Increased nighttime temperatures were associated with significant declines in crop yield at the International Rice Research Institute Farm in the Philippines, according to a report in Monday's issue of Proceedings of the National Academy of Sciences.
Indeed, an average daily temperature increase of 1 degree Celsius resulted in a 10 percent reduction in the rice crop, according to the researchers. One degree Celsius equals about 1.8 degree Fahrenheit.
Increasing temperatures, thought to be a result of heat trapped by industrial and other chemicals in the atmosphere, have caused mounting concern in recent years. Scientists have argued over the potential effects of climate change on crops, largely basing their contentions on laboratory tests and computer models of climate and crop yield.
This new study was a direct measurement of yields under field conditions using practices that good farmers would employ, said lead researcher Kenneth G. Cassman of the University of Nebraska.
Cassman and colleagues studied 12 years of rice yields at the farm, along with weather data, to reach their conclusions.
The results are generally similar to findings reported last year following a 17-year study of U.S. crop yields. That study indicated that increases in temperature resulted in reductions in corn and soybean yields.
Cassman said researchers are working to determine the cause of the reduction, but they speculate that it is because the hotter nights make the plants work harder just to maintain themselves, diverting energy from growth.
"If you think about it, world records for the marathon occur at cooler temperatures because it takes much more energy to maintain yourself when running at high temperatures. A similar phenomenon occurs with plants," he said.
Tim L. Setter, a professor of soil, crop and atmospheric science at Cornell University, commented that higher nighttime temperatures "could consume carbohydrates in a nonproductive way, and by reducing the reserves of carbohydrates, particularly at time of flowering and early grain filling, would decrease the number of kernels that would be set."
Setter, who was not a part of the rice research team, said a recent study in corn had shown that the number of kernels was decreased when researchers experimentally increased the night temperatures.
Cassman commented that rice production has stagnated in recent years in some key producing regions of Asia.
"That's something that hasn't been on the radar screen and needs to get more attention," he said.
The new study was funded by the International Rice Research Institute and the Agricultural Research Division of the University of Nebraska.
By Randolph E. Schmid