On matters of substance, international environmental treaty making generally require agreement in consensus. This article explores strategies in consensus-making processes in international environmental diplomacy. Specifically it examines the consensus-making politics, in the case of negotiating historic responsibility within the UN Framework Convention on Climate Change (UNFCCC). In doing so, analytical concepts from the discourse theory of Laclau and Mouffe are utilized to look for rationales that underpin agreement. To conclude, three rationales have dealt with conflicts over historic responsibility. While the first rationale hid conflict behind interpretative flexibility, the second reverted to “reasoned consensus,” excluding perspectives commonly understood as political rather than scientific. The third rationale has enabled equivocal use of the concept of historic responsibility in several parallel discourses, yet negotiators still stumble on how to synthesize these with a potential to foster future, more policy-detailed, consensuses with higher legitimacy. Understanding the history and current situation of negotiations on historic responsibility from this perspective can help guide policy makers towards decisions that avoid old pitfalls and construct new rationales that generate a higher sense of legitimacy.