Abstract: In September 2011, the US Environmental Protection Agency asked 12 economists how the benefits and costs of regulations should be discounted for projects that affect future generations. This paper summarizes the views of the panel on three topics: the use of the Ramsey formula as an organizing principle for determining discount rates over long horizons, whether the discount rate should decline over time, and how intra- and intergenerational discounting practices can be made compatible.The panel members agree that the Ramsey formula provides a useful framework for thinking about intergenerational discounting. We also agree that theory provides compelling arguments for a declining certainty-equivalent discount rate. In the Ramsey formula, uncertainty about the future rate of growth in per capita consumption can lead to a declining consumption rate of discount, assuming that shocks to consumption are positively correlated. This uncertainty in future consumption growth rates may be estimated econometrically based on historic observations, or it can be derived from subjective uncertainty about the mean rate of growth in mean consumption or its volatility. Determining the remaining parameters of the Ramsey formula is, however, challenging.

Abstract: Should governments, in discounting the future benefits and costs of public projects, use a discount rate that declines over time? The argument for a declining discount rate is a simple one: if the discount rates that will be applied in the future are persistent, and if the analyst can assign probabilities to these discount rates, this will result in a declining schedule of certainty-equivalent discount rates. A growing empirical literature estimates models of long-term interest rates and uses them to forecast the declining discount rate schedule. I briefly review this literature, focusing on models for the United States. This literature has, however, been criticized for a lack of connection to the theory of project evaluation. In cost-benefit analysis, the net benefits of a project in year t (in consumption units) are to be discounted to the present at the rate at which society would trade consumption in year t for consumption in the present. With simplifying assumptions, this leads to the Ramsey discounting formula. The Ramsey formula results in a declining certainty-equivalent discount rate if the rate of growth in consumption is uncertain and if shocks to consumption are correlated over time. Using the extended Ramsey formula to estimate a numerical schedule of certainty-equivalent discount rates is, however, challenging.

Abstract: Nearly all discussions about the appropriate consumption discount rate for climate change policy evaluation assume that a single discount rate concept applies. We argue that two distinct concepts and associated rates apply. We distinguish between a social-welfare-equivalent discount rate appropriate for determining whether a given policy would augment social welfare (according to a postulated social welfare function) and a finance-equivalent discount rate suitable for determining whether the policy would offer a potential Pareto improvement. Distinguishing between the two rates helps resolve arguments as to whether the choice of discount rate should be based on ethical considerations or empirical information (such as market interest rates), and whether the discount rate should serve a prescriptive or descriptive role. Separating out the two rates also helps clarify disputes about the appropriate stringency of climate change policy. We find that the structure of leading numerical optimization models used for climate policy analysis may have helped contribute to the blurring of the differences between the two rates. In addition, we indicate that uncertainty about underlying ethical parameters or market conditions implies that both rates should decline as the time horizon increases.