## 6 Discussion  
The results reveal several intriguing findings. Firstly, the TFP of ports has a positive and significant impact on the CO\({}_{2}\) emissions of port cities, consistent with the research outcomes of [PERSON], [PERSON], and [PERSON] (2019), [PERSON], [PERSON], and [PERSON] (2019), and [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] (2019). Their studies also confirmed that improving transportation infrastructure contributes to the growth of urban CO\({}_{2}\) emissions. However, this study is the first to propose this perspective from the standpoint of port infrastructure. As critical nodes in the global supply chain, ports are not only logistics hubs but also significant sources of energy consumption, environmental pollution, and CO\({}_{2}\) emissions that contribute to climate change ([PERSON] and [PERSON], 2019). Ports themselves generate pollution, primarily from dust and exhaust emissions during loading and unloading operations and from fuel combustion by ships. As the core of maritime shipping, the high fuel consumption and harmful emissions of ships pose a significant challenge to the environment, affecting air quality and public health in marine, coastal, and even inland regions ([PERSON] and [PERSON], 2019; [PERSON] and [PERSON], 2019). Furthermore, we argue that the enhancement of port production efficiency represents a higher volume of goods handled and promotes the expansion of logistics and related industrial chain activities, increasing the demand for energy sources such as oil and coal. This, in turn, significantly raises the CO\({}_{2}\) emissions of port cities. Secondly, the impact of port TFP on CO\({}_{2}\) emissions intensifies with the increase in CO\({}_{2}\) emission levels, indicating that the effect of port TFP on CO\({}_{2}\) emissions is particularly significant in high-pollution areas. Some studies have shown that improvements in transportation infrastructure can significantly enhance the production efficiency of enterprises. In this regard, enterprises in high-pollution regions tend to have more robust CO\({}_{2}\) emission capabilities, meaning that while improving port efficiency increases these enterprises' production efficiency, it does not effectively reduce their CO\({}_{2}\) emissions. On the contrary, this enhancement gives enterprises a greater production capacity, which may lead to more pollution. Therefore, this finding provides essential implications for policymaking. If policies overly focus on enhancing port efficiency while neglecting the importance of reducing CO\({}_{2}\) emissions, they may exacerbate environmental pressures. This could have a more severe environmental impact than if port efficiency had not been improved. Hence, when formulating policies, there needs to be a balanced consideration of the relationship between improving port efficiency and reducing CO\({}_{2}\) emissions to ensure a better balance between economic development and environmental protection. Thirdly, the study indicates that the port's size significantly influences port TFP's impact on CO\({}_{2}\) emissions. Specifically, only when the size of a port--measured by the length of its production berths--exceeds 7867 m does the enhancement of port TFP lead to \begin{table}
\begin{tabular}{c c} \hline \hline
**Variables** & **Threshold Model** \\ \hline \(TFP(d_{i,t}\leq 7867)\) & \(-0.008\) \\ \(TFP\big{(}d_{i,t}>7867)\) & \(0.062\) *** \\ \(TFP\big{(}d_{i,t}>7867)\) & \((0.021)\) \\ \(P\) & \(-0.121\) * \\ \(P\) & \((0.064)\) \\ \(A\) & \(0.174\) *** \\ \(A\) & \((0.059)\) \\ \(T\) & \(0.083\) *** \\ \(T\) & \((0.018)\) \\ \(Constant\) & \(1.815\) \\ \((0.736)\) \\ \hline \(F\) & \(49.78\) \\ \(N\) & \(176\) \\ \(R^{2}\) & \(0.164\) \\ \hline \hline \end{tabular} Note: standard errors in parentheses, ***\(p<0.01\), *\(p<0.1\).  
\end{table} 
Table 6: Threshold regression results. an increase in the CO\({}_{2}\) emissions of the city in which it is located.