Aerosol mitigation matters to future water availability in the global monsoon region.

Water availability, as measured by precipitation minus evaporation (P-E), is projected to increase in the 21st century across the global monsoon region. However, while the impacts of increased greenhouse gas (GHG) concentrations are highlighted in existing studies, the contribution of reduced anthropogenic aerosol (AA) emissions is likely to be overlooked. Here, utilizing single-forcing projections under the SSP2-4.5 scenario, we elucidate the fingerprints of GHG and AA forcings on future P-E evolution. We reveal that future P-E changes are primarily driven by changes during the wet season, with contrasting trends between the Asian-African and American monsoon domains. The escalation of GHG concentrations is projected to increase P-E over Asian-African monsoon domains while decreasing it over the American monsoon domains. Conversely, aerosol reductions will drive a transition from current widespread drying to future wetting, with contributions comparable to GHG forcing over the Asian-African monsoon domains. While GHG increases and AA reductions can elevate atmospheric moistening through radiative warming, the disparate P-E responses arise from dynamic processes that favor drying trends in the American monsoon domains under GHG forcing. In contrast, strengthened monsoon circulations contribute to a wetting trend in the Asian-African monsoon domains under AA reductions, attributable to greater interhemispheric thermal contrast. Our findings highlight the importance of considering aerosol mitigation in climate risk assessments for densely populated monsoon regions.