Valuation of ultra-long investments and sustainable development.

Summary Do we do enough for the future? This question is related to many different current issues, from the reduction of sovereign debt in Europe, to the pension reform, the fight against climate change, the preservation of natural resources, the level of public investment in infrastructure, or the fiscal treatment of long-term savings for example. Our social responsibility towards future generations is decentralized through the choice of the discount rate, which determines the tradeoff between present sacrifices and future benefits. How should we define the efficient level of long-termism? In this paper, I summarize the most important evolutions in the economic analysis of this question in recent years. Given the strong uncertainties that prevail concerning the long term evolution of our society, I recommend to use a discount rate ranging from 2 times the anticipated growth rate of consumption for short time horizons, going down to 1 % for time horizons above 100 years. The systematic risk premium should also have a term structure, starting at around 1 % for short maturities to 3 % for extra-long ones.