Fargione J.E., R.J. Plevin, J.D. Hill. 2010. The ecological impact of biofuels. Annual Review of Ecology, Evolution, and Systematics 41:351–77
Sometimes Science Chronicles contains reviews of recent scientific articles from a clear-eyed and critical scientific perspective. Full disclosure: This is no such review, since I wrote both the scientific article and this review of it. So you won’t be surprised to hear that I think this paper is an excellent overview of the ecological impact of biofuels.
More surprising, however, may be some of the paper’s findings. We attempted to provide quantitative answers to many of the most commonly asked questions about biofuels — including questions on land use, GHG impacts from land use and fossil fuel use, water use and biodiversity impacts.
First, some context: In 2008, biofuel production required about 33.3 million ha, or about 2.2 percent of global cropland, in order to produce about 1.7 percent of global liquid fuel production (on an energy basis). The World Energy Outlook predicts that biofuel production will increase over 2008 levels by at least 170 percent by 2020, and that this increase will come almost exclusively from first-generation biofuels (i.e., the five food crops: corn, sugarcane, soy, oil palm, and rapeseed). Current U.S. law mandates the blending of 36 billion gallons of ethanol by 2022, which would increase global ethanol production by 150 percent over 2008 levels, even if ethanol production in the rest of the world did not increase. (So-called second-generation biofuels include cellulosic ethanol made from biomass, but the estimated date that cellulosic ethanol will be commercially available keeps retreating into the future.)
Turning food into fuel on such a large scale raises several issues for conservation, even leaving aside ethical issues associated with the potential for biofuels to compete with food production:
Increasing stress on water supplies: Irrigated corn requires about 643 gallons of water for every gallon of ethanol. Because rain-fed cropland in the United States is already largely in use, new corn production is occurring disproportionately on irrigated lands — 34 percent of the new corn production between 2003 and 2008 came from irrigated lands. This is not good news for the Ogallala aquifer
Increasing land conversion to agriculture: The amount of land required for biofuel production is a function of conversion efficiencies, crop yields, unharvested areas and co-products. The paper reviews current and likely future values for all of these variables for current and proposed biomass crops. One notable result is that, although crop yields are increasing throughout the globe, the increases are consistently linear. These empirical trends suggest that yield increases will not be a panacea — increased biofuel production will mean converting more land to agriculture.
One piece of good news is that, for some crops, biofuel production generates coproducts that can offset a substantial chunk of their land use. Specifically, corn produces distillers grains and solubles (DGS), which are fed to livestock, replacing some of the corn and soybean meal in livestock diets. Because soy has lower yields and therefore requires more land to grow than does corn, when DGS replaces soybean meal, that replacement makes for a lot of cropland that you don’t need for growing soy. In total, DGS may offset about 60 percent of the land needed to grow corn for ethanol, so that the net increase in agricultural land required to meet our food and fuel demand is only about 40 percent of the land used for corn ethanol.
The carbon debt of corn ethanol: There have now been a handful of studies that estimate the net GHG impact of first-generation biofuels, taking their land demand into account. For example, the EPA estimates that, for every hectare of corn used for ethanol, there are about 0.43 hectares put into new cropland (the figure is lower than 1 because of DGS co-products and because ethanol demand raises corn prices, which suppresses demand for corn). Global average emissions from new cropland are about 200 Mg CO2 per hectare. This means that every hectare of new corn ethanol is responsible for about 83 Mg of CO2 emissions due to land-use change somewhere in the world. In 2008, we coined the term “carbon debt” to describe these one-time emissions associated with land-use change (Fargione et al. 2008). If the corn ethanol produced on that hectare eliminated an equivalent amount of gasoline consumption, it would take about 70 years of corn production to reduce CO2 emissions to repay this carbon debt. In the meantime, the net effect of corn ethanol production is to increase atmospheric CO2 concentrations.
Effects on biodiversity: The direct impacts to biofuels on biodiversity are, in general, not very well studied — but there are several notable exceptions. Fletcher et al. (2010) report that animal diversity in row crops such as corn and soy (as measured with, e.g., species richness or Shannon’s Index) was reduced by about 60 percent compared to reference habitat. Studies in oil palm plantations find that 85 percent of animal species found in paired primary forests were absent in oil palm plantations (Fitzherbert et al. 2008). We expect that sugarcane plantations are similarly depauperate compared to native cerrado and coastal Brazilian rainforests, both of which could be impacted by increased sugarcane and soy production to meet biofuel demands.
Solutions: The Nature Conservancy has pioneered Development by Design approaches that could be fruitfully applied to biofuel production — especially in Brazil, Indonesia and Malaysia, where biofuels are contributing to habitat loss on the agricultural frontier. To apply this approach to biofuels, areas that need to be avoided should be defined and areas already converted or degraded should be identified and targeted for new biofuel production. Companies should be encouraged to pay compensatory mitigation to offset residual impacts. For example, companies that purchase food crops from existing cropland for biofuel production could calculate the likely indirect land-use change impact associated with displacing food production and pay for an equivalent amount of habitat restoration or for habitat protection that saves an equivalent amount of habitat from conversion.
Future advances in technology may (or may not) bring super-algae that produces fuel from a significantly smaller land footprint and could see society get much better at using biomass wastes to produce fuel. But such solutions, if they are forthcoming, are a ways off. The reality is that globally we are producing biofuels almost exclusively through food crops, and we will continue to do so for the next decade, if not longer. This fact makes biofuels into a globally significant contributor to habitat conversion and a good candidate for conservation strategies that the Conservancy has successfully applied to other forms of development.
Joe Fargione is a Lead Scientist at The Nature Conservancy in North America
Image credit: Zermie/Flickr through a Creative Commons license.
Fargione, J., J. Hill, D. Tilman, S. Polasky, and P. Hawthorne. 2008. Land clearing and the biofuel carbon debt. Science 319:1235-1238.
Fitzherbert, E. B., M. J. Struebig, A. Morel, F. Danielsen, C. A. Brulh, P. F. Donald, and B. Phalan. 2008. How will oil palm expansion affect biodiversity? Trends in Ecology & Evolution 23:538-545.
Fletcher, R. J., B. A. Robertson, J. Evans, P. J. Doran, J. R. R. Alavalapati, and W. Schemeske. 2010. Biodiversity conservation in the era of biofuels: risks and opportunities. Frontiers in Ecology and the Environment.