A Symphony of Efforts: Voluntary Climate Initiatives and Subnational Climate Action

Cities have emerged as central actors in global climate governance, increasingly adopting voluntary climate mitigation policies through transnational networks. Yet, the conditions under which these Voluntary Climate Initiatives (VCIs) foster high-integrity climate action at the city level remain insufficiently understood. This study investigates the causal impact of VCI participation, particularly of the Global Covenant of Mayors (GCoM), on the adoption and ambition of urban climate mitigation targets across 1,180 large cities (population >500,000) in 142 countries. Using a proportional hazards model, we find that cities joining GCoM are significantly more likely to adopt climate targets over time, even after controlling for national and regional factors. While GCoM membership is positively associated with target adoption, most cities—members and non-members alike—have yet to set measurable goals. To assess the role of functional specialisation, we analyse the effects of VCIs focused on enhanced standard-setting, knowledge dissemination, and institutional capacity-building. Our results show that VCIs with a core function of target-setting significantly raise the ambition and formalisation of city targets, advancing high-integrity voluntary climate action. VCIs that prioritise knowledge-sharing also show a modest association with the adoption of more innovative targets, while results for participatory and monitoring-focused initiatives are mixed. This study offers three contributions: (1) it clarifies the functional roles of VCIs in driving ambitious and credible subnational climate action; (2) it leverages novel time-series data to isolate network effects; and (3) it presents the first global analysis of VCI influence across a broad sample of large cities. These findings inform ongoing efforts to strengthen the effectiveness, accountability, and integrity of transnational climate governance at the urban level.