Corn should be used for food, not motor fuel, and yet the United States is committed to a policy that encourages farmers to turn an increasing amount of their crop into ethanol. This may save the nation a bit of the cost of imported oil, but it increases global-warming gases and contributes to higher food prices.

Candidates for president need to tell Americans the truth about ethanol, but they are falling over themselves in pursuit of the farm belt vote. Barack Obama and Hillary Clinton want more ethanol factories built than even President Bush envisaged when he called for 15 percent of US gasoline consumption to be replaced by alternative fuels by 2017. John McCain, who correctly called the ethanol push a boondoggle in 2000, now says that it is "a very important way to achieve energy independence."

Ethanol consumes almost a quarter of US corn production. The energy self-sufficiency that all the candidates seek should not come at the expense of the environment or the food supply.

Increased ethanol production isn't the only reason for the spike in food costs, but it's more controllable than drought in Australia, higher fertilizer prices, or increased meat consumption by the Chinese. Unlike those other cost-drivers, ethanol production is encouraged by federal subsidies.

And it's not as though ethanol improves the environment. When emissions inherent in the production process are included, ethanol consumption generates more carbon dioxide per gallon than gasoline, according to a recent report in Science magazine. Conversion of other cultivated biomass, such as sugarcane or soy, presents the same problem. The only biofuel that produces a net benefit is agricultural waste, an uncertain source. The best way for American motorists to use less gasoline is to drive fewer miles in lighter vehicles, rather than rely on the false promise of biofuels.

Ethanol is now usually sold as 10 percent of a fuel mixture that includes 90 percent gasoline. The government is thinking of ordering refiners to raise the blend to 15 or 20 percent. Ethanol generates fewer miles per gallon than regular gasoline. And it's not yet clear, according to the Consumer Reports website, how the higher blends would affect engine reliability or longevity. Before the government insists on a new fuel blend, it ought to examine all the hidden costs.

Greater use of ethanol means more greenhouse gases and more expensive food for people and livestock, hardly a fair exchange. There's a limited role for biofuels, excluding corn, in reducing oil imports from volatile regions, but they are not the answer to the world's need for energy on the go.

© 2008 The Boston Globe