Abstract: As an independent chapter for the first time, international cooperation on mitigation is assessed comprehensively in the Fifth Assessment Report of IPCC. The major conclusions include: United Nation Framework Convention on Climate Change (UNFCCC) is the main multilateral forum, however, the basic principle of UNFCCC should be evolved as pattern of development, emission and impacts change, and the Kyoto Protocol together with CDM has not been as successful as intended; international cooperation on climate change has become more institutionally diverse over the past decade, varying in the degree to which their authority is centralized; the lessons from existing agreements are participation and compliance. A “bottom-up” mechanism and finance/technology incentives encourage broad participation, so does trade measures, which can also be used to enhance compliance. Through policy linkage, the diversified mechanisms outside UNFCCC can complement a binding international agreement on climate change. The assessment has significant impacts on international cooperation after 2020.

Key words: climate change mitigation, international cooperation, IPCC AR5, WGIII