New Studies Identify Change in Land Use Associated with Biofuel Production as Major Contributor of Greenhouse Gases, Far Offsetting Benefits of Most Current Biofuels
Two separate studies published in the current online edition of the journal Science identify land use change—the conversion of rainforests, peatlands, savannas, or grasslands to produce food-based biofuels or to replace existing cropland diverted to biofuel crop production—as a major source of increased carbon dioxide emissions, far offsetting the presumed greenhouse gas benefits of using most current biofuels.
The studies stress the importance of using biomass waste or biomass grown on non-agricultural lands as feedstock for biofuel production to avoid this problem.
Timothy Searchinger from Princeton and colleagues at Woods Hole Research Center and Iowa State University used a worldwide agricultural model to estimate emissions produced by the change in land use as farmers worldwide respond to higher prices and convert forest and grassland to new cropland. They concluded that corn-based ethanol, instead of producing a net 20% savings in greenhouse gases, nearly doubles greenhouse emissions over 30 years and increases greenhouse gases for 167 years. Biofuels from switchgrass, if grown on US corn lands, increase emissions by 50%.
In their study, Joseph Fargione from the Nature Conservancy and colleagues at the University of Minnesota, calculated that land use conversion in Brazil, Southeast Asia, and the United States creates a ‘biofuel carbon debt’ by releasing 17 to 420 times more CO2 than the annual greenhouse gas (GHG) reductions these biofuels provide by displacing fossil fuels. In contrast, they said, biofuels made from waste biomass or from biomass grown on abandoned agricultural lands planted with perennials incur little or no carbon debt and offer immediate and sustained GHG advantages.
In the worst case Fargione and his colleagues examined, converting peatlands in Indonesia into palm oil plantations ran up a carbon debt that would take 423 years to pay off. The next worst was soybeans in the Amazon, at 319 years.
Searchinger et. al. note in their paper that “land use change emissions” refers to all of the carbon storage and ongoing sequestration that is foregone by devoting land to the production of biofuels. Using land to produce a biofuel feedstock forgoes some of that storage and ongoing sequestration, causing offsetting emissions in a variety of ways.
A forest or grassland can be directly converted to grow a biofuel such as corn, resulting in the direct loss of the carbon in the standing trees and grasses and a fair chunk of the carbon after plowing up the soils. Soils store major quantities of carbon in forests and grasslands.
The same land, if not devoted to biofuels, could continue to sequester carbon. For example, a young, growing forest will continue to sequester carbon as the forest grows for many years. This ongoing sequestration is lost if the land is converted to a biofuel for ethanol. (Although land converted to grow the biofuel, such as corn, will continue to sequester carbon, the typical biofuel analysis already takes account of that carbon.)
Both of these effects can occur indirectly. For example, if corn in the United States is diverted to ethanol production, grasslands or forest could be converted anywhere in the world to replace the corn. Complicating this analysis, these indirect effects can pass through many steps. For example, soybean land in the US can be planted in corn, and forest or grassland plowed up in Brazil to replace the soybeans.
In essence, under typical biofuel calculations, the carbon withdrawn from the atmosphere by growing the feedstock becomes a greenhouse gas credit. We call this credit a feedstock uptake credit, which we treat as part of the overall land use effect. But the world’s land already exists, and that land is for the most part removing carbon from the atmosphere each year and in most cases has stored substantial amounts of carbon for decades that may be lost if used to produce biofuels.
The proper focus must be on the net change in carbon removed from the atmosphere that is either stored by land or used to replace fossil fuels. (Replacing fossil fuels is a form of storage because the unneeded fossil fuel remains stored underground.) An accurate accounting must subtract the emissions from land use change from the feedstock uptake credit to produce a proper net estimate of the overall land use effect—the effect of using land to produce biofuels.
Timothy Searchinger et. al. Use of US Croplands for Biofuels Increases Greenhouse Gases Through Emissions from Land Use Change. Science 7 February 2008 DOI: 10.1126/science.1151861
Joseph Fargione, et. al. Land Clearing and the Biofuel Carbon Debt. Science 7 February 2008 DOI: 10.1126/science.1152747