Quantifying the efficiency of stratospheric aerosol geoengineering at different altitudes

Stratospheric aerosol injection (SAI) of reflective sulfate aerosols has been proposed to temporarily reduce the impacts of global warming. In this study, we compare two SAI simulations which inject at different altitudes to provide the same amount of cooling, finding that lower-altitude SAI requires 64% more injection. SAI at higher altitudes cools the surface more efficiently per unit injection than lower-altitude SAI through two primary mechanisms: the longer lifetimes of SO2 and SO4 at higher altitudes, and the water vapor feedback, in which lower-altitude SAI causes more heating in the tropical cold point tropopause region, thereby increasing water vapor transport into the stratosphere and trapping more terrestrial infrared radiation that offsets some of the direct aerosol-induced cooling. We isolate these individual mechanisms and find that the contribution of lifetime effects to differences in cooling efficiency is approximately five to six times larger than the contribution of the water vapor feedback.