Ethanol made from corn grown across millions of acres of American farmland has become the country’s premier renewable fuel, touted as a low-carbon alternative to traditional gasoline and a key component of the country’s efforts to reduce greenhouse gas emissions.

But a new study, published this week, finds that corn-based ethanol may actually be worse for the climate than fossil-based gasoline, and has other environmental downsides.

“We thought and hoped it would be a climate solution and reduce and replace our reliance on gasoline,” said Tyler Lark, a researcher with the Nelson Institute for Environmental Studies at the University of Wisconsin, Madison, and lead author of the study. “It turns out to be no better for the climate than the gasoline it aims to replace and comes with all kinds of other impacts.”

The study, published in the Proceedings of the National Academy of Sciences, looks specifically at the effect of the Renewable Fuel Standard (RFS), which was first passed by Congress in 2005 and updated in 2007 (RFS2). The standard requires that blenders add billions of gallons of renewable fuel to the country’s transportation fuel supply every year, creating the world’s biggest biofuels program.

At the time, lawmakers and proponents hailed the standard as a major victory for the climate and part of an overall effort to reduce dependence on foreign oil.

But in the 15 years since, its promises have yet to be fulfilled, critics say, and a mounting pile of studies shows corn ethanol has not dampened demand for fossil fuels, as expected, but has instead forced the conversion of grasslands and forests into croplands, both domestically and internationally, releasing carbon in the process.

In the new study, Lark and his colleagues found that after the RFS took effect, farmers expanded corn production on nearly 7 million acres each year, causing the conversion of lands to cropland “such that the carbon intensity of corn ethanol produced under the RFS is no less than gasoline and likely at least 24% higher.” The policy, the study said, also resulted in increased fertilizer use, water pollution and habitat loss.

In a previous study, from 2019, Lark and his colleagues found that cropland expansion in the United States, mostly for corn and soybeans, has led to increased greenhouse gas emissions, but did not connect that expansion to the RFS.

After the current standard took effect in 2007, the Environmental Protection Agency, which is in charge of running the program, determined that ethanol from corn met the requirement that any renewable fuel under the program had to demonstrate a 20 percent reduction in greenhouse gasses compared to gasoline.

But the following year, researchers published a study in the journal Science projecting that corn ethanol would double greenhouse gas emissions over 30 years because demand for corn would push farmers to plow up more carbon-rich forest and grassland. That study triggered an ongoing debate about ethanol’s carbon benefits.

In the following years, the U.S. Department of Agriculture and researchers at prominent agriculture-focused universities, produced studies showing that corn-based ethanol reduced greenhouse gas emissions by 40 to 45 percent.

In response to the new paper, the Renewable Fuels Association (RFA) pointed to those studies and said: “The claims in this report simply don’t align with reality and the facts on the ground, and the paper reads more like a fantasy novel than a genuine piece of academic literature.”

Tim Searchinger, a researcher at Princeton University who authored the Science study in 2008 projecting the doubling of greenhouse gas emissions over 30 years, said the research the RFA uses to support the climate virtues of ethanol fail to adequately account for land-use change, or it underestimates the carbon emission from converting forests and grasslands to cropland. 

“Their numbers are invented out of whole cloth,” Searchinger said.

The RFA points out that cropland for corn has not expanded since the onset of the standard, but the study’s authors say they instead proved the “counterfactual.”

“What’s important is what would have happened without the RFS, without this ethanol boom,” Lark said. “Without this policy, there would have been a big decrease in corn.”

The intent of the RFS and RFS2 was for other forms of renewable fuels, especially cellulosic ethanol from plant and wood fiber, to increasingly become part of the fuel mix. But that has yet to happen. Instead, corn has become the backbone of the program.

The Biden administration is set to revamp the “renewable volume obligations”—the percentage of renewable fuels required in the fuel mix under the law—in the coming months. The Senate Committee on Environment and Public Works is scheduled to hold a hearing on the RFS Wednesday. 

Aaron Smith, one of the co-authors of the study and a professor of agricultural economics at the University of California, Davis, said they chose to analyze the impacts of the RFS now, in part because of the upcoming reviews and potential changes to the program.

“It was written in the legislation that we should be looking at the environmental impact of the RFS,” Smith said. “And what stood out to us was the carbon impact.”

John Reilly, a co-director emeritus at the MIT Joint Program on the Science and Policy of Global Change and a longtime Department of Agriculture researcher, called the study “impressive work” that will likely trigger yet more debate between environmental groups and the biofuels industry.

“The real supposed benefit of the RFS2 was to spur production of second-generation biofuels from cellulosic material, which was supposed to be much more environmentally beneficial,” Reilly noted. “The regulation is an impressive failure in that regard. If further research verifies the results of this study with respect to corn ethanol, then the RFS2 will have failed spectacularly on two fronts.”