Life cycle assessment (LCA) provides the basis for quantifying the environmental impact of these systems in their entirety, from seed to biofuel. Current disagreements about the performance of biofuels rest on different approaches and assumptions used by the investigators performing these assessments. Standardized LCA methods and agreement on the most relevant metrics for assessing different biofuel systems are needed to forge a consensus in the scientific community, general public, and industry about the contributions to both petroleum replacement potential and environmental goals. Such a consensus would help advance public policy initiatives to encourage development of “green” biofuel industries.
The Energy Independence and Security Act of 2007 requires that life-cycle GHG emissions of corn grain ethanol, cellulosic ethanol, and advanced biofuels are evaluated to ensure they meet 20%, 60%, and 50% GHG emissions reductions relative to gasoline, respectively. Moreover, producers will need to verify life-cycle GHG reductions to import their biofuel into regulated markets that require GHG certification, such as under the Low Carbon Fuel Standard in California. A biofuel certification process based on LCA would enable the biofuel industry to participate in emerging markets for GHG emissions trading, which could provide additional industry revenue and encourage investment in GHG emissions reducing technologies. Life-cycle methods are also crucial for ex ante evaluation of alternative technologies to improve the performance of new biorefinery designs, and to assist in research prioritization for development of new feedstock crops and production systems.
Given this need for standardized methods and scientific consensus, this article reviews life-cycle methods, assumptions, and metrics used to assess biofuel systems, compares several studies to identify inconsistencies, and proposes approaches for more accurate and standardized LCA methodology and applications for biofuel systems. It focuses on two categories of biofuels systems:
(i) established, large-scale systems, with a focus on state of-the-art corn grain-ethanol systems, and (ii) experimental systems under development but not yet deployed on a commercial scale. Systems are evaluated for their impacts on net GHG emissions for mitigating climate change, and net energy yield as an indicator of contributions to energy self-sufficiency, land requirements, and potential competition with food production.

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