More evidence is piling up that biofuels may not be the boon to energy resources that they've been touted to be.

In recent years, studies have pointed out several potential problems with using biofuels, such as ethanol, as energy sources. Some research has indicated it may take more energy to make the ethanol than it would ultimately provide as a fuel. Some types of ethanol when burned may not cut down on the release of greenhouse gases as much as was hoped. And devoting more land to growing biofuel crops can strain water resources, other studies have found.

A new study, detailed in the Feb. 8 issue of the journal Science, concludes that biofuels aren't the way to go with alternative energy. The researchers found that much more carbon is lost when natural ecosystems, such as rainforests, grasslands and peatlands, are converted to croplands than is saved by using biofuels.

"It's not really the full solution that some people seem to think it is," said study leader Joseph Fargione of the Nature Conservancy, an environmental advocacy group.

Carbon storage

Natural ecosystems store enormous amounts of carbon in trees, leaves, grasses and soils. In fact, there's three times as much carbon in the plants and soils of the earth as there is in Earth's atmosphere.

"Carbon is the main building block of life, so plants are 50 percent carbon by dry weight," Fargione said. "So when you're looking at a rainforest, there's tons and tons of carbon stored in the plant biomass and in the soils."

When land is cleared either by cutting trees down or by burning, much of that stored carbon is released into the atmosphere.

"Fire releases the carbon directly, as carbon dioxide, and decomposition, when plants decay, that also releases the carbon as carbon dioxide," Fargione explained. "And this carbon dioxide goes into the air as an important greenhouse gas and contributes to global warming."

Large amounts of carbon in these ecosystems are released each year through deforestation and other land conversion.

"Over the last 150 years, 25 percent of our carbon emissions have come from land clearing," Fargione said.

Carbon debt

Biofuels from crops such as corn, sugarcane, soybeans and palms require land to grow on. Most of this land must either directly or indirectly come from the destruction of natural ecosystems, because "right now we're asking the world's farmers to feed 6 billion people, and they're doing it on some fixed amount of land," Fargione said. "And if we're also going to produce energy, that requires new land, and that new land has to come from somewhere."

Clearing natural ecosystems, either for farming food crops or growing biofuel crops, creates what Fargione calls a "carbon debt." The initial clearing of the land releases an amount of carbon dioxide that could take decades or centuries to make up for by using biofuels.

For example, converting peatlands to a biofuel crop results in a net release of carbon dioxide. Decomposing peat, which is nearly all organic matter, releases 55 tons of carbon dioxide per hectare per year. (One hectare is about 2.5 acres.) Palm oil biodiesel saves only about 7 tons of carbon dioxide per hectare per year. Fargione and his colleagues concluded that with this kind of land replacement, the carbon debt could take more than 800 years to repay.

"Any potential benefit of biofuels has to first pay off this carbon debt, otherwise it's just making global warming worse," Fargione said.

Other options

The carbon debt that would result from most biofuel production isn't worth the cost to our climate, Fargione says.

"Our natural ecosystems, if they're left intact, provide an incredibly valuable service of carbon storage and climate regulation," he told LiveScience.

Fargione says there are other options for producing biofuels, including using land that is too degraded for growing food crops. Planting a perennial crop on that land would actually start to build up the carbon in the land again.

"So producing biofuels on that land would not only have the benefit of producing a biofuel, which you could use to offset fossil fuels, but it also has a benefit of sequestering and building up carbon in that natural ecosystem," Fargione said.

Biofuels can also be produced in ways that don't require land, either by using agricultural waste biomass, manure waste from feedlots or steam heating through burning biomass, Fargione said.

But the main implication of their study, Fargione said, is that any climate change regulations that consider using biofuels must look at how that biofuel is produced as well as what it releases when it is burned, otherwise it may overestimate the benefits of using biofuel.

"The clear and dramatic policy implications of this work is that any climate change policy that does not take land use change into account will not work," Fargione said.