Open Access

Climate change and thus low-carbon transitions are global challenges, which require commitment and effort on all political levels. As international climate politics has approached its limits over the last two decades, the role of cities has simultaneously gained in importance. Many cities1 worldwide have committed to ambitious climate protection targets, which often exceed national targets. However, cities cannot act in isolation. Their opportunities for action are embedded in an (inter)national policy framework, which may either support or hinder local actions. This gives rise to the question: which opportunities for climate protection do cities really have in a political system of multi-level governance? This question can be illustrated using the city of Hamburg as an example for the German climate policy regime. The city aims to reduce its annual CO2 emissions by 2 million metric tons and attempts to quantify the impact of local and national policies and actions using a bottom-up monitoring approach. We therefore analyse more than 400 local actions with respect to the induced CO2 emission reductions. We also take a closer look at national and European policies and their impacts on local energy use and emissions. In total, 15 policies and instruments - broadly ranging from instruments to foster energy efficiencyin residential and non-residential buildings, in appliances and in the transport sector, to support renewable energy sources (including biofuels) and to uptake CHP - are considered. Our approach consists in measuring separately the impact of local and national policies and actions on urban CO2 emissions. While the city of Hamburg has implemented many policies and actions, our results show that, a significant proportion of its CO2 reduction is due to national policies, in the context of the German "Energiewende", which cannot or can only indirectly be influenced by the city. The results imply that local commitment and effort is essential in addressing the global challenge, yet ambitious targets can only be met in the presence of a supportive national policy framework. The analysis shows that many policies and measures implemented at national level require supportive structures and activities at local level in order to bridge information and implementation gaps of these measures.