Published: August 25, 2011 r2011 American Chemical Society 8197 dx.doi.org/10.1021/es2016236 | Environ. Sci. Technol. 2011, 45, 8197–8203 POLICY ANALYSIS pubs.acs.org/est Relevance of Emissions Timing in Biofuel Greenhouse Gases and Climate Impacts Stefan Schwietzke,* ,† W. Michael Griffin, †,‡ and H. Scott Matthews †,§ † Department of Engineering and Public Policy, Carnegie Mellon University, Baker Hall 129, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Tepper School of Business, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States § Department of Civil and Environmental Engineering, Carnegie Mellon University, Porter Hall 123A, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States b S Supporting Information 1. INTRODUCTION Corn ethanol is currently the prevailing biofuel in the U.S. Domestic production increased from 1.8 to 10.6 Bgal/yr over the past decade. 1 The federal government established a mandate to blend gasoline with corn ethanol, which will increase to 15 Bgal/ yr (7% of U.S. gasoline consumption) by 2016. 2 Corn ethanol is currently subsidized at $0.45/gal. 2,3 These policies are aimed toward mitigating climate change impacts from the transporta- tion sector and to overcome the dependence on foreign oil among others. 3 Currently, the perceived effectiveness of corn ethanol or other biofuels to mitigate climate change is mainly based on green- house gas (GHG) emissions. Policy makers rely increasingly on greenhouse gas accounting (GHGA) or life cycle assessment (LCA) to support complex performance evaluations of emerging technologies. 4 However, as the accounting procedures within LCA are improved to provide a better representation of reality, LCA results sometimes change signi ficantly. 5 This can dramatically alter prior results and beliefs, influencing policies, and amending decisions that determine billions of dollars of public and private investments. 1.1. Estimating Corn Ethanol Life Cycle GHG Emissions. GHGA or LCA of corn ethanol and other biofuels includes establishing a GHG inventory of all emissions that occur during the fuel ’s production and use and is usually compared to a gasoline baseline. 5 Typically, emissions are accounted for during all life cycle stages including agricultural feedstock production, conver- sion into liquid fuel, combustion, transportation of the feedstock and the fuel, and ancillary processes such as production of fertilizer and machinery. 6 These are sometimes referred to as well-to- wheel (WTW) emissions. 6 Other effects may be taken into account including emissions from land use change (LUC). 7 LUC emissions can occur directly when the biofuel feedstock displaces other biomass (e.g., previous crops or forests). The above- and belowground biomass may be burned and rapidly decayed, respectively. Emissions from LUC may also be released indirectly through market-mediated e ffects (see the SI for details). Received: May 12, 2011 Accepted: August 25, 2011 Revised: July 14, 2011 ABSTRACT: Employing life cycle greenhouse gas (GHG) emissions as a key performance metric in energy and environ- mental policy may underestimate actual climate change impacts. Emissions released early in the life cycle cause greater cumulative radiative forcing (CRF) over the next decades than later emis- sions. Some indicate that ignoring emissions timing in traditional biofuel GHG accounting overestimates the effectiveness of policies supporting corn ethanol by 1090% due to early land use change (LUC) induced GHGs. We use an IPCC climate model to (1) estimate absolute CRF from U.S. corn ethanol and (2) quantify an emissions timing factor (ETF), which is masked in the traditional GHG accounting. In contrast to earlier analyses, ETF is only 2% (5%) over 100 (50) years of impacts. Emissions uncertainty itself (LUC, fuel production period) is 12 orders of magnitude higher, which dwarfs the timing effect. From a GHG accounting perspective, emissions timing adds little to our understanding of the climate impacts of biofuels. However, policy makers should recognize that ETF could significantly decrease corn ethanol’s probability of meeting the 20% GHG reduction target in the 2007 Energy Independence and Security Act. The added uncertainty of potentially employing more complex emissions metrics is yet to be quantified.