WASHINGTON — It takes a long time — and some thick skin — to buck conventional environmental lore, but two new U.S. studies that challenge biofuels’ reputation as a green-friendly alternative to fossil fuels are getting some attention.

According to researchers at Princeton University and Iowa State University, the widespread use of ethanol from corn and plant-based biodiesel could produce nearly twice the greenhouse gas emissions as the gasoline and petrol diesel it would replace because of land-use changes, Associated Press reports.

The researchers said that past studies showing the benefits of ethanol in combating climate change have not taken into account almost certain changes in land use worldwide if ethanol and biodiesel — and in the future from other feedstocks — become high-demand commodities.

The report suggests that as demand for alternative fuels grows, farmers are plowing under forests and grasslands releasing formerly stored carbon into the atmosphere. Therefore, the carbon lost by converting rainforests, peatlands, savannas and grasslands outweighs the carbon savings from biofuels.

This “carbon debt” theory has been echoed by similar research conducted by the University of Minnesota for the Nature Conservancy.

“Using good cropland to expand biofuels will probably exacerbate global warming,” concludes the study published in Science magazine.

However, the Renewable Fuels Association, which represents ethanol and biodiesel producers, said the theories in the studies are too “simplistic” and “fail to put the issue in context.”

U.S. President George Bush recently signed legislation that mandates ethanol usage to 136 billion litres a year by 2022.

In Canada, Ottawa is reviewing a proposal that would require a B2 (2 percent biodiesel) standard sold at pumps across the country.

— with files from Associate Press