The Paris Agreement establishes a cycle where parties submit their nationally determined contributions (NDCs) every five years. First-round NDCs fail to put emissions on a path consistent with achieving the Agreement’s 2°C goal. This article presents a formal, dynamic model of reciprocity-based collective action among states and investigates the conditions under which the Paris process might deliver sufficient ambition ratchet-up to achieve the 2°C goal. The model is run under various assumptions about parties’ (1) willingness to increase ambition as a function of what others promise and deliver, (2) compliance with promises, (3) trust in the outcome of the review process, and (4) trust in the outcome of the periodic global stocktake. The results show that the Paris process delivers sufficient ambition ratchet-up to achieve the 2°C goal only under a very restricted set of conditions. At minimum, parties need to increase ambition by 4 percent of global emissions when they submit, confirm, or revise their 2030 targets in 2020
