The main biofuel in the United States is ethanol made by fermenting corn. But the process is inefficient, requiring a lot of land, energy, water, and fertilizer. Turning food crops into fuels may also cause food prices to increase. And corn-based ethanol only reduces greenhouse-gas emissions by a relatively modest amount over gasoline.
The ultimate goal is to use plant waste or dedicated energy crops—plants like switchgrass that grow easily and quickly. Biofuels from these sources generate significantly less greenhouse gases than gasoline. But breaking down cellulose— the tough molecular chain that forms a plant’s cell walls—into sugars that can be fermented is complicated and costly.