How can we narrow down the uncertainty in Afro-Asian monsoon projection?

The Afro-Asian summer monsoon (AfroASM) sustains billions of people living in many developing countries covering West Africa and Asia, vulnerable to climate change. Future increase in AfroASM precipitation has been projected by current state-of-the-art climate models, but large inter-model spread exists. Here we show that the projection spread is related to present-day interhemispheric thermal contrast (ITC). Based on 30 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6), we find models with a larger ITC trend during 1981-2014 tend to project a greater precipitation increase. Since most models overestimate present-day ITC trends, emergent constraint indicates precipitation increase in constrained projection is reduced to 70% of the raw projection, with the largest reduction in West African (49%). The land area experiencing significant increases of precipitation is 57% of the raw projection. Given that the emergent constraint improves the reliability in AfroASM precipitation projections, we further investigate the impacts of the constrained projection on the potential water availability. The fractions of land area that will experience a significant increase of potential water availability are about 66% of the raw projection. We find that global surface air temperature warming plays a dominant role in the emergent constraint on precipitation changes, while the contribution from hydrological sensitivity should not be neglected. The smaller increase of potential water availability in the constrained projection than the raw projection may pose a challenge to climate change adaptation and mitigation activities related to water management and food security, although a smaller than expected increase in rainfall will also reduce the risk of extreme precipitation and flooding.