The article discusses the political results of the Copenhagen Conference and evolutions in the international climate arena including geopolitical shifts, new issues on the agenda and a changing cartography of the main actors. As recent attacks on the climate regime concern both its political governance and the peculiar relationship between science and politics that developed through its main institutions (IPCC and the Conference of the Parties), we retrace in a first part the construction of the climate arena and in a second part the framing of the problem between climate science, expertise, and politics. Drawing on this historical sketch, we suggest the years 2000 were characterized by a convergence of top-down approaches in climate expertise and policies, structuring action and discourse around quantified reduction targets, temperature and concentration thresholds, and carbon budgets. The bottom-up character of the voluntary reduction commitments in the Copenhagen Accord is a serious setback to this approach. A central figure in this context is the threshold of “dangerouswarming” of two degrees. The Copenhagen Accord – endorsed in the Cancun compromise – elevates this figure to an official target of the UN negotiation process, thereby accentuating the tension between a newly assumed “Realpolitik” and an alarming expertise. The article analyzes the construction of the two-degrees target and the role it plays in the climate regime. We conclude by discussing several contributions to the Post-Copenhagen debate.