Global relationships between cropland intensification and summer temperature extremes over the last 50 years

Conversion of native ecosystems to cropland and the use of irrigation are considered dominant pathways through which agricultural land-use change alters regional climate. Recent research proposes that increases in cropland productivity, or intensification, also influences climate through increasing evapotranspiration. Increases in evapotranspiration are expected to have the greatest temperature influence on extremely hot summer days with high vapor pressure deficits. Here, the generalizability and importance of such relationships are assessed by examining historical land-use and climate trends in seven regions across the globe, each containing a major temperate or subtropical cropping area. Trends in summer high-temperature extremes are sequentially compared against trends in cropland area, area equipped for irrigation, precipitation, and summer cropping intensity. Trends in temperature extremes are estimated using quantile regression of weather station observations, and land-use data are from agricultural inventories and remote sensing. Intensification is the best predictor of trends in extreme temperatures among the factors that are considered and is generally associated with trends that are 0.2°- 0.4°C decade^-1 cooler than in adjacent regions. Neither cropland area nor precipitation trends are systematically associated with extreme temperature trends across regions, although high temperatures are suppressed over those portions of central North America and East Asia experiencing growth in irrigation. Both the temperature trends associated with intensification and increased irrigation can be understood as a consequence of increased latent cooling. These results underscore that the weather experienced by crops is not entirely external but also depends on agricultural practices.