*4.4. Nonlinear Effects of Tourism Development on Carbon Emission Intensity and Urban Green Economic Efficiency*

To investigate whether there is a similar phenomenon of tourism development on urban green economic efficiency and carbon emission intensity [64], this paper introduces a quadratic term of the logarithm of tourism development level based on the spatial Durbin econometric model to test the nonlinear characteristic relationship between tourism development and carbon emission intensity and urban green economic efficiency. The results are shown in Table 7. The positive coefficient of the quadratic term of tourism development on urban green economic efficiency indicates that there is a positive U-shaped nonlinear characteristic relationship between tourism development and urban green economic efficiency, and similarly, there is an inverse U-shaped nonlinear characteristic relationship between tourism development and urban carbon emission intensity [32]. Similarly, there is an inverse U-shaped nonlinear relationship between tourism development and urban carbon emission intensity. Furthermore, to accurately measure the degree of nonlinear effects, a semiparametric spatial lag model is introduced, and the nonlinear relationship between tourism development and urban green economic efficiency and carbon emission intensity can be visually observed by drawing the partial derivatives of G(lnTC) in the model, as shown in Figures 3 and 4, where the horizontal coordinates indicate the level of tourism development and the vertical coordinates indicate the marginal effects.

**Table 7.** Nonlinear effects of tourism development on urban carbon intensity and green economic efficiency.


Note: \*\*\*\*, \*\*, and \* represent significance levels of 1%, 5%, and 10%, respectively.

From Figure 3, it can be seen that tourism development has a positive "U" shaped nonlinear effect on urban green economic efficiency, but the curve is always above 0, which means that the marginal effect of tourism development on urban green economic efficiency is always positive, i.e., it always shows a facilitating effect. The curve in Figure 3 shows that although the tourism industry has always had a positive effect on the green economic efficiency of the city, there are roughly three stages. The first stage is lnTC between −8 and −7, with decreasing marginal effects, which indicates that the development of tourism does produce certain pollution in the initial stage or will attract a large number of manufacturing industries to gather in the tourist destination, thus causing some suppression of the marginal effect of tourism development on green economic efficiency. This causes a certain suppression of lnTC from −7 to −2. The second stage is between lnTC from −7 to −2, and the positive marginal effect of tourism development in this stage grows slowly and represents the exploration stage of the green tourism development model. The third stage is the stage after lnTC-2, which fully demonstrates the positive effect of tourism development on urban green economic efficiency. Therefore, hypothesis H2 is verified.

Figure 4 shows that tourism development has an "M" shape on urban carbon intensity, which is different from the inverted "U" shape obtained in the previous paper, and the "U" shape may only be part of the "M" shape. In contrast, the marginal effect of tourism development on urban carbon intensity is overwhelmingly below 0, indicating that tourism development is beneficial for cities to reduce carbon emissions. Similarly, it can be seen that the mitigation effect of tourism development on urban carbon emission intensity is relatively stable until lnTC is −3, with a brief rise in the curve between −3 and −2. The marginal effect ushers in a rapid decline after −2. As with the marginal effect of green economic efficiency, tourism development, after a certain point, has a positive impact. Therefore, hypothesis H4 is verified.

**Figure 3.** Partial derivative of tourism development on urban green economic efficiency.

**Figure 4.** Partial derivative of tourism development on urban carbon emission intensity.

*4.5. Mediating Effect of Urban Carbon Emission Intensity on the Role of Tourism Development on Green Economic Efficiency*

To examine whether urban carbon emission intensity has mediating utility in the effect of tourism development on urban green economic efficiency, the mediating utility model is used, and a second test is conducted by bootstrapping to ensure the robustness of the test results. Tourism development has a significant positive impact on urban green economic efficiency, which is consistent with the previous results, while urban carbon emission intensity has a significant negative impact on urban green economic efficiency, and urban carbon emissions have an inhibitory effect on the improvement of green economic efficiency, which also confirms the robustness of the mediated utility model. The test result of model (7) shows that urban carbon emission intensity has a significant mediating effect on the influence of tourism development on urban green economic efficiency (Table 8); meanwhile, the result of the bootstrap test shows that the upper and lower bounds of BC do not contain 0 between them, and the test is passed, which proves the mediating utility of urban carbon emission intensity. Therefore, hypothesis H5 is verified.


**Table 8.** Results of the mediating effect test of urban carbon emission intensity.

Note: \*\*\*\*, \*\*, and \* represent significance levels of 1%, 5%, and 10%, respectively.

#### *4.6. Exogenous Shock Impacts of Low-Carbon City Pilot Policies*

The parallel trend test allows the use of multiperiod DID for policy assessment analysis. In this paper, the low-carbon city pilots enacted in China in 2010 and afterward are used as dummy variables, which are low-carbon city pilots recorded as 1 and 0 otherwise. First, the traditional multiperiod DID is used for estimation, as shown in Table 9. The traditional DID results show that with or without control variables, the exogenous shock of the low-carbon city pilot policy has a significant positive effect on city green economic efficiency and carbon emission intensity, and under the spatial DID model, the low-carbon pilot city policy shock on both city green economic efficiency and carbon emission intensity shows a significant spillover effect. As shown in Figures 5 and 6, the kernel density distribution of the estimated coefficients by 1000 randomly generated treatment groups shows that the red curves are all above the horizontal line of 0.1, and the blue curves have *p* values of mostly approximately 0. The values of the true coefficients are shown as red dashed lines, which are significantly different from the red curves on the left, and the placebo test indicates that the estimation results are not biased. The low-carbon city pilot reflects the realistic path of tourism development to adjust the industrial structure, and tourism development promotes green economic efficiency and reduces the carbon emission intensity of the city in the construction of the low-carbon pilot city, so the exogenous test in this study further verifies the robustness of the effect of tourism development on urban green economic efficiency and urban carbon emission intensity.


**Table 9.** Exogenous impacts of low-carbon city pilot policies.

Note: \*\*\*\*, \*\*, and \* represent significance levels of 1%, 5%, and 10%, respectively.

**Figure 5.** Placebo test of the impact of low-carbon city pilot policies on urban green economic efficiency.

**Figure 6.** Placebo test of the impact of low-carbon city pilot policies on urban carbon emission intensity.

#### **5. Conclusions and Discussion**

## *5.1. Conclusions*

In this paper, based on the mechanism of the influence of tourism development on urban green economic efficiency and carbon emission intensity, we measured the levels of tourism development and urban green economic efficiency and carbon emission intensity of 280 prefecture-level cities from 2007 to 2019 using entropy TOPSIS and superefficiency EBM and tested the spatial spillover effect of tourism development on urban green economic efficiency and carbon emission intensity using the spatial Durbin model. The spatial heterogeneity and nonlinear characteristics of the spillover effects from cities are further analyzed. Finally, the mediating role of urban carbon emission intensity in the impact of tourism development on urban green economic efficiency is examined.

Tourism development has a significant enhancing effect on urban green economic efficiency and carbon emission intensity mitigation, and it also decomposes the role effect. It is found that the green economic effect and carbon emission reduction effect of tourism development are mainly manifested as spillover effects. From the perspective of regional heterogeneity, it is found that the green economic effect and carbon emission reduction effect of tourism development are much less effective in the eastern region than in the central, western and northeastern regions. The results show that the green economic effect and carbon emission reduction effect of tourism development are not affected by urban agglomeration [60], which is basically consistent with Tong Yun's conclusion.

The green economic effect and carbon emission reduction effect of tourism development are nonlinear, with a positive "U" shape and an "M" shape, respectively. It is found that although tourism development has a certain degree of negative impact on green economic efficiency and carbon emission intensity at the early stage, the overall impact is positive. At the same time, the green economic effect and carbon emission reduction effect of tourism development significantly increase after the level of tourism development reaches 0.135 or above, and by calculation, only 13% of the cities have reached this level in 2019.

The results of the intermediary effect show that carbon emission intensity has a significant intermediary effect in the influence of tourism development on the green economic efficiency of cities. Tourism development can achieve a green economic effect through its carbon emission reduction effect. Moreover, the low-carbon city policy not only verifies the positive effect of the policy on carbon emission reduction and the green economic development of cities but also proves that the green economic effect and carbon emission reduction effect of tourism development are robust.

#### *5.2. Discussion*

This study complements and enriches the impact and spillover effects of tourism development on urban green economic efficiency and carbon emission intensity. It also examines the spatial heterogeneity of urban clusters and different regions and demonstrates the nonlinear characteristic relationship between tourism development and urban green economic efficiency and carbon emission intensity strength.

The possible marginal contributions of this study are as follows: this study is supported by the data of 280 prefecture-level cities in China from 2010 to 2019 and uses the "lowcarbon city pilot" as an exogenous shock to test the spatial spillover effects of tourism development on urban green economic efficiency and carbon emission intensity. At the same time, this study measures the development of the tourism industry in terms of both tourism scale and tourism economy, and the results are more representative. This study explores the nonlinear characteristic relationship between tourism development on urban green economic efficiency and carbon emission intensity, further improving and enriching the study of the spillover effect of tourism development on urban green economic efficiency. Moreover, this paper confirms the mediating role of carbon emission intensity in tourism development for urban green economic efficiency spillover.

Meanwhile, the following policy insights are obtained from this paper. First, the spillover effect of tourism development on urban green economic efficiency and carbon emission intensity has significant spatial heterogeneity, and different regions should develop differentiated strategies according to their development conditions [66]. The marginal spillover effect of tourism development in the eastern region is weakened, and tourism development should be shifted to high-end and low-carbon sectors, strengthening the linkage and integration with other industries, encouraging technological innovation, and providing technical support for resource-saving development models. Second, there is regional heterogeneity in the spillover effects of tourism development on urban green economic efficiency and carbon emission intensity, which highlights the importance of mutual coordination and cooperation within strategic alliances for regional tourism cooperation [67]. On the premise of breaking down administrative barriers, we actively promote the rational matching of resource elements between regions. Third, to promote the green development of cities, it is necessary to focus on both the green economic effects of tourism development and to explore the mode of green development of tourism itself. It is important to focus on the negative impact of tourism development on the environment, and to focus on the sustainable development of tourism at a reasonable pace [60].

In the future, the green economy effect and carbon emission reduction effect of tourism development will be more prominent, but the intermediary role of carbon emission intensity in the impact of tourism development on green economy efficiency may not be clear. On the one hand, global enthusiasm and efforts in carbon reduction will be maintained, and the green transformation of industries is a trend. Meanwhile, the tourism industry, contributing to the implementation of carbon emission reduction in several regions and countries, has outstanding green economy attributes. On the other hand, in many more backward developing regions, tourism is a rough economic pattern due to the lack of experience in tourism development and late development history. Although these countries or regions attract a large number of tourists by virtue of their unique tourism resources, they cause more damage to the ecological environment than before development. Likewise, the issue of carbon emissions is not taken into account in the process of tourism development, resulting in a potentially unsatisfactory relationship between tourism development and green economic effects and carbon reduction effects. In addition, as the COVID-19 epidemic continues to impact the tourism industry, new forms of tourism are emerging. Many real tourism activities are shifted to virtual tourism activities, in which case the negative environmental externalities of the tourism industry itself may be weakened, as well as the EKC effect, positive environmental externality effect and indirect effect of tourism development.

Compared with previous studies, this paper refines the traditional subject of "tourism, carbon dioxide and economic growth" [6] to "tourism, carbon emissions and green economic efficiency", and further confirms Tong's conclusion on the green economic effect of tourism [60]. On the basis of this paper, we further find that the green economy effect of tourism has a positive "U" nonlinear characteristic, which is one of the important points of innovation in this paper. This paper also confirms that tourism development has a significant carbon reduction effect [6,32], and also finds that there is an "M" type nonlinear relationship between tourism development and carbon emission intensity, which is different from the "inverted U" type relationship obtained by Reza et al. [32]. To be more precise, the same "inverted U" type was found in this paper, but after the accurate measurement of the nonlinear effect by the semi-parametric spatial lag model, it was found that the "inverted U" type is only a vague form of the "M". The "inverted U" shape is only a fuzzy form of the "M" shape. In order to further confirm whether the carbon emission reduction of tourism affects the green economy effect of tourism, this paper innovatively uses carbon emission intensity as a mediating variable and concludes that tourism development affects green economy efficiency through carbon emission reduction. Although the article has conducted a detailed study on the relationship between tourism development, carbon emission intensity and green economic efficiency, there are also the following shortcomings. First, the article takes 280 prefecture-level cities in China as examples, but the tourism

development of these cities is uneven, which may cause some interference in the results. Subsequent research can further discuss the formation mechanism among the three factors. Second, the impact mechanism is not thoroughly explored in this paper, which leaves room for further discussion of the formation mechanisms between the three. Instead, this paper only discusses the relationship between tourism development, carbon emission intensity and green economy efficiency in detail.

**Author Contributions:** Conceptualization, X.H. and H.X.; methodology, J.S. and X.H.; validation, X.H.; formal analysis, J.S. and C.C.; investigation, J.S. and X.H.; resources, J.S. and C.C.; writing original draft preparation, J.S. and X.H.; writing—review and editing, X.H. and J.S.; supervision, Q.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Social Science Foundation, grant number 21FGLB070, and Natural Science Foundation of Hunan Province, grant number 2022JJ30387.

**Institutional Review Board Statement:** Ethical review and approval were waived for this study. The national law "Regulations for Ethical Review of Biomedical Research Involving Humans" does not require the full review process for data collection from adults who have adequate decision-making capacity to agree to participate.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.
