Global Warming Reduces Expected Yields of Harvests in Some Countries, Study Says

http://www.nytimes.com/2011/05/06/science/earth/06warming.html?_r=1&ref=world

By JUSTIN GILLIS
Published: May 5, 2011

Global warming is already cutting substantially into potential crop yields in some countries — to such an extent that it may be a factor in the food price increases that have caused worldwide stress in recent years, researchers suggest in a new study.

Wheat yields in recent years were down by more than 10 percent in Russia and by a few percentage points each in India, France and China compared with what they probably would have been without rising temperatures, according to the study.

Corn yields were off a few percentage points in China, Brazil and France from what would have been expected, said the researchers, whose findings were published in Friday’s issue of the journal Science.

Some countries saw small gains from the temperature increases, however. And in all countries, the extra carbon dioxide that humans are pumping into the air acted as a fertilizer that encouraged plant growth, offsetting some of the losses from rising temperatures caused by that same greenhouse gas.

Consequently, the study’s authors found that when the gains in some countries were weighed against the losses in others, the overall global effect of climate change has been small so far: losses of a few percentage points for wheat and corn from what they would have been without climate change. The overall impact on production of rice and soybeans was negligible, with gains in some regions entirely offsetting losses in others.

But the authors of the study — David Lobell and Justin Costa-Roberts of Stanford University, and Wolfram Schlenker of Columbia University — pointed out that temperature increases were expected to accelerate in coming decades, making it likely that the challenges to food production will grow in an era when demand is expected to rise sharply.

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The authors of the new study specifically excluded the effects of extreme weather like brief heat waves and flash floods because of limitations in the data that they used. For that and other reasons, Dr. Lobell said, the study’s estimate of the impact of climate change is probably conservative.

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