**6. Conclusions and Policy Implications**

#### *6.1. Conclusions*

Based on 108 Chinese cities from Yangtze River Economic Belt between 2003 and 2017, this study examines the impact of the city cluster policy on cities' carbon emissions using the difference-in-differences method. The main conclusions are as follows.

(1) The city cluster policy has significantly reduced the level of cities' carbon emissions. During our sample period, carbon emissions from cities declined by an average of 7.4% after cities were classified as the city cluster. After a series of robustness tests, the conclusion remains robust.

(2) Productivity, technological innovation, and industrial structure optimization are three essential mechanisms for the city cluster policy to reduce carbon emissions. We find that the TFP level of the city increased by 22%, the city R&D input increased by 61.1%, and the ISO index decreased by 0.6% after cities were classified as the city cluster. It means that cities are more productive, innovative, and have a more reasonable industrial structure.

(3) There is a difference in the effect of the positioning level of the city cluster on the reduction of carbon emissions. The effect of city cluster policies on carbon emission reduction is significant only in the national-level city cluster. The carbon emissions from the national-level city cluster declined by an average of 10.3%.
