Impacts of Agricultural Intensification on Extreme Temperatures in the US Midwest.

Changes in land cover and land use can modify surface energy balances and influence temperature variability. Across the Midwestern United States, such factors are hypothesized to have influenced an observed cooling in summer maximum temperatures. Here we examine historical land cover and land use changes as they correlate spatially with the pattern of Midwest cooling: changes in total cropland area, changes in irrigated area, and changes in crop production. Daily maximum temperature data from the United States Historical Climatology Network (USHCN) weather stations are analyzed by month and percentile for recent years (1998-2012) and earlier portions of the 20th century (1911-1950). We find the most pronounced effect for extremely hot days, with a 2°C cooling in the 95^th percentile of July-August maximum temperatures. While model results in the literature have focused on albedo changes from cropland conversion, we find that the broad spatial pattern of cooling is closely related to the increase in crop production observed between the two periods. These results suggest that changes in evapotranspiration associated with agricultural intensification have driven the temperature shift, 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. Our findings imply that the intensification of agriculture in the Midwest may itself lead to more favorable temperature conditions – with the major exception that the moderating effect played by evapotranspiration is lost during severe drought conditions.