In the wake of the new studies, a group of 10 of the United States’s most eminent ecologists and environmental biologists today sent a letter to President Bush and the speaker of the House, Nancy Pelosi, urging a reform of biofuels policies. “We write to call your attention to recent research indicating that many anticipated biofuels will actually exacerbate global warming,” the letter said.

The European Union and a number of European countries have recently tried to address the land use issue with proposals stipulating that imported biofuels cannot come from land that was previously rain forest.

But even with such restrictions in place, Dr. Searchinger’s study shows, the purchase of biofuels in Europe and the United States leads indirectly to the destruction of natural habitats far afield.

For instance, if vegetable oil prices go up globally, as they have because of increased demand for biofuel crops, more new land is inevitably cleared as farmers in developing countries try to get in on the profits. So crops from old plantations go to Europe for biofuels, while new fields are cleared to feed people at home.

Likewise, Dr. Fargione said that the dedication of so much cropland in the United States to growing corn for bioethanol had caused indirect land use changes far away. Previously, Midwestern farmers had alternated corn with soy in their fields, one year to the next. Now many grow only corn, meaning that soy has to be grown elsewhere.

Increasingly, that elsewhere, Dr. Fargione said, is Brazil, on land that was previously forest or savanna. “Brazilian farmers are planting more of the world’s soybeans  and they’re deforesting the Amazon to do it,” he said.

International environmental groups, including the United Nations, responded cautiously to the studies, saying that biofuels could still be useful. “We don’t want a total public backlash that would prevent us from getting the potential benefits,” said Nicholas Nuttall, spokesman for the United Nations Environment Program, who said the United Nations had recently created a new panel to study the evidence.