Stratospheric aerosol injection

Stratospheric aerosol injection is a proposed method of solar geoengineering (or solar radiation modification) to reduce global warming. This would introduce aerosols into the stratosphere to create a cooling effect via global dimming and increased albedo, which occurs naturally from volcanic winter. It appears that stratospheric aerosol injection, at a moderate intensity, could counter most changes to temperature and precipitation, take effect rapidly, have low direct implementation costs, and be reversible in its direct climatic effects. The Intergovernmental Panel on Climate Change concludes that it "is the most-researched [solar geoengineering] method, with high agreement that it could limit warming to below 1.5 °C (2.7 °F)." However, like other solar geoengineering approaches, stratospheric aerosol injection would do so imperfectly and other effects are possible, particularly if used in a suboptimal manner.

Various forms of sulfur have been shown to cool the planet after large volcanic eruptions. However, as of 2021, there has been little research and existing natural aerosols in the stratosphere are not well understood. So there is no leading candidate material. Alumina, calcite and salt are also under consideration. The leading proposed method of delivery is custom aircraft.