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Farming fuel crops may drive far-off deforestation and CO2 release

In a 2008 paper in the journal Science, Timothy Searchinger and several colleagues argued that growing crops for fuel instead of food in one part of the world might create ripple effects in lands far away—for example, the clearing of forests or plowing of pastures to make way for more food crops. Clearing land to farm releases carbon dioxide, and such land changes contribute about 20% of the globe’s annual greenhouse gas emissions.  Overall, Searchinger and colleagues argue that this connection creates an indirect "carbon debt" for the biofuels.1

Here’s how it is thought to work. Thanks to increased ethanol demand, US farmers are incentivized to plant more acres in corn and fewer in soybeans. A reduced global supply of soybeans would then drive up soy prices, all other factors being equal. If the price were to go high enough, farmers from other countries might decide to plant more soy — Brazil, for instance, which is already a major producer. To expand acreage there, some farmers might decide to convert cattle pastureland into soy production. This would result in carbon releases from the soil — but may not be the end of the story. 

Those farmers—or others—might want to make up for the lost pastureland. So they might cut down forests to create new pasture — a common practice in Brazil. This last act would make for a large carbon debt. Of course, the same argument would hold if the forest were cut in order to plant more soybeans — a domino effect with one less domino.

This story seems simple and logical enough, and may even be true. The problem is that it's difficult to draw a line of cause and effect, or to physically measure how land-use decisions in one country influence those in another. As a result, researchers use computer models of the global economy to try to quantify the carbon debt that biofuels incur as a result of these indirect land-use changes. Because researchers make many assumptions about how land-use decisions are made when running their models, it is not surprising that there is plenty of disagreement about the results.2 3 4 5 6 7 8 9

  1. Searchinger, Timothy, Ralph Heimlich, R.A. Houghton, Fengxia Dong, Amani Elobeid, Jacinto Fabiosa, Simla Tokgoz, Dermot Hayes, and Tun-Hsiang Yu. “Use of croplands for biofuels increases greenhouse gases through emissions from land-use change.” (PDF) Science 319, no. 5867 (2008).
  2. Klein, Br, and Virginia H. Dale. “Biofuels: Effects on Land and Fire.” (PDF) Science 321 (2008): 199.
  3. Khosla, Vinod. “Biofuels: Clarifying Assumptions.” (PDF) Science 322, no. 5900 (2008): 1238-1240.
  4. Cockerill, Sam, and Chris Martin. “Biotechnology for Biofuels: Are biofuels sustainable? The EU perspective.” (Abstract) Biotechnology for Biofuels 1, no. 9 (2008).
  5. Robertson, G. Philip. “Sustainable Fuels Redux” (Abstract) 322, no. 5898 (2008): 49-50.
  6. Food and Agriculture Organization of the United Nations. The State of Food and Agriculture 2008. (Abstract) FAO, 2008.
  7. Executive summary of the Gallagher review.”
  8. Goldemberg, Jose, and Patricia Guardabassi. “Are biofuels a feasible option?.” (Abstract) Energy Policy 37, no. 1 (January 2009).
  9. Morton, Douglas C., Ruth S. DeFries, Yosio E. Shimabukuro, Liana O. Anderson, Egidio Arai, del Bon Espirito-Santo Fernando, Freitas Ramon, and Morisette Jeff. “Cropland expansion changes deforestation dynamics in the southern Brazilian Amazon.” (Abstract) PNAS 103, no. 39 (September 2006): 14637-14641.