Extreme temperature trends in major cropping systems and their relation to agricultural land use change
High temperature extremes during the growing season can reduce agricultural production. At the same time, agricultural land cover and land use changes can modify temperatures by altering the surface energy budget. Here we examine a centennial cooling trend in the hottest temperatures of the summer growing season in the US Midwest, and investigate its association with land use change drivers using historical USDA data. Temperature trends are calculated across percentiles of summer temperatures using GHCN weather station data. Cooling is compared to changes in cropland area, irrigation, and an agricultural intensity index that integrates harvested area and productivity from historical USDA data. Relationships identified in the US are then examined globally in major cropping systems using digitized agricultural census records available starting in 1961.
In the US Midwest, we identify an association between cooling and the intensification of primarily rainfed agriculture. Increased irrigation is also associated with a distinct cooling effect over a small area, and land conversion to cropland has no identifiable influence over the time period examined. Intensification has been achieved through widespread changes in crop management and cultivar characteristics that generally increase evapotranspiration rates, which has likely driven the observed shift. During severe drought, this moderating influence of evapotranspiration is lost and temperatures revert back to historically high values. Preliminary results indicate these relationships between temperature extremes, irrigation, and intensification are also observed in other major cropping systems, including northern China, Argentina, and the Canadian Prairies. These findings demonstrate that land management changes impact the coupling between agroecosystems and the climate system, and have contributed to observed climate change.