Abstract : Climate change is representative of a general class of environmental issues where decisions have to be taken under controversies. The policy framework for these kinds of decisions is defined by three important traits: scientific ignorance, mediatization and the need for innovation. Scientific ignorance is an issue here because decisions must be taken before the end of scientific controversies about the predictability of future climate. Mediatization is key because agents can't have a sensible experience of the global climate change, and some interest-holders (future generations, distant countries) cannot participate directly in the decision. Third, the need for innovation is crucial because today's technology offers the only alternative between fossil fuels and nuclear power as a main primary energy source.In the case of climate change, the institutional context is the United Nations Framework Convention on Climate Change. The making of global environmental policy is framed not upon a hypothetical code of international law (there is no such a thing), but upon a body of doctrine arising from consistent reference to a given set of principles. The key principles are sustainability (satisfying the need of present generations without preventing future generations to satisfy theirs), precaution (ignorance is not an excuse for inaction), the common but differentiated responsibility (developed countries take the lead in action against climate change), and economic efficiency (which lead to prefer flexible instruments over blind regulation).Given the scientific controversies and the fuzziness of guiding principles, no clear-cut demonstration could justify the choice of a theoretically optimum course of action, even in the short term. Historically, climate negotiations can be seen as an oscillation between two regulation modes. On one side is coordinated policies and measures, where countries adopt an uniform international rate of carbon tax. On the other side is emission trading, where a defined emission reduction target is allocated to each country.