Biofuels could 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 new study.
"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," said 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.
"Biofuels have caused alarm because of how quickly production has been growing: Global ethanol production increased by four times and biodiesel by 10 times between 2000 and 2007," Gibbs said.