A University of Wisconsin-Madison study shows that the shift of more than 7 million acres into cropland led to huge releases of carbon emissions into the atmosphere after a 2007 federal law mandated ethanol in gasoline.
The increased carbon emissions are equivalent to 20 million new cars driving down American roads every year, according to the researchers' estimates in the study.
The findings show big changes in land use across the Midwest and other parts of the United States between 2008 and 2012. That coincided with a change in federal law that required blending ethanol from crops like corn and soybeans into gasoline.
The federal Energy Information Agency reported that 10 percent of 143 billion gallons of gasoline came from ethanol in 2016.
The study shows the unintended consequences of a federal policy meant to reduce U.S. reliance on fossil fuels.
While adding ethanol means burning fewer fossil fuels, the study found that the benefits were lost as even larger amounts of carbon held in the soil were released into the atmosphere in newly cultivated farm fields.
The researchers used satellite images and other data to identify landscape changes over time and used computer modeling to estimate the carbon that had been stored in soil.
The results drew criticism from an industry group. The Renewable Fuels Association, based in Washington, said other university studies have shown that claims of land use change after the ethanol mandate is "grossly overstated."
The study examined climate impacts after grassland, forests and other types of land were plowed under after passage of the ethanol mandate, also known as the renewable fuel standard.
The authors of the study were Seth A. Spawn and Tyler J. Lark, two graduate students in the Department of Geography, and the project's principal investigator, Holly K. Gibbs, an associate professor of geography.