Evaluating investments with long-term consequences using discount rates that decline with the time horizon, (Declining Discount Rates or DDRs) means that future welfare changes are of greater consequence in present value terms. Recent work in this area has turned towards operationalising the theory and establishing a schedule of DDRs for use in cost benefit analysis. Using US data we make the following points concerning this transition: i) model selection has important implications for operationalising a theory of DDRs that depends upon uncertainty; ii) misspecification testing naturally leads to employing models that account for changes in the interest rate generating mechanism. Lastly, we provide an analysis of the policy implications of DDRs in the context of climate change for the US and show that the use of a state space model can increase valuations by 150% compared to conventional constant discounting.JEL classification: C13, C53, Q2, Q4

Convergence club, Carbon dioxide emissions, Nonlinear time-varying factor model, Relative transition, C23, Q53,

Should economic policy target immediate problems like malaria and AIDS? Or should it target climate change, which may have even more dramatic life-threatening effects in the very long term? We discuss how the pattern of discount rates over the planning horizon bears on this important issue. A declining pattern of discount rates is theoretically justified by uncertainty about future economic conditions, and its shape and relevance can be estimated from historical data. We analyse empirically long-term interest rate data from nine countries, construct a weighted average representing a possible global discount rate pattern for a very long range of future dates, and assess its implications for the valuation of carbon mitigation policies. -Christian Gollier, Phoebe Koundouri and Theologos Pantelidis D e c l i n i n g d i s c o u n t r a t e s DECLINING DISCOUNT RATES 759