Recent shift from energy- to moisture-limitation over global croplands

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 in r(T, ET) over the past ∼40 years, indicating a shift to a more moisture-limited state. In some regions, especially Europe, the sign of r(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 in r(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.