To prevent severe climate change, greenhouse gas emissions must stabilize in less than 10 years and then be reduced by 80 percent or more by midcentury. Some people say the costs of such actions will devastate the economy. Others say cutting emissions before the problem becomes extreme will have moderate costs or may actually benefit the economy. Whom should we believe?

Having managed a number of environmental policy economic assessments over the years, I know that the assumptions one makes at the start shape the conclusions reached at the end. Follow the assumptions, and you can usually predict the outcome.

Robert Repetto, an economist working with the Yale University School of Forestry, recently analyzed thousands of policy simulations from 25 economic models being used to predict the economic impacts of reducing U.S. carbon emissions. The analysis found that seven key assumptions account for most of the differences among the models in their predicated outcomes.

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