68: Two-Degree Rise Drops Rice Yield By 10 Percent

By Robert Kunzig|Sunday, January 02, 2005

Is global warming good or bad for farming? The answer, no doubt, depends on the crop and the location of the farm. But a pioneering study reported in 2004 gave an alarming answer for one of the world’s most important crops: rice, the staff of life for billions of people around the world.

After an 11-year inquiry, a group of Asian and American researchers found a 10 percent drop in rice-crop yields for every increase in nighttime temperatures of 1.8 degrees Fahrenheit. The scientists, led by agronomist Kenneth Cassman of the University of Nebraska and Shaobing Peng of the International Rice Research Institute in the Philippines, believe hotter nights may speed up respiration, causing the plant to work harder and waste energy. The rice loss is particularly strong during the dry season, the higher yielding of the year’s two crops.

Global warming does not necessarily lead to agricultural disasters. Heat might be expected to harm a crop (or force farms to move poleward), but added carbon dioxide, which all plants use to make organic matter, might act as a fertilizer. “The big issue is, how do those two forces play out?” says Cassman. “What’s the net effect?”

Previously, most work on that question has been done in greenhouses or computer models. Cassman and Peng organized something different. For 11 years, from 1992 to 2003, they and their colleagues grew rice the way farmers do, controlling everything that might prevent a good yield—everything, that is, except the temperature and carbon dioxide. With average nighttime temperatures up 1.8 degrees F at the institute’s farm in the Philippines from 1979 to 2003, average yields were down 10 percent.

So far, rice yields outside the lab aren’t falling yet—farmers are clever, Cassman says, and keep finding ways to improve output. But with the Green Revolution a distant memory, yields are flattening, and Asian populations are growing, raising prospects of future food shortages. “Yes, I’m concerned about climate change,” says Cassman. “It’s like a headwind scientists must fight to increase yields.”

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