( dpa ) - The US rush to plant more corn for biofuel is already being blamed for soaring food prices.
Now, two new US studies show that cultivated biofuel crops may actually increase instead of decrease the carbon emissions that ethanol and other biofuels were supposed to reduce.
The studies used a worldwide agricultural model to calculate how corn-based ethanol nearly doubles greenhouse emissions over 30 years. Planting switchgrass and other wild crops on farm land, as advocated by US President George W Bush, would increase emissions by 50 per cent.
The studies from the University of Minnesota and the Nature Conservancy published Friday are the latest warning against an all - out US rush into corn ethanol at the expense of food for humans and tilling of uncultivated ground.
The authors said their research is the most extensive yet to probe the environmental costs of producing biofuels.
Ploughing up rainforests, peatlands, savannas and grasslands to plant corn, sugarcane and other crops for biofuel releases 17 to 240 times more carbon than the annual savings from replacing fossil fuels, the researchers at the University of Minnesota and the Nature conservancy warned.
That's because the plants and soil are a giant carbon-storage system. During cultivation, the carbon escapes as carbon dioxide, a greenhouse gas blamed for global warming, the studies published in Science magazine this week found.
Similar "carbon debt" has already mounted up in Indonesia, where the authors calculated it will take 423 years to pay off the loss of peatlands to create palm oil plantations, the authors calculated.
The second largest debt is in the Amazon, where it will take 319 years to recoup the carbon emissions to create diesel-soybean plantations, the study said.
The projected debt figure for ambitious corn-biofuel production targets would be 167 years - the time it would take for biofuels to stop contributing to climate change, the authors said.
"We don't have proper incentives in place because landowners are rewarded for producing palm oil and other products but not rewarded for carbon management," said one of the authors, Stephen Polasky, of the University of Minnesota.
The big push for corn-biofuel came in early 2007 when Bush called for the US to displace 15 per cent of its annual petrol use with alternative fuels by 2017 - a vast increase over the current 3-per- cent level. He got congressional backing for his tax-incentive plan.
Energy self-sufficiency drove Bush's interest, but he also benefited from the popular image of biofuel as environmental- friendly.
The Washington-based Earth Policy Institute has predicted that ethanol distilleries would need 139 million tons of maize by the current year - almost half of the US harvest - to keep up with US demand, and that's just the beginning.
That means not only rising corn prices, but the displacement of corn and other grain-growing to other countries, including the Amazon, as farmers chase rising prices, the study said.
"What they react to is an increase in price," the authors wrote.
The researchers said it made no sense to convert more land for biofuel production because "all the biofuels we use now cause habitat destruction."
"Global agriculture is already producing food for 6 billion people. Producing food-based biofuel, too, will require that still more land be converted to agriculture," the Nature Conservancy's Joe Fargione said in a statement.
The authors urged use of more agricultural and forest waste and native grasses and woody biomass from marginal lands. Affordable technology for conversion to ethanol from these sources is however still out of reach.
The US is the second-largest producer of ethanol, after Brazil, which gleans more than 40 per cent of its transportation fuel from renewable sources, mostly sugar cane.
Corn is a less efficient. Sugar cane produces 5.3 times as much ethanol as does corn: one unit of energy input produces only 1.5 units of ethanol from corn, but 8 units of ethanol from sugar.
The studies challenge previous studies that show substituting biofuels for gasoline will reduce greenhouse gases because the feedstock sequesters carbon as it grows.