Environ. Sci. Technol. 2010, 44, 5330–5331
Biofuels and Greenhouse Gas Emissions: Green or Red? MARK O. BARNETT* Department of Civil Engineering, Auburn University, Auburn, Alabama
RHONDA SAUNDERS
Authors’ Viewpoint
As recently as five years ago, the promise of biofuels was seen as virtually limitless. Biofuels represented a source of secure, independent, and sustainable energy that promised to reinvigorate American agriculture, reduce soil erosion, expand wildlife habitat, and significantly reduce greenhouse gas (GHG) emissions. To be sure, biofuel, particularly corn-based ethanol, has always had its detractors. Among other negatives, corn-based biofuels drive up food prices, promote environmental degradation and losses in biodiversity, and contribute to water shortages. Biofuels currently consume >25% of U.S. corn production, leading to significant increases in the price of corn and related foods and products. Although it is widely recognized that cellulosic feedstocks have a much lower environmental footprint, the U.S. Environmental Protection Agency (EPA) recently adjusted the congressionally mandated 2010 100 million gallon yr-1 cellulosic biofuel mandate to 6.5 million gallons, a ∼95% reduction, based on the lack of progress in bringing cellulosic biofuels to the marketplace. In the near term, the production of corn-based biofuel will likely remain significant. *
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 14, 2010
There have even been fundamental questions about the net energy and GHG balances of biofuels. In 2006, Farrell et al. (1) addressed these criticisms by critically reviewing and re-evaluating the results of six extant studies of biofuels production. Their analyses revealed that some studies had incorrectly ignored the positive benefits of ethanol coproducts in calculating the net energy and carbon emissions of biofuels. When corrected, corn ethanol led to a significant yet marginal net reduction in green-house gas emissions of 13%. In contrast, cellulosic ethanol led to reductions as high as 88%. Two studies in 2008, however, fundamentally challenged these conclusions relative to GHG. Searchinger et al. (2) claimed that most previous life-cycle studies had incorrectly omitted or systemically underestimated the effect of landuse changes when calculating the net GHG effects of biofuels. As large-scale biofuels subsidies and mandates are enacted in the future, more and more forests, grasslands, etc., will be cleared, either directly or indirectly, releasing their tremendous stores of carbon (soils and plant biomass contain almost three times as much carbon as the atmosphere). When properly accounting for these land-use changes, Searchinger et al. (2) estimated that rather than reducing GHG, cornbased ethanol doubles emissions for over thirty years and results in increased emissions for 167 years. Switchgrassbased biofuels, even if grown on U.S. corn fields, would still increase GHG emissions by 50% over thirty years. In the same issue of Science, Fargione et al. (3) defined the amount of carbon released during the first 50 years after native ecosystems were converted to biofuel production as the “carbon debt.” They then calculated a pay-back period, the time that it takes the reduction in carbon emissions produced by the biofuels relative to petroleum to repay the initial carbon debt. Ethanol produced from corn on converted U.S. grassland, for example, would have a carbon payback period of 93 years. In other words, ethanol produced from corn on converted U.S. grassland would produce more carbon than the equivalent amount of petroleum fuel for almost 100 years. Even producing corn ethanol on abandoned cropland resulted in a payback period of 48 years. Only producing prairie biomass ethanol on marginal or abandoned cropland produced payback periods of
