A scientific committee of the European Union has published a report arguing that EU policies favoring biofuels are based on a “serious” error in calculating the overall greenhouse gas emissions associated with the fuels. The result, says the committee, is an underestimation that could have “immense” climate-related consequences.

The policies, which include the EU’s emissions trading system and renewable energy targets, do not adequately take into account the effects of land-use changes, the committee argues. It concludes that as a result, biofuels are often considered carbon-neutral when they may actually be adding carbon to the atmosphere.

Under the current rules, if an energy source is derived from plants, the greenhouse gas emissions from its combustion are considered equal to the carbon the plants took from the atmosphere as they grew, and thus do not count. Similar reasoning has led many governments around the world to view biofuels as carbon-neutral and to promote biofuel production and use in order to reduce greenhouse gas emissions. In recent years, however, many in the scientific community have disputed the premise, pointing out that it fails to take into account increases in emissions that result from changes in land use associated with growing crops for fuel.

Some of the current EU policies, in particular those aimed at encouraging the production and use of biofuels for transportation, do attempt to account for direct land-use changes. But this calculation “is highly uncertain and ends up being based on very coarse estimates,” says John DeCicco, a senior lecturer at the University of Michigan, who studies energy use and transportation-related greenhouse gas emissions. Too often, the policies assume that simply planting biofuel crops will increase the amount of carbon absorbed from the atmosphere.

None of the EU policies count indirect land-use effects, which are often convoluted and hard to track. For instance, consider a wheat field that is converted to energy crops. Land elsewhere might in turn be converted into farmland to replace the wheat and meet demand for food—possibly through the clearing of more forest or grassland.

Ultimately, questions about how biofuels affect climate boil down to whether there is a net addition of biomass, and thus carbon absorption, says Timothy Searchinger, a lecturer on international public affairs at Princeton University whose work the EU committee cites in the new report. “If you want an excuse to ignore the carbon emitted from tailpipes, figure out if it was offset by something else,” Searchinger says. “What happened to reduce the carbon in the air?”