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Published February 18, 2008 09:17 AM

Crop biofuels 'create carbon debt'

Two studies have shown that changes in land use to produce crop-based biofuels can actually result in more greenhouse-gas emissions than burning fossil fuels.

The studies, both published in Science last week (8 February), estimate the impact of converting forests and grasslands into cropland for the production of biofuels.

Both conclude that the resulting carbon emissions, released through decomposition or burning of biomass, create a 'carbon debt' that takes decades or even centuries to be paid back through biofuel usage.

This finding undermines previous claims that substituting fossil fuels with biofuels should offset greenhouse-gas emissions because biofuels sequester carbon while they grow.

According to Timothy Searchinger, researcher at Princeton University and the lead author of one of the studies, previous assessments did not include the carbon storage and sequestration sacrificed when diverting land from its existing use.

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Searchinger and colleagues looked at the use of US cropland to produce corn-based ethanol and calculated it would take 167 years to repay carbon emissions resulting from land-use change, and that in 30 years greenhouse-gas emissions from corn ethanol could be nearly double those from gasoline.

"Biofuels in the US and Europe are increasing the price of crops, which naturally results in more efforts to clear land. In that way, farmers make more money," he says.

Much of this land clearing will occur in Brazil, China and India, the authors write.

In the other study, by the Nature Conservancy and University of Minnesota, researchers estimate carbon debts and pay back years for different cases of conversion from native vegetation.

They found soybean biodiesel produced on converted Amazonian rainforest would take around 320 years to gain a 'carbon benefit' over petroleum diesel. For biodiesel and sugarcane-based ethanol produced on Brazilian cerrado — tropical savannah — the estimations are 37 and 17 years, respectively. 

Improving the productivity of agricultural land, creating biofuels from waste biomass and municipal waste, or from biomass grown on abandoned agricultural land, are all ways to avoid the need for a change in land use, the authors suggest.

The results of the studies do not surprise Roberto Schaeffer, researcher at the Federal University of Rio de Janeiro. "Nobody thought deforestation for biofuel production would be a good solution," he told SciDev.Net.

"Biofuels are only effective in specific situations, as in the case of Brazilian ethanol. It is possible to increase production without devastating forests."

Link to the article by Searchinger et al [85kB]

Link to article by Fargione et al [93kB]

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