The social cost of carbon, defined as the present value change in consumption due to an incremental change in carbon emissions, is used by federal agencies in cost-benefit analysis of any regulation that changes emissions. In 2009, the Oce of Management and Budget, through an interagency process, estimated the social cost of carbon and required all agencies to use their estimate. Their central value was $21.40/tCO2 with range of -$2.7/tCO2 to $142.4/tCO2.

The government’s estimate of the social cost of carbon, which is consistent with estimates by private researchers, implicitly assumes that the economic growth continues even with substantial temperature increases. In one of the models, temperatures increase by 6.3 by the year 2300. With this temperature increase, the global economy is roughly 30 times larger than it is today on a per capita basis for the model. The apparent reason for this estimate is that damages from climate change do not affect growth. They are modeled as reducing usable output in a given year with exogenously specified growth continuing regardless of climate damages.

We estimate the social cost of carbon when climate change reduces the growth rate of the economy. We use the same model as the OMB but modify it so that a fraction of damages from climate change affect the growth rate rather than simply reducing usable output. Growth might be reduced, for example, because resources are diverted from research to adaptation to climate change. Even relatively small growth effects produce substantial change in the social cost of carbon suggesting that (1) the estimates of the social cost of carbon are not robust to modest changes in the estimating model and (2) research into the impacts of climate change should focus on growth effects rather than level effects.