The committee estimated the amounts of cellulosic biomass that could be produced sustainably in the United States and result in fuels with significantly lower greenhouse gas emissions than petroleum produces. For the purpose of this study, the committee considered biomass to be produced in a sustainable manner if it met the following criteria: (1) croplands would not be diverted for biofuels (so that land would not be cleared elsewhere to grow the crops thus displaced); and (2) the growing and harvesting of cellulosic biomass would incur minimal adverse environmental impacts—such as erosion, excessive water use, and nutrient runoff—or even reduce them.

The committee estimated (1) that about 400 million dry tons (365 million dry tonnes) per year of biomass could potentially be made available for the production of liquid transportation fuels using technologies and management practices of 2008 and (2) that the cellulosic biomass supply could increase to about 550 million dry tons (500 million dry tonnes) each year by 2020 (Table 5.1). A key assumption in the committee’s analysis was that 18 million acres of land currently enrolled in the Conservation Reserve Program (CRP) would be used to grow perennial grasses or other perennial crops for biofuel production, and that the acreage would increase to 24 million acres by 2020 as knowledge increased with time. Other key assumptions were that (1) harvesting methods would be developed for efficient collection of forestry or agricultural residues; (2) improved