The Afro-Asian summer monsoon (AfroASM) is crucial for billions of people, yet climate models show large discrepancies in future precipitation projections. This study reveals a link between the projection spread and the present-day interhemispheric thermal contrast (ITC). Models with larger ITC trends project greater precipitation increases. Emergent constraint, accounting for model overestimation of ITC, reduces the projected precipitation increase to 70% of the raw projection, most significantly in West Africa (49%). The land area experiencing significant precipitation/runoff increases is 57%/66% of the raw projection. Reduced precipitation increases may lower flood risk but challenge future water resource management.
