risks both between SRM and non-SRM scenarios and between different SRM scenarios. To address this gap, we begin by outlining some general considerations on scenario design for SRM. We then describe a specific set of scenarios to capture a range of possible policy choices and uncertainties and present corresponding SAI simulations intended for broad community use. [ABSTRACT FROM AUTHOR]

Climate intervention through deliberate injection of sulfate aerosols into the stratosphere is one of the proposed solar radiation modification options to counteract some of the adverse effects of climate change. Although this approach can offset global mean temperature change, several studies have shown that there will be large residual and overcompensating regional changes. In this study, we estimate the impact of varying the latitudinal position of aerosol injection on the global monsoon precipitation in the RCP8.5 scenario by analyzing single point injection simulations from CESM1 where 12 teragram (Tg) of sulfur dioxide (SO2) are injected each year into the stratosphere at latitudes 30° S, 15° S, equator, 15° N, and 30° N. During the period 2043–2049, relative to RCP8.5, the hemispheric mean summer monsoon precipitation decreases in the hemisphere where aerosols are injected but increases in the opposite hemisphere. The hemispheric mean monsoon precipitation changes by up to ± 10% depending on the injection location. The changes in precipitation are linked to the changes in interhemispheric temperature difference and shifts in the intertropical convergence zone. The summer monsoon precipitation over India decreases by about 21% for 15° N and 29% for 30° N injections. Thus, adverse effects are likely for regions such as India when aerosols are injected at 15° N, though injection at 15° N has been shown to be more efficient in cooling the global climate in a recent study. Our study highlights the likelihood of climate interventions leading to large regional disruptions while attempting to keep the global mean climate within a safe limit. [ABSTRACT FROM AUTHOR]

Solar geoengineering is gaining prominence in climate change debates as an issue worth studying; for some it is even a potential future policy option. We argue here against this increasing normalization of solar geoengineering as a speculative part of the climate policy portfolio. We contend, in particular, that solar geoengineering at planetary scale is not governable in a globally inclusive and just manner within the current international political system. We therefore call upon governments and the United Nations to take immediate and effective political control over the development of solar geoengineering technologies. Specifically, we advocate for an International Non‐Use Agreement on Solar Geoengineering and outline the core elements of this proposal. This article is categorized under:Policy and Governance > International Policy Framework [ABSTRACT FROM AUTHOR]

Solar geoengineering appears able to reduce climate change risks but raises controversy, the leading cause of which is the concern that its research, development, and evaluation might inappropriately obstruct efforts to cut greenhouse gas emissions ('mitigation'). Describing how policies could effectively and feasibly manage such possible mitigation obstruction has proven elusive. One option would be to strategically link the international policies of mitigation and solar geoengineering. Here I explore this by disaggregating states based on their relevant characteristics. I propose linkages of mitigation policy with: (1) solar geoengineering research and development, (2) decision-making regarding whether to deploy solar geoengineering, and (3) how to deploy solar geoengineering. Based on the incentives that states would face under them, these linkages are assessed on whether they can be expected to effectively increase mitigation and are seem minimally feasible. Linkages in each of the three categories have potential and could occur sequentially. In the linkage which I believe has the greatest potential, one or more states would proclaim their right to deploy solar geoengineering if and only if they meet their own mitigation goals and the rest of the world insufficiently mitigates, and would promise to forego deployment if either condition is not met. I identify this proposed linkage's possible challenges, including legitimacy, credibility, optimal size, relations among targets of the linkage, stringency of mitigation goals, and potential counterproductivity. Limitations to this exploration and assessment include the speculative nature, the assumption that states' preferences regarding mitigation and solar geoengineering are properly related, and the use of noncooperative linkage. Key policy insights Solar geoengineering could reduce the impacts of climate change but is controversial, in large part because of concerns that its research, development, or use might obstruct efforts to cut greenhouse gas emissions. I explore and assess whether linking international policies of greenhouse gas emissions reductions and solar geoengineering could feasibly and effectively increase emissions reductions. In the linkage that I believe has the greatest potential, one or more states would proclaim their right to deploy solar geoengineering if and only if they meet their emissions reductions goals and other countries do not. [ABSTRACT FROM AUTHOR]

Regional geoengineering, by reflecting sunlight over a very limited spatial domain, might be considered as a means to target specific regional impacts of climate change. One of the obvious concerns raised by such approaches is the extent to which the resulting effects would be detectable well beyond the targeted region (e.g. in neighbouring countries). A few studies have explored this question for targeted regions that are still comparatively large. We consider idealized simulations with increased ocean albedo over relatively small domains; the Gulf of Mexico (0.23% of Earth's surface) and over the Australian Great Barrier Reef (0.07%), both with negligible global radiative forcing. Applied over these very small domains, the only statistically significant non-local changes we find are some limited reduction on summer precipitation in Florida in the Gulf of Mexico case (adjacent to the targeted region). The lack of transboundary effects suggests that governance needs for such targeted interventions are quite distinct from those for more global sunlight reflection.

Solar radiation modification, particularly stratospheric aerosol injection, holds the potential to reduce the impacts of climate change on sustainable development, yet could itself generate negative impacts and is subject to intense scholarly debate based on relatively little evidence. Based on expert elicitation involving over 30 individuals with backgrounds across the domains of the United Nations’ Sustainable Development Goals (SDGs), we identify a broad range of potential implications of solar radiation modification for the SDGs. Depending on design and application scenarios, applications could potentially assist in the pursuit of several of the goals by limiting temperature rise and limiting acceleration in atmospheric water cycles as well as extreme weather events. However, by adding to particulates, introducing an additional layer of complexity and potential for conflict in global governance, as well as otherwise altering planetary environments, they might also detract from the pursuit of SDGs and introduce novel risks. The overall impact of solar radiation modification on sustainable development is currently highly uncertain and dependent on climate change mitigation pathways and governance. We identify key areas for further transdisciplinary research the pursuit of which might reduce some uncertainty and help inform emerging governance processes. [ABSTRACT FROM AUTHOR]

Abstract Solar radiation modification (SRM) is known to strengthen both land and ocean carbon uptake because of its impacts on surface temperature, solar radiation, and other potential drivers of the global carbon cycle. However, the magnitude and timing of the response of both land and ocean carbon uptake to SRM and its consequence on allowable CO2 emissions remain poorly understood. Here we use the results of six Earth system models simulating a continuous stratospheric injection of 5 Tg of sulfur dioxide per year between 2020 and 2069 under the representative concentration pathways 4.5 to investigate the impact of SRM on land and ocean carbon uptake. We find that 50 years of SRM under this protocol increases the allowable CO2 emissions by 40 ± 19 GtC; 85% of this additional uptake of carbon is stored in the land biosphere and 15% in the ocean. This increase in allowable CO2 emissions is however not sustainable after the stoppage of SRM. Earth system models predict a mean release of 8 ± 11 GtC of the carbon back to the atmosphere 20 years after the stoppage which is dominated by large uncertainties in the response of the simulated land carbon cycle to rising temperature and solar radiation. We demonstrate that the time scales of carbon dioxide removal (CDR) potential of SRM are smaller than the time scales of the geological storage assumed in well‐established CDR options. This shows that the CDR potential of SRM should be compared to well‐established CDR options with caution.

Solar radiation modification (SRM) could greatly reduce climate change and associated risks. Yet it has not been well‐received by the climate change expert community. This is evident in the authoritative reports of the Intergovernmental Panel on Climate Change (IPCC), which emphasize SRM's governance, political, social, and ethical challenges. I find seven such challenges identified in the IPCC reports: that SRM could lessen mitigation; that its termination would cause severe climatic impacts; that researching SRM would create a "slippery slope" to its inevitable and unwanted use; that decisions to use it could be contrary to democratic norms; that the public may not accept SRM; that it could be unethical; and that decisions to use SRM could be unilateral. After assessing the extent to which these challenges are supported by existing evidence, scholarly literature, and robust logic, I conclude that, for six of the seven, the IPCC's claims variously are speculative, fail to consider both advantages and disadvantages, implicitly make unreasonable negative assumptions, are contrary to existing evidence, and/or are meaninglessly vague. I suggest some reasons for the reports' failure to meet the IPCC's standards of balance, thoroughness, and accuracy, and recommend a dedicated Special Report on SRM. This article is categorized under:Integrated Assessment of Climate Change > Assessing Climate Change in the Context of Other Issues [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection (SAI) might alleviate some climate risks associated with accumulating greenhouse gases. Reduction of specific process uncertainties relevant to the distribution of aerosol in a turbulent stratospheric wake is necessary to support informed decisions about aircraft deployment of this technology. To predict aerosol size distributions, we apply microphysical parameterizations of nucleation, condensation, and coagulation to simulate an aerosol plume generated via injection of calcite powder or sulfate into a stratospheric wake with velocity and turbulence simulated by a three‐dimensional (3D) fluid dynamic calculation. We apply the model to predict the aerosol distribution that would be generated by a propeller wake in the Stratospheric Controlled Perturbation Experiment (SCoPEx). We find that injecting 0.1 g s−1 calcite aerosol produces a nearly monodisperse plume and that at the same injection rate, condensable sulfate aerosol forms particles with average radii of 0.1 µm at 3 km downstream. We test the sensitivity of plume aerosol composition, size, and optical depth to the mass injection rate and injection location. Aerosol size distribution depends more strongly on injection rate than injection configuration. Comparing plume properties with specifications of a typical photometer, we find that plumes could be detected optically as the payload flies under the plume. These findings test the relevance of in situ sampling of aerosol properties by the SCoPEx outdoor experiment to enable quantitative tests of microphysics in a stratospheric plume. Our findings provide a basis for developing predictive models of SAI using aerosols formed in stratospheric aircraft wakes. Key Points: Improved models of stratospheric solar geoengineering require accurate predictions of aerosol formation in aircraft wakesA new 3D aerosol microphysics model predicts distribution and optical density of sulfate and calcite aerosols in the SCoPEx propeller wakeThis model is a step toward quantitative comparison between models and measurements of aerosol formation in stratospheric wakes [ABSTRACT FROM AUTHOR]

Solar radiation modification (SRM) reduces the CO2‐induced change to the mean global hydrological cycle disproportionately more than it reduces the CO2‐induced increase in mean surface temperature. Thus, if SRM were used to offset all CO2‐induced mean warming, global‐mean precipitation would be less than in an unperturbed climate. Here, we show that the mismatch between the mean hydrological effects of CO2 and SRM may partly be alleviated by spectrally tuning the SRM intervention (reducing insolation at some wavelengths more than others). By concentrating solar dimming at near‐infrared wavelengths, where H2O has strong absorption bands, the direct effect of CO2 on the tropospheric energy budget can be offset, which minimizes perturbations to the mean hydrological cycle. Idealized cloud‐resolving simulations of radiative‐convective equilibrium confirm that spectrally tuned SRM can simultaneously maintain mean surface temperature and precipitation at their unperturbed values even as large quantities of CO2 are added to the atmosphere. Plain Language Summary: It may be possible to partly counteract CO2‐driven climate change by solar radiation modification (SRM) that intentionally reduces the amount of sunlight absorbed by the Earth. But different wavelengths of the solar spectrum are absorbed at different altitudes within the surface‐atmosphere system, so different climatic effects are to be expected depending on which wavelengths of sunlight are affected by an SRM intervention. Here, we show that if the goal is to minimize perturbations to the mean hydrological cycle, the ideal spectrally tuned SRM intervention may need to focus on near‐infrared wavelengths. This study clarifies the basic physics underlying the effects of SRM on atmospheric energetics and the mean hydrological cycle. Key Points: Spectrally flat solar geoengineering reduces CO2‐induced change in mean rainfall disproportionately more than mean temperatureA spectrally tuned sunshade restores mean temperature and rainfall simultaneously in an idealized modelEmerging technologies could preferentially scatter sunlight in the near‐infrared, providing a spectral sunshade [ABSTRACT FROM AUTHOR]

Solar radiation modification (SRM) reduces the CO2‐induced change to the mean global hydrological cycle disproportionately more than it reduces the CO2‐induced increase in mean surface temperature. Thus, if SRM were used to offset all CO2‐induced mean warming, global‐mean precipitation would be less than in an unperturbed climate. Here, we show that the mismatch between the mean hydrological effects of CO2 and SRM may partly be alleviated by spectrally tuning the SRM intervention (reducing insolation at some wavelengths more than others). By concentrating solar dimming at near‐infrared wavelengths, where H2O has strong absorption bands, the direct effect of CO2 on the tropospheric energy budget can be offset, which minimizes perturbations to the mean hydrological cycle. Idealized cloud‐resolving simulations of radiative‐convective equilibrium confirm that spectrally tuned SRM can simultaneously maintain mean surface temperature and precipitation at their unperturbed values even as large quantities of CO2 are added to the atmosphere. Plain Language Summary: It may be possible to partly counteract CO2‐driven climate change by solar radiation modification (SRM) that intentionally reduces the amount of sunlight absorbed by the Earth. But different wavelengths of the solar spectrum are absorbed at different altitudes within the surface‐atmosphere system, so different climatic effects are to be expected depending on which wavelengths of sunlight are affected by an SRM intervention. Here, we show that if the goal is to minimize perturbations to the mean hydrological cycle, the ideal spectrally tuned SRM intervention may need to focus on near‐infrared wavelengths. This study clarifies the basic physics underlying the effects of SRM on atmospheric energetics and the mean hydrological cycle. Key Points: Spectrally flat solar geoengineering reduces CO2‐induced change in mean rainfall disproportionately more than mean temperatureA spectrally tuned sunshade restores mean temperature and rainfall simultaneously in an idealized modelEmerging technologies could preferentially scatter sunlight in the near‐infrared, providing a spectral sunshade [ABSTRACT FROM AUTHOR]

Stratospheric aerosol injection (SAI) is proposed as a means of reducing global warming and climate change impacts. Similar to aerosol enhancements produced by volcanic eruptions, introducing particles into the stratosphere would reflect sunlight and reduce the level of warming. However, uncertainties remain about the roles of nucleation mechanisms, ionized molecules, impurities (unevaporated residuals of injected precursors), and ambient conditions in the generation of SAI particles optimally sized to reflect sunlight. Here, we use a kinetic ion-mediated and homogeneous nucleation model to study the formation of H 2 SO 4 particles in aircraft exhaust plumes with direct injection of H 2 SO 4 vapor. We find that under the conditions that produce particles of desired sizes (diameter ∼200-300 nm), nucleation occurs in the nascent ( t < 0.01 s), hot ( T = 360-445 K), and dry (RH = 0.01-0.1%) plume and is predominantly unary. Nucleation on chemiions occurs first, followed by neutral new particle formation, which converts most of the injected H 2 SO 4 vapor to particles. Coagulation in the aging and diluting plumes governs the subsequent evolution to a narrow (σ g = 1.3) particle size distribution. Scavenging by exhaust soot is negligible, but scavenging by acid impurities or incomplete H 2 SO 4 evaporation in the hot exhaust plume and enhanced background aerosols can matter. This research highlights the need to obtain laboratory and/or real-world experiment data to verify the model prediction.

Solar radiation modification (SRM) is known to strengthen both land and ocean carbon uptake because of its impacts on surface temperature, solar radiation, and other potential drivers of the global carbon cycle. However, the magnitude and timing of the response of both land and ocean carbon uptake to SRM and its consequence on allowable CO2 emissions remain poorly understood. Here we use the results of six Earth system models simulating a continuous stratospheric injection of 5 Tg of sulfur dioxide per year between 2020 and 2069 under the representative concentration pathways 4.5 to investigate the impact of SRM on land and ocean carbon uptake. We find that 50 years of SRM under this protocol increases the allowable CO2 emissions by 40 ± 19 GtC; 85% of this additional uptake of carbon is stored in the land biosphere and 15% in the ocean. This increase in allowable CO2 emissions is however not sustainable after the stoppage of SRM. Earth system models predict a mean release of 8 ± 11 GtC of the carbon back to the atmosphere 20 years after the stoppage which is dominated by large uncertainties in the response of the simulated land carbon cycle to rising temperature and solar radiation. We demonstrate that the time scales of carbon dioxide removal (CDR) potential of SRM are smaller than the time scales of the geological storage assumed in well‐established CDR options. This shows that the CDR potential of SRM should be compared to well‐established CDR options with caution. Key Points: The global carbon uptake is reinforced by 40 ± 19 GtC after 50 years of solar radiation modificationEighty‐five percent of this global uptake is driven by the land carbon sink; only 15% is controlled by the oceanThe largest model uncertainties consist in the response of the land carbon cycle to the stoppage of solar radiation modification [ABSTRACT FROM AUTHOR]