Three of the most important agricultural crops produced in the United States—corn, soybeans and cotton—are predicted to suffer a devastating decline if temperatures continue to rise as a result of climate change. The study appears online this week in Proceedings of the National Academy of Sciences.

 

Experts debate whether crops will suffer or flourish with rising temperatures caused by human-induced climate change. Research conducted by Wolfram Schlenker, assistant professor in Columbia’s department of economics and School of International and Public Affairs, and Michael Roberts, North Carolina State University agricultural and resource economist, however, show that yields for these important crops would increase gradually between roughly 50 and 86 degrees Fahrenheit (F), then fall precipitously after reaching a crop-specific temperature ceiling.

 

“Our findings show that temperature increases are only beneficial for yields up to approximately 86° F,” said Schlenker. “Current crop varieties are very sensitive to temperature increases above this temperature. The problem is that the damaging effects from additional days with temperatures above 86° F far outweigh the benefits from warming below it.”  

 

He adds that U.S. crop yields would decrease by 30 to 46 percent over the next century under slow global warming scenarios. Under a rapid warming scenario, these crops could experience a devastating 63 to 82 percent failure. The warming scenarios used in the study are taken from Hadley III climate models used in assessments by the Intergovernmental Panel on Climate Change.

 

To conduct their analysis, the researchers used data comprised of new fine-scale weather outcomes merged with a large panel of crop yields that spans most U.S. counties from 1950 to 2005. The new weather data focuses on the length of time each crop is exposed to every one-degree Celsius temperature interval in each day. This was then tallied across all days of the growing season and estimated for specific locations within each U.S. county where crops are grown. Their analysis revealed that days with more temperature variation above a set average critically affected crop yields.

 

The researchers’ findings show that such drastic declines in crop yields could have reverberating global  impacts. Many developing countries depend on food exports from the U.S., the world's biggest producer and exporter of corn and soybeans. Steep decreases in critical crops would drive up prices, which would particularly affect poor areas of the world.

 

Schlenker and Roberts believe that estimating the correct relationship between weather and yields for these major crops is a critical first step before more elaborate models can be used to examine how crop-planting choices, food and fiber supply, and prices will ultimately shift in response to climate change. According to their study, under the current climate forecasts, the best adaptation strategy would be to develop more heat-resistant crops.

 

Strikingly, the researchers found that the warmer southern areas of the United States exhibit the same sensitivity to heat as places further north. “This is troublesome, as one would have expected to learn from farmers in the warmer south how to deal with hotter temperatures,” said Schlenker, “but we found no evidence that they did.”  He and Roberts believe that future research should focus on examining more closely how heat tolerance evolves over time.

 

“While crop yields depend on a variety of factors, extreme heat is the best predictor of yields,” said Roberts. “There hasn’t been much research on what happens to crop yields over certain temperature thresholds, but this study shows that temperature extremes are not good.”

 

In their analysis, the researchers do not account for carbon dioxide (CO2) concentrations. According to the paper, plants use CO2 as an input in the photosynthesis process; increasing CO2 levels might increase plant growth and yields, however, the magnitude of this effect is still being debated.