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Science: Biofuels Come With Their Own Emission Costs

While biofuels are an attractive alternative to fossil fuels for meeting global energy demands, the picture isn't all rosy, scientists say.

Growing crops that could be converted into biofuels will indirectly cause substantial greenhouse gas emissions, if farmers clear forests and use more fertilizer in order to cultivate the land, according to a new study published online by Science, at the Science Express Web site.

At the same time, a major “accounting” flaw in national cap-and-trade laws and the rules of the international Kyoto Protocol prevent a realistic assessment of how much carbon is released to the atmosphere by biofuel production, according to a Policy Forum appearing in this week's Science.

In the first study, Jerry Melillo of the Marine Biological Laboratory and colleagues used a model that simulated economic and environmental changes to predict the effects of growing cellulosic biofuel crops on overall land use. These crops consist of grassy or woody plants and have lately been considered a better alternative than corn or soy, in part because they don't need fertilizer and shouldn't be affected by global food prices.

If biofuel crops replace food crops on current farmlands, Melillo and colleagues report, the clearing of forested land for additional food crops will release more carbon from the soil than in the areas where the biofuel crops themselves are being grown. Using fertilizer to sustain the growth of these new food crops would release important amounts of nitrous oxide--a far more effective heat-trapping molecule than carbon dioxide.

“A global greenhouse gas emissions policy that protects forests and encourages best practices for nitrogen fertilizer use can dramatically reduce emissions associated with biofuels production,” the authors write in their Science report.

In the Policy Forum, Timothy Searchinger of Princeton University and colleagues warn that rules for applying the Kyoto Protocol and national cap-and-trade laws contain a major but fixable "accounting" flaw in assessing bioenergy. If not corrected, this flaw will severely undermine greenhouse-gas reduction goals, they say.

These rules exempt carbon dioxide that is emitted via bioenergy use, regardless of the source of the biomass. But they also do not count the large net releases of carbon from activities involved in generating the biomass, such as clearing forest to provide fuel wood or to grow energy crops.

This approach essentially treats all bioenergy as “carbon neutral,” regardless of the source of the biomass. If applied globally, the authors warn, this discrepancy could lead companies or countries to clear much of the world's forests as carbon caps tighten.

"When forests or other plants are harvested for bioenergy, the resulting carbon release must be counted either as land-use emissions or energy emissions," said Melillo, who is also an author on the Policy Forum. "If this is not done, the use of bioenergy will contribute to our greenhouse gas problem rather than help to solve it."

The authors call for a system that would count all carbon dioxide emissions, whether from fossil fuels or bioenergy. Then, biomass should receive credits to the extent that its use truly results in an overall reduction of carbon in the atmosphere.


Kathy Wren

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