For this reason, if every one of the 70 million acres on which corn was grown in 2006 was used for ethanol, the amount produced would displace only 12 percent of the U.S. gasoline market. Moreover, the "new" (non-fossil) energy gained would be very small — just 2.4 percent of the market. Car tune-ups and proper tire air pressure would save more energy.

There is another problem with relying on a food-based biofuel, such as corn ethanol, as the poor of Mexico can attest. In recent months, soaring corn prices, sparked by demand from ethanol plants, have doubled the price of tortillas, a staple food. Tens of thousands of Mexico City's poor recently protested this "ethanol tax" in the streets.

In the United States, the protests have also begun — in Congress. Representatives of the dairy, poultry and livestock industries, which rely on corn as a principal animal feed, are seeking an end to subsidies for corn ethanol in the hope of stabilizing corn prices. (It takes about three pounds of corn to produce a pound of chicken, and seven or eight pounds to grow a pound of beef.) Profit margins are being squeezed, and meat prices are rising.

U.S. soybeans, which are used to make biodiesel, may be about to follow corn's trajectory, escalating the food vs. fuel conflict. The National Biodiesel Board recently reported that 77 biodiesel production plants are under construction and that eight established plants are expanding capacity.

In terms of environmental impact, all biofuels are not created equal. Ethanol is the same chemical product no matter what its source. But ethanol made from prairie grasses, from corn grown in Illinois and from sugar cane grown on newly cleared land in Brazil have radically different impacts on greenhouse gases.

Corn, like all plants, is a natural part of the global carbon cycle. The growing crop absorbs carbon dioxide from the atmosphere, so burning corn ethanol does not directly create any additional carbon. But that is only part of the story. All of the fossil fuels used to grow corn and change it into ethanol release new carbon dioxide and other greenhouse gases. The net effect is that ethanol from corn grown in the Corn Belt does increase atmospheric greenhouse gases, and this increase is only about 15 percent less than the increase caused by an equivalent amount of gasoline. Soybean biodiesel does better, causing a greenhouse gas increase that is about 40 percent less than that from petroleum diesel.

In Brazil, ethanol made from sugar cane produces about twice as much ethanol per acre as corn. Brazilian ethanol refineries get their power from burning cane residue, in effect recycling carbon from the atmosphere. The environmental benefit is large. Sugar-cane ethanol grown on established soils releases 80 percent less greenhouse gases than gasoline.