Scientists Respond to Carbon Debt Issue

Scientists Respond to Carbon Debt Issue

Reaction to the ScienceXpress articles by Searchinger et al. and Tilman et al. has focused on the assumptions the teams of authors used in measuring greenhouse gas emissions from changes in land use. As Michigan State University Professor of Chemical Engineering Bruce Dale points out,

Both of these papers are modeling studies and are therefore completely dependent on the validity of the models themselves and also of the basic assumptions and data input to the models. If the models, assumptions and data are appropriate, then the predictions are useful, if not, the predictions may have little or no value.”

Dale is editor of Biofuels, Bioproducts & Biorefining (Biofpr) and head of the Biomass Conversion Research Laboratory at MSU. Michigan State University Bruce Dale Letter to Science

Michael Wang of Argonne National Laboratory and Zia Haq of the U.S. DOE provide a useful definition of “greenhouse gas emissions from land use change” in a published response to Searchinger.

Direct land use changes involve direct displacement of land for farming of the feedstocks needed for biofuel production. Indirect land use changes are those made to accommodate farming of food commodities in other places in order to maintain the global food supply and demand balance.”

Wang developed the GREET model used in the Searchinger analyis. Michael Wang, Zia Haq, Letter to Science

According to Wang and Haq, Searchinger et al. assume that use of corn in the United States for biofuel production will cause Brazil, China and India to convert forest and grassland to corn production to replace what they would have imported from the United States. Wang and Haq write that Searchinger’s assumption about indirect land use change is “seriously flawed” because it relies on data from the 1990s. Brazil and China have since that time adopted policies to prevent deforestation and conversion of grassland. Wang and Haq also criticize Searchinger for ignoring improvements in the use of land for growing crops and in technology to convert crops and crop residues as well as other biomass for biofuels.

The 25×25 coalition  notes that the ScienceXpress articles highlight the need for energy crops and production of cellulosic ethanol. They point to a study published in the Proceedings of the National Academies of Science that shows “that switchgrass grown for biofuel production produced 540 percent more energy than that needed to grow, harvest and process it into cellulosic ethanol.” 25×25 concludes, “Developing new sources of renewable fuels is a far better alternative to trying to squeeze petroleum from a depleting resource of fossil fuels at a demonstrably appalling environmental cost.”