Introduced as part of the Energy Independence and Security Act (EISA) of 2007, the Renewable Fuel Standard version 2 (RFS2) committed the USA to producing 15 billion gallons of corn ethanol a year by 2015.

The study, from the University of Minnesota Duluth, the University of Wisconsin-Madison and the National Wildlife Federation, is the first comprehensive analysis of land-use change across the entire US ethanol sector as production surged under EISA.

Chris Wright, from the Natural Resources Research Institute at the University of Minnesota Duluth, led the study. He said: "As environmental policy, EISA was designed to reduce greenhouse-gas emissions by the US transportation sector. However, the immediate greenhouse-gas benefit of corn ethanol largely disappears if the crop production required to produce more ethanol leads to counteracting land-use change, thereby releasing the carbon stocks held in previously untilled soils.

"So although the US has pursued rapid development of corn ethanol as a matter of national biofuel policy, relatively little is known about this policy’s comprehensive impact in terms of land-cover land-use change surrounding ethanol refineries. We believe this knowledge gap hampers the ability of policy makers to identify and mitigate potentially negative environmental impacts of ethanol production."

The Environmental Protection Agency (EPA) was tasked with monitoring the environmental impact of biofuels by submitting triennial reports to Congress. In its initial report in 2011, the EPA identified the potential expansion of corn production onto land enrolled in the Conservation Reserve Program or used as pasture (both grassland conversions) as the most significant source of negative environmental impacts potentially arising from ethanol development; for example, soil erosion, fertilizer runoff and loss of wildlife habitat.

Wright said: "However, the EPA noted a key uncertainty over whether feedstock demand would be met through cropland expansion, or by intensification of corn production on existing cropland. Because the EPA didn’t produce a report in 2014, policy makers were further hampered in considering how to mitigate the adverse impact of renewable fuels."

The research team examined land-use impacts of RFS2 as a whole, targeted within 100-mile neighbourhoods surrounding US refineries. They found that 4.2 million acres of arable land within these areas had been converted to grow crops, including the 3.6 million acres of grassland. Though nearly twice the area of Yellowstone National Park, this new cropland could supply, at most, 25% of mandated increases in ethanol use, they estimated.

The majority of this cropland expansion took place around the margins of the Corn Belt and along the Ogallala Aquifer, including states such as North Dakota, South Dakota and Kansas, where surviving native prairie is extensive. However, the team found the regulations in place to prevent the conversion of native prairie ineffective.

As US ethanol production surged under RFS2, domestic gasoline consumption actually declined due to higher fuel-efficiency standards for new vehicles. With most ethanol consumed in a 10% mixture with gasoline, concerns regarding the transportation sector’s ability to absorb higher ethanol volumes, the so-called E-10 ‘blend wall’, led the EPA to revise the RFS2 schedule downward, beginning in 2014 and ending with a 2016 standard set at 14.5 billion gallons.

Recently, the EPA increased the corn ethanol mandate to 15 billion gallons for 2017. Finding that the US ethanol industry is well positioned to utilize an under-developed feedstock capacity on grasslands in the Mid-West and Great Plains, the team concluded that additional grassland conversion could be reasonably expected.

Wright said: "While the recent adjustments to RFS2 centred on ethanol demand, the EPA has not considered the comprehensive environmental impact of ethanol as a basis for revision. We hope this information will allow policy makers to make a reasoned judgment of the broader environmental effect of biofuels."

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