The idea that global warming might be a boon for crop production seems less likely in light of a long-term study showing just the opposite effect for one key crop.
Rice is considered the most important food staple, consumed daily by about half the world's population. Researchers say that production worldwide needs to rise by 1% annually to meet the demands of a growing population. But there's no consensus on how climate change will affect rice cultivation, says Kenneth Cassman, an agronomist at the University of Nebraska, Lincoln, because of "our relatively crude understanding" of how environmental changes affect the growth and yield of crops.
The accepted wisdom is that hotter, sunnier days and an increase in atmospheric carbon dioxide should boost photosynthesis and thus growth. At the same time, hotter nights are known to increase plant respiration, which cuts into grain production. Cassman and a team led by Shaobing Peng, an agronomist at the International Rice Research Institute in Manila, the Philippines, analyzed 25 years' of weather data recorded at the institute and rice harvests of its experimental plots to see which factors predominated.
Their results, published online this week in the Proceedings of the National Academy of Sciences, showed that the only clear relationship between the yield of rice and the changing environment was a negative one. Yield dropped by about 10% for every 1oC rise in nighttime maximum temperature, the researchers found. (Daytime and nighttime maximum temperatures at the site increased by 0.35oC and 1.13oC, respectively, over the period.) The current models used to predict climate impact on crop production will have to be revised, says Cassman, because they lump together daytime and nighttime temperatures into a single average temperature.
Global warming could have a "highly significant impact" on food supply, says Chris West, director of the Environmental Change Institute at the University of Oxford, U.K., if that relationship holds for all crops in all locations. Current climate models predict an overall rise of 1.5oC to 4.5oC degrees by 2100, most of that representing higher overnight temperatures. One response to such a change, suggests West, is to breed new cultivars of rice that can better handle those hot nights.