Authors' Summary: Solar geoengineering is a proposed set of technologies to help lessen the impacts of climate change by reducing the amount of sunlight received by the Earth. Stratospheric aerosol injection is a method of solar geoengineering that reduces sunlight by increasing the amount of aerosol particles in the stratosphere, a process which can also cause stratospheric ozone depletion. Nearly all studies of stratospheric aerosol injection have focused exclusively on the direct impacts of increased stratospheric aerosol on climate. However, changes in sunlight also alter the rates of chemical reactions throughout the atmosphere, changing the concentrations of greenhouse gases that affect climate like methane and tropospheric ozone.

Our results show that these changes in greenhouse gases due to geoengineering chemical feedbacks can substantially alter the climate effect of geoengineering, especially on regional and seasonal scales. Our results also show that geoengineering-induced stratospheric ozone depletion can lead to net global health benefits, as the impacts on mortality from overall improvements in surface air quality due to chemical feedbacks outweigh those from increases in UV exposure. These same chemical feedbacks can also improve crop yields and overall plant growth.

Our results underscore the risk of surprises that could arise from solar geoengineering.

Abstract: We present results from large ensembles of projected 21st century changes in seasonal precipitation and near-surface air temperature over Africa and selected sub-continental regions. These ensembles are a result of combining Monte Carlo projections from a human-Earth system model of intermediate complexity with pattern-scaled responses from climate models of the Coupled Model Intercomparison Project Phase 6. These future ensemble scenarios consider a range of global actions to abate emissions through the 21^st century. We evaluate distributions of surface-air temperature and precipitation change. In all regions, we find that without any emissions or climate targets in place, there is a greater than 50% likelihood that mid-century temperatures will increase threefold over the current climate’s two-standard deviation range of variability. However, scenarios that consider more aggressive climate targets all but eliminate the risk of these salient temperature increases. A preponderance of risk toward decreased precipitation exists for much of the southern Africa region considered, and this is also compounded by enhanced warming (relative to the global trajectory). Over eastern and western Africa, the preponderance of risk in increased precipitation change is seen. Strong climate targets abate evolving regional hydroclimatic risks. Under a target to limit global climate warming to 1.5˚C by 2100, the risk of precipitation changes within Africa toward the end of this century (2065-2074) is commensurate to the risk during the 2030s without any global climate target. Thus, these regional hydroclimate risks over much Africa could be delayed by 30 years, and in doing so, provide invaluable lead-time for national efforts to prepare, fortify, and/or adapt.