This is Bob Doughty with the VOA Special English Agriculture Report.

Scientists have had theories that climate change could harm crops. Now, a study offers what it calls direct evidence that rising temperatures at night can shrink（使萎縮）harvests of rice.

Scientists from China, the Philippines and the United States did the study for the International Rice Research Institute. The institute is based outside Manila. The report appeared in the United States in the Proceedings of the National Academy of Sciences.

The researchers used weather information gathered over twenty-five years at the institute's own farm. They also used information on rice harvests at the research farm from nineteen-ninety-two until last year.

The scientists say every increase of one degree Celsius（攝氏度）in the average daily temperature would decrease rice harvests by fifteen percent. Earlier studies found half that estimate. That was because they did not consider the effects of temperatures at different times of the day.

The new study found that the average daily temperature in the twenty-five year period increased by seven-tenths of one degree. But nighttime temperatures increased more than a full degree. In fact, the increase was three times greater than the increase in daytimes temperatures. The scientists found that the daytime increase had no clear effect on productivity（產量）.

Professor Kenneth Cassman of the University of Nebraska-Lincoln took part in the study. He says the new findings could be explained by the theories of scientists about the effects of global warming.

The scientists say industrial gasses trap heat in the atmosphere, so the ground cannot cool as much at night. Climate studies suggest that average temperatures could increase as much as four-point-five degrees in the next one hundred years.

The researchers in the new study say higher nighttime temperatures may cause the rice plants to spend less energy on growing. Other studies have suggested that grains like wheat and maze（迷宮）act the same way. But scientists say more work must be done to understand how plants act under conditions of climate change.

Improved crops developed in the nineteen-sixties and seventies mean that rice harvests are now two times greater. Rice production has kept up with population growth so far. But Professor Cassman says the gains needed in the future could be more difficult.

This VOA Special English Agriculture Report was written by Mario Ritter. This is Bob Doughty.