Once heralded as the answer to oil, biofuels have become increasingly controversial because of their impact on food prices and the amount of energy it takes to produce them.
They could also be responsible for pumping far more carbon dioxide into the atmosphere than they could possibly save as a replacement for fossil fuels, according to a study released Saturday.
"If we run our cars on biofuels produced in the tropics, chances will be good that we are effectively burning rainforests in our gas tanks," warned Holly Gibbs, of Stanford’s Woods Institute for the Environment.
Gibbs studied satellite photos of the tropics from 1980 to 2000 and found that half of new cropland came from intact rainforests and another 30 percent from disturbed forests.
"When trees are cut down to make room for new farmland, they are usually burned, sending their stored carbon to the atmosphere as carbon dioxide," Gibbs said.
For high-yield crops like sugar cane it would take 40 to 120 years to pay back this carbon debt.
For lower yield crops like corn or soybeans it would take 300 to 1,500 years, she told reporters at a meeting of the American Association for the Advancement of Science.