• Eli is a contributing correspondent for Science magazine.

Tropical Biofuels Getting Less and Less Green

9 July 2008 (All day)

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Knockdown fight. New research suggests that using tropical rainforests to grow biofuels could be even trickier than previously estimated.

A new analysis suggests that biofuels grown in the tropics are not a much greener source of energy than drilling for oil--at least in the short term. The research paints an even gloomier picture of biofuels than previous studies, which have begun to cast doubts on the greenhouse gas benefits that these alternatives to petroleum might provide.

Proponents see plant-based biofuels as a carbon-friendly alternative to fossil fuels. That's because plants that produce, say, palm oil or corn ethanol recycle carbon dioxide from the atmosphere as they grow. In contrast, petroleum production introduces new carbon into the air that was previously sequestered deep within Earth.

Two studies published earlier this year in Science, however, suggest that the carbon benefits from biofuels are delayed for centuries when farmers knock down carbon-absorbing forests in order to grow the plants. One paper, for example, estimated that cutting down Brazilian rainforest to grow soybeans for diesel fuel would result in a so-called carbon debt that would take 319 years to repay--essentially rendering the fuel as carbon-unfriendly as gasoline in the short term.

But critics, including the U.S. Department of Energy, have charged that such analyses underestimate the yield of biofuel crops, especially those grown in the tropics.

To get a better sense of just how green biofuels are, Holly Gibbs, a tropical land-use scientist at the University of Wisconsin, Madison, and colleagues used newly available data on the yields of 10 crops in various seasons and ecosystems within South American, African, and Asian rainforests. Even when Gibbs assumed that the plants would perform in the top 10% of all global varieties--with corn ethanol varieties, for example, producing more than seven times as much ethanol as current species in Africa--the picture remained gloomy. For example, corn grown with high yields in tropical forests (thanks to fertilizer, irrigation, and sophisticated farming) repaid its carbon debt in 100 years: five times faster than corn grown at normal yields. But the improvement is still a "losing proposition," Gibbs says, given the goal of stemming carbon emissions immediately. What's more, she notes, such yields would be hard to achieve throughout the developing world anyway given the cost of world-class agriculture.

To make biofuels more efficient, Gibbs suggests growing biofuel crops in places where trees wouldn't have to be cut down, such as in West African scrublands where cocoa plantations once grew. Otherwise, growing, say, oil palm trees on land that was previously carbon-rich peatland forests in Southeast Asia can create a nearly millennium-long carbon debt, the team reports online today in Environmental Research Letters. The paper highlights the short-term risk of ramping up production of tropical biofuels and cutting down carbon-rich forests to grow them, says Gibbs.

The new studies offer a set of estimates more precise than before for the carbon tradeoff that biofuels entail, says Princeton University agricultural expert Timothy Searchinger, an author of one of the Science papers. "Forests and grasslands have a lot of carbon, so there is really no way to transform those lands into biofuel crops that produce net benefits," he says.

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