A new study indicates that rising nighttime temperatures cause a decline in the yield of rice plants. The results of the study have serious implications for half of the world's people, who depend on rice as a major part of their diet. Scientists are analyzing the results to figure why this is happening and what can be done to stop it.

According to a study conducted by the International Rice Research Institute, an average nighttime temperature increase of just one degree Celsius causes a 10 percent reduction in rice crops.

The spokesman for the institute, Duncan Macintosh, says he hopes this study will encourage further research into why this phenomenon is happening.

"Here's something significant," he said. "We really need now to get to work and go into detail and understand why it's happening, the scale of its impact globally. All these sorts of issues are what we're hoping now will follow on in terms of research."

The scientists involved in the study looked at rice production on a farm in the Philippines over a 12-year period, and combined their observations with climate data. They were surprised to find that higher nighttime temperatures resulted in lower rice production. Most previous research had focused on the impact of daytime temperature changes.

Mr. Macintosh says it takes years for average temperatures to rise even one degree, so rice farmers will probably not notice the decline in their farms' output.

"At the end of the day, even though much of the world and certainly western societies are concerned about an issue like this, it has absolutely no impact almost at the farmer level," he said. "Let's not fool ourselves, the rice farmers of the world are concerned about their next meal, about generating enough income to feed their families, and getting out of the poverty that literally millions of them are stuck in."

But the International Rice Research Institute is worried about the problem. In its report, the institute blames the rising nighttime temperatures on global warming. The controversial theory of global warming says that pollution from the burning of fossil fuels in cars and furnaces is causing a long-term increase in temperatures worldwide.

The director of the United Kingdom Climate Impact Program, Chris West, says the rice production study is further evidence that people need to do more to reduce pollution.

"As this rice study shows, we need to start to adapt our human systems to the new conditions that we now recognize are coming and are happening now," he said. "But at the same time, we have got to recognize that if we don't start taking action now to address the causes of climate change, the changes in the latter part of this century and in subsequent centuries will be much greater and much more dramatic."

Mr. West says if rising nighttime temperatures cause a decline in rice production, they likely have other effects as well.

But a professor of global agriculture and economics at Cambridge University, Alister McFarquhar, does not believe there is evidence that global warming even exists. He says the increased temperatures observed in this study were probably caused by other factors.

"They keep repeating the assumption that there is global warming in their paper," he said. "If they had simply said that we find nighttime temperatures affect rice yield more than daytime temperatures that would be interesting. But it is very curious that they haven't considered the effect of volcanic activity, particularly [Mount] Pinataubo, which is not very far away."

But the spokesman for the rice research institute, Duncan Macintosh, does not want the debate over global warming to overshadow the results of the study.