Journal article: The paper starts reviewing some basics of the climate science and economics of the greeenhouse effect. It stresses the nature and extent of uncertainty on both sides as well as the political background of present climate policies. It then comes to an assesment of some of the main debates surrounding climate policies. When to act ? How to act ? two question that lead to confront the the promises of research and the merit of early abatments. The question of the urgency and intensity of action calls for some costbenefit analysis of the kind presented in the Stern report, which is critically examined. The last section is concerned with the difficult question of designing the institutions of climate policies, and particularly with the key issue of the involvement of developing countries.