A final strategy for reducing transportation GHG emissions is reducing the GHG emissions associated with the use of each unit of transport energy, typically through the development and deployment of vehicles that run on electricity or liquid or gaseous transportation fuels not based on petroleum, such as biofuels or hydrogen. In ad dition to propulsion and energy storage technologies themselves, this requires the development of ways to manufacture and distribute the new fuel or energy sources. While some of these vehicle and fuel combinations would significantly reduce or completely eliminate tailpipe GHG emissions, the GHG emissions generated as a result of fuel production and distribution could be significant and offset all or some of these benefits. Indeed, in some circumstances, the resulting "well-to-wheels" GHG emissions—emissions resulting from the extraction, production, and distribution of fuel plus the emissions resulting from its use by the vehicle—can exceed the well-to-wheels emissions generated by current transport vehicles using petroleum-based fuels. For example, some biofuels, especially corn-based ethanol but also certain forms of biodiesel, may not yield a net reduction in GHG emissions (Campbell et al., 2009; Searchinger et al., 2008).
In its analysis of the well-to-wheels impacts of alternative liquid transportation fuels, the America's Energy Future panel on this topic found that CO2 emissions from corn grain ethanol are only slightly lower than those from gasoline (NRC, 2009b). In contrast, CO2 emissions from cellulosic ethanol (biochemical conversion) are much lower (NRC, 2009b). However, cellulosic processes are not yet economical and production of corn-based ethanol may be encouraged for other reasons, such as bolstering domestic agricultural markets and building the market for biofuels (see NRC, 2009b). Similar concerns have been raised about battery- and hydrogen-powered vehicles, especially if the feedstock used to make the hydrogen or electricity that charges the batteries comes from GHG-intensive energy sources. In addition, the production of alternative fuel sources may carry unintended negative consequences for other resources, environmental concerns, trade issues, and human security issues, and the trade-offs and life-cycle costs and benefits of these alternatives have to be evaluated (see Chapter 14).
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