The climate response to stratospheric aerosol geoengineering can be tailored using multiple injection locations

By injecting different amounts of SO2 at multiple different latitudes, the spatial pattern of aerosol optical depth (AOD) can be partially controlled. This leads to the ability to influence the climate response to geoengineering with stratospheric aerosols, providing the potential for design. We use simulations from the fully coupled whole-atmosphere chemistry climate model CESM1(WACCM) to demonstrate that by appropriately combining injection at just four different locations, 30 degrees S, 15 degrees S, 15 degrees N, and 30 degrees N, then three spatial degrees of freedom of AOD can be achieved: an approximately spatially uniform AOD distribution, the relative difference in AOD between Northern and Southern Hemispheres, and the relative AOD in high versus low latitudes. For forcing levels that yield 1-2 degrees C cooling, the AOD and surface temperature response are sufficiently linear in this model so that the response to different combinations of injection at different latitudes can be estimated from single-latitude injection simulations; nonlinearities associated with both aerosol growth and changes to stratospheric circulation will be increasingly important at higher forcing levels. Optimized injection at multiple locations is predicted to improve compensation of CO2-forced climate change relative to a case using only equatorial aerosol injection (which overcools the tropics relative to high latitudes). The additional degrees of freedom can be used, for example, to balance the interhemispheric temperature gradient and the equator to pole temperature gradient in addition to the global mean temperature. Further research is needed to better quantify the impacts of these strategies on changes to long-term temperature, precipitation, and other climate parameters. Plain Language Summary Solar geoengineering, by adding aerosols to the stratosphere that reflect some sunlight, could be used to partially offset the climate change from increased carbon dioxide (CO2). However, one of the concerns is that this does not affect the climate the same way that increased CO2 does, leading to some regions cooling more than others. Previous simulations only injected aerosols at a single latitude. We show that if you were to inject aerosols at a combination of multiple different latitudes, you could better tailor the resulting climate response, providing a way of designing solar geoengineering to better meet climate goals. One could, for example, adjust not only the injection rate to maintain the global mean temperature at some desired value but also the temperature difference between Northern and Southern Hemispheres (which influences tropical precipitation) and the temperature difference between tropics and high-latitude regions.