Impacts of cropland intensification on extreme temperatures

Background/Question/Methods

Changes in land cover and land use can modify regional surface energy balances and influence temperature variability. Across the United States, conversion of natural ecosystems to cropland and the use of irrigation are thought to have had a cooling effect on summer temperatures, due to changes in albedo and evapotranspiration. We hypothesize that agricultural intensification could also influence temperatures, as increases in crop productivity are related to increases in evapotranspiration. We use weather station data to analyze whether cooling is observed in recent years (1998-2012) across agricultural regions of the United States relative to earlier portions of the 20^th century (1911-1950). Using county-level USDA crop data, we examine four hypotheses to explain agriculture-related cooling: increases in irrigated farmland, changes in cultivated area, intensified production on existing farmland, and a null hypothesis of internal variability unrelated to land use.

Results/Conclusions

Our analysis of ~850 weather stations shows a strong cooling in summer maximum temperatures across much of the Midwestern United States. The cooling effect is most pronounced for 95^th percentile maximum temperatures, with heavily cropped areas showing a ~2°C drop in these extreme temperature events. The broad spatial pattern of cooling is closely related to the increase in crop production observed between the two periods, suggesting that intensification-driven changes in evapotranspiration are more important for controlling the temperature shift than solely changes in crop area or irrigation. These results 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.