Expanding Irrigation Intensifies the Humid Heat Stress in China: Evidence From Observations and Modeling

China is the world's second-largest agricultural irrigation country, with profound influences on the escalating heat stress. This study combines historical weather records and 10-year regional climate simulations, in conjunction with optimized irrigation algorithm, to disentangle the dual impacts of irrigation on heat stress in China. Application of province-specific soil moisture thresholds generates improved simulation of irrigation water amount and a lower model error by 30%. The substantial cooling and moistening effects by irrigation are revealed by weather records and optimized irrigation-climate simulations, that is, −0.13°C/decade (−0.01°C to −0.72°C) for daily maximum air temperature and +1.80%/decade (0.02%–3.96%) for relative humidity. Despite alleviating the extreme high-temperature events, agricultural irrigation intensifies the occurrence (0.93 days yr⁻¹) and population exposure (0.46 million person-days yr⁻¹) of humid heat. This study suggests that irrigation may not serve as an optimal land management adaptation strategy to mitigate the growing humid heat under climate change.