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Abstract Hot and dry conditions pose a substantial risk to global crops. The frequency of co-occurring heat and drought depends on land–atmosphere coupling, which can be quantified by the correlation between temperature and evapotranspiration (r(T, ET)). We find that the majority of global croplands have experienced declines inr(T, ET) over the past ∼40 years, indicating a shift to a more moisture-limited state. In some regions, especially Europe, the sign ofr(T, ET) has flipped from positive to negative, indicating a transition from energy-limitation to moisture-limitation and suggesting a qualitative shift in the local climate regime. We associate stronger declines inr(T, ET) with faster increases in annual maximum temperatures and larger declines in soil moisture and ET during hot days. Our results suggest that shifts towards stronger land–atmosphere coupling have already increased the sensitivity of crop yields to temperature in much of the world by 12%–37%, as hot days are not only hotter, but also more likely to be concurrently dry. 

The impacts of extreme heat events depend on the concurrent humidity level. Hot and dry conditions harm crops and vegetation, while hot and humid conditions are particularly dangerous for human health. Whether a place tends to experience heat waves that are hot and dry, hot and wet, or both, depends on the local correlation between temperature and humidity. This paper uses reanalysis, observational data, and climate models to show that the global temperature‐humidity correlation has steadily declined since 1981 at a somewhat faster rate than climate models simulate. This decline in the temperature‐humidity correlation has occurred because humidity is increasing less quickly (or decreasing) on days with high dry‐bulb temperatures as compared to days with high wet‐bulb temperatures. The relative drying of hot and dry days is associated with those days warming faster than hot and humid days. Overall, the decline in the temperature‐humidity correlation has exacerbated the risk from hot and dry conditions while slightly slowing increases in hot and humid conditions.