In order to decide whether a comprehensive treaty covering all greenhouse gases is the best next step after UNCED, one needs to distinguish among the four questions about the international justice of such international arrangements: (1) What is a fair allocation of the costs of preventing the global warming that is still avoidable?; (2) What is a fair allocation of the costs of coping with the social consequences of the global warming that will not in fact be avoided?; (3) What background allocation of wealth would allow international bargaining (about issues like 1 and 2) to be a fair process?; and (4) What is a fair allocation of emissions of greenhouse gases (over the long-term and during the transition to the long-term allocation)? In answering each question we must specify from whom any transfers should come and to whom any transfers should go. As the grounds for the answers we usually face a choice between fault-based principles and no-fault principles.