The new studies, published in the journal 'Science,' add to the growing controversy about the environmental impacts of growing massive amounts of food crops to produce biofuels.
Although such fuels emit less greenhouse gases than fossil fuels, clearing native ecosystems causes a subsequent release of carbon dioxide from plants and soil through fire or decomposition over a period of decades. Furthermore, cropland absorbs less carbon than the native ecosystems it replaces.
One of the studies, conducted by researchers with The Nature Conservancy and the University of Minnesota, quantified emissions of carbon dioxide that result from clearing native ecosystems in order to grow food-based crops for biofuel production.
Lead author Joe Fargione, a scientist with The Nature Conservancy, said the study he conversion of land for biofuels and 'asks the question ‘is it worth it?’'
'And surprisingly, the answer is no,' Fargione said. 'These natural areas store a lot of carbon, so converting them to croplands results in tons of carbon emitted into the atmosphere. If you're trying to mitigate global warming, it simply does not make sense to convert land for biofuels production.'
Fargione and his coauthors found that converting native ecosystems, such as rainforests, savannas and grasslands, into fields of corn, sugarcane, palm trees or soybeans, would release 17 to 420 times more carbon than would be gained by replacing fossil fuels.
The researchers identify this release as a 'carbon debt,' which must be paid before biofuels produced on the land can truly be credited with reductions in greenhouse gases.
They found that the size of these debts could be too great, predicting that converting native ecosystems to produce biofuels could actually accelerate the release of gases linked to global warming.
For example, it would take more than four centuries to pay off the carbon debt incurred by converting Indonesian peatlands for palm oil plantations.
Soybean production in the Amazon would take some 319 years to offset the environmental costs of production and converting U.S. grasslands to corn production for ethanol would require 93 years to offset.
The second study notes that diverting food crops into biofuel production also leads to the clearing of more native ecosystems for crop production.
Diversion of U.S. corn supplies to ethanol is having a global effect, the researchers say, requiring the conversion of more land to corn production and driving up prices.
Annual U.S. production of corn ethanol has risen from to nearly seven billion gallons from 1.5 billion gallons since 2000, driven by mandates enacted by the federal government. The Bush administration and many U.S. lawmakers have promoted the environmental benefits of ethanol.
But estimates that corn-based ethanol would result in 20 percent reductions in greenhouse gases compared to fossil fuels are wildly inaccurate, according to the study by researchers at Princeton University, Iowa State University and the Woods Hole Research Center.
Their analysis finds corn ethanol 'nearly doubles greenhouse gas emissions over 30 year and increases greenhouse gases for 167 years.'
Industry groups reacted with skepticism to the new reports, saying they ignore the realities of the market forces propelling the demand for biofuels.
The studies 'fail to put the issue in context,' said Bob Dineen, president of the Renewable Fuels Association, which represents the U.S. ethanol industry.
'Biofuels alone are not the silver bullet to the energy or environmental challenges our planet faces,' Dineen said in a statement. 'But they do offer a pathway forward … the alternative is to continue to exploit increasingly costlier fossil fuels for which the environmental price tag will be great.'
But the studies echoing growing concern about the rush to biofuels. Last month the European Union announced a review of its plan to boost the use of biofuels in transport and said it was considering a ban on imports of biofuels that are not produced sustainably.
Professor John Pickett, chair of the UK Royal Society biofuels study, noted that 'one biofuel is not the same as another.'
Pickett said the new research underscores the need for criteria to assess the environmental benefits of biofuels.
'This must happen at an international level so that we do not just transfer any potentially negative effects of these fuels from one place to another,' he said.
The researchers involved in the two studies echoed that view, noting some biofuels do not contribute to global warming because they do not require the conversion of native habitat, such as waste from agriculture and forest lands and native grasses as well as woody biomass grown on marginal lands unsuitable for crop production.
'We will need to implement many approaches simultaneously to solve climate change,' Fargione said. 'There is no silver bullet, but there are many silver BBs. Some biofuels may be one silver BB, but only if produced without requiring additional land to be converted from native habitats to agriculture.'