## 3 Literature Review
### The Impact of Transportation Infrastructure on Urban CO\({}_{2}\) Emissions  
Current research on the relationship between transportation infrastructure and urban CO\({}_{2}\) emissions mainly focuses on roads and railways. The conclusions regarding whether the impact of transportation infrastructure on urban CO\({}_{2}\) emissions is positive or negative are inconsistent.  
On one hand, some studies indicate that the development of transportation infrastructure leads to an increase in urban CO\({}_{2}\) emissions. For example, [PERSON], [PERSON], and [PERSON] [22] used data from 283 Chinese cities between 2003 and 2013, with the length of highways as an indicator of transportation infrastructure, and found that the construction of transportation infrastructure increased carbon emissions, especially in larger cities. This phenomenon may stem from the urban economic growth and technological innovation promoted by transportation infrastructure. Similarly, [PERSON], [PERSON], and [PERSON] [25], by analyzing the per capita road area in cities, also found that transportation infrastructure has a negative impact on the urban environment. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [26] studied the passenger and freight flow on railways and highways, as well as railway length, from the perspective of regional spatial spillover, considering these as significant contributors to urban carbon emissions. Meanwhile, [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [24] pointed out that road infrastructure, through the effect of industrial agglomeration, has exacerbated carbon emissions in Chinese prefecture-level cities.  
On the other hand, there are studies that believe the development of transportation infrastructure can help reduce urban CO\({}_{2}\) emission levels. [PERSON] and [PERSON] [27], through the study of urban road areas, found that improvements in transportation infrastructure significantly reduced the level of urban carbon emissions. [PERSON] et al. [39], when considering the impact of transportation factors in China's megacities on CO\({}_{2}\) emissions, also found that an increase in urban road density helps to reduce carbon emissions. In terms of railways, [PERSON] and [PERSON] [23] regarded the operation of urban high-speed rail as a \"quasi-natural experiment\" and found that the operation of high-speed rail significantly reduced CO\({}_{2}\) emissions in cities along the line.  
In addition to studies directly focused on transportation infrastructure itself, there has been research considering policies to improve the operational efficiency of transportation infrastructure, assessing the impact of these policies on urban CO\({}_{2}\) emissions. For example, [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [29] and [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [28] studied the implementation of smart transportation policies and smart logistics policies, respectively, finding that both policies effectively curbed urban CO\({}_{2}\) emissions.  
Although existing research has extensively discussed the impact of roads and railways as transportation infrastructures on urban CO\({}_{2}\) emissions, these studies often overlook the critical role of ports in this process. Our research aims to fill this gap, exploring the relationship between port productivity and CO\({}_{2}\) emissions in port cities.