Renewable fuels such as ethanol have long been hailed as a cleaner-burning alternative to fossil fuels and a potent way to reduce the climate-changing gases pumped out of car tailpipes.

The 2007 Energy Act signed by President Bush last December doubles the nation's use of corn-based ethanol. Ethanol production in Minnesota, a pioneer of the technology, is expected to double during the next three years.

But research by Minnesota scientists is challenging the underpinnings of the biofuel rush. Ethanol and similar products may do more harm than good because of the changes they bring to the landscape, some scientists say.

The exploding demand for ethanol, soy diesel and other products is causing farmers to clear forests, grasslands and peat lands on a massive scale, unleashing far more carbon dioxide than is saved by the lower emissions of the biofuels, the study said.

"If we keep moving to get large amounts of energy by growing it on newly cleared land, which is what's happening around the world, we're going to be releasing much more greenhouse gas than the benefits we get from those biofuels," said David Tilman, ecology professor at the University of Minnesota and one of the study's authors.

The study, published Thursday in the online version of the journal Science, was funded by the National Science Foundation and the University of Minnesota. Another study published Thursday in Science, with estimates from economists at Iowa State University, concluded that corn-based ethanol could double greenhouse gas emissions, rather than reducing them, during the next 30 years because of the dramatic changes in land use.

Ethanol industry officials called the latest studies a simplistic analysis that does not include the economic benefits for those who grow biofuel crops in the United States and abroad, or the environmental cost of continuing to rely on petroleum.

And the focus of renewable fuels will ultimately lead to even more environmentally friendly products, they say.

But a growing number of scientists are questioning the ecological benefits of biofuels. A policy report last month by the British Royal Society indicated that biofuels have been described as "carbon neutral," meaning that the carbon they emit to the atmosphere when burned is offset by the carbon that plants absorb from the atmosphere while growing.

The problem is that those benefits assume the world can turn large amounts of crops into biofuels, the report said, without needing to use more land to make up for lost food production. Clearing tropical forests and growing crops on natural peat lands in Malaysia, Indonesia and elsewhere "risk releasing enough greenhouse gases to negate any of the intended future climate benefits," the report said.

The reason for scientists' concern, said Tilman, is that soil and plants hold three times more carbon than air. Clearing trees to grow more corn or bulldozing tropical forests to grow more sugarcane emits large amounts of carbon dioxide, either quickly through the burning of the wood, or more gradually through the decomposition of carbon stored in plants and soil.

Tilman calculated that converting natural ecosystems to raise corn or sugarcane for ethanol, or soybeans or palms for biodiesel, will release 17 to 420 times more carbon than the annual savings from replacing fossil fuels. The Minnesota study estimated that in the United States, it will take 93 years for the carbon losses from plowing one acre of healthy grassland to equal the carbon savings from corn-based ethanol produced on that land.

Using set-aside land

Farmers in the Great Plains are rapidly cultivating lands once set aside for conservation, mostly because of strong demand and higher prices for corn. The U.S. Department of Agriculture estimated that in Minnesota, Iowa, North Dakota and South Dakota, about 750,000 acres of conservation grasslands were converted to crops last year, and that 1.1 million more acres will be eligible for conversion by 2010.

Bob Dinneen, president of the Renewable Fuels Association, a national trade group, said that assigning blame for rainforest deforestation and grassland conversion to agriculture ignores factors such as population growth and demand for food from rising middle classes in China and India.

"By adopting the use of biofuels today and encouraging the development of next-generation technologies for the future, the road can be paved for the future fuels and technologies to come," Dinneen said in a statement.

Robert White, interim executive director for the Omaha, Neb.-based Ethanol Promotion and Information Council, said that studies critical of ethanol do not credit the industry with advances in farming that yield more corn per acre, and improved technology that uses less natural gas and water.

White said that the industry is moving rapidly toward producing ethanol from wheat straw, wood chips and other wastes. "The benefits of ethanol will be consistent and will continue to grow as evolution of feedstocks continues," White said. "Ethanol is not the sole solution, but it's one that's available today."

Tom Meersman • 612-673-7388