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n the United States, both the federal Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) are struggling with the issue of indirect land use change (ILUC) and its potential eff ects on the greenhouse gas performance of biofuels. A similar struggle is going on within the European Union (EU). As the time of writing, the regulatory outcome is far from certain. US law requires that ILUC estimates meet standards of scientifi c signifi cance and life cycle analysis.
Th e ILUC theory refers to bringing new lands into agricultural production because of market forces. For example, corn rendered unavailable to the world market because of ethanol production in the USA could hypothetically be replaced by converting new land to agriculture. Th e land conversion event might create a large, one-time carbon 'debt' (e.g., by burning tropical forests) which the inherent greenhouse gas benefi ts of the biofuel system could require centuries to 'repay'. A single paper in Science in February 2008 describing potential greenhouse gas impacts of ILUC, along with intense, related media coverage, has cast a pall over all biofuels and chilled investment in advanced non-grain-based biofuels. But that paper does not meet standards for scientifi c signifi cance or life cycle analysis.
No actual data connect corn ethanol production with, to cite a widely quoted example, clearing the Amazon rainforest. Instead, the paper's conclusions depend entirely on seven linked models and many assumptions. Th e validity of these assumptions and the reliability of the models are now being explored. Th us far, the paper is not holding up well to scrutiny. For example, other modeling predicts that hypothetical ILUC will occur primarily in US grasslands and commercial forests, not in tropical forests. My research group has shown that appropriate land-management strategies following land use change can greatly reduce the carbon debt. Dr Lee Lynd's group (Dartmouth College) has demonstrated that