Farmers and farm state politicians have been excited about biofuels for years. Farmers are planting record amounts of corn, and a lot of it is being turned into ethanol.
In Brazil, ethanol made from sugar cane has replaced nearly a third of the gas burned in cars and trucks. In Malaysia and Indonesia, huge plantations are turning palm oil into bio-diesel.
But there's a cost to these crops, one that many policy-makers haven't acknowledged yet.
"In the top chunk of soil, we lose 40% of the carbon from that soil when we convert it to agriculture."
Joe Fargione says there's almost three times as much carbon in plants and soil as there is in the air. So when the soil is disturbed -- when rain forests are cut to plant sugar cane, or prairie is plowed to plant corn, or tropical peatlands are drained to plant palm trees -- it releases carbon from the existing plants and the soil into the air.
Fargione is a biologist. He did the study for the University of Minnesota and The Nature Conservancy.
He combed through reports on land use changes around the world, and calculated what he calls a "carbon debt" from land clearing. He says clearing land to plant crops releases more carbon than is saved by replacing fossil fuels with biofuels.
"It's like taking out a loan and then trying to save money, but you can't save any money until you've paid off your debt. And the debts are so large it'll take decades or centuries for us to pay off that debt."
The US Agriculture Department says this year farmers may convert seven million acres from conservation land to corn. Fargione says that conversion racks up a carbon debt that will take about 90 years to repay.
He says converting tropical rainforests to palm plantations will take more than 400 years to pay off.
And because both food and energy are global commodities, what Midwestern farmers do affects the Amazon. American farmers traditionally alternate each year between corn and soybeans. But now some are planting corn every year to meet the growing demand for ethanol. Those choices are prompting Brazilian farmers to clear more land for soybeans.
"We can't ask the world's farmers to feed six billion people and then say 'also produce energy,' without them requiring more land," he says. "That land has to come from somewhere; there's no free lunch."
Fargione says there are some bio-fuels that don't contribute to global warming. Waste products from agriculture and forestry can be turned into fuel. And farmers could plant perennial grasses like switchgrass, which grows well on poor land.
"We can take land that's been degraded, and plant it to perennials," he says. "There's carbon storage in the soil, and then we can harvest those perennials, and use those for biomass. That could have a real benefit."
Research on that so-called next-generation, cellulosic ethanol is underway. But Fargione says we also need policies that will make growing those fuel stocks profitable for farmers.
Nathaniel Greene is a policy analyst at the Natural Resources Defense Council. He says the information on carbon debt is key to developing biofuels that really help address climate change.
"We can do biofuels smart, or we can do them stupid, and we have to really choose," Greene says.
The federal energy bill Congress passed in December includes a greenhouse gas performance standard for renewable fuels. To meet the standard, biofuels would have to emit significantly less carbon than fossil fuels, on a life-cycle basis. That includes carbon emissions from land conversion, and fossil fuel use to grow and process the crop. The Environmental Protection Agency is using studies like Fargione's to set up a way of accounting for the total carbon emissions of various fuels.