Growing crops to make biofuels may accelerate global warming, not slow down its effects, a new study says.

When farmers clear native ecosystems such as forests or grasslands to grow crops, this gives off substantial amounts of carbon dioxide, the primary greenhouse gas that fuels climate change.

Biofuels such as ethanol from corn and biodiesel from palm oil typically start out with a "carbon debt."

Before these biofuels could reduce individual carbon dioxide emissions, they would first have to pay off this debt, which would take decades or centuries.

"I was surprised that with so many of the crops, it takes so long before you break even [on carbon emissions]," said study co-author David Tilman of the University of Minnesota in St. Paul. The university and the nonprofit group the Nature Conservancy conducted the study.

"I don't think we can afford to make biofuels if we have to wait 50 years for any benefit," he added.

Many scientists believe the planet is at a crucial juncture, and that carbon dioxide emissions must be reduced sharply over the next 50 years to avoid the catastrophic consequences of climate change.

But there are some kinds of biofuels that could be helpful, such as those made from grasses or algae, as they avoid the debt problem, Tilman said.

The study will be published online tomorrow in the journal Science.

Carbon Dioxide Release

Biofuels release roughly as much carbon dioxide when burnt as regular gasoline or diesel. But since biofuel crops also soak up carbon dioxide as they grow, they were thought to reduce overall emissions as compared with fossil fuels.

However, few earlier studies into biofuel viability factored in carbon released at the start, when land is converted to grow crops.

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SOURCES AND RELATED WEB SITES Stockholm Environment Institute U.S.: Roel Hammerschlag University of Leicester: Sue Page University of Minnesota: David Tilman Science

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