Biofuels can’t get any respect these days. Science magazine (subs. req’d) recently published an article whose abstract reads, simply,
The carbon sequestered by restoring forests is greater than the emissions avoided by the use of the liquid biofuels.
The article, “Carbon Mitigation by Biofuels or by Saving and Restoring Forests?” notes:
Two issues need to be addressed before the efficacy of biofuels can be assessed: the net reduction in fossil carbon emissions (avoided emissions) arising from use of agriculturally derived biofuels and the effect of alternative land-use strategies on carbon stores in the biosphere.
What happens when you do this analysis?
In all cases, forestation of an equivalent area of land would sequester two to nine times more carbon over a 30-year period than the emissions avoided by the use of the biofuel. Taking this opportunity cost into account, the emissions cost of liquid biofuels exceeds that of fossil fuels.
Does any source of biomass justify using land for biofuels? Yes:
[O]nly conversion of woody biomass may be compatible with retention of forest carbon stocks. Woody biomass can be used directly for fuel or converted to liquid fuels. Although still in a development stage, avoided emissions in temperate zones appear similar to assimilation by forest restoration. Moreover, it may be possible to avoid environmental problems associated with extensive monoculture by harvesting from standing forests. In this case, soil and above-ground carbon stocks may be built up in parallel with sustainable harvesting for fuel production.
So over the longer term, cellulosic ethanol may still turn out to be a good idea. Still, the authors have a clear conclusion:
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. In addition to reducing net carbon dioxide flux to the atmosphere, conversion of large areas of land back to secondary forest provides other environmental services (such as prevention of desertification, provision of forest products, maintenance of biological diversity, and regional climate regulation), whereas conversion of large areas of land to biofuel crops may place additional strains on the environment. For the longer term, carbon-free transport fuel technologies are needed to replace fossil hydrocarbons.