title: Relevance of emissions timing in biofuel greenhouse gases and climate impacts.
abstract: Employing life cycle greenhouse gas (GHG) emissions as a key performance metric in energy and environmental 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 emissions. 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.
authors.affiliation: Department of Engineering and Public Policy, Carnegie Mellon University, Baker Hall 129, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States. sschwiet@andrew.cmu.edu
authors.forename: Stefan
authors.initials: S
authors.lastname: Schwietzke
authors.forename: W Michael
authors.initials: WM
authors.lastname: Griffin
authors.forename: H Scott
authors.initials: HS
authors.lastname: Matthews
mesh_terms: D000393:Air Pollutants / Q000032:analysis*; D056804:Biofuels / Q000032:analysis*; D002980:Climate; D000431:Ethanol / Q000032:analysis; D005740:Gases / Q000032:analysis*; D005742:Gasoline / Q000032:analysis; D017752:Greenhouse Effect; D013997:Time Factors; D003313:Zea mays / Q000737:chemistry
chemical_list: D000393:Air Pollutants; D056804:Biofuels; D005740:Gases; D005742:Gasoline; D000431:Ethanol