## 7 Conclusions  
This study analyzed the total factor productivity (TFP) of the Yangtze River inland ports from 2009 to 2019 and its impact on port cities' carbon emissions to reveal how improvements in port production efficiency affect CO\({}_{2}\) emissions in port cities. The results show that the overall improvement in the TFP of the Yangtze River inland ports reflects a significant increase in operational efficiency, primarily due to technological progress. However, analyses using the panel data model with two-way fixed effects indicate that improved port production efficiency has concurrently led to increased CO\({}_{2}\) emissions in port cities. Further investigation through panel quantile regression analysis reveals that the impact of TFP on CO\({}_{2}\) emissions varies among port cities with different emission levels and is significantly more pronounced in cities with medium and higher CO\({}_{2}\) emission levels. Moreover, this study also identifies a significant threshold effect of port size on the relationship between TFP and CO\({}_{2}\) emissions: in smaller ports, improvements in TFP have a negligible impact on CO\({}_{2}\) emissions; however, once the scale exceeds a certain threshold, the enhancement in TFP significantly promotes an increase in CO\({}_{2}\) emissions.  
In light of these findings, it is recommended that policymakers promote the improvement of port production efficiency while simultaneously adopting measures to mitigate the growth of carbon emissions. Specific measures include promoting the application of green port technologies, enhancing energy efficiency management, encouraging the use of clean energy, and optimizing the logistics chain between ports and cities to reduce carbon emissions. For larger ports, given their greater potential environmental impact, more stringent environmental policies and standards should be developed and enforced to ensure that port development does not come at the expense of environmental quality.  
The contribution of this paper is that it offers a new perspective on understanding the environmental impacts of transportation infrastructure, particularly port activities. It fills a research void concerning the relationship between carbon emissions and port activities in cities along the Yangtze River. It provides an empirical foundation for developing policies to balance improving port productivity with reducing carbon emissions.  
This study is somewhat limited, mainly due to the lack of detailed data on ports, such as the specific number of pieces of equipment, yard areas, and the number of employees. The absence of these detailed data is due to difficulties in data collection, which has led us to rely on traditional input variables to calculate port TFP, thus limiting our research to some extent.  
Future research should consider a more comprehensive analysis of the impact of the internal interactions within transportation infrastructure on urban carbon emissions. This includes the role of individual transportation infrastructures like roads, railways, and ports and the exploration of their synergistic effects. Moreover, future studies could extend to coastal ports in China, further exploring the mechanisms and strategies affecting them to balance the enhancement of port production efficiency with reducing carbon emissions.  
**Author Contributions:** Conceptualization, X.D. and Y.-J.C.; Methodology, X.D.; Formal analysis, X.D.; Writing--original draft, X.D.; Writing--review & editing, X.D.; Supervision, Y.-J.C.; Project administration, Y.-J.C.; Funding acquisition, Y.-J.C. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available from the authors upon request.  
**Conflicts of Interest:** The authors declare no conflict of interest.