WASHINGTON — A global-warming trend will reduce farm yields and make food supplies scarcer over the next century, an environmental group said yesterday, citing data from the United Nations and the National Academy of Sciences and trends in the world rice market.

"The combination of rising temperatures and falling water tables is likely to lead to a tightening of world grain supplies," said Lester Brown, president of the Earth Policy Institute. "This is already evident with world rice prices, which have risen over 30 percent in the last year."

Last year's grain harvest was 2 billion tons, the most ever and 26 million tons more than was consumed, thanks to unusually good weather. But demand outstripped supply in the four previous years as crops withered under severe heat in the United States, Europe and India, according to the Agriculture Department.

"If stocks go down, we could see a scramble, and I think we're likely to see a politics of food scarcity beginning to emerge," Brown said. "We're already seeing some of it."

Despite depressed prices for wheat and corn, the world price for rice — a crop particularly vulnerable to water shortages — has climbed by more than 30 percent to $260 a ton in the past year, according to government figures.

In research published by the National Academy of Sciences last year, a team of nine scientists concluded that rice yields typically decline by 10 percent with each 2-degree Fahrenheit increase in temperature during the growing season.

In the Philippines, where the research was done, temperatures rose an average of 2 degrees between 1979 and 2003, the scientists said.

Brown's group noted that Earth's average temperature rose by 1 degree in the past three decades, and that the United Nations' Intergovernmental Panel on Climate Change predicts it will rise by 2 to 10 degrees by 2100.

Compounding the problem is that half the world's population lives in countries where water tables are falling and wells are going dry, Brown said.

These include the big three grain producers — China, India and the United States — which account for nearly half the world grain harvest.

Brown calculated that the population in countries where wells are drying up will increase by nearly 3 billion people by 2050.