Submitted by: Mick Kulikowski, NC State News Services

Yields of three of the most important crops produced in the United States - corn, soybeans and cotton - are predicted to fall off a cliff if temperatures rise due to climate change.

In a paper published online this week in Proceedings of the National Academy of Sciences, North Carolina State University agricultural and resource economist Dr. Michael Roberts and Dr. Wolfram Schlenker, an assistant professor of economics at Columbia University, predict that U.S. crop yields could decrease by 30 to 46 percent over the next century under slow global warming scenarios, and by a devastating 63 to 82 percent under the most rapid global warming scenarios. The warming scenarios used in the study - called Hadley III models - were devised by the United Kingdom's weather service.

The study shows that crop yields tick up gradually between roughly 10 and 30 degrees Celsius, or about 50 to 86 degrees Farenheit. But when temperature levels go over 29 degrees Celsius (84.2 degrees Farenheit) for corn, 30 degrees Celsius (86 degrees Farenheit) for soybeans and 32 degrees Celsius (89.6 degrees Farenheit) for cotton, yields fall steeply.

"While crop yields depend on a variety of factors, extreme heat is the best predictor of yields," Roberts says. "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."

Roberts adds that while the study examined only U.S. crop yields under warming scenarios, the crop commodity market's global reach makes the implications important for the entire world, as the United States produces 41 percent of the world's corn and 38 percent of the world's soybeans.

"Effects of climate change on U.S. crop production will surely be felt around the globe, especially in developing countries,"he says.

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Note: An abstract of the paper follows.

"Nonlinear temperature effects indicate severe damages to U.S. crop yields under climate change"

Authors: 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

Abstract: The United States produces 41% of the world's corn and 38% of the world's soybeans. These crops comprise two of the four largest sources of caloric energy produced and are thus critical for world food supply. We pair a panel of county-level yields for these two crops, plus cotton (a warmer-weather crop), with a new fine-scale weather dataset that incorporates the whole distribution of temperatures within each day and across all days in the growing season. We find that yields increase with temperature up to 29 degrees C for corn, 30 degrees C for soybeans, and 32 degrees C for cotton but that temperatures above these thresholds are very harmful. The slope of the decline above the optimum is significantly steeper than the incline below it. The same nonlinear and asymmetric relationship is found when we isolate either time-series or cross-sectional variations in temperatures and yields. This suggests limited historical adaptation of seed varieties or management practices to warmer temperatures because the cross-section includes farmers' adaptations to warmer climates and the time-series does not. Holding current growing regions fixed, area-weighted average yields are predicted to decrease by 30-46% before the end of the century under the slowest (B1) warming scenario and decrease by 63-82% under the most rapid warming scenario (A1FI) under the Hadley III model.