Warming temperatures over the past quarter century have cut production of cereal crops worldwide by millions of tons and caused annual losses of roughly $5 billion, according to a study released Friday.
The news wasn't completely dire. The report from the Lawrence Livermore National Laboratory also said the damage has been offset by production gains from genetically modified crops and better farming techniques. Researchers also said they did not account for farmers making their own adjustments to cope with the rising temperatures.
"The warming we've already experienced since 1980 (about 0.7 degrees Fahrenheit) is having a major impact on the production of crops," the lead author, David Lobell, said in a telephone interview. "It is true that the impacts of climate change have been relatively small compared to the overall increase because of technology."
The study from the Livermore, Calif.-based lab is the first to examine how much global food production has been harmed by climate change, Lobell said.
The study estimated that the global temperature increase from 1980 to 2002 means 40 million fewer metric tons of barley, corn, rice, sorghum, soybeans and wheat are produced around the world each year, leading to an annual loss of $5 billion.
The report does not include national or regional breakdowns. California officials said there would be little effect on the nation's top agricultural state, which is not a major producer of grains other than rice.
Farm researchers were skeptical of the study's findings.
"That kind of flies in the face of reality," said Terry Francl, a senior economist with the Washington, D.C.-based American Farm Bureau Federation. "The reality is we've had improved trends in yields during all of that time. It's hard to see how you would calculate global warming's effect on that."
Longer growing seasons and an increase in carbon dioxide also encourage plant growth. Yields have improved due largely to genetic modification of grain crops, and farmers have adjusted to changing conditions by planting crops where they grow best, Francl said.
Lobell said changes in growing seasons and carbon dioxide levels have tended to cancel each other out when it comes to crop yields. He said researchers did not attempt to account for farmers' crop adaptation.
"Farmers have the ability to adjust," Lobell said. "But over the last 20 years, there has been little evidence that farmers are adjusting."
Lawrence Livermore is operated by the University of California for the U.S. Department of Energy's National Nuclear Security Administration. The peer-reviewed report was published online in Friday's Environmental Research Letters, published by the London-based Institute of Physics.
Lobell wrote the report with Christopher Field, who directs the Carnegie Institutions' Department of Global Ecology at Stanford University.
They compared changes in yields with variations in average temperature and precipitation in major crop-growing regions. They concluded that yields have been dropping about 3 percent to 5 percent for every 1 degree Fahrenheit increase.