Abstract:The previous chapter argued that a policy approach to reducing GHG emissions associated with transportation fuel use should, among other things, take account of all greenhouse gas emissions associated with the production,  distribution, and use of a fuel. But as mentioned in that chapter, some areas of scientific uncertainty exist when it comes to quantifying the climate impacts of biofuels. This chapter explores four key measurement uncertainties that create challenges in accounting for such impacts—uncertainties that transportation policies designed to encourage low-carbon fuels should consider addressing. First, instead of treating emissions that occur at
different times equally, an accounting of the climate impacts of GHG emissions should consider the effect of emissions over time. Second, there is a need to account for non-GHG global warming factors such as albedo, and the effect of non-Kyoto gases and pollutants such as aerosols and black carbon. Third, more work needs to be done on the question of how to account for indirect land-use effects, which can be large for crop-based feedstocks. And fourth, when forest wastes are used as feedstock for biofuel production, the impacts on forest systems, especially changes in the fire behaviors, forest sinks, soil emissions, and other forest carbon pools should be considered. The last two uncertainties relate to what are often called the leakage and indirect effects that occur when there are dynamic linkages between different carbon pools.

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