Focusing on a nine-state region (Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, South Dakota and Wisconsin) that produces roughly two-thirds of national maize and soybean output, Lobell and study co-author Jennifer Burney, an associate professor of environmental science at the University of California, San Diego, set out to measure the impact on crop yields of ozone, particulate matter, nitrogen dioxide and sulfur dioxide.

Ozone is the result of heat and sunlight-driven chemical reactions between nitrogen and hydrocarbons, such as those found in car exhaust. Particulate matter refers to large particles of dust, dirt, soot or smoke. Nitrogen dioxide and sulfur dioxide are gases released into the atmosphere primarily through the burning of fossil fuels at power plants and other industrial facilities.

“This has been a tricky problem to untangle because historically our measurements of different types of air pollutants and our measurements of agricultural yields haven’t really overlapped spatially at the necessary resolution,” explained Burney. “With the new high spatial resolution data, we could look at crop yields near both pollution monitors and known pollutant emissions sources. That revealed evidence of different magnitudes of negative impacts caused by different pollutants.”

Lobell and Burney extended their analysis back to 1990, when Congress passed Clean Air Act amendments that resulted in significant air quality improvements across the country. The researchers looked through air pollution data from hundreds of monitoring stations around the region, federal data on power plant emissions, satellite-based observations of nitrogen dioxide around those power plants, crop yield data from federal surveys and satellite imagery, as well as weather data to account for growing season conditions known to explain crop yield variations.

Surprising findings

What Lobell and Burney discovered surprised them. Among their findings: negative effects of each of the four pollutants on corn and soybean yields, and a clear yield increase the farther away from power plants – particularly coal-burning facilities – crops were grown. The unique spatial patterns of each pollutant allowed them to disentangle the effect of each pollutant in a way that past studies could not.

The researchers estimated that total yield losses from the four pollutants averaged 5.8 percent for maize and 3.8 percent for soybean over the past two decades. Those losses declined over time as the air grew cleaner. In fact, the reduction in air pollution contributed to an estimated 4 percent growth in corn yields and 3 percent growth in soybean yields – increases that equal 19 percent of corn’s overall yield gains during the timeframe and 23 percent of soybeans’ overall yield gains.

“We already know that the Clean Air Act resulted in trillions of dollars of benefits in terms of human health, so I think of these billions in agricultural benefits as icing on the cake,” Lobell said. “But even if it’s a small part of the benefits of clear air, it has been a pretty big part of our ability to continue pushing agricultural productivity higher.”

Lobell is also a professor of Earth system science in Stanford’s School of Earth, Energy & Environmental Sciences, the William Wrigley Senior Fellow at the Stanford Woods Institute for the Environment and a senior fellow at the Freeman Spogli Institute for International Studies and the Stanford Institute for Economic Policy Research. Burney also holds the Marshall Saunders Chancellor’s Endowed Chair in Global Climate Policy and Research at UC San Diego and is a research affiliate at UC San Diego’s Policy Design and Evaluation Laboratory, a fellow at the Stanford Center on Food Security and the Environment, and head of the Science Policy Fellows Program at UC San Diego.

This research was funded by NASA and the National Science Foundation.

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