by Caroline Hays
Food security is an ever growing concern in a world with an expanding population, an expanding demand for land-intensive meat production, and a finite amount of cropland. Crop productivity is a primary concern for food security and is largely affected by precipitation and temperature. Extreme temperatures caused by global warming and the subsequent climate change are a particular concern for crop productivity. In addition to global effects, local climate patterns are important for crop productivity. As it turns out, climate’s effects on crops are not a one way street: agricultural practices also exert their own influences on rainfall and temperature. Mueller et al. (2015) looked at this feedback loop in the US Midwest and found that cropland intensification has contributed to more desirable conditions for crops, leading to higher precipitation rates and more moderate extreme high temperatures.
In the US Midwest, a major agricultural hub, the authors looked particularly at trends in the hottest temperatures, which are important because of their potential to harm crops. Over the last century, the Midwest has seen a decrease in these extreme hot temperatures. The authors found that cropland intensification and increased irrigation were significantly correlated with this cooling trend, while conversion of undeveloped land to cropland was not. Specifically, the authors found that a growth in irrigation of greater than 7% of county area per decade was associated with 0.3 °C cooling per decade.
Cropland intensification had a stronger effect on cooling than irrigation. To explain this effect, the authors hypothesized that cropland intensification causes greater evaporation from the soil and from plants leaves. Newer tilling techniques, fertilizers, and shifts in crop types are some of the factors that may lead to greater evaporation from the soil and plants, which has an overall cooling effect.
To test this theory the authors looked at high temperatures during periods of drought. During a drought, higher rates of evaporation would not be possible, cutting off the proposed cooling mechanism. The results showed that in rain-fed areas during periods of drought there is no cooling trend present as there is during periods of non-drought. In areas with significant irrigation, drought did not have any effect on cooling trends. This result supports the theory that increased evaporation from soil and plants is the mechanism by which cropland intensification led to cooler temperatures. This result also predicts that as temperatures are lowered by this mechanism, droughts may lead to more extreme relative temperatures by inhibiting the cooling effects of cropland intensification that go on outside of periods of drought.
Mueller, Nathaniel D., Ethan E. Butler, Karen A. McKinnon, Andrew Rhines, et al. 2015. Cooling of US Midwest Summer Temperature Extremes from Cropland Intensification. Nature Climate Change. doi:10.1038/nclimate2825.