1. Introduction
Despite significant global development in industrialization and urbanization, demand for Earth’s resources has exceeded reasonable limits. Global warming, environmental pollution, and serious resource depletion have caused severe problems worldwide. The current ecological and environmental crisis threatens the sustainable development of humans and the regional environment [
1]. A continuous increase in the global ecological footprint caused by economic growth, industrial structure changes, and increased energy consumption was revealed by measuring the ecological footprint of 144 countries from 1988 to 2008 [
2]. China’s Ecological Footprint Report 2015, published by the World Wide Fund for Nature [
3], reported that China is consuming at a rate of 2.2× its ecological resources. Indeed, China’s ecological footprint now accounts for one-sixth of the world’s total, more than any other country. Thus, considerable research should be conducted into China’s ecological footprint.
The ecological footprint concept was first proposed by the Canadian ecological economist Rees in 1992 [
4]. It describes the area of biologically productive land required by humans to produce the necessary resources and absorb the generated waste under certain demographic and economic conditions. In other words, it reflects the impact of human activities on the natural ecological environment. The ecological footprint not only reflects individual or regional resource consumption intensity but can also objectively measure and compare temporal and spatial sustainability. Therefore, it can measure human-induced stress to the ecological environment [
5]; the larger the footprint, the more serious is the damage to the ecological environment [
6].
Previous studies have suggested that the impact of technological innovation on the ecological environment exhibits complex “duality” [
7]. On the one hand, improvements in technological innovation can improve energy efficiency (e.g., by replacing fossil fuels with clean energy) and CO
2 emission treatment technology, thereby improving the efficiency of natural resource utilization, reducing environmental pollution caused by carbon emissions, and contributing to the “green” development of the ecological environment [
8]. Similar studies have shown that increasing technological innovation has significantly reduced the deterioration of the ecological environment in various provinces in China [
9]. However, as in developed countries, China’s ecological problems are largely due to industrialization [
10]. Technological innovation promotes production efficiency and accelerates the large-scale expansion of industry, bringing economic benefits but also increasing the excessive consumption of resources [
11]. Therefore, industrialized regions are impacted by resource depletion and the deterioration of the ecological environment. In addition, although technological innovation can improve energy efficiency, it only slightly reduces energy consumption, as it is impossible to reduce most of the energy use. For example, if energy prices fall because of improved energy efficiency, lower prices may encourage humans to use more energy, which in turn places greater pressure on the ecological environment [
12]. Therefore, research has not yet reached a clear conclusion on the relationship between technological innovation and the ecological environment.
Innovation efficiency refers to the allocation and utilization efficiency of scientific and technological resources over space and time [
13], which reflects the strength of regional technological innovation capabilities [
14]. The ecological footprint, as a comprehensive indicator reflecting the quality of the ecological environment, can measure human pressure on the environment. Therefore, it is important to evaluate the impact of innovation efficiency on the ecological footprint, the relationship between the two measures, and the effect of regional economic development on this relationship. Although studies have investigated many of the factors influencing the ecological footprint [
15], few have analyzed the mechanism of these impacts from the perspective of innovation efficiency. In addition, as the ecological footprint is not simply a local ecological problem, China’s per capita ecological footprint is not completely random and exhibits significant spatial agglomeration [
16]. Therefore, when discussing the impact of innovation efficiency on the ecological footprint, it is necessary to consider the spatial correlation of the ecological footprint itself to obtain more accurate research results.
Based on the foregoing, this study uses the super-efficiency slack-based measure (SBM) model to calculate the innovation efficiency of 280 cities in China from 2014–2018 and then conducts an empirical analysis of the impact of innovation efficiency on the ecological footprint using the generalized spatial two-stage least squares (GS2SLS) model, which can control for both the spatial effect and the endogenous effect. This study thus contributes to environmental science and public health research in the following three aspects. First, the impact of urban innovation efficiency on the ecological footprint is systematically analyzed. Considering that the core spatial carriers of the national innovation system are cities, 280 Chinese cities above the prefecture level are used as the research objects. Second, a threshold regression model is employed to investigate the regional and staged impact of innovation efficiency on the ecological footprint under different economic development levels. Night light data are used as a threshold variable to characterize the economic development level of a region because it can test real economic growth and measure economic agglomeration, urbanization, population mobility, energy consumption, and other economic activities [
17]. Finally, following [
18], this study constructs a mediating effect model composed of three regression equations to identify the transmission mechanism of innovation efficiency on the ecological footprint.
2. Literature Review
Research on the relationship between technological innovation and the ecological environment is predominantly conducted from three viewpoints. The first is that technological innovation can improve the ecological environment. According to the Porter hypothesis, stimulating the “innovation compensation” effect of enterprises through technological innovation is a key method of reducing pollution emissions [
19,
20]. The IPAT model (environmental impact (I) = population (P) × affluence (A) × technology (T)) links the environmental impact with the population size, per capita wealth, and technological level, suggesting that technological innovation and progress can alleviate the environmental pollution caused by population growth [
21]. One study found that the technological innovation capabilities of 30 provinces in China have a positive effect on the structure of regional economic growth, resource utilization, and the ecological environment; simultaneously, the spillover effect of regional technological innovation also has a positive effect on the ecological environment for three main reasons [
22]: (1) it improves energy efficiency and reduces energy consumption by using a more environmentally friendly combination of production methods, (2) it develops new energy sources and reduces the over-exploitation of resources, and (3) it promotes low-carbon technology and improves the efficiency of pollution control. Therefore, increased technological innovation can protect and improve the ecological environment [
23,
24,
25].
The second viewpoint is that not all technological innovation can improve the ecological environment. The greater the technological innovation, the higher is the degree of environmental pollution in areas with more advanced industrial development. This is observed in eastern and central China, where the quality of the ecological environment is far below the national average [
26]. Several studies have shown that technological innovation has a significant destructive effect on the ecological environment, as enterprises in the initial stage of industrial agglomeration do not accumulate a large amount of human capital and there is insufficient motivation for the innovation of clean production technology in the agglomeration area. Hence, in terms of technology research, development, and implementation, greater focus is placed on how to improve the level of product technology to increase firm profits, neglecting environmental protection technology [
27,
28,
29]. Therefore, the pollution suppression effect of technological innovation is offset or even concealed by the environmental damage effect of industrial enterprises. In addition, when industrial agglomeration is small, the infrastructure is not yet complete, public pollution control facilities have not yet been built, and resource allocation has not yet reached the optimal state, which is not conducive to reducing the marginal pollution control costs of enterprises [
30,
31].
The third perspective is that the impact of technological innovation on the ecological environment has an inverse U-shaped relationship. For example, in eastern and central China, the relationship between technological innovation and ecological pollution has a clear inverse U-shaped curve, whereby technological innovation first promotes then inhibits environmental pollution [
32]. Technological innovation and progress can also curb carbon emissions in the long term, but not in the short term [
33]. Therefore, technological innovation may exhibit a nonlinear relationship with initial destruction followed by an improvement in the ecological environment. Hence, the impact of innovation efficiency on the ecological footprint may also have an inverse U-shaped relationship characterized by inhibition then promotion. Hence, the following hypothesis is proposed:
Hypothesis 1 (H1). The impact of innovation efficiency on the ecological footprint exhibits an inverse U-shaped trend.
China has a vast territory and the intensity of natural resource utilization and ecological environment structure exhibit clear spatial heterogeneity between regions. China’s ecological footprint has risen rapidly since 2000 [
34], with the highest ecological footprint in the east, followed by a “stepped” spatial distribution in central and western regions. In addition, China’s regional innovation efficiency is characterized by heterogeneity and agglomeration [
35]. Moreover, the innovation efficiency of provinces in China declines from east to west [
36,
37]. Therefore, innovation efficiency may have a heterogeneous impact on the ecological footprint of different regions; however, previous or current research has not fully resolved this issue.
In addition, as economic development is relatively high in eastern cities, technological innovation has improved the ecological environment in eastern China more significantly than that in other regions [
38]. The inhibitory effect of technological innovation on China’s carbon emission reductions is positively affected by the regional economic development, as when the economic development is high, the greater financial support required for the development and application of technological innovation is available, which is conducive to vigorously developing, promoting, and utilizing clean energy, reducing carbon emissions, and suppressing pollutant emissions [
39]. Further, a higher economic development leads to a greater awareness of social environmental protection as well as a gradual shift in consumers’ focus from the price of final products to environmental protection and energy conservation during the production process, which has a positive impact on the ecological environment [
40]. Thus, under different levels of regional economic development, technological innovation may result in varying degrees of improvement to the ecological environment. The second hypothesis is therefore proposed:
Hypothesis 2 (H2). The impact of innovation efficiency on the ecological footprint exhibits regional differences; under different economic development levels, the impact of innovation efficiency on the ecological footprint exhibits a threshold effect.
Changes in the ecological footprint are affected by many social, economic, and natural factors such as population, consumption, land, climate, technology, and management, each with complex nonlinear characteristics. Therefore, a review of the previous literature [
41], as shown in
Figure 1, suggests that innovation efficiency may affect the ecological footprint in four ways: population aggregation, the industrial structure, the energy structure, and energy efficiency.
First, the improvement in urban innovation efficiency leads to a more rapid urban population agglomeration supported by R&D and service industry personnel, which affects the ecological footprint [
42]. Population agglomeration refers to an increase in urban population density, which translates into shorter commuting distances, reduced car usage per capita, and fewer pollutant gas emissions [
43]. In addition, population agglomeration leads to the concentration of enterprises and public facilities, which is conducive to the centralized construction of infrastructure [
44]. In particular, this results in sharing environmental pollution control facilities in the centralized infrastructure, reducing the effect of pollution diffusion, and taking advantage of economies of scale and agglomeration to improve the ecological environment [
45]. Therefore, an improvement in innovation efficiency may reduce the ecological footprint through population agglomeration.
Second, the industrial structure typically refers to the proportions of the primary, secondary, and tertiary industries in the economy, with the consumption of ecological resources by the primary and secondary industries serving as an important driving force for the continuous increase in the ecological footprint [
46]. This is mainly because the primary and secondary industries account for a relatively higher degree of natural resources and pollution when compared to the tertiary industry. However, the process of improving innovation efficiency guides the flow of innovation resources such as research and development (R&D) funds and R&D capital to more efficient departments and then promotes a further accumulation of innovation resources. Simultaneously, this also improves the industrial technology and output quality, which continuously increase the proportion of the tertiary industry (characterized by high added value and low energy consumption) [
47] and gradually reduce the proportion of the primary and secondary industries (characterized by high pollution, high energy consumption, and low added value). This in turn promotes the greater rationalization of the regional industrial structure and improves the ecological environment. Therefore, improved innovation efficiency may restrain the ecological footprint by optimizing and upgrading the industrial structure.
Third, optimizing China’s energy structure aims to gradually reduce its dependence on coal and increase the use of cleaner and more sustainable energy [
48]. Keeping all other conditions constant, a 1% increase in technological innovation in China’s 30 provinces from 1997 to 2014 reduced the average proportion of coal consumption in China by 0.732% [
49,
50]. Therefore, increasing technological innovation can significantly reduce the consumption of traditional coal energy in the production process [
51], thereby improving the ecological environment. In addition, technological innovation can promote the development of renewable energy and increase the supply capacity of renewable energy [
52] to meet energy demand and optimize the energy structure [
53], which again improves the ecological environment. As innovation efficiency is an important driving force for improved technological innovation, the ecological footprint is an effective indicator of the ecological environment. As such, an optimized energy structure also suppresses increases in the ecological footprint.
Fourth, during economic development or industrial production, technological innovation is an important factor affecting the energy efficiency of a region [
54] because improvements in technological innovation promote clean environmental energy in the production process, thereby reducing pollutant emissions [
55]. Moreover, energy savings and effective energy use increase with increasing energy efficiency, reducing excessive energy consumption and helping limit the ecological footprint. Therefore, the third hypothesis is proposed:
Hypothesis 3 (H3). Innovation efficiency can affect the ecological footprint through population aggregation, the industrial structure, the energy structure, and energy efficiency.
5. Conclusions
This study used panel data from 280 Chinese cities from 2014 to 2018 and the GS2SLS method to investigate the relationship between innovation efficiency and the ecological footprint. Moreover, the impact of innovation efficiency on the ecological footprint and its transmission mechanism were discussed at different economic development levels. The following three main conclusions were obtained.
First, the ecological footprint of cities across China as well as those in the eastern, central, and western regions exhibits significant spatial spillover effects, whereas that in the cities in northeastern China does not. After considering the spatial spillover effect of the ecological footprint and controlling for endogeneity, a significant inverse U−shaped relationship is observed between innovation efficiency and the ecological footprint of cities across China as well as in the eastern and central regions. That is, innovation efficiency first promotes and then inhibits the ecological footprint. However, there is no inverse U-shaped relationship between innovation efficiency and the ecological footprint in western and northeastern China, but a positive and significant relationship instead. Therefore, China should continue to adhere to the innovation-driven economic development strategy through mutual promotion and continuous improvement in innovation and economic development. Moreover, the western and northeastern regions should take the national regional economic strategy as an opportunity to integrate various innovative elements; strengthen exchanges and cooperation between scientific and technological resources, enterprises, and governments in the eastern, central, western, and northeastern regions; achieve good synergy effects; and steadily improve innovation efficiency, which will limit the growth of the ecological footprint.
Second, the impact of innovation efficiency on the ecological footprint has a double-threshold effect. Typically, with an improvement in economic development, the coefficient of the impact of innovation efficiency on the ecological footprint changes from −0.0067 to −0.0313 for the whole country, which indicates a gradual increase in the inhibitory effect of innovation efficiency on the ecological footprint. The eastern and central regions exhibit the same pattern as the entire country. However, in the western and northeastern regions, which have a greater economic development, improved innovation efficiency increases the ecological footprint, although this promoting effect has gradually weakened in the former. Therefore, China should formulate different innovation efficiency strategies according to regional economic development, actively open up the innovation chain between cities, comprehensively promote China’s innovation efficiency and technological innovation, and optimize and upgrade the industrial structure. This will gradually reduce the dependence of economic development on natural resources, promote its sustainable development, and promote greener development, thereby preventing the ecological footprint from growing further.
Finally, an improvement in innovation efficiency affects the ecological footprint through three mediating factors: the industrial structure, the energy structure, and energy efficiency. With increasing urban innovation efficiency, the population agglomeration effect does not inhibit the growth of the ecological footprint in China; instead, it increases it. Therefore, China should focus on the positive externalities of innovation efficiency to promote green technological innovation and implement relevant fiscal and tax policies to encourage and guide the development of green technology R&D activities, improve energy efficiency, reduce dependence on coal resources, and optimize the industrial structure. In addition, it is also necessary to aggressively attract high-tech industries and high-end talents, promote the joint effect of optimizing the industrial structure and energy structure and technological innovation progress, and maximize the mediating effect of innovation efficiency to reduce the ecological footprint.