Commitment Ambiguity and Ambition in Climate Pledges

International review mechanisms can help states overcome collective action problems by revealing accurate information about their cooperative intent and performance. However, many existing review mechanisms have lenient informational requirements, leading to ambiguous reporting that impedes mutual verification of efforts and potentially undermines cooperation. This article evaluates how commitment ambiguity affects cooperation under the Paris Agreement on climate change, which features a pledge-and-review system where governments decide unilaterally on the depth of their commitments. We develop a decision-theoretic model of ambiguity and risk behavior in climate pledges that delineates the relationship between commitment ambiguity and ambition. In our model, commitment ambiguity is a sum of structural uncertainty and strategic ambiguity. We argue that structural uncertainty—information constraints that prevent governments from perfectly gauging their commitment potential—reduces ambition in climate pledges. This prudence effect is driven by compliance concern: The anticipated international and domestic audience costs arising from noncompliance induce policymakers to adjust ambition downward. Our empirical analysis of all climate pledges under the Paris Agreement demonstrates that ambiguous pledges are less ambitious than precise pledges, in line with our prudence conjecture. We also show that democracies are more prudent than autocracies, reflecting systemic variations in domestic audience costs. Overall, this article contributes an original theory of how ambiguity affects cooperation in international institutions and produces empirical findings that shed light on the effectiveness of international climate cooperation.