Warmer climate, decreased rice yield

Agricultural data gathered over a dozen years at a rice paddy in the Philippines suggest that climate changes brought about by global warming could significantly diminish rice yields.

At the International Rice Research Institute in Los Baños, scientists grow two crops of rice each year, one of them during the dry season from January through April. On some plots, researchers have grown the same variety of rice with the same agricultural techniques since 1992, says Kenneth G. Cassman, an agronomist at the University of Nebraska in Lincoln.

From 1992 through 2003, dry-season crop yields declined roughly 10 percent for each 1°C increase in the average minimum temperature, which typically occurred at night. Cassman and his colleagues suggest that these higher temperatures boost the plant’s nighttime respiration, decreasing the amount of energy available to store in grain. Although changes in average minimum temperature apparently didn’t affect the length of the growing season or the size of grains, rice plants that grew during warmer years were smaller at harvest time and produced fewer grains.

Crop yields during the wet season, from July to September, showed no significant variations with temperatures between 1992 and 2003. The researchers report their findings in the July 6 Proceedings of the National Academy of Sciences.

Average maximum temperatures at the site rose 0.35°C over the past 25 years, but the average nighttime lows over the same period jumped 1.13°C. Similar warming patterns have been observed at many sites worldwide in recent years, and they don’t bode well for future rice yields. Average global temperature is projected to increase between 1.5°C and 4.5°C by the end of this century.