How Does Polycentric Engagement Relate to Countries' NDC Ambition and Mitigation Policy Effort?

Under the Paris Agreement, global climate governance has become decidedly more polycentric. However, it is still debated whether the turn toward greater polycentricity leads to more effectiveness in delivering public goods like climate change mitigation. This article argues that engagement in polycentric climate governance is linked to both more ambitious mitigation targets and stronger mitigation policy effort at the national level. Empirically, we analyze the extent to which countries' mitigation ambition and policy effort are associated with state-level memberships in transnational climate governance initiatives and with sub- and nonstate actors' memberships, while controlling for other potential explanations. We find that while polycentric engagement is not associated with higher ambition of countries' mitigation targets, it does correlate with greater policy effort. This is particularly the case for former non–Annex I countries, that is, countries without mitigation commitments under the old Kyoto regime. We explain these synergies with a discussion of polycentric systems' contribution to supporting knowledge exchange and learning.