Stratospheric aerosol injection does not cause stronger Asian monsoon drying than greenhouse gas mitigation

Abstract Stratospheric aerosol injection (SAI) is a potential approach to limit global mean surface temperature (GMST) rise, buying time for mitigation. Yet, concerns persist that SAI may substantially reduce Asian monsoon (ASM) rainfall, a vital water resource for nearly half of the global population. Using idealized simulations from six climate models participating in the Geoengineering Model Intercomparison Project Phase 6 (GeoMIP6), we show that both greenhouse gas (GHG) mitigation and equatorial SAI lead to an overall reduction in ASM rainfall, primarily through thermodynamic effects associated with decreased atmospheric moisture as GMST cools. Importantly, equatorial SAI produces no extra drying over the ASM region compared with GHG mitigation for the same magnitude of GMST cooling. Relative to a baseline with comparable GMST, all six models show no substantial drying over the ASM region beyond internal variability. A major part of the inter-model spread arises from differences of the changes in meridional insolation gradient, which may depend on the spatial distribution of aerosols. Smaller (larger) insolation reductions over the ASM region relative to the equator favor enhanced (suppressed) ascent air motion and rainfall over the ASM region, a relationship confirmed by single-model simulations with aerosol injections at varying latitudes. These results suggest that well-designed SAI deployment strategies could minimize risk of monsoon failure.