May 062011
 
Wheat.

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A crop-yield analysis reveals that warming temperatures have already diminished the rate of production growth for major cereal crop harvests during the past three decades.

The people of the world get 75 percent of their sustenance—either directly, or indirectly as meat—from four crops: maize (corn), wheat, rice and soybeans. The world’s rising population—now predicted by the United Nations to reach 10.1 billion by century’s end—has been fed thanks to rising yields of all four of these crops during the past century. Humanity’s predilection for burning fossil fuels, however, is now contributing to the slowing of such rising yields, cutting harvests of wheat 5.5 percent and maize 3.8 percent from what they could have been since 1980, according to a new analysis of yields.

“On a global scale, we can see pretty clearly significant changes in the weather for most places where we grow crops,” explains agricultural scientist David Lobell of Stanford University’s Woods Institute for the Environment, who led the analysis published in the May 6 issue of Science. “Those changes are big enough to sum up to pretty big losses for wheat and corn.”

Using U.N. Food and Agriculture Organization data going back to 1980 for crop yields in all major crop-growing regions of the world, and pairing that with temperature and precipitation data for their growing seasons, Lobell and his colleagues found that warming temperatures were reducing yields—although changes in precipitation did not appear to be having an effect, yet.

Those temperature changes are the result of increasing concentrations of atmospheric greenhouse gases, such as carbon dioxide (CO2), largely as a result of burning fossil fuels and agricultural practices. But CO2 also helps rice, soybeans and wheat grow. In fact, the researchers suggest the extra CO2 boosted yields for these crops by roughly 3 percent during the period studied. Unfortunately, in the case of wheat, that wasn’t enough to overcome the loss in yields resulting from warming temperatures. “Temperature effects are already overriding CO2 effects,” Lobell notes.

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