There is a fairly broad consensus among both the philosophers who write about climate change and the majority of the climate-policy community that efforts to reduce greenhouse-gas emissions—“mitigation” in the jargon—should not harm the ability of poor countries to grow economically and to reduce as rapidly as possible the widespread poverty their citizens suffer. Indeed, this principle of a “right to development” has been substantially embraced in the United Nations Framework Convention on Climate Change (UNFCCC) itself. Yet as the evidence of the risks from climate change has continued to mount and calls have grown for more stringent mitigation targets, the need to give substance to this right has come into conflict with the evident unwillingness of already “developed” countries to pay the costs of adequately precautionary mitigation. The long and the short of it is that almost any reasonable ethical principles lead to the conclusion that, as Henry Shue (1999) put it straightforwardly, “the costs [of mitigation] should initially be borne by the wealthy industrialized states.” In the words of the UNFCCC, “the developed country Parties should take the lead in combating climate change and the adverse effects thereof,” and this point is embodied in practical terms in the Kyoto Protocol itself, in which only the 40 developed “Annex I” countries have binding emissions limits. Yet particularly because of the rejection of Kyoto by the United States but also because of the weak efforts at mitigation that have taken place so far in Europe, Japan, and other industrialized countries, we find ourselves in a situation in which precaution requires that emissions be reduced extremely soon in poor countries, too, but the rich countries can’t yet be said to have fulfilled their obligations to “take the lead.” The delay in taking action so far, the increasing evidence of current climate-change impacts and greater risks than previously estimated, and the speed with which we must now move all imply substantially greater costs for adequately precautionary action than were previously estimated.