http://www.ucsusa.org/clean_vehicles/technologies_and_fuels/biofuels/the-truth-about-ethanol.html
Accurate lifecycle emissions accounting requires that all of the energy and inputs associated with growing, producing, delivering, and using any biofuel are tracked, including emissions associated with changes in land use. Most analyses conducted before 2008 indicate that corn ethanol delivers a 10 to 20 percent reduction in global warming emissions over its lifecycle compared with gasoline, but these analyses did not include land use changes. The reduction is modest because corn production requires a significant amount of fossil fuel inputs for farm operations, processing and distilling, and fertilizer production (generally natural gas). Fertilizers used for corn production also generate a substantial amount of nitrous oxide, a potent global warming pollutant, as unused fertilizer breaks down in the field.
In addition, many corn ethanol production facilities operate on natural gas; if new production facilities use coal instead, the emission benefits of corn ethanol could be reduced or eliminated.
Several new studies have focused on the contribution of land use changes to lifecycle global warming pollution. If land is converted from forest to cropland, there can be a significant increase in global warming pollution. Recent estimates suggest that the emissions from these changes in land use may be huge and could dramatically shift the balance of risks and rewards for some kinds of ethanol.
For example, when lifecycle analysis of corn ethanol includes land use changes caused by using corn for ethanol rather than food or animal feed, the lifecycle emissions can end up as high as gasoline or potentially much higher (see the UCS Land Use Fact Sheet for more details).