1. Introduction
With the continuous improvement of urbanization and industrialization, the intensity of development and utilization of various types of energy is also increasing. The use of energy has laid a foundation for the life of residents and the production of enterprises, and has effectively promoted rapid economic growth. However, it has also caused environmental problems such as greenhouse gas emissions and air pollution. In particular, China’s energy consumption structure is mainly based on fossil energy. The excessive consumption of polluting fossil energy generates a large amount of carbon dioxide, making China the world’s largest carbon emitter. Based on the requirements of green development, China is striving to change the mode of production, constantly promote the adjustment of industrial structures, and make its mode of economic development more environmentally friendly. However, due to the obvious path dependence of air pollutants, it is difficult to significantly reduce carbon dioxide emissions in a short time. As a responsible country, China will take more effective measures to reach peak CO
2 emissions before 2030 and strive to be carbon neutral before 2060 [
1]. The report of the 19th National Congress of the Communist Party of China also pointed out that the construction of ecological civilization is a millennium plan for the sustainable development of the Chinese nation. It is necessary to establish and practice the concept that green water and mountains are golden and silver mountains, and adhere to the basic national policy of resource conservation and environmental protection [
2]. Although China is facing enormous pressure on carbon emission reduction, adhering to green and low-carbon development has become the only way to achieve high-quality development.
The transportation industry is a major contributor to carbon emissions, accounting for more than 10% of China’s total carbon emissions. The transportation industry not only has a direct impact on carbon emissions, but also indirectly affects the regional carbon emission level through the agglomeration effect of transport infrastructure. Therefore, the internal structural adjustment and high-quality development of the transportation industry have a significant impact on the economic development model and pollutant emissions. Relevant studies have shown that the technological upgrading of transportation tools and the green transformation of transportation industry can effectively reduce pollutant emissions and promote sustainable development. For example, Chen and Whalley [
3] found that the operation of Taipei Metro reduced the usage of motor vehicles, thus effectively reducing CO emissions. Liang and Xi [
4] comprehensively used various policy assessment methods to confirm that cities with rail transit have better air quality. According to the research of Zhang and Feng [
5], the opening of high-speed railway (HSR) can reduce haze pollution through scale, structural, and technical effect, and the opening of HSR can also reduce haze pollution by improving the status of urban transport network [
6]. In view of this, we infer that efficient and intensive transportation can promote carbon emission reduction by affecting the production and living mode of economic subjects. Thus, a natural question arises: as a green, environmentally friendly, and efficient railway transit infrastructure, can the construction and development of HSR reduce urban carbon emissions?
HSR is a typical representative of high-quality development of China’s transportation industry. China’s HSR construction has entered a stage of rapid development since 2008, when the Beijing–Shanghai and Beijing–Guangzhou HSR and other special passenger lines were started. In 2018, a state-planned “four vertical and four horizontal” operation network was formed. Compared with traditional railway transport and road transport, HSR, as a new mode of transportation, can not only optimize the internal structure of the transport industry and improve the efficiency of transport services, but also promote the sustainable development of the economy and society through the spatial–temporal compression effect and the factor flow effect [
7]. In theory, the above two aspects are conducive to indirectly reduce carbon emissions. Identifying the net effect of HSR network on regional carbon emissions and analyzing its internal impact mechanism will not only help to understand the internal relationship between HSR development and carbon emissions, but also be of great significance to exploring the environmental benefits brought by the construction of HSR. At present, the green environmental effect of HSR has gradually emerged, but the impact mechanism of transportation infrastructure on regional carbon emissions is relatively complex, and there are relatively few empirical studies on CO
2 emissions caused by the opening of HSR. Among them, Sun and Ge [
8] used the difference-in-difference (DID) model to prove that the opening of HSR can reduce urban industrial carbon emissions by affecting technological upgrading and production costs of enterprises. However, we think that the measurement method of carbon emissions may not be authoritative and accurate enough. Lin et al. [
9], based on the annual passenger and freight flow monitoring information of China’s expressways and ordinary national highways from 2009 to 2016, as well as the information about HSR stations and mileage, used the DID model to confirm the CO
2 emission reduction effect of HSR in terms of traffic substitution for expressways, but ignored the impact of the indirect economic effect behind the opening of HSR on carbon emissions. In addition, Jia et al. [
10] focused on analyzing the impact of HSR service intensity on carbon emission reduction from the perspective of spatial spillover, providing a future research direction for the author.
In view of this, based on the panel data of 285 cities in China from 2004 to 2017, this paper mainly uses the time-varying DID model to empirically analyze the effect of HSR development on urban carbon emissions. The possible contribution of this study mainly includes the following aspects. First, most of the studies focus on the growth effect and structural effect of HSR, while few studies focus on the environmental effect of HSR. Under the background of “double carbon”, this paper also enriches the research on HSR and carbon emissions. Second, this paper uses the carbon emissions data calculated based on night light data. These data are more reliable than the existing algorithm, which is more conducive to identifying the role of HSR development on CO2 emission reduction. Third, from the perspective of system theory, this paper uses the mediation effect model to explore the indirect CO2 emission reduction effect of the opening of HSR. The above has certain guiding significance for local governments to achieve regional low-carbon development through high-quality transportation infrastructure.
2. Literature Review and Research Hypothesis
The whole cycle of HSR from construction, opening, operation to maintenance involves different energy consumption, which is closely related to regional carbon emission. During the construction stage, the construction of HSR project needs to consume a lot of energy and raw materials, and the use of cement and steel is the main source of carbon emissions in this stage. According to the China High-speed Railway Development Report issued by the World Bank, CO2 emissions generated by HSR construction are about 25,000 to 30,000 tons per kilometer. During the operation stage, HSR is not “green” because it is driven by electricity. At present, coal-fired power plays a dominant role, and this power production mode emits a large amount of greenhouse gases. However, compared with other transportation modes, railway transportation has the advantages of large carrying capacity and long transport mileage, and its unit carbon emission is still significantly lower than that of other modes of transportation. According to the relevant statistics of the Ministry of Transport in the current carbon emission structure of China’s transportation field, roads are the main body, accounting for 87%; shipping and aviation are about 6%; and railways have the lowest proportion, 0.68%. Moreover, in the process of HSR construction, the mode of leading the road by bridge is widely used, and the construction of the green belt is strengthened along the way, which also makes the HSR transportation network have an additional carbon reduction effect.
From the perspective of the internal structure of the transportation industry, HSR, as a new public transportation tool, plays an important role in replacing the traditional mode of transportation. The study of Lin et al. [
9] confirms that after the cities were connected by HSR, the traffic flow of passengers and goods on expressways decreased significantly. Moreover, passengers switch from the traditional railway to HSR, which also releases the freight transport capacity of the traditional railway, thus promoting the transfer of road freight transport to the more environmentally friendly traditional railway. Under the same transportation volume, the energy consumption of HSR is far lower than that of highway and aviation, and the carbon dioxide emission is less than one tenth of that of aircraft. It is reported that from 2008 to 2016, the operation of HSR has helped China reduce greenhouse gas emissions equivalent to 11.18 million tons of CO
2 each year, accounting for about 1.33% of the total greenhouse gas emissions of China’s transportation industry [
9]. Dalkic et al. [
11] also confirmed that HSR consumes less fossil fuels in passenger and freight transportation, thus contributing to carbon emission reduction. Moreover, the railway department has also actively promoted the electronic ticketing service to reduce paper consumption and make railway development more low-carbon and environmentally friendly. Therefore, the substitution effect caused by the opening of HSR can effectively reduce CO
2 emissions. In addition, the relevant literatures also point out that the traditional transportation mode has a high level of energy consumption, and the carbon emission level of the transport industry should be reduced by using advanced transportation equipment, providing high-quality transportation services, and adjusting the transportation structure [
12]. In the future, with the technological transformation of the transportation industry, positive environmental externalities caused by this substitution effect will continue to appear. Given this, we propose the following hypothesis:
Hypothesis 1. The development of HSR can effectively reduce the level of urban CO2 emissions.
Regarding the influencing factors of carbon emissions, some studies are analyzed based on the IPAT equation (I = P × A × T, where I, P, A, and T, respectively, represent Impact, Population, Affluence and Technology). The STIRPAT (Stochastic Impacts by Region on Population, Affluence, and Technology) model optimizes the IPAT equation and allows economic, environmental, social, and other factors to be incorporated into the driving factors, expanding the scope and core of influencing factors [
13,
14]. Some scholars also pointed out that the structure of transportation industry and transportation volume are important factors affecting regional carbon emissions through the research framework of ASIF (A, S, I, F represent Activity, Structure, Intensity, and Fuels, respectively) [
15]. On this basis, Chinese scholars have conducted research on China’s carbon emissions. Relevant literature shows that regional energy structure, urbanization process, technological innovation, industrial structure, and industrial production scale are all key factors affecting CO
2 emissions [
16,
17].
Considering the system theory, the social system is a typical complex system composed of many subsystems such as population, resources, capital, technology, and the environment. As a subsystem, the impact of transportation infrastructure on regional carbon emissions is produced under the joint action of other influencing factors in the economic and social systems. As the key infrastructure for high-quality development of the transportation industry, HSR not only represents the technological upgrading of the railway transportation industry, but also deeply affects the industry, innovation, and other subsystems in the economic and social systems. For example, the opening of HSR can affect economic growth [
18], capital flow [
19], regional economic integration [
20], industrial structure upgrading [
21], and enterprise innovation [
22]. The above studies confirm that the positive external spillover effect of HSR development is obvious. This means that the opening of HSR may affect the regional green and low-carbon development in many aspects, thus forming the occurrence mechanism of environmental pollution change [
23].
First of all, from the perspective of industrial structure, the opening of HSR may reduce carbon emissions by promoting the adjustment of industrial structure. The development model of high input, high pollution, low output, and unreasonable industrial layout of the secondary industry will significantly increase carbon emissions. Compared with industry, the tertiary industry has the characteristics of knowledge intensive and low pollution intensity. Therefore, under the pressure of industrial structural contradictions and green development, the upgrading of industrial structure is an effective way to stabilize growth and promote emission reduction [
6]. The opening of HSR has weakened the spatial flow barriers of factors and promoted the optimal allocation of production factors on a larger scale [
24]. As the HSR mainly focuses on passenger transport, it will have a more obvious impact on the service industry with strong mobility of production factors, which will help strengthen the agglomeration development of the service industry in cities along the route [
25]. The agglomeration of service industry is closely related to the adjustment of the industrial structure, which not only helps to increase the proportion of tertiary industry, but also has a crowding-out effect on industrial production factors [
23]. The research of Liu and Li [
26] has shown that the flow of resources brought by the opening of HSR can promote the development of the tertiary industry while leading to the decline of the proportion of the secondary industry. Deng et al. [
21] further pointed out that the opening of HSR can realize the optimization and upgrading of industrial structures through improving market potential, promoting technological innovation, and optimizing resource allocation. The upgrading of industrial structure reduces the dependence of economic development on energy and resources and promotes the flow of factors to sectors with higher efficiency, thus helping to improve the efficiency of energy use and build a green production system. Yu [
27] also pointed out that the optimization and upgrading of industrial structures has a significant emission reduction effect, which is an important part of green and low-carbon development. Given this, we propose the following hypothesis:
Hypothesis 2a. The opening of HSR can reduce the level of urban carbon emissions by optimizing the industrial structure.
Secondly, this can also be studied from the perspective of technological innovation. On the one hand, the opening of HSR shortens commuting time, improves accessibility, effectively alleviates the obstacles to the flow of elements [
28], reduces the unnecessary loss of knowledge and technology in the dissemination, and then helps to improve innovation ability [
29]. On the other hand, due to the advantages of high transportation efficiency, safety, and comfort, HSR has become the first choice for time-sensitive technical talents, who are an important carrier of factor flow and technological innovation [
30]. Moreover, the opening of HSR will not only make innovation elements flow efficiently among regions, but also cause a knowledge spillover effect, which will generate a learning effect, an imitation effect, and an incentive effect among innovation subjects. Such effects will effectively promote the generation and development of intellectual property rights and the innovation mode, and ultimately help to improve the innovation level of cities and enterprises [
22,
31]. Regarding the impact of technological innovation on energy conservation and emission reduction, Grossman and Krueger [
32] earlier put forward the conclusion that technological innovation is helpful to reduce the negative environmental output brought about by economic development, thus improving the urban environmental quality. As an important part of technological innovation, green technology innovation was first proposed by Braun and Wield [
33]. They believe that green technology is different from previous technologies, which can not only increase the total economic output, but also effectively promote sustainable development. As an important way to achieve low-carbon development [
34], green technology innovation can not only promote clean production of enterprises and improve energy efficiency, but also promote the transformation and development of traditional industries to green low-carbon industries, thus reducing energy consumption from the production side and achieving a source control of carbon emissions. Yuan et al. [
35] also confirmed that green technology innovation has a significant effect on carbon emission reduction. Therefore, we propose the following hypothesis:
Hypothesis 2b. The opening of HSR can reduce urban carbon emissions by promoting green technology innovation.