Biofuel crops expanded onto 7 million acres of new land in the U.S. over a recent four-year period, replacing millions ofacres of grasslands, according to new research from the University of Wisconsin. Using high-resolution satellite imagery, the researchers calculated that converting grasslands to croplands for corn and soy biofuels could have emitted as much carbon dioxide as 34 coal-fired power plants operating for one year, or the equivalent of an additional 28 million cars on the road. Nearly 80 percent of cropland expansion replaced grasslands, which store large amounts of carbon in their soils, according to the report published in Environmental Research Letters. The study is the first comprehensive analysis of land-use change across the U.S. between 2008 and 2012, following the passage of the federal Renewable Fuel Standard.