By Amanda Peterka
Researchers affiliated with the Agriculture Department and Department of Energy are pushing back against a study from earlier this year that questioned the greenhouse gas benefits of producing ethanol from cornstalks, husks and other agricultural wastes.
In a series of correspondence published online today in the November issue of Nature Climate Change, the scientists charge that the study out of the University of Nebraska, Lincoln, was based on unrealistic assumptions about the amount of crop waste that cellulosic ethanol companies intend to use. They also faulted the study’s scope and modeling and argued that it overlooked carbon savings from co-produced electricity.
The study in question was published in April in Nature Climate Change and estimated the effects of removing corn crop residues from 128 million acres across 12 Corn Belt states. It found that total annual emissions from producing biofuel out of those residues would be 7 percent higher than those of gasoline averaged out over five years.
“The claim … that corn stover-derived ethanol can be worse than gasoline has generated lots of media interest, but offers little value to the research community or to policymakers,” a group of researchers from USDA’s Agricultural Research Service wrote today. “They have merely demonstrated that if you model an irresponsible and unsustainable scenario, the results will look irresponsible and unsustainable.”
Using a computer model, University of Nebraska researchers estimated that removing about 2.7 tons an acre of corn stover per year would add an average of 50 to 70 grams of carbon dioxide per megajoule of biofuels. The states of Minnesota, Iowa and Wisconsin showed the highest net loss of carbon because they have cooler temperatures and have more carbon in the soil.
The findings meant not only that cellulosic biofuels would emit more CO2 than gasoline but that they would fall far short of a statutory requirement that cellulosic biofuels reduce greenhouse gas emissions by at least 60 percent compared to a 2005 gasoline baseline (Greenwire, April 21).
The study was backed by a $500,000 DOE grant.
Adam Liska, lead author of the study and assistant professor of biological systems engineering at the University of Nebraska, defended the results in a comment published today along with the criticisms and in a series of emails over the past few weeks with Greenwire. Liska expressed disappointment in the response to the study and in its being ignored by the Obama administration in life-cycle analyses of greenhouse gases.
The researchers, he said, specifically designed the study so U.S. EPA could incorporate it into its analyses of biofuels.
“Assuming that the process we identify is real and our estimates are roughly correct, it would be appropriate that the EPA use these values in their analysis, as they are the best scientific estimates we have at this time,” Liska wrote in an email. “To not recognize this process is simply to ignore real significant CO2 emissions.”
When the study was first released, biofuel producers immediately pushed back against the results, saying they unfairly characterized their operations. Some government researchers were also quick to point to alleged flaws (ClimateWire, April 28).
Three separate groups of scientists today published letters to the editor disputing the study’s results: the USDA researchers, a group that included scientists from the Department of Energy’s Great Lakes Bioenergy Research Center and Pacific Northwest National Laboratory, and a trio of researchers from Denmark.
“While we agree with the motivation that underlies their analysis — there is a pressing need to understand the full climate impact of biofuels in general and stover removal in particular — we believe that this is best achieved with efforts that are based on our full understanding of carbon turnover in agricultural soils, and not on models that unduly simplify important relationships,” said the scientists associated with DOE’s labs and research centers in their comments.
According to their letter, the model used for the study was based on data from a single site — a highly productive irrigated no-till corn field in Nebraska — that was not representative of the rest of the Corn Belt. That field’s characteristics caused it to experience high losses of soil organic carbon that the study authors then translated to the rest of the country, they said.
DOE submitted an additional critical response to the study, but it was rejected by the scientific journal because it had too many similarities to the other letters. The department made it available online instead.
In their published letter, USDA scientists also faulted the study for using a “simplistic soil carbon model” that ignored soil moisture and soil texture and did not give companies a credit for electricity produced as a byproduct of the biofuel production process. They said the study failed to account for management practices that could reduce the rate of carbon loss associated with removing corn crop residues, such as no-till farming and covering fields with alternate crops during the winter.
The USDA letter also charged that residue removal rates used in the Liska study were unreasonable. The finding that cellulosic ethanol would result in 7 percent higher greenhouse gas emissions than gasoline was based on a scenario where 75 percent of the crop residue was collected and processed for biofuels production.
The companies that are opening big cellulosic ethanol plants in the Midwest this year — POET-DSM Advanced Biofuels LLC, Abengoa Bioenergy and DuPont Co. — are all collecting about a quarter of the crop waste left on farm fields after harvest from about a 30-mile radius around the plants.
“No one who has given serious thought to crop residues for biofuels would find their proposed across-the-board [75 percent] collection rate in the U.S. Corn Belt at all reasonable,” the USDA comment says.
In their letter, the Denmark researchers faulted the five-year timeline of the study, arguing that the U.N. Intergovernmental Panel on Climate Change recommends a 20-year perspective for judging greenhouse gas impacts.
Liska and other authors of the study say the criticism is unfounded.
In their own letter to the scientific journal, they write that critics are ignoring the main finding of the study: that removing residue that normally sits on fields over wintertime increased carbon dioxide emissions at all removal rates. In other words, the carbon intensities remained the same, whether 1 or 6 tons of crop waste was removed from a field.
“At a 25 percent removal level, cellulosic ethanol can still be more carbon intense than gasoline,” Liska wrote in a follow-up email. “Even at a 1 percent, 5 percent or 10 percent residue removal level, cellulosic ethanol can still be more carbon intense than gasoline.”
And the University of Nebraska authors said that while the companies that are building up cellulosic ethanol plants this year are providing some energy for their own operations, there are currently no electricity exports or other co-products.
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