Converting corn to ethanol in Iowa not only leads to clearing more of the Amazonian rainforest, researchers report in a pair of new studies in Science, but also would do little to slow global warming—and often make it worse.

"Prior analyses made an accounting error," says one study's lead author, Tim Searchinger, an agricultural expert at Princeton University. "There is a huge imbalance between the carbon lost by plowing up a hectare [2.47 acres] of forest or grassland from the benefit you get from biofuels."

Growing plants store carbon in their roots, shoots and leaves. As a result, the world's plants and the soil in which they grow contain nearly three times as much carbon as the entire atmosphere. "I know when I look at a tree that half the dry weight of it is carbon," says ecologist David Tilman of the University of Minnesota, coauthor of the other study which examined the "carbon debt" embedded in any biofuel. "That's going to end up as carbon dioxide in the atmosphere when you cut it down."

By turning crops such as corn, sugarcane and palm oil into biofuels—whether ethanol, biodiesel, or something else—proponents hope to reap the benefits of the carbon soaked up as the plants grow to offset the carbon dioxide (CO2) emitted when the resulting fuel is burned. But whether biofuels emit more or less CO2 than gasoline depends on what the land they were grown on was previously used for, both studies show.

Tilman and his colleagues examined the overall CO2 released when land use changes occur. Converting the grasslands of the U.S. to grow corn results in excess greenhouse gas emissions of 134 metric tons of CO2 per hectare—a debt that would take 93 years to repay by replacing gasoline with corn-based ethanol. And converting jungles to palm plantations or tropical rainforest to soy fields would take centuries to pay back their carbon debts. "Any biofuel that causes land clearing is likely to increase global warming," says ecologist Joseph Fargione of The Nature Conservancy, lead author of the second study. "It takes decades to centuries to repay the carbon debt that is created from clearing land."

Diverting food crops into fuel production leads to ever more land clearing as well. Ethanol demand in the U.S., for example, has caused some farmers to plant more corn and less soy. This has driven up soy prices causing farmers in Brazil to clear more Amazon rainforest land to plant valuable soy, Searchinger's study notes. Because a soy field contains far less carbon than a rainforest, the greenhouse gas benefit of the original ethanol is wiped out. "Corn-based ethanol, instead of producing a 20 percent savings [in greenhouse gas emissions], nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years," the researchers write. "We can't get to a result with corn ethanol where we can generate greenhouse gas benefits," Searchinger adds.

Turning food into fuel also has the unintended consequence of driving up food prices, reducing the access of the neediest populations to grains and meat. "It's equivalent to saying we will try to reduce greenhouse gases by reducing food consumption," Searchinger says. "Unfortunately, a lot of that comes from the world's poorest people."

"We are converting their food into our fuel," Tilman notes. " The typical driver of an SUV spends as much on fuel in a month as the poorer third of the world spend on food."

The studies do find some benefit from biofuels but only when planted on agricultural land too dry or degraded for food production or significant tree or plant growth and only when derived from native plants, such as a mix of prairie grasses in the U.S. Midwest. Or such fuels can be made from waste: corn stalks, leftover wood from timber production or even city garbage.

But that will not slake a significant portion of the growing thirst for transportation fuels. "If we convert every corn kernel grown today in the U.S. to ethanol we offset just 12 percent of our gasoline use," notes ecologist Jason Hill of the University of Minnesota. "The real benefit to these advanced biofuels may not be in displacement of fossil fuels but in the building up of carbon stores in the soil."

Of course, there is another reason for biofuels: energy independence. "Biofuels like ethanol are the only tool readily available that can begin to address the challenge of energy security," Bob Dinneen, president of industry group the Renewable Fuels Association said in a statement. "The alternative is to continue to exploit increasingly costlier fossil fuels for which the environmental price tag will be great."

But the environmental price tag of biofuels now joins the ranks of other, cheaper domestic fuel sources—such as coal-to-liquid fuel—as major sources of globe-warming pollution as well as unintended social consequences. As a result, 10 prominent scientists have written a letter to President Bush and other government leaders urging them to "shape policies to assure that government incentives for biofuels do not increase global warming."

"We shouldn't abandon biofuels," Searchinger says. But "you don't solve global warming by going in the wrong direction."