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grow in more ideal conditions for using sunlight and water and because they are often cultivated manually, with fewer fossil energy requirements and fewer inputs of fertilizer and pesticides. Temperate biofuel production pathways have become significantly more efficient in recent decades as agricultural practices have improved and fuel production mills have streamlined their operations. However, it is generally acknowledged that biofuels produced from temperate oil seeds, sugar beets, wheat, and corn have limited ability to displace other fuels, because of either their low yields or their high input requirements.

Since transportation energy accounts for only a small share of a biofuel’s overall energy use, the above factors suggest that it would be more energetically efficient for countries with temperate climates to import biofuels (e.g. made from sugar cane or palm oil) than to produce them at home. It would be more efficient to transport the final fuel, rather than the feedstock, because the fuel is more energetically dense.

With regard to next-generation feedstock, the energy cost of producing biofuels from lcellulosic biomass will likely continue to exceed that of producing biofuels with conventional starch, sugar, and oil, considering all of the energy inputs (including the biomass) required for the conversion process. While conversion technologies will improve over time, in the near term cellulosic biomass has the greatest potential as a source of processing energy for conventional (first-generation) biofuels, providing a means to significantly improve the overall fossil energy balance of these fuels. As cellulosic conversion becomes more viable, analysts should continue to evaluate the most-efficient uses of cellulosic biomass, raising the importance of “energy efficiency” metrics as opposed to measures of fossil energy.

When considering strategies for slowing the pace of climate change, the fossil energy balance of different biofuel production pathways can be a useful measure of their relative effectiveness. It is worth emphasizing that the fossil energy balance of biofuels could theoretically approach infinity, but only if renewable energy alone is used to cultivate, harvest, refine, and deliver biofuels. However, fossil energy balance does not take into account other ways that biofuel production contributes to climate change, such as changes in land use.

Greenhouse Gas Emissions

The global transportation sector is responsible for 25 percent of the world’s energy- related greenhouse gas (GHG) emissions, and this share is rising. A dramatic increase in the production and use of biofuels has the potential to significantly reduce those emissions, particularly with the development of advanced biofuel technologies that rely on agricultural wastes and dedicated cellulosic crops such as switchgrass. However, if biofuels are produced from low-yielding crops, are grown on previously wild grasslands or forests, and/or are produced with heavy inputs of fossil energy, they have the potential to generate as much or more GHG emissions than petroleum fuels do. For example, a key issue today is the transition from natural gas to coal energy at ethanol refineries in the United States: of the two energy sources, coal releases substantially more carbon per joule.

Figure 10 shows the range of potential GHG emissions reductions from the use of wastes and other next-generation feedstock, relative to current-generation feedstock.

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