Cities are crucial climate change actors, developing largely voluntary action plans and emission reduction targets. There is limited evidence, however, of their impact through transnational climate initiatives and little empirical support linking mitigation strategies and emissions reductions. Here, we show that 60% of more than 1,000 EU Covenant of Mayors’ cities are on track to achieve their 2020 emission reduction targets. Assessments of cities’ mitigation outcomes and the determinants of performance show that on-track cities tend to have less-ambitious targets and higher baseline emissions and are in countries with more-ambitious national climate policies and greater realized emissions reductions than cities that are not on track. Automated textual analysis and a regression model find city emissions reduction is influenced by plan-level, city-level and country-level characteristics. Greater emissions reductions are associated with plans targeting energy efficiency. These results of city-level achievement provide empirical support for the theorized subnational actors’ contribution to global climate mitigation.

城市是关键的气候变化参与者，主要制定自愿行动计划和减排目标。然而，仅有有限的证据表明它们通过跨国气候倡议产生的影响，并且很少有将缓解战略与减排联系在一起的经验支持。在这里，我们表明，在1000多个欧盟市长城市中，有60％有望实现其2020年减排目标。对城市减缓成果和绩效决定因素的评估表明，在轨城市中的目标往往雄心勃勃，基准排放量更高，并且在国家气候政策雄心勃勃且实际减排量较大的国家中，未减排城市跟踪。自动化的文字分析和回归模型发现城市减排量受计划水平的影响，城市级和国家级的特征。更大的减排量与针对能效的计划有关。这些城市成就的结果为理论上的地方行动者对全球减缓气候变化的贡献提供了经验支持。