of greenhouse gases and others have found it to be lower. Much of the variation is caused by the studies’ underlying assumptions, he said.

At first glance, this variation seems somewhat surprising, he said, because LCA is standardized by the International Organization for Standardization (ISO). He noted, however, that the ISO standard itself is more concerned with standardizing terminology and reporting than it is in the particular steps one takes when conducting an LCA. To offer a specific, detailed example of how and why LCAs vary, he described how studies of the same phenomenon—ethanol production—might be carried out from three very different perspectives.

Three Approaches to Life-Cycle Assessments

Hill noted that the three approaches he would describe are summarized from a lengthier discussion in a National Research Council report, Renewable Fuel Standard: Potential Economic and Environmental Effects of U.S. Biofuel Policy (NRC, 2011). The approaches examine how three different people might use an LCA, approaching the subject from the point of view of an ethanol plant owner, an ethanol industry analyst, and a government regulator.

To begin with, these individuals might be interested in very different physical volumes of ethanol, Hill said. “An ethanol plant owner may be wondering about the greenhouse gas profile of the fuel being produced in his or her facility. The ethanol industry analyst may be wondering about the average corn ethanol produced in the United States in a given year. And the regulator may be interested in the marginal increase in ethanol production that occurs as a result of a policy, say RFS2 [Renewable Fuel Standard, version 2.0],” he said.

Furthermore, each of the individuals would likely choose different data as model inputs. Consider yield (mass of crop per acres harvested), for example. The ethanol plant owner might choose the yield of the corn that feeds into his or her facility. The ethanol industry analyst might choose the average yield for all U.S. corn production. The government regulator might focus on the future yield that is expected as a result of a certain policy. “All of those methods are correct, but they need to be used only for their intended purposes,” Hill said. “Otherwise, you run into trouble.”

The three different individuals might also differ in which “flavor” of LCA they chose. Attributional LCA looks at the overall impact of a production process—the total emissions, for example—and divides that impact among the different inputs and processes that went into the

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