Written on March 4, 2008
I feel a little bit like a traitor writing this post. I was trained as a maize geneticist and the recent interest, scientific and financial, in corn-based ethanol as a biofuel has been a boon to anyone in the corn business.
The problem is that an increasing number of scientific studies are indicating that corn-based ethanol may not be the green giant that folks thought. Just before Christmas a letter to Science by a respected ecologist suggested that the jump in corn farming in the U.S. has led to increased Amazonian deforestation. Grain prices hit record highs in the summer of 2007 due at least in part to the massive increase in demand for ethanol. This week, two papers in Science propose that the real costs of the ethanol boom may not be an increase in food prices but, ironically, an increase in atmospheric carbon.
The first of these studies stems from a collaboration between scientists from The Nature Conservancy and the University of Minnesota. Led by ecologist David Tilman, they predict that the conversion of uncultivated land in Southeast Asia, Brazil and the U.S. that would be required to deal with the increased demand for biofuels will result in a “biofuel carbon debt”. Atmospheric carbon is the problem in global climate change. The hope for biofuels is that their use reduces atmospheric carbon because growing more plants sucks up more carbon dioxide from the atmosphere. The problem with that theory is that fallow lands sequester a lot of carbon. This is both due to existing plants which remove carbon from the air but also carbon that is sequestered in the soil and biomass. In other word, if you clear an acre of forest to plant corn you remove the photosynthetic trees and understory plants and then burn them, releasing their fixed carbon into the atmosphere. Tilman’s group claims that conversions like these would release 17 to 420 times more carbon dioxide than the annual greenhouse gas reductions that ethanol would provide.
The companion paper, written by a collaboration of scientists from Princeton University, Woods Hole Research Center and Iowa State University generally confirms the Tilman group’s results. These researchers, led by Princeton’s Timothy Searchinger, used a worldwide agricultural model to predict greenhouse emissions over the next several decade under a variety of conditions. They found that corn-based ethanol production will double greenhouse gas emissions over 30 years rather than producing a 20% reduction as previous models predicted. Other crops are slightly better – switchgrass, George Bush’s favorite, would increase emissions by 50% and sugar cane only induces a four year carbon debt. Of course, the latter only applies if tropical cattle ranchers conveniently disappear.
In summary, these studies suggest that if we are to sustain world food production at current levels, crop based ethanol production will produce more atmospheric carbon then what we are currently generating. It is worth noting that both of these studies rely on complicated ecological models rather than any empirical data and I’m not enough of a modeler to properly critique their methods. That being said, most climate science and most of our fears about climate change are based on predictive models. This is what allows the climate change deniers to maintain any level of credibility.
It is not, however, all bad news. There are other sources for biofuel production beyond corn, switchgrass and sugar cane. Municipal and crop waste is a ripe source of energy that otherwise will be released as carbon dioxide into the atmosphere. Use of these materials would also reduce landfill inputs. Perennial plants grown on abandoned agricultural lands are another source that would incur little carbon debt. The take home message from both of these papers is that we should pour resources into improving the efficiency of ethanol or other biofuel production from these waste products – effectively killing two birds with one stone.
Filed in: Science.