A University of Michigan study published in the journal Climatic Change challenges a fundamental assumption of the life cycle analyses underpinning the EPA’s Renewable Fuel Standard, California’s Low Carbon Fuel Standard, and indeed all climate policies promoting the substitution of biofuels for petroleum fuels.
Life cycle analysis (LCA) compares the carbon dioxide (CO2) emissions of biofuels and petroleum fuels not just when the fuels are combusted but also throughout their respective supply chains: “farms and oil wells, bio-refineries and petroleum refineries, operations that transport feedstocks and distribute fuels, and operations that provide inputs such as fertilizer and purchased energy.” The study, led by Prof. John DeCicco, does not find fault with standard LCA estimates of supply-chain emissions.
The problem, rather, is the a priori assumption that each year’s CO2 emissions from the fermentation and combustion of ethanol are taken up (“fixed” or “sequestered”) in next year’s biofuel crop. DeCicco et al. treat the seldom-questioned belief that biofuel combustion is “inherently carbon neutral” as an empirical hypothesis to be tested by observation. According to the researchers, data from actual locations where biofuels are grown shows a substantial “gap” between biofuel fermentation and combustion emissions and subsequent “carbon uptake” by biofuel crops and soils.
Specifically, during 2005-2013, “additional carbon uptake on [U.S.] cropland was enough to offset only 37% of the biofuel-related biogenic [combustion] CO2 emissions,” not 100% as assumed by EPA, the U.S. Department of Energy, and the California Air Resources Board.
Main takeaway: During the nine-year period of study, corn ethanol was 27% more carbon-intensive than gasoline even before considering any life-cycle process emissions or indirect emissions due to biofuel-induced conversions of forests and grasslands to croplands.