Cellulosic ethanol made from corn residue emits more carbon dioxide than gasoline, according to a newly published, federally funded peer-reviewed study. The findings are a severe blow to the biofuel industry, as fuels must reduce carbon dioxide emissions by 60 percent in relation to conventional gasoline to qualify as a renewable fuel for federal subsidy programs.
Funded by the U.S. Department of Energy, agricultural and environmental scientists at the University of Nebraska-Lincoln conducted a comprehensive life-cycle carbon analysis for biofuels made from leftover residue after corn harvests. The study was among the first to measure how much carbon is released into the atmosphere when corn stalks, cobs, and leaves are turned into ethanol instead of left on the ground to naturally replenish the soil. The scientists found that turning the leftover residue, known as stover, into ethanol caused a net 7 percent increase in carbon dioxide emissions versus conventional gasoline.
To qualify for $1-per-gallon federal subsidies designed to reduce carbon dioxide emissions, alternative fuels must reduce carbon dioxide emissions by 60 percent compared to conventional gasoline. Cellulosic biofuels have already benefited from $1 billion in federal subsidies.
The study, titled “Biofuels from crop residue can reduce soil carbon and increase CO2 emissions” and published in Nature Climate Change, noted agricultural soils release carbon into the atmosphere. Removing stover from the ground for ethanol production eliminated a key source of carbon absorption that kept the soil in balance. Taking this and other life-cycle factors of the process into consideration, the net result of cellulosic ethanol production from corn stover was a 7 percent increase in carbon dioxide emissions.
The scientists examined 10 years of data in a variety of soil, weather, and stover removal conditions throughout 12 Corn Belt states.
Emissions Increase with Stover Removal
“The actual amount of SOC [soil organic compound] loss to CO2 on average across the region could be greater than or less than estimated here, but these results indicate the likely direction of change and relative magnitudes,” the study reported.
“The resulting map indicates that Minnesota, Wisconsin and Iowa have the highest loss of SOC,” the study added.
“Importantly … the average intensity of CO2 emissions per amount of residue removed is roughly the same for all removal levels; less residue removed causes less decrease in SOC but is associated with a smaller biofuel energy yield. On a relative basis, biofuels from crop residue yield a low amount of energy and oxidize a large C [carbon] pool, producing high CO2 emissions per unit of energy,” the study observed.
Stover Provides other Environmental Benefits
The scientists also noted leaving stover on the ground is a long-established practice to reduce soil erosion and increase soil fertility.
“Crop residue has conventionally been left on the field after harvest to reduce soil erosion and maintain the SOC stocks and soil fertility of the Corn Belt,” the authors noted.
EPA Criticizes Federally Funded Study
The study’s results refute U.S. Environmental Protection Agency assertions that cellulosic ethanol from corn stover reduces carbon dioxide emissions sufficiently to qualify for federal renewable fuel subsidies. Responding to the findings, the Obama administration took the unusual step of criticizing its own federally commissioned and federally funded study.
The peer-reviewed study “does not provide useful information relevant to the life cycle greenhouse gas emissions from corn stover ethanol,” said EPA spokeswoman Liz Purchia in a press statement.
Purchia’s statement indicated EPA will likely ignore the study and continue to operate under the assumption cellulosic ethanol from corn stover dramatically reduces carbon dioxide emissions.
Authors Documented Accuracy
The study authors, however, documented the accuracy of their data and analysis, noting the close agreement between a model they devised from their findings and real-world carbon measurements.
“The model predicted annual measured net CO2 emissions to the atmosphere from soil and residue with an error of 12.4% on average.… The global character of the model assumptions combined with these regional tests indicates the model has enough accuracy to confidently estimate the average direction of change in net CO2 emissions and SOC from residue removal across the Corn Belt,” the study reported.
James M. Taylor ([email protected]) is managing editor of Environment & Climate News.
Liska, A. et al, “Biofuels from crop residue can reduce soil carbon and increase CO2 emissions,” Nature Climate Change, April 20, 2014, http://www.nature.com/nclimate/journal/v4/n5/full/nclimate2187.html