86 Degrees Is Ceiling for Corn, Soybeans, Cotton, Study Says

A Columbia, North Carolina State study finds that current crop varieties are sensitive to increases in temperature above 86 degrees Fahrenheit.

Corn, soybeans and cotton are predicted to suffer a devastating decline if temperatures continue to rise as a result of climate change. The study appeared online recently 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, then fall precipitously after reaching a crop-specific temperature ceiling.

"Our findings show that temperature increases are only beneficial for yields until 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 model runs, one of the major 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 include the length of time each crop is exposed to each one-degree Celsius temperature interval in each day, summed across all days of the growing season, all estimated for the specific locations within each county where crops are grown. Holding the average temperature constant, days with more variation will include more exposure to extreme outcomes, which can critically influence yields.

The researchers' findings show that drastic declines could have reverberating impacts around the world as the United States is by far the biggest producer and exporter of corn and soybeans. Many developing countries depend on the food exports from the U.S., and food price spikes between 2005 and 2008 have shown that a shortage in world production is experienced most by developing countries.

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.

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.

"Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change," by Wolfram Schlenker, Columbia University and Michael Roberts, North Carolina State University, published Aug. 24, 2009, in the online version of Proceedings of the National Academy of Sciences.

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