technological changes, policies that encourage development of one option rather than another, and market forces could alter the conclusions.

To examine the potential supply of liquid transportation fuels from nonpetroleum sources, the panel developed estimates of the unit costs and quantities of various cellulosic biomass sources that could be produced sustainably as discussed in Chapter 2. The panel’s analysis was based on land that is not now used for growing foods although the panel cannot ensure that none of that land will be used for food production in the future. The estimates of biomass supply were combined with the amount of corn grain that would probably be used to produce fuels to satisfy the current legislative requirement to produce 15 billion gallons of ethanol per year. The panel’s analysis allowed it to estimate a supply function for biomass that shows the quantities of cellulosic biomass feedstocks that would potentially be available at the various unit costs. The panel assumed that coal would not be limiting in that it would be available in sufficient quantities at a constant unit cost if used with biomass in thermochemical conversion processes. The panel developed quantitative comparative analyses of alternative pathways to convert biomass, coal, or combinations of coal and biomass to liquid fuels (either ethanol or synthetic diesel and gasoline). Pathways, in principle, could include any combination of the various biomass feedstocks and coal and could include either thermochemical or biochemical conversion processes.² However, rather than treating all possible combinations, the panel first examined the cost of and the CO₂ emissions associated with each of the various thermochemical and biochemical conversion processes that would use one biomass feedstock and then examined the costs, supplies, and CO₂ emissions associated with one thermochemical conversion process and one biochemical conversion process that would use each of the biomass feedstocks.

The first set of analyses compared the costs and greenhouse gas emissions from fuels produced by biochemical and thermochemical conversion. The panel