Résumé en anglais

This article discusses the discount rate to be used in projects that aimed at improving the environment. The model is quite stylized and involves two different goods, one is the usual consumption good whose production increases exponentially, the other is an environmental good whose quality remains limited. We define an ecological discount rate and we exhibit different relations that link this discount rate with the usual interest rate and the growth rate. We also discuss, mainly in the long run, the sensibility of this ecological discount rate to the parameters.