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Biofuels and Carbon Debt

February 14, 2008
The Energy Independence and Security Act of 2007 establishes an ambitious renewable fuel standard that increases biofuel production and use to 36 billion gallons by 2022. The standard also mandates that renewable fuels produced in new facilities constructed after enactment of the Act, which occurred in December 2007, "achieve at least a 20 percent reduction in lifecycle greenhouse gas emissions compared to baseline lifecycle greenhouse gas emissions." The Act does not specify a specific instrument for measuring lifecycle greenhouse gas emissions, but it does define lifecycle greenhouse gas emissions as "including direct emissions and significant indirect emissions such as significant emissions from land use changes and all stages of fuel and feedstock production and distribution, from feedstock generation or extraction through the distribution and delivery and use of the finished fuel to the ultimate consumer."

Recently, two teams of researchers proposed a modification of the GREET model (see Biofuels and GHG Lifecycle) to include greenhouse gas emissions from land use change, as mentioned in the Act, and their work was published in Science on Feb. 8, 2008. David Tilman of the University of Minnesota and colleagues proposed a "carbon debt" for biofuel feedstocks as a corollary to their carbon capture.This debt is defined as "the amount of CO2 released during the first 50 years" of the process of clearing land for production of biofuel feedstocks. The carbon debt is repaid primarily by the uptake of carbon dioxide by the feedstocks. Tilman et al. conclude that "If biofuels are to mitigate global climate change, our results suggest that they need to be produced with little reduction of the storehouses of organic carbon in the soils and vegetation of natural and managed ecosystems. Tilman et al. allow that "improvements in biofuel production could reduce payback times" though these improvements are not calculated in the present paper.

Searchinger and colleagues go further than Tilman in calculating this "carbon debt," by including indirect increases in carbon emissions such as the loss of future storage of carbon in converted land.  They also calculate an expected increase in croplands in Central and South America -- at the expense of rainforests and grasslands -- due to population growth and increased demand for food in these regions. They claim that diversion of U.S. crops to biofuels must carry a "carbon debt" for this phenomenon.

As the EPA considers implementation of the lifecycle reduction of greenhouse gases required in the new Energy Security Act, they should include a full accounting of the carbon balance of land use including possible beneficial changes. Better agronomic practices can increase carbon sequestration in cropland while allowing for collection of crop wastes for biofuel production.