By JUSTIN GILLIS – New York Times.

Global warming is already cutting substantially into potential crop yields in some countries — to such an extent that it may be a factor in the food price increases that have caused worldwide stress in recent years, researchers suggest in a new study.

Wheat yields in recent years were down by more than 10 percent in Russia and by a few percentage points each in India, France and China compared with what they probably would have been without rising temperatures, according to the study.

Corn yields were off a few percentage points in China, Brazil and France from what would have been expected, said the researchers, whose findings were published in Friday’s issue of the journal Science.

Some countries saw small gains from the temperature increases, however. And in all countries, the extra carbon dioxide that humans are pumping into the air acted as a fertilizer that encouraged plant growth, offsetting some of the losses from rising temperatures caused by that same greenhouse gas.

Consequently, the study’s authors found that when the gains in some countries were weighed against the losses in others, the overall global effect of climate change has been small so far: losses of a few percentage points for wheat and corn from what they would have been without climate change. The overall impact on production of rice and soybeans was negligible, with gains in some regions entirely offsetting losses in others.

But the authors of the study — David Lobell and Justin Costa-Roberts of Stanford University, and Wolfram Schlenker of Columbia University — pointed out that temperature increases were expected to accelerate in coming decades, making it likely that the challenges to food production will grow in an era when demand is expected to rise sharply.

Over the period covered by the study, 1980 to 2008, temperatures increased briskly in many of the world’s important agricultural regions. A notable exception was the United States: for reasons climate scientists do not fully understand, temperatures in the Midwestern corn and soybean belt during the summer crop-growing season have not increased in recent decades.

“One way to think of it is that we got a pass on the first round of global warming,” Dr. Lobell said.

However, the study found that in virtually all of Europe, large parts of Asia and some parts of Africa and South America, temperatures during the growing season have warmed by an average of several degrees since 1980, increasing the likelihood of extremely hot summer days. The study also looked at rainfall, but changes were relatively minor compared with the temperature increases.

Plants are known to be sensitive to high temperatures, especially if the hot days occur when they are flowering. “In many of these countries, a typical year now is like a very warm year back in 1980,” Dr. Lobell said.

Wheat, rice, corn and soybeans account for the majority of calories consumed by the human race, either directly or as meat from animals raised on grains. Because demand for these grains is inflexible and rising, the losses from climate change probably accounted for price increases of about 6 percent in the four major commodities, the study’s authors found.

At today’s grain prices, that calculation implies that climate change is costing consumers, food companies and livestock producers about $60 billion a year.

“We aren’t talking about the sky falling,” Dr. Lobell said. “But we are talking about billions of dollars of losses. Every little bit of production is valuable when we’re trying to feed the world.”

If the price estimate is correct, it makes climate change a small contributor to a large trend. The prices of many foodstuffs have doubled or tripled in recent years as a result of a host of factors, including rapidly rising food demand in Asia, government mandates to use crops for biofuel production and extreme weather that may or may not be linked to climate change.

The authors of the new study specifically excluded the effects of extreme weather like brief heat waves and flash floods because of limitations in the data that they used. For that and other reasons, Dr. Lobell said, the study’s estimate of the impact of climate change is probably conservative.