The promise of biofuels like ethanol is that they will someday help the world grow its way out of its addiction to oil. Nine billion gallons of corn ethanol were produced in the U.S. in 2008, while countries like Brazil have already widely replaced gasoline with ethanol from sugar cane and countless start-ups are working to bring cellulosic and other second-generation biofuels to market. The reasoning is that if we use greener biofuels in place of gasoline, it will significantly enhance our effort to reduce greenhouse-gas emissions.

But the question is, Are biofuels really green? A pair of new studies in the Oct. 22 issue of Science damningly demonstrate that the answer is no, at least not the way we currently create and use them. In the first study, a team of researchers led by Jerry Melillo of the Marine Biological Laboratory in Woods Hole, Mass., projected the effects of a major biofuel expansion over the coming century and found that it could end up increasing global greenhouse-gas emissions instead of reducing them. In the second paper, another team of researchers led by Tim Searchinger of Princeton University uncovered a potentially damaging flaw in the way carbon emissions from bioenergy are calculated under the Kyoto Protocol and in the carbon cap-and-trade bill currently being debated in Congress. If that error in calculation goes unfixed, a future increase in biofuel use could end up backfiring and derailing efforts to control global warming, according to the paper. “Biofuels can be an important part of the portfolio of climate-change activities,” says Steve Hamburg, chief scientist for the Environmental Defense Fund and a corresponding author on the second Science paper. “But we have to make sure we incentivize the right way, or we could end up with perverse outcomes.”

(Watch a video about the environmental cost of biofuel in Indonesia.)