Stratospheric aerosol
Executive summary
Stratospheric aerosol intervention (SAI) is a proposed form of solar geoengineering that would inject particles (most commonly sulfur-derived aerosols) into the stratosphere to reflect sunlight and cool the surface; recent model experiments prescribe sulfur injections beginning in 2025 in coordinated intercomparison projects (e.g., SSP2‑4.5 SAI) to study climate responses [1] [2]. Scientific literature in 2025 emphasizes both potential rapid cooling and broad uncertainties and risks — impacts on precipitation, stratospheric heating, ozone, crop nutrition, and distributional effects remain contested across studies and model ensembles [2] [3] [4] [5].
1. What SAI is and how researchers model it
Stratospheric aerosol injection aims to form a persistent layer of particles in the lower stratosphere that scatters incoming sunlight back to space, producing global dimming and a cooling influence similar in mechanism to large volcanic eruptions (the Pinatubo analogy) — this is the basic premise used in coordinated modelling experiments such as the SSP2‑4.5 SAI runs that begin prescribed SO2 injections in 2025 for intercomparison studies [2] [1]. Researchers use retrieval tools and model frameworks (for example, REMAP and derived aerosol optical properties) to translate aerosol distributions into the radiation and chemistry inputs required by chemistry‑climate models [6].
2. Evidence of efficacy: rapid cooling but imperfect compensation
Multiple sources underline that SAI can produce rapid cooling and, at moderate intensity, can largely counter temperature and some precipitation changes in models — which explains why it is called "the most‑researched" solar geoengineering method in assessments [2]. However, modelling also shows SAI would not perfectly reverse all aspects of climate change: it does not remove atmospheric CO2 and hence creates a novel climate state where temperature and CO2 are decoupled [4] [2].
3. Key physical uncertainties — residence time, injection latitude and height
How long aerosols persist depends strongly on where and how they are injected: tropical injections and injection height control stratospheric residence time and thus climatic influence, with studies showing sensitivity in lifetime and plume dynamics that affect distribution and duration of cooling [7] [8]. These transport and size‑distribution uncertainties complicate attempts to predict regional climate responses and optimal deployment strategies [8].
4. Chemistry and ozone risks: competing material choices
Sulfur‑based aerosols can cool but warm the stratosphere and participate in heterogeneous chemistry that may deplete ozone; recent work explores solid particles (alumina, calcite) as alternatives that could reduce stratospheric heating but whose ozone impacts are uncertain — authors explicitly warn that solid particle injection "entails great uncertainties" and could still affect ozone in models [3]. The literature therefore frames material choice as a trade‑off between radiative efficiency, stratospheric heating, and chemical side effects [3] [2].
5. Regional climate and ecological side effects
Regional impacts show divergence across model experiments: some multi‑model ensembles find no stronger Asian monsoon drying from SAI compared with greenhouse mitigation under certain configurations, while others flag risks to precipitation patterns and storm behavior — model design, aerosol spatial distribution, and insolation gradients drive much of the intermodel spread [9] [5]. Agricultural impacts are also a concern: studies predict SAI could change crop protein concentrations and nutrient outcomes because it alters the temperature–CO2 relationship and radiation environment [4] [10].
6. Governance, deployment feasibility, and socio‑political issues
Technical modelling aside, analysts stress governance and coordination as decisive: uncoordinated or unilateral deployment could shift regional rainfall, exacerbate droughts or monsoon disruptions, and create transboundary harms that require international institutional arrangements — commentators and reports urge extreme caution and international cooperation [11] [12]. Cost and who could deploy SAI are active policy debates referenced in public commentary and review pieces [13].
7. Where the science converges and where it does not
There is convergence that SAI can cool quickly and that many physical processes (residence time, transport, aerosol microphysics) critically determine outcomes — hence the proliferation of coordinated experiments and retrieval tools to reduce modeling uncertainties [1] [6] [7]. There is marked disagreement or uncertainty about regional precipitation responses, ozone impacts across proposed particle types, ecological effects on crops and nutrition, and the geopolitical risks of deployment — these topics remain debated and model‑dependent [3] [4] [9] [5].
8. What reporting often omits or understates
Many popular summaries emphasize "rapid cooling" or "a quick fix" but available technical literature emphasizes imperfect compensation for CO2‑driven changes, substantial uncertainties in regional outcomes, and distinct trade‑offs among aerosol materials [2] [4] [3]. Also, retrieval and modelling advances (e.g., REMAP) are essential background work not always covered in media stories but critical for reducing scientific uncertainty [6].
Limitations: this analysis summarizes peer‑reviewed studies, model intercomparison experiments, and recent commentary included in the provided set of sources; available sources do not mention some policy specifics (e.g., enacted treaties) or operational deployment plans beyond modelling and scenario work (not found in current reporting).