Impacts of agricultural intensification on extreme temperatures in the US Midwest

Monday, 15 December 2014
Nathaniel D Mueller, Ethan E Butler, Noel M Holbrook and Peter J Huybers, Harvard University, Cambridge, MA, United States
Changes in land cover and land use can modify surface energy balances and influence temperature variability. Agricultural land management changes are widespread across the US Midwest, and here we investigate whether historical land cover and land use changes can explain observed reductions in daily maximum temperatures across the region. We find cooling is most pronounced for the hottest temperatures, with intensively cropped regions showing a 2ºC cooling in the 95th percentile of late summer temperatures (1998-2012 relative to 1911-1950). We examine changes in total cropland area, changes in irrigated area, and intensification of crop production (represented as the change in crop NPP) as candidate mechanisms and show that only the latter significantly corresponds to the observed cooling of temperature extremes. These findings are consistent with agricultural intensification leading to a greater capacity for evapotranspiration that manifests most clearly on the hottest days, a hypothesis we further investigate using a one-dimensional energy balance model driven by hourly data from the Illinois Water and Atmospheric Resources Monitoring Program. Intensification of agriculture in the Midwest may itself lead to more favorable growing conditions, but with the major exception that the moderating effects of evapotranspiration will be lost during drought conditions.