Do biofuels slow global warming—or speed it up? Daniel Kammen, head of the University of California Berkeley’s Renewable and Appropriate Energy Laboratory, and colleagues found more modest gains for corn ethanol, about 13% on average. That jibes with results from ecologists Jason Hill and David Tilman of the University of Minnesota (Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 11,206–11,210). ERIK A ENGELHAUPT
It seems like a simple question: are biofuels better for the climate than fossil fuels? One recent study says greenhouse gas emissions from cornbased ethanol are 40% lower than those from gasoline—not bad. But another new study says ethanol’s total emissions can be as much as 50% higher than those from the gasoline it replaces—not good. And an informal ES&T survey of more than half a dozen scientists who study the issue drew a resounding “it depends.” As Tad Patzek of the University of California Berkeley and David Pimentel of Cornell University point out, it takes energy to run the tractors that harvest the plants, to make the fertilizer that nourishes the plants, and to haul the ethanol and biodiesel to pumping stations. And that means making CO2. In addition, “about 150 metric tons of carbon per hectare is released over 30 years from tropical forests converted to cropland,” says Nathanael Greene of the Natural Resources Defense Council. Microbes in farm soils produce nitrous oxide (N2O), a greenhouse gas almost 300 times more potent than CO2. The use of nitrogen fertilizer increases N2O production, so fertilizer-intensive crops like corn make the problem even worse. Several life-cycle analyses have tallied emissions from farm to pump, with varying results. “We found a net greenhouse gas benefit for all biofuels compared to fossil fuel,” says soil scientist Stephen Del Grosso of the U.S. Department of Agriculture, referring to his recent study in Ecological Applications (2007, 17 [3], 675–691). Corn ethanol and soy biodiesel came out about 40% ahead of fossil fuels, and some cellulosic ethanol sources had negative net emissions. A review of six other studies by
A refinery in Brazil turns sugarcane into ethanol; cane ethanol is thought to have a smaller carbon footprint than corn ethanol.
Chemist Paul Crutzen and colleagues muddied the waters further when they submitted a study in August to an online discussion forum of the journal Atmospheric Chemistry and Physics. According to the paper, biofuel crops release several times more N2O indirectly than previously thought. Biofuel critics latched onto the paper, and news stories reported that the Nobel Prize-winning Crutzen had found that biofuels worsen global warming. However, the paper had not yet cleared peer review, and Crutzen says that a negative review recently prompted the journal to request major revisions. He declined to comment on the paper’s content but said the authors plan to proceed with publication. “Come what may, if you shove 100 kilograms of nitrogen into the biosphere as fertilizer, you get 3–5% of it coming back out at some point as N2O,” says Crutzen’s coauthor, Keith
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Smith of the University of Edinburgh (U.K.). “Many of the people who do life-cycle analyses use the Intergovernmental Panel on Climate Change’s [IPCC’s] default value, which is 1% or so; then they calculate how much fuel was used, et cetera, and those numbers vary a lot,” he says. But Del Grosso counters that his group has used N2O numbers similar to Crutzen’s in a life-cycle analysis and still found that biofuels can come out ahead. It depends on a number of factors, he says, such as how far grains must be transported to refineries. “There is a danger that [some biofuels] are being oversold in terms of climate-change-tackling abilities,” notes David Reay, who studies the role of nitrogen in climate change at the University of Edinburgh. His back-of-the-envelope calculation based on Crutzen’s numbers for N2O indicates that, in a worst-case scenario, the U.S. Senate’s targeted sevenfold increase in corn ethanol by 2022 could increase greenhouse gases by the equivalent of 345 million metric tons of CO2 annually. “It’s safe to say cellulosic ethanol will be much more efficient than grain-based ethanol,” says Charles Rice of Kansas State University, a lead author on the IPCC report. The fermentation process is more efficient, he notes, and uses more of the plant. “In the face of climate change, what we have to be doing is looking for low-carbon fuels, not just any alternative fuel,” says Judi Greenwald of the Pew Center on Global Climate Change. California is the first state to adopt a low-carbon fuel standard, which will cut life-cycle greenhouse gas emissions by 10%. But before California can brag about its cuts, Greenwald notes, it will have to calculate the life-cycle emissions of all fuels, including biofuels. “That’s the challenge,” she says. —ERIKA ENGELHAUPT © 2008 American Chemical Society
