Published online 11 July 2006 | Nature | doi:10.1038/news060710-4

Corrected online:

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Corn ethanol and soybean biodiesel are not future energy solutions.

You can squeeze energy from corn, but not enough to displace gasoline. © Getty

Producing biofuels such as ethanol from food crops isn't worth the effort. That's the conclusion of a new and painstaking study published this week in the Proceedings of the National Academy of Sciences1. Researchers should instead concentrate either on producing ethanol from indigestible plant material such as cellulose, or on synthetic hydrocarbon fuels.

The comprehensive study finds that if all the corn (maize) produced in the United States last year were removed from food supplies and turned into ethanol, just 12% of US gasoline demand would be offset. Turning soybeans into diesel would account for only 6% of US diesel demand.

Environmentally friendly?

Contrary to some recent studies, this review finds that corn ethanol and soybean biodiesel do supply more energy than is needed to produce them; soybean biodiesel wins out over ethanol, providing 93% more energy than the amount used in growing, reaping and converting the soy crop to fuel, as compared with 25% for ethanol. Both also reduce levels of greenhouse gas emissions over fossil fuels; biodiesel much more so. But neither biofuel could replace much petroleum without a serious impact on food supplies, the researchers note.

"Corn ethanol is a first-generation biofuel," says study co-author David Tilman, of the University of Minnesota in St Paul. "It's as likely to realistically meet US energy demand as a Wright brothers' plane would be to fly across the Atlantic." Alex Farrell, an energy expert at the University of California, Berkeley, agrees. "Corn ethanol comes at the price of soil erosion and nutrient runoff," he adds. "Producing ethanol from cellulose is a much more environmentally preferable option."

Cellulose, an inedible plant fibre, could be obtained from switchgrass, a prairie grass that could be grown on abandoned agricultural land. This would avoid destroying natural habitats and would require small pesticide input. Cellulosic ethanol plants are on trial in Spain and Canada, says Tilman, who also recommends investigating synthetically manufactured fuels.

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A US Department of Energy report released on Friday 7 July said that biofuels such as cellulosic ethanol could displace 30% of the fuel consumed in US transportation by 2030, which Tilman thinks is a long-term viable goal; although it would take significant technological advances.

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Corrected: Due to an error in the draft version of the PNAS paper, this article originally incorrectly stated that soybean biodiesel would account for 9%, rather than 6%, of US diesel demand.