Compounding future escalation of emissions- and irrigation-induced increases in humid-heat stress

Irrigation has been investigated as an important historical climate forcing, but there is no study exploring its future climatic impacts considering possible changes in both extent and efficiency. Here, we address these issues via developing irrigation efficiency scenarios in line with the Shared Socioeconomic Pathways (SSPs), implementing these in the Community Earth System Model, and applying them to generate projections over the period 2015-2074. We project that annual irrigation water withdrawal decreases under SSP1-2.6 (from  ~2100 to  ~1700 km³ yr^-1) but increases under SSP3-7.0 (to  ~2400 km³ yr^-1), with some new irrigation hot spots emerging, especially in Africa. Irrigation is projected to reduce the occurrence of dry-heat stress under both scenarios, but cannot reverse the warming trend due to greenhouse gas emission (e.g., increasing from  ~90 to around 600 and 1200 hours yr^-1 in intensely irrigated areas, under two scenarios). Moreover, moist-heat extreme event frequency increases more substantially (by ≥1600 hours yr^-1 under SSP3-7.0 in tropical regions), and irrigation further amplifies the hours of exposure (for example, by ≥100 hours yr^-1 in South Asia), thereby raising the risk of moist-heat-related illnesses and mortality for exposed communities. Our results underscore the importance of reducing greenhouse gas emissions, limiting irrigation expansion and improving irrigation efficiency to preserve water resources and decelerate escalating exposure to dry- and moist-heat stress.