Life cycle assessment (LCA) is the standard approach used to evaluate the greenhouse gas (GHG) benefits of biofuels. However, the need for the appropriate use of LCA in policy contexts is highlighted by recent findings that corn-based ethanol may actually increase GHG emissions. This is in contrary to most existing LCA results. LCA estimates can vary across studies due to heterogeneities in inputs and production technology. Whether marginal or average impacts are considered can matter as well. Most important of all, LCA is product-centered. The determination of the impact of biofuels expansion requires a system wide approach (SWA) that accounts for impacts on all affected products and processes.

This paper presents both LCA and SWA for ethanol based on Iowa corn. LCA was conducted in several different ways. Growing corn in rotation with soybean generates 35% less GHG emissions than growing corn after corn. Based on average corn production, ethanol's GHG benefits were lower in 2007 than in 2006 because of an increase in continuous corn in 2007. When only additional corn was considered, ethanol emitted about 22% less GHGs than gasoline. SWA was applied to two simple cases. Using 2006 as a baseline and 2007 as a scenario, corn ethanol's benefits were about 20% of the emissions of gasoline. If geographical limits are expanded beyond Iowa, then corn ethanol could generate more GHG emissions than gasoline. These results highlight the importance of boundary definition for both LCA and SWA.