Can Agricultural Intensification Explain Unexpected Cooling of Extreme Heat in the Midwestern United States?

Abstract

Despite climate change, data show decreasing maximum temperatures in a portion of the midwestern United States during the mid-summer. The cause of this “warming hole” is unknown. Previous research has discussed the importance of aerosols, atmospheric circulation, and agriculture as contributors to this phenomenon. In this research I examine the effect of corn and soybean production, the two most prevalent crops in the Midwest, on maximum temperature anomalies during the summer. I develop a novel test of the hypothesis, motivated by the research of Mueller et al. (2016), that the unusual cooling is a result of changes in agriculture via the biological process of evapotranspiration. Specifically, using data on crop progress from 1981 to 2019, I compare year-to-year variability in timing of peak transpiration to the corresponding temperature anomalies. I find mixed results. Soybeans seem to have a cooling effect on the maximum temperatures while corn seems to have a warming effect. This challenges what one would assume from the literature that corn, which has a slightly greater evapotranspiration rate, should have greater or similar cooling effects as soybeans. These findings suggest that while agricultural activity may influence the climate, the link is more complex than simple evapotranspirative cooling. While the mechanism is not clear, the empirical evidence suggests that growth of soybean production may have contributed to the Midwestern warming hole.