This is the text of the ?Jean-Jacques Laffont lecture?, closing the 2010 Congress of the afse, ?Association Française de Science Économique?. The first part comes back on the standard economic viewpoint on expectational coordination, a viewpoint that the recent events have challenged. The...

The paper considers a model with two goods: a private good and an ??environmental?? good. The analysis has two basic ingredients: the two goods are imperfectly substitutable and the long run characteristics of substitution are uncertain. An ecological discount rate is defined. It is argued that...