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Heat, not drought, will drive lower crop yields, researchers say

New research from Ariel Ortiz-Bobea, Toby Ault and Carlos Carrillo in Environmental Research Letters looks at how heat stress remains the primary climatic driver of lower future agriculture yields under climate change. Photo provided. 

Cornell researchers have weighed in on a high-stakes debate between crop experts and scientists: Which of climate change’s challenges – higher temperature or stress from drought – poses the greater threat to U.S. rain-fed agriculture?

“There is a big divide in this field, and we thought there must be some way to resolve this puzzle,” said Ariel Ortiz-Bobea, assistant professor of applied economics and management and CoBank/Farm Credit East Sesquicentennial Faculty Fellow in Production Economics and Sustainability.

Using decades’ worth of data from government and other sources, the researchers predict that climate change-induced heat stress will play a larger role than drought stress in reducing the yields of several major U.S. crops later this century. That has major implications for crop management as well as plant breeding.

The researchers’ findings are reported in “Unpacking the Climatic Drivers of U.S. Agricultural Yields,” published May 24 in Environmental Research Letters. Contributors included Toby Ault, assistant professor of earth and atmospheric sciences; postdoctoral associate Carlos Carrillo; and Haoying Wang, assistant professor of management at New Mexico Tech.

While simulations based on mechanistic crop models anticipate water supply will substantially drive future crop yields, statistical models – which capture the relationship between observed crop yields and weather – have not. 

“My hypothesis was that empiricists like myself have not been able to accurately measure the water available to plants,” Ortiz-Bobea said. “If one cannot measure water availability, then one cannot measure its effect on yield properly. … [D]rought tolerance and heat tolerance are associated with different traits in plant breeding, and it requires significant time to incorporate them into new varieties.”

Ortiz-Bobea

Ortiz-Bobea and his team incorporated information from three sources to develop a statistical crop yield model for six crops: maize, cotton, sorghum, soybeans, spring wheat and winter wheat. They tapped more than three decades of crop yield records from the U.S. Department of Agriculture, weather data from the PRISM Climate Group at Oregon State University, and hourly snapshots of soil moisture content at nine-mile intervals across all of North America from the National Oceanic and Atmospheric Administration and NASA.

The researchers’ analysis revealed that soil moisture alone was the best predictor of year-to-year variations in yield across the past four decades. Harvests were particularly sensitive to drought stress in the middle portion of the growing season. For example, reducing soil moisture by 30% from historical averages was linked to an 18% reduction in maize yield. However, slightly dry conditions appeared beneficial for some crops around harvest.

“When we improved how we measure water availability, we were able to tease out nuances within the growing season, such as that wet conditions and dry conditions have different effects, depending on the crop’s growth stage,” said Ortiz-Bobea, a 2017-2018 Social Sciences, Humanities and Arts fellow with Cornell’s Atkinson Center for a Sustainable Future.

The team then applied its statistical model to climate change scenarios ranging from mild to severe. The analysis projects that temperature, which the authors interpret as heat stress, will be the primary climatic driver of crop yields in 2050 and 2100. Under the mildest scenario, yields for the six crops are predicted to decrease by 8% to 19%, relative to a world without climate change. Under the most severe scenario, the projected yield reductions range from 20% to 48%.

The greatest losses are forecasted for maize and spring wheat, but more resilient crops such as sorghum, which is half as sensitive to high temperature as maize, will experience less damage.

Co-author Ault noted climate change projections show that many of the food-producing counties in the United States could become drier in the summer even if rainfall increases. In a changing climate, this could motivate farmers to plant earlier, but that approach to adaptation can be thwarted by heavy rains during the late spring, as many regions are experiencing this year.

“The work highlights two major challenges for adapting to a changing climate,” Ault said. “First, how do we deal with increases in temperature that through higher evaporation rates could surpass increases in precipitation? And second, how can we start to envision an agricultural system of the 21st century that is equipped to handle the remarkable shifts in seasonality that might occur?”

The study was partially funded by the Atkinson Center through its Faculty Fellowship for the Social Sciences, Humanities and the Arts program.

Amanda Garris is a freelance writer for the Atkinson Center.

This article also appeared in the Cornell Chronicle.