Quantifying the Efficiency of Stratospheric Aerosol Geoengineering at Different Altitudes
Geoengineering News
Abstract
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.
Key Points
We compare two stratospheric aerosol injection strategies which inject SO2 at different altitudes to meet the same temperature target
The low altitude strategy requires two thirds more injection to provide the same amount of cooling
We isolate and quantify the different factors which cause the high altitude injection strategy to cool the surface more efficiently
Plain Language Summary
Stratospheric aerosol injection (SAI)—the artificial introduction of reflective droplets, called aerosols, into the middle atmosphere–could reflect a small portion of sunlight and cool the planet in order to temporarily reduce the impacts from global warming. Injecting the aerosols at higher altitudes would be more expensive, but it would also be more efficient because the aerosols would last longer before falling out of the atmosphere. Additionally, injecting at a lower altitude would cause more water vapor to enter the middle atmosphere; since water vapor is a greenhouse gas, this would increase the greenhouse effect, meaning more aerosols would be needed to cool the surface to a desired temperature. In this study, we directly compare high-altitude SAI to low-altitude SAI to determine how much more efficient it is to inject at a higher altitude, and we break down the different factors that effect efficiency to see which has the biggest effect.
Source: AGU