What are the projected climate effects and limits of SAI on global temperatures and precipitation patterns?
Executive summary
Stratospheric aerosol injection (SAI) can rapidly reduce global-mean surface temperature by reflecting sunlight, and many model experiments show it can stabilize or lower average temperatures relative to high‑CO2 futures [1] [2]. However, precipitation and regional climate responses are heterogeneous: SAI generally reduces the magnitude of future precipitation increases globally but produces large spatial and seasonal shifts in rainfall, circulation patterns, and other side effects that vary with injection strategy [3] [4] [5].
1. How SAI cools the planet and the projected limits on global temperature control
SAI works by increasing stratospheric aerosol loading (typically sulfate from SO2) to scatter incoming solar radiation, producing a “global dimming” that can take effect rapidly and lower global-mean temperature in model experiments [6] [1]. Multiple modelling intercomparisons and scenario studies find that many SAI strategies can offset several degrees of greenhouse warming at relatively low direct implementation cost compared with some mitigation pathways [6] [1]. Yet this capacity has limits: modelled temperature control depends on sustained, repeated injections — termination would lead to rapid rebound warming — and effectiveness depends on injection amount, latitude and aerosol type, so SAI cannot “solve” CO2-driven ocean warming or acidification [6] [7] [1].
2. Precipitation: overall damping but large regional divergence
Across model ensembles, SAI tends to reduce the global-mean changes in precipitation projected under unabated warming — the decrease in incoming solar radiation lowers evaporative cooling, which mutes the water‑cycle amplification expected with warming — but this global damping coexists with substantial regional redistribution of rainfall [4] [3]. Studies show SAI can lessen future increases in wet-season rainfall and soil‑moisture declines in some vulnerable regions, yet other areas may experience drying or altered monsoon timing, with consequences for agriculture and water security [8] [3].
3. Circulation shifts drive much of the regional variability
The dominant mechanism for heterogeneous precipitation responses is dynamical: SAI alters stratospheric and tropospheric heating, which in turn modulates large‑scale circulations such as the Hadley and Walker cells, the Intertropical Convergence Zone (ITCZ), and annular modes — effects that change where and when rain falls and can strengthen or weaken storm tracks [9] [5]. Model experiments find that injection latitude is a first‑order control: equatorial injections often produce stronger undesirable circulation and precipitation side effects, while multi‑latitude or high‑latitude strategies can mitigate some regional disparities [5] [9].
4. Timescales, variability, and perceived “failure” risks
Even if global-mean temperatures are stabilized, internal climate variability can dominate SAI’s signal at regional scales for years to decades, meaning some regions could still experience anomalously high temperatures or drought shortly after deployment — a dynamic that could be perceived politically as failure even when global targets are met [2]. Moreover, abrupt cessation (“termination shock”) would cause rapid warming with attendant hydrological disruption, making continuous commitment a practical and ethical constraint [7].
5. Non‑climatic limits and tradeoffs that constrain SAI’s usefulness
Beyond temperature and precipitation, SAI affects stratospheric chemistry (including ozone), changes the ratio of direct to diffuse sunlight with ecological consequences, and poses governance, health and ethical challenges; these non‑climatic limits constrain how and whether SAI could be used even if models show temperature control is feasible [5] [6] [10]. Scientific assessments therefore call for comprehensive, inclusive frameworks for research and monitoring to detect and manage regional impacts and to weigh risks against mitigation and adaptation options [11] [1].
Conclusion: controllable at the global mean, unpredictable at local scales
The accumulated model evidence frames SAI as a potentially powerful lever for rapidly shaving global temperatures, but with clear limits: it cannot reverse CO2‑driven ocean and biogeochemical changes, it requires continuous deployment to maintain cooling, and it redistributes precipitation and circulation in complex, strategy‑dependent ways that create winners and losers regionally [2] [6] [4]. Policymaking must therefore confront scientific uncertainty, unequal regional risks, and governance questions before treating SAI as a practical climate solution [11] [10].