Summer Climate Change in the Midwest and Great Plains due to Agricultural Development during the Twentieth Century

Agricultural development is among the most significant forms of land-use change globally. In central North America it has consisted of cropland expansion in the early 1900s, yield intensification starting in the 1930s, and the development of large irrigated areas beginning in the 1950s. The area of this study encompasses the Midwest and Great Plains of the United States not only because significant agricultural change has occurred here but also because of the significant cooling (warming hole) there in the midcentury. This study investigates the relative contribution of agricultural development and greenhouse gas (GHG) emissions on the observed patterns of regional changes in summer temperature, precipitation, and evapotranspiration using a long-term twentieth- century reanalysis dataset (CERA-20C) as boundary conditions for simulations with the MIT Regional Climate Model (MRCM). Temperatures in the Great Plains (33–43°N, 95–109°W) and the Midwest (38–48°N, 82– 109°W) would have been significantly higher in the second half of the twentieth century without the influence of agricultural development, largely due to an increase in evaporative cooling. The simulations of precipitation changes reflect a significant influence of global SST teleconnections at decadal time scales. Numerical simu- lations also demonstrate the competing effects of cropland expansion and yield intensification on shaping the observed pattern of increases in precipitation. Ultimately, a combination of agricultural development and de- cadal variability of global sea surface temperatures (SST) explains most of the observed variability of summer temperature and precipitation during the twentieth century over central North America.