Chicago ? Scientists had thought that there was one potential upside to global warming: more food to feed the world.

Years of laboratory tests led them to believe that more carbon dioxide in the atmosphere could fertilize food crops such as corn, soybeans, wheat and rice, offsetting the plant-damaging effects of higher global temperatures and less rainfall.

But a new study with field tests in Illinois and other spots around the globe is challenging that assumption, suggesting that any increase in crop yields due to the buildup of greenhouse gases would be modest or nonexistent.

Lower-than-expected yields could have dire consequences for the world’s food supply, the study’s authors concluded. They called for more research into plant varieties that could withstand the atmospheric assault.

The prevailing scientific wisdom has been cited repeatedly in government projections on food supplies and by Bush administration officials who oppose mandatory limits on emissions of heat-trapping gases.

Authors of the new University of Illinois study, recently published in the journal Science, said their findings are more accurate because they mimic predicted atmospheric changes in farm fields. Instead of growing plants in a greenhouse, the researchers set up plots surrounded by rings of tubes that spray carbon dioxide and ozone over the crops.

They found that corn yields didn’t increase at all when the air over the plots contained the amount of carbon dioxide projected to be lingering in the atmosphere by 2050. Increases in wheat and soybean yields were about half of what was previously thought.

“These results are very important,” said Bert Drake, a plant pathologist at the Smithsonian Environmental Research Center who was not involved in the study. “There hasn’t been much of an effort to develop plants that will respond to projected conditions.”

By the middle of the century, cars, power plants, factories and other sources are expected to boost the amount of carbon dioxide in the atmosphere by about 50 percent from current levels. Although that is expected to lead to higher global temperatures, it also could increase the photosynthesis of plants.

Tests conducted at University of Illinois plots in Illinois, Arizona, New Zealand, Japan and Switzerland found that those potential benefits are limited by the ability of many crop varieties to absorb more carbon.

The new study, financed in part by the U.S. Department of Agriculture, comes a few days after the U.S. Supreme Court agreed to consider whether carbon dioxide should be regulated as an air pollutant.