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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.

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 intervention We find MCB impacts that have qualitative similarities to prior work, but there are discrepancies that suggest key inter‐model uncertainties MCB simulations generally show reduced tipping element risk overall, but certain MCB patterns may exacerbate some tipping element changes

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 microphysics Accounting for settling and agglomeration of solid particles can substantially reduce the radiative forcing (RF) per unit of injected material Injection of diamond particles (r = 150 nm) instead of SO2 largely reduces stratospheric temperature, circulation and water vapor anomalies

Persistently rising atmospheric greenhouse gas concentrations challenge dominant Liberal hopes that science and multilateralism might deliver rational, global climate outcomes. Emerging Realist climate approaches that take geopolitics and national interests more seriously have yet to explore Morgenthau’s concern that ‘scientism’ – exaggerated faith in scientific rationality to solve political problems – would lead to disastrous underestimations of power and irrationality. Recently, Realists have mooted ‘solar geoengineering’ designs as a ‘lesser evil’ option to deliberately cool the Earth independently of emissions reductions. However, assessments of solar geoengineering prospects barely factor in Realist concerns, focusing instead on idealised scientific modelling of bio-physical effects and Liberal governance scenarios. To explore how geoengineering techno-science would be ‘translated’ into security assessments, geopolitical logics were elicited through interviews and group discussions with (mainly Arctic-oriented) national security professionals. Security experts reframe solar geoengineering in three significant ways: (a) from a climate ‘global public good’ to a source of geopolitical leverage and disruption; (b) from a risk-reduction tool to a potential source of distrust and escalation; and (c) from a knowledge-deficit problem solvable by more research, to a potential disinformation vector. This expands Realist scholarship on climate change and identifies serious risks to ongoing scientific and commercial pursuit of such technologies.

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.

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.

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.

The deliberate addition of sulfur dioxide in the stratosphere to form reflective sulfate aerosols, reflect sunlight, and reduce surface temperatures is increasingly being considered as an option for minimizing the impacts of climate change. This strategy would create an unprecedented climate where the relationship between surface temperature and carbon dioxide concentration is decoupled. The implications of stratospheric aerosol intervention (SAI) for global crop protein concentrations have not yet been explored. While elevated CO2 concentrations are expected to reduce crop protein, higher temperatures may increase crop protein concentrations. Here we report changes of maize, rice, soybean, and wheat protein concentrations under a medium emissions climate change scenario and a SAI scenario to maintain global average temperatures at 1.5 °C above preindustrial levels, as simulated by three global gridded crop models. We show that using SAI to offset surface temperature increases would create decreases in the global protein concentrations of maize and rice, with minimal impact on wheat and soybean. Some already protein-deficient and malnourished nations that rely heavily on these crops to meet protein demands would show large decreases in protein intake under SAI with the current diet pattern, which could exacerbate their nutrient scarcity. The range of results between crop models highlights the need for a more comprehensive analysis using additional crop models, climate models, a broader range of climate intervention scenarios, and advancements in crop models to better represent protein responses to climate changes.

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.