But that isn't the case for sugar-cane ethanol or soybean biodiesel from Brazil's newly cleared lands, including tropical forests and savannas. Clearing land releases immense amounts of greenhouse gases into the air, because much of the material in the plants and soil is broken down into carbon dioxide.

Plants and soil contain three times more carbon than the atmosphere. The trees and soil of an acre of rainforest — which, once cleared, is suitable for growing soybeans — contain about 120 tons of organic carbon. An acre of tropical woodland or savanna, suitable for sugar cane, contains about half this amount. About a fourth of the carbon in an ecosystem is released to the atmosphere as carbon dioxide when trees are clear-cut, brush and branches are burned or rot, and roots decay. Even more is lost during the first 20 to 50 years of farming, as soil carbon decomposes into carbon dioxide and as wood products are burned or decay.

This means that when tropical woodland is cleared to produce sugar cane for ethanol, the greenhouse gas released is about 50 percent greater than what occurs from the production and use of the same amount of gasoline. And that statistic holds for at least two decades.

Simply being "renewable" does not automatically make a fuel better for the atmosphere than the fossil fuel it replaces, nor guarantee that society gains any new energy by its production. The European Union was recently shocked to learn that some of its imported biodiesel, derived from palm trees planted on rain-forest lands, was more than twice as bad for climate warming as petroleum diesel. So much for the "benefits" of that form of biodiesel.

Although current Brazilian ethanol is environmentally friendly, the long-term environmental implications of buying more ethanol and biodiesel from Brazil, a possibility raised recently during President Bush's trip to that country, are cloudy. It could be harmful to both the climate and the preservation of tropical plant and animal species if it involved, directly or indirectly, additional clearing of native ecosystems.

Concerns about the environmental effects of ethanol production are starting to be felt in the United States as well. It appears that American farmers may add 10 million acres of corn this year to meet booming demand for ethanol. Some of this land could come from millions of acres now set aside nationwide for conservation under a government-subsidized program. Those uncultivated acres absorb atmospheric carbon, so farming them and converting the corn into ethanol could release more carbon dioxide into the air than would burning gasoline.