After a fierce battle over agricultural incentives in a landmark climate bill, Congress plans to ask the National Academy of Sciences to study how biofuel production in the Midwest can shift food production abroad, stimulating a wave of deforestation.

Tomorrow's expected vote in the House of Representatives on the climate bill would move the nation a step closer to a cap-and-trade system that would limit greenhouse gas emissions. But the bill's sponsors have made significant concessions to Agriculture Committee Chairman Colin Peterson (D–Minn.), who threatened to torpedo the legislation as it was written. President Obama is pressing for its passage, which would still have to work its way through the Senate in July to become law.

Environmentalists have walked a fine line between encouraging growth of the renewable energy sector and regulating it enough to ensure that it does not lead to increased greenhouse emissions as forests are cleared abroad for food production. Scientists believe that market forces and government incentives will cause domestic farmers to shift from food production to biofuels creating a demand for foreign agriculture to fill in the food gap. The technical term for this phenomenon is "indirect land-use change".

Taking into account indirect land-use change, the U.S. Environmental Protection Agency (EPA) estimates that corn-based ethanol will reduce greenhouse gas emissions by just 16 percent. That means it won't meet the threshold of 20 percent required to be classified as a renewable fuel and cannot receive the economic incentives the federal government promised to provide under a 2007 energy law. Biodiesel producers would also lose because they say that vegetable oils would not qualify under the restrictions, leaving only recycled restaurant grease and animal fats.

In sticking up for farm states, Peterson has now proposed an amendment to the legislation that would prevent such requirements from becoming part of renewable fuel or electricity standards for at least six years. At the end of a five-year period, the National Academy of Sciences will study the issue, and the EPA and the U.S. Department of Agriculture will decide whether indirect land-use changes can be accurately measured. In a statement yesterday, the American Coalition for Ethanol heralded the delay in ruling on biofuels, noting that the "agreement ensures that science, not politics, will determine whether the EPA can go forward with this highly controversial theory."

For many scientists, however, the question is not whether there are indirect impacts but rather how big they are. Environmentalists think it would be a mistake for the EPA to lower the bar for biofuels so soon. "We want the EPA to use the best science and economics to establish regulation," says Nathanael Greene of the National Resources Defense Council in New York City. "We recognize that the regulation is going to be imperfect so let's update it."

At the same time, there's rising sentiment that growing new crops to quench the thirst for biofuels is not the best strategy for reducing greenhouse emissions.

David Tilman, an ecologist at the University of Minnesota, who recently chaired a panel on alternative fuels for the National Research Council in Washington, D.C., says that the new standards should encourage the use of the half billion tons of biomass, such as forest slash, that is already available per annum. "We could have a policy that uses biomass sources with a minimal impact on indirect land use," he says.