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What is stratospheric aerosol injection and how does it work?
Executive summary
Stratospheric aerosol injection (SAI) is a proposed form of solar radiation modification that would add reflective or otherwise radiatively active particles or precursor gases into the stratosphere to reduce incoming sunlight and cool Earth’s surface [1] [2]. Modeling and analogues from volcanic eruptions suggest SAI could lower global temperatures rapidly and at relatively low direct cost, but experiments and models show large uncertainties and potential side‑effects including stratospheric heating, ozone impacts, changes to precipitation and regional climate, and health/ecosystem risks [1] [3] [4].
1. What SAI is, in plain terms
SAI aims to mimic the cooling effect of big volcanic eruptions by intentionally increasing fine particles (aerosols) or releasing precursor gases like sulfur dioxide (SO2) into the stratosphere; those substances form aerosols that scatter sunlight back to space, reducing the solar energy that reaches the surface and thereby lowering temperatures [1] [2] [4].
2. How it would be done — methods and materials
Proposed delivery methods include directly injecting particles or releasing precursor gases (e.g., SO2) that oxidize to sulfate aerosols, and researchers have also considered alternatives such as calcium carbonate, aluminum oxide (alumina), or engineered solid particles (sometimes called “diamond dust” in proposals) [2] [5] [3]. Studies discuss using aircraft — from specially designed high‑altitude jets to modified existing large aircraft for certain latitudes — and seasonal or latitudinal deployment strategies to optimize effects [6] [7].
3. The basic physics: why scattering aerosols cool
Small aerosol particles in the lower stratosphere increase planetary albedo — the fraction of sunlight reflected back to space — so less shortwave radiation reaches and warms the surface; that mechanism is supported by observed post‑volcanic cooling such as after Mount Pinatubo [2] [8]. Some recent modeling also explores putting absorptive particles higher in the stratosphere to increase infrared emission and weaken CO2’s greenhouse effect, a different radiative pathway than traditional sulfate SAI [9].
4. What models and experiments say about effectiveness
Multiple modeling studies indicate SAI could swiftly offset a substantial portion of greenhouse warming and “peak‑shave” temperatures at moderate intensities, and it is one of the most‑researched solar geoengineering ideas [1] [2]. However, model intercomparison and systematic reviews stress large uncertainties — outcome depends on particle type, injection altitude/latitude/timing, and climate model differences — so confidence in regional impacts and side effects remains limited [8] [10].
5. Known and potential risks and tradeoffs
SAI is not risk‑free: models and lab‑calibrated simulations report possible stratospheric heating, perturbations to ozone chemistry (leading to depletion risk), altered precipitation patterns, and regional climate shifts that could produce winners and losers geographically [3] [10] [4]. Health, ecosystem, and agricultural effects are active research topics; for example, some recent work questions whether SAI could preserve suitable climates for certain crops [11]. Studies underline that side‑effects generally grow with the amount of cooling attempted [7].
6. Alternatives, uncertainties, and research gaps
Researchers are exploring alternatives to sulfate aerosols (solid particles, absorptive particles higher up) to reduce particular side effects like stratospheric warming, but such alternatives introduce their own uncertain chemistry and impacts [3] [9]. Systematic literature reviews call for robust uncertainty quantification and more coordinated modeling and observational experiments before any deployment could be considered [8].
7. Governance, feasibility and who could act
Technical studies show various injection strategies could be feasible with different aircraft types and amounts — some analyses even suggest existing jets could deliver meaningful injections in certain regions — but feasibility does not resolve legal, diplomatic, or ethical questions about who would decide to deploy SAI and when [6] [7] [12]. Policy and governance remain central unresolved issues in the literature [12].
8. Bottom line for readers
SAI is a scientifically grounded but contentious proposal with documented potential to lower global temperatures quickly, and with many modeled benefits; yet it carries substantial uncertainties and plausible harmful side‑effects, and alternatives or optimizations are still being explored in the literature [1] [2] [3]. Available sources do not mention any implemented, sustained planetary deployment of SAI — reporting and research emphasize modeling, limited field experiment planning, and the need for governance and further study [1] [8].