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.