The goal of the Paris Agreement to keep global warming well below 2°C can only be reached if countries commit to and adopt ambitious climate change mitigation action. However, we often witness a gap between international commitments and the policies and instruments aimed at implementing them domestically. This project aims to quantify and qualify the gap between international promises and national implementation. We first measure this gap by developing an index of vertical climate policy harmonization, which reflects the divergence between international commitments and national implementation. In addition, we seek to explain what drives climate policy harmonization by advancing and empirically testing an interdisciplinary theory on the factors that contribute to explain the harmonization of policies in multi-level systems. To this aim, we probe relevant macro-economic and political factors. Further, the project examines normative questions linked to vertical policy harmonization, like democratic legitimacy at both levels of decision-making.We combine several research methods including policy network analysis, econometric analysis, and forecasting political negotiation outcomes. We use both qualitative and quantitative methods to collect new data, including from through computer-assisted analysis of political texts, surveys and expert interviews.