Social discount rates (SDRs) are crucial for evaluating the costs of climate change. We show that the fundamental anchor for market-based SDRs is the equilibrium or steady-state real interest rate. Empirical interest rate models that allow for shifts in this equilibrium real rate find that it has declined notably since the 1990s, and this decline implies that the entire term structure of SDRs has shifted lower as well. Accounting for this new normal of persistently lower interest rates substantially boosts estimates of the social cost of carbon and supports a climate policy with stronger carbon mitigation strategies.

The downtrend in U.S. interest rates over the past two decades may partly reflect a decline in the longer-run equilibrium real rate of interest. We examine this issue using dynamic term structure models that account for time-varying term and liquidity risk premiums and are estimated directly from prices of individual inflation-indexed bonds. Our finance-based approach avoids two potential pitfalls of previous macroeconomic analyses: structural breaks at the zero lower bound and misspecification of output and inflation dynamics. We estimate that the longer-run equilibrium real rate has fallen about 2 percentage points and appears unlikely to rise quickly.

Estimates of the Taylor rule using historical data from the past decade or two suggest that monetary policy in the U.S. can be characterized as having reacted in a moderate fashion to output and inflation gaps. In contrast, the parameters of optimal Taylor rules derived using empirical models of the economy often recommend much more vigorous policy responses. This paper attempts to match the historical policy rule with an optimal policy rule by incorporating uncertainty into the derivation of the optimal rule and by examining plausible variations in the policymaker's model and preferences.