The Great Plains of the central United States—the Corn Belt—is one of the most fertile regions on Earth, producing more than 10 billion bushels of corn each year. It’s also home to some mysterious weather: Whereas the rest of the world has warmed, the region’s summer temperatures have dropped as much as a full degree Celsius, and rainfall has increased up to 35%, the largest spike anywhere in the world. The culprit, according to a new study, isn’t greenhouse gas emissions or sea surface temperature—it’s the corn itself.
This is the first time anyone has examined regional climate change in the central United States by directly comparing the influence of greenhouse gas emissions to agriculture, says Nathan Mueller, an earth systems scientist at the University of California (UC), Irvine, who was not involved with this study. It’s important to understand how agricultural activity can have “surprisingly strong” impacts on climate change, he says.
The Corn Belt stretches from the panhandle of Texas up to North Dakota and east to Ohio. The amount of corn harvested in this region annually has increased by 400% since 1950, from 2 billion to 10 billion bushels. Iowa leads the country for the most corn produced per state.
To see whether this increase in crops has influenced the region’s unusual weather, researchers at the Massachusetts Institute of Technology in Cambridge used computers to model five different 30-year climate simulations, based on data from 1982 to 2011. First, they compared simulations with high levels of intense agriculture to control simulations with no agricultural influence. Unlike the real-life climate changes, the control simulations showed no change in temperature or rainfall. But 62% of the simulations with intense agriculture resulted in temperature and rainfall changes that mirror the observed changes, the team reports this week in Geophysical Research Letters.
The team then compared its results to historical global simulations from the World Climate Research Programme (WCRP), an international program for the coordination of global climate research sponsored by the International Council for Science, the World Meteorological Organization, and the Intergovernmental Oceanographic Commission of UNESCO. WCRP’s models take into account greenhouse gas emissions and other natural and humanmade influences, but do not consider agricultural land use. When researchers ran the numbers for the Corn Belt, the global models fell short of reality: They predicted both temperature and humidity to increase slightly, and rainfall to increase by up to 4%—none of which matches the observed changes.
Other climate simulations that use sea surface temperature variation didn’t match observed changes, either. Those simulations matched historical data until 1970; after that, the simulations predicted temperatures to keep increasing, rather than decreasing as they did in reality. This is a strong indication that agriculture, and not changing sea surface temperature, caused the regional changes in climate during the last third of the 20th century, the researchers say.
“The [influence] of agriculture intensification is really an independent problem from greenhouse gas emissions,” says Ross Alter, lead author of the study and now a meteorologist with the U.S. Army Corps of Engineers in Hanover, New Hampshire. In fact, Alter says, heavy agriculture likely counteracted rising temperatures regionally that might have otherwise resulted from increasing greenhouse gas emissions. One other place that shows a similar drop in temperatures, he notes, is eastern China, where intensive agriculture is widespread.
But how does agriculture cause increased rainfall and decreased temperatures? The team suspects it has to do with photosynthesis, which leads to more water vapor in the air. When a plant’s pores, called stomata, open to allow carbon dioxide to enter, they simultaneously allow water to escape. This increases the amount of water going into the atmosphere and returning as rainfall. The cycle may continue as that rainwater eventually moves back into the atmosphere and causes more rainfall downwind from the original agricultural area.
Rong Fu, a climate scientist at UC Los Angeles, agrees with the team’s assessment. She also thinks that though human influence might be “greater than we realize,” this regional climate change is probably caused by many factors, including increased irrigation in the region.
“This squares with a lot of other evidence,” says Peter Huybers, a climate scientist at Harvard University, who calls the new study convincing. But he warns that such benefits may not last if greenhouse gas emissions eventually overpower the mitigating effect of agriculture.
Alter agrees, and says it’s unlikely that the large increases in U.S. crop production during the 20th century will continue. Other scientists have voiced concern that agricultural production could soon be reaching its limit in many parts of the world.
“Food production is arguably what we’re more concerned about with climate change,” Mueller says. And understanding how agriculture and climate will continue to affect one another is crucial for developing projections for both climate and agricultural yields. “It’s not just greenhouse gasses that we need to be thinking about.”
*Correction, 20 February, 9:45 a.m.: This article has been updated to correct amount of temperature drop and rainfall increase, the years of the simulations, the number of WRCP models used, and results of the control simulations. Ross Alter's quote has been updated.