Cooling of US Midwest summer temperature extremes from cropland intensification

Abstract

High temperature extremes during the growing season can reduce agricultural production. At the same time, agricultural practices can modify temperatures by altering the surface energy budget. Here we identify centennial trends towards more favourable growing conditions in the US Midwest, including cooler summer temperature extremes and increased precipitation, and investigate the origins of these shifts. Statistically significant correspondence is found between the cooling pattern and trends in cropland intensification, as well as with trends towards greater irrigated land over a small subset of the domain. Land conversion to cropland, often considered an important influence on historical temperatures, is not significantly associated with cooling. We suggest that agricultural intensification increases the potential for evapotranspiration, leading to cooler temperatures and contributing to increased precipitation. The tendency for greater evapotranspiration on hotter days is consistent with our finding that cooling trends are greatest for the highest temperature percentiles. Temperatures over rainfed croplands show no cooling trend during drought conditions, consistent with evapotranspiration requiring adequate soil moisture, and implying that modern drought events feature greater warming as baseline cooler temperatures revert to historically high extremes. I ncreasing population, rising per capita food demand, and limited availability of arable land all point to a need to achieve greater crop productivity 1 . Climate change, however, may compromise the ability to sustain growth in crop yields 2 , in part owing to expected increases in damaging extreme temperatures 3-5 . Yet agricultural areas are subject to substantial local, as well as global, climate forcings, as changes in agricultural land cover and land management can alter the surface energy balance and influence temperatures 6-20 . Against this backdrop, it is relevant to examine historical trends in growing-season climate, especially in the most important growing regions. We focus on the US Midwest because it exhibits the most vigorous crop growth anywhere on the planet during the peak of the growing season ( Fig. 1), and because of the availability of detailed weather and crop data. Centennial trends in Midwest summer climateAlthough overall US temperature trends are towards warming over the past century, the hottest temperatures observed during the growing season in the US Midwest have actually cooled. We examine temperature since 1910 as a balance between duration and availability of continuous data, and use quantile regression of daily maximum temperature records from weather stations to assess trends across multiple percentiles (see Methods). Trends in hot summer temperatures are evaluated using the 95th percentile, and are of particular interest because of the negative effects of high temperatures on yield 3-5 . Midwest cooling is less evident in median temperature trends, and temperatures are generally warming at the 5th percentile (Fig. 2a,b). The...

Aerosol trends as a potential driver of regional climate in the central United States: evidence from observations

“…We also do not consider microclimate feedbacks from irrigation at the Ameriflux station. Although irrigation has been shown in other studies to suppress extreme temperatures in the Midwest (Mueller et al, 2016), we find that the monthly-average temperatures recorded at the Ameriflux Bondville site nearly match the temperature readings at the nearest airport in Champaign, IL (not shown). Also, since the Ameriflux temperatures and SWdn data correspond closely to these value in the 1/8 • NLDAS dataset (Fig.…”

Biomass sorghum and maize have similar water-use-efficiency under non-drought conditions in the rain-fed Midwest U.S.

“…Dense crop canopies and intensely managed agroecosystems have been shown to reduce maximum summertime temperatures in the Corn Belt (i.e. warming hole) to the extent that water is available (Mueller et al, 2015). Therefore, if water becomes more limited in future climates, the ability for maize to regulate summertime temperatures may be inhibited.…”

“…Therefore, if water becomes more limited in future climates, the ability for maize to regulate summertime temperatures may be inhibited. Sustained ET associated with biomass sorghum may preserve the observed warming hole in the Midwest, US (Mueller et al, 2015).…”

The impact of clouds, land use and snow cover on climate in the Canadian Prairies

“…The large increase in the area of cropland has increased summer transpiration, which has reduced maximum temperatures in the growing season over the Prairies (Gameda et al, 2007). Other analyses of US Midwest summer temperature maxima also show a cooling from land-use change to cropland (Bonan, 2001) and cropland intensification (Mueller et al, 2016). Figure 5 summarizes the long-term climate impact of the reduction of summer fallow in Saskatchewan (Betts et al, 2013b).…”

“…Early case studies showed that snow cover reduces surface temperatures by about 5 • C on both the short-term timescale and on monthly timescales (Namias, 1960(Namias, , 1985Cohen and Rind, 1991;Dewey, 1977;Wagner, 1973). However there has been surprisingly little quantitative observational analysis of the impact of snow on local and regional climate.…”