Direct comparisons of gas emissions resulting from the combustion of anhydrous ethanol, ethanol-gasoline blends, and gasoline are straightforward to perform but are poor indicators of the overall consequences of substituting ethanol for gasoline. Instead, from the early 1990s, full fuel-cycle analyses were performed to estimate gas emissions (projected beyond 2000) throughout the production process for bio­ethanol to gauge the direct and indirect consequences of gasoline replacement, including

• Changes in land use and the replacement of native species by energy crops

• Agricultural practices and the potential for utilizing agricultural wastes

• Materials manufacture and the construction of facilities

• Bioethanol production

• Transportation of feedstocks and bioethanol

• Fuel usage per mile driven

With the three major greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), a major difference between corn-derived and cellulosic ethanol was immediately apparent.89 Figure 1.27 compares the projected CO2 emissions for a range of fuels; noncarbon-based fuels (i. e., electric vehicles powered using electricity generated by nuclear and solar options) were superior, whereas corn ethanol showed no net advantage. More recent estimates place corn ethanol production as giving modest reductions in greenhouse gas emissions, 12-14%.58 81 The GREET model of the Argonne National Laboratory indicates steeper reductions (figure 1.28).95

An unavoidable complication in any such calculation, however, is that they assume that harvested crops are from land already under cultivation; converting “native” ecosystems to biofuel production would inevitably alter the natural carbon balance and reduce the potential savings in greenhouse gas emissions.58

As with energy balance computations, including combined heat and power systems into cellulosic ethanol production displaces fossil fuel use and accounts for the reductions in greenhouse gas emissions.8188 In some corn ethanol production scenarios, the use of fossil fuels such as coal to power ethanol distillation plants and diesel to enable the long-distance transportation of corn can actually increase total greenhouse gas emissions.81 96 97 If the NEB is less than 1, the total ethanol production cycle would also increase net greenhouse emissions unless renewable energy sources rapidly supplanted fossil fuels in power generation.78,97,98 Corn sto­ver ethanol can, if its production process shares facilities with grain production, exhibit lower life cycle emissions than switchgrass-derived ethanol in projections up to 2010.99

For other priority pollutants, bioethanol production (even from cellulosic resources) and the use of bioethanol as a transportation fuel have ambiguous effects: increased nitric oxides, particulate matter, carbon monoxide, and volatile organic carbon but decreased sulfur oxides when measured in total in both urban and rural locations (figure 1.28). Combined fuel cycle and vehicle life cycle analyses also show major reductions in greenhouse gases but not in other emissions with cellulosic ethanol as an alternative fuel.100

For production sources other than sugarcane, therefore, the actual reductions in greenhouse gas emissions resulting from the adoption of biofuels may be considerably less than anticipated. One estimate for corn ethanol concluded that, by itself, fuel alcohol use in the United States would struggle to reduce transportation — dependent emissions by more than 10%.101 This study used the following data and arguments:

• For U. S. gasoline consumption of 460 x 109 L/year, corn ethanol replaced 0.8% of this while using 1% of the total cropland.

• To replace 10% of the gasoline consumption, corn ethanol would need to be produced on 12% of the total U. S. cropland.

• Corn ethanol only avoids 25% of the CO2 emissions of the substituted gaso­line emissions when the fossil fuel-dependent energy consumed to grow and process the corn is taken into account.

• Offsetting 10% of the CO2 emissions from gasoline consumption would require a fourfold higher production of corn ethanol, that is, from 48% of U. S. cropland.

• “A challenge for providing transportation fuels is to be able to substitute a joule of energy in harvestable biomass for a joule of primary energy in fossil fuel and to do this without significant fossil energy consumption. If a joule for joule consumption can be achieved, dedicated biomass plantations may be able to displace some 15% of the CO2 emissions expected from all uses of fossil fuels globally by 2030. For biofuels to replace more than about 15% of fossil-fuel CO2 emissions in 2030 will require a more rapid improvement in the energy efficiency of the global economy than is appar­ent from past trends in efficiency.”101

This pessimistic — or realistic — view of the quantitative impacts of biofuels on greenhouse gas scenarios in the twenty-first century was strongly reinforced by a mid-2007 study of alternatives to biofuels, specifically conserving and (if possible) extending natural forests, savannahs, and grasslands: compared with even the best biomass-to-biofuel case, forestation of an equivalent area of land was calculated to sequester at least twice the amount of CO2 during a 30-year period than the emis­sions avoided by biofuel use.102 As the authors concluded:

If the prime object of policy on biofuels is mitigation of carbon dioxide-driven global warming, policy makers may be better advised in the short term (30 years or so) to focus on increasing the efficiency of fossil fuel use, to conserve the existing forests and savannahs, and to restore natural forest and grassland habitats on cropland that is not needed for food.

Present national policies have, however, more diverse “prime object” aims and goals, including those of ensuring continuity of supply of affordable transportation fuels and (increasingly important on the geopolitical stage) establishing some degree of “energy independence.” Nevertheless, the assessment of potential savings in green­house gas emissions remains a powerful argument for the adoption of a considered, balanced raft of biofuels options.