[en] What shall we do in front of climatic change? Considering the potential seriousness of global risks, shall we start drastic policies of abatement of greenhouse gas emissions? Taking into account the cost of these policies and the uncertainties about the reality of risks, should we postpone the action? Do the efforts of emissions abatement make a good use of rare resources when the stake is only to gain few degrees of temperature and while we need today urgent developments to fight against poverty? One of the priorities of CIRED, a mixed research unit involved in the sustainable development issue, is to clarify the terms of the debate in order to avoid its transformation into a conflict which would paralyze any significant action. To do so, CIRED has elaborated 'integrated' models which allow to study the optimal answer to be adopted considering the size of uncertainties and the forcefulness of the controversies which characterize this subject. (J.S.)