A key factor in America’s prodigious agricultural output turns out to be something farmers can do little to control: clean air. A new Stanford-led study(link is external) estimates pollution reductions between 1999 and 2019 contributed to about 20 percent of the increase in corn and soybean yield gains during that period – an amount worth about $5 billion per year.

The analysis, published this week in Environmental Research Letters, reveals that four key air pollutants are particularly damaging to crops, and accounted for an average loss of about 5 percent of corn and soybean production over the study period. The findings could help inform technology and policy changes to benefit American agriculture, and underscore the value of reducing air pollution in other parts of the world.

“Air pollution impacts have been hard to measure in the past, because two farmers even just 10 miles apart can be facing very different air quality. By using satellites, we were able to measure very fine scale patterns and unpack the role of different pollutants,” said study lead author David Lobell, the Gloria and Richard Kushel Director of the Center on Food Security and the Environment.

The research highlights the considerable power of satellites to illuminate pollution impacts at a scale not possible otherwise. That power could be of even greater value in countries with less access to air monitors and yield data.

Reading the air

Scientists have long known that air pollution is toxic to plant life in high doses, but not how much farmers’ yields are actually hurt at current levels. The impact of pollution on agriculture overall, as well as the effects of individual pollutants, has also remained unknown.

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.

Despite important agricultural advancements to feed the world in the last 60 years, a new study shows that global farming productivity is 21% lower than it could have been without climate change. This is the equivalent of losing about seven years of farm productivity increases since the 1960s.

The future potential impacts of climate change on global crop production has been quantified in many scientific reports, but the historic influence of anthropogenic climate change on the agricultural sector had yet to be modeled. Now, a new study published April 1 in Nature Climate Change provides these insights. 

David Lobell, professor of earth system science at Stanford University and coauthor of the study, said that the results show clearly that adaption efforts must look at the whole supply chain, including labor and livestock. “They also show that even as agriculture becomes more mechanized and sophisticated, the sensitivity to weather does not go away,” he said. “This is counter-intuitive for most people, and we need a deeper understanding of why.”

“We find that climate change has basically wiped out about seven years of improvements in agricultural productivity over the past 60 years,” said Ariel Ortiz-Bobea, associate professor in the Charles H. Dyson School of Applied Economics and Management at Cornell University and lead author of the study. “It is equivalent to pressing the pause button on productivity growth back in 2013 and experiencing no improvements since then. Anthropogenic climate change is already slowing us down.”

The scientists and economists developed an all-encompassing econometric model linking year-to-year changes in weather and productivity measures with output from the latest climate models over six decades to quantify the effect of recent human-caused climate change on what economists call “total factor productivity,” a measure capturing overall productivity of the agricultural sector.

Ortiz-Bobea said they considered more than 200 systematic variations of the econometric model, and the results remained largely consistent. “When we zoom into different parts of the world, we find that the historical impacts of climate change have been larger in areas already warmer, including parts of Africa, Latin America and Asia,” he said.

Humans have already altered the climate system, Ortiz-Bobea said, as climate science indicates the globe is about 1 degree Celsius warmer than without atmospheric greenhouse gases.

“Most people perceive climate change as a distant problem,” Ortiz-Bobea said. “But this is something that is already having an effect. We have to address climate change now so that we can avoid further damage for future generations.”

Ortiz-Bobea and Robert G. Chambers, professor of production economics at the University of Maryland, have been pioneering new productivity calculations in agriculture to include weather data that has not been addressed historically, aiming to bring new accuracy to climate models.

“Productivity is essentially a calculation of your inputs compared to your outputs, and in most industries, the only way to get growth is with new inputs,” Chambers said. “Agricultural productivity measurement hasn’t historically incorporated weather data, but we want to see the trends for these inputs that are out of the farmer’s control.” 

“My sense is that we are just getting better at eliminating all the non-weather constraints on production, but we need to scrutinize various possible explanations,” said Lobell, who examines the impact of climate change on crop production and food security. “This study is a big leap beyond the traditional focus on a few major grain crops,” he said. “By looking at the whole system – the animals, the workers, the specialty crops – we can see that the entire agricultural economy is quite sensitive to weather. It seems that in agriculture, practically everything gets harder when it’s hotter.”

In addition to Ortiz-Bobea, Chambers and Lobell, the co-authors are Toby R. Ault, professor of earth and atmospheric sciences in the College of Agriculture and Life Sciences; and Carlos M. Carrillo, research associate in the Department of Earth and Atmospheric Science. 

Funding was provided by USDA National Institute of Food and Agriculture and the National Science Foundation.

Media Contacts: 

Blaine Friedlander, bpf2@cornell.edu, 607-254-8093

Devon Ryan, devonr@stanford.edu, 650-497-0444