Nonfossil fuels -- ethanol made from corn or sugarcane and biodiesel made from palm trees or soybeans -- are meant to lessen dependence on petroleum products, which release the greenhouse gas carbon dioxide when they burn.

However, biofuels can release carbon even before they are burned, depending on how they are made, said study co-author Jason Hill of the University of Minnesota.

As demand for these alternative fuels grows, farmers are plowing under forests and grasslands that used to store carbon and keep it from getting into the atmosphere, and using these lands to grow the food crops that now can be used for ethanol or biodiesel.

Biofuels grown this way come with a "carbon debt," the researchers found. Instead of cutting greenhouse pollution, the net effect is to increase it.

The carbon lost by converting rainforests, peatlands, savannas and grasslands outweighs the carbon savings from biofuels. Some conversions release hundreds of times more carbon than the annual savings from replacing fossil fuels.

CENTURIES OF CARBON DEBT

For example, the scientists wrote, Indonesia's conversion of peatlands for palm oil plantations had the world's greatest carbon debt, one that would take 423 years to repay.

The next worst case was the planting of soybeans in the Amazon, which would not pay for itself in renewable soy biodiesel for 319 years.

There are biofuel sources that do not rack up these formidable carbon debts, Hill said: nonfood plants including perennial grasses that only have to be harvested, without plowing under existing species that hold on to carbon.

"Our group has looked at using diverse mixtures of native species ... (on) prairie land, land that's restored back into prairies," Hill said in a telephone interview. "We essentially have no native prairies left in this nation but we can restore land into prairies, thereby restoring an ecosystem that was natural and also getting the biofuel benefit from it."

Biofuels, whether made from prairie plants, corn or soybeans, lack the potential to satisfy US fuel needs, Hill said.

"If we take every corn kernel we produce in this nation and convert it to ethanol, we would offset only 12 percent of our gasoline use," he said. "And that doesn't include the energy it took to produce that ethanol in the first place.