Research on Evolution Pattern and Spatial Correlation between Economic Development and Environmental Pollution Centers of Gravity

: The coordinated development of environment and economy is an important way to achieve sustainable development. As the Guangdong-Hong Kong-Macao Greater Bay Area has been included in the national agenda, Guangdong province faces a turning point in its economic, social, and environmental development. Taking Guangdong province as an example, this paper analyzes the spatial evolution and correlation of economic development and environmental pollution by means of center of gravity (COG) and geo-information system (GIS). The results show the shift of economic development COGs are smaller than that of environmental pollution. Environmental pollution COGs are negatively correlated with economic scale and quality COGs, whereas it is positively correlated with economic growth COG, which depends on the industrial structure and local policies. The continuous transformation of the industrial structure of the Pearl River Delta Region (PRD) is conducive to improving its environment and promoting economic development of Non-Pearl River Delta Region in Guangdong province (Non-PRD) through bilateral causality. As the receiving place of industrial transfer, eastern Guangdong has obvious e ﬀ ects of environmental pollution transfer from the secondary industries. In this study, the logical spatial evolution path of the economic development and environmental pollution COGs is established. It provides theoretical and practical references for the study of interrelationship between economy and environment.


Introduction
As China's development enters the new normal stage and regional integration deepens, interregional economic and environmental linkages are producing more profound impact. The coordinated development of environment and economy is an important part of the theory of sustainable development, while environmental deterioration will restrict the progress and quality of economic development. As the forefront of the reform and opening up, Guangdong province's economy has developed rapidly; however, the ecological environment has deteriorated. The contradiction between the two will restrict sustainable development of the region. At present, Guangdong province, especially the Greater Bay Area, leads the country in overall ecological and environmental quality, but the coordination between economic growth and environmental protection is still far behind that of New between economic agglomeration and environmental pollution in China [26]. At present, scholars mostly use the center of gravity method to study the spatial differences and correlations among regional attributes [27,28]. Some scholars combine the center of gravity method with GIS and statistical analysis to improve the logicality and visibility of such spatial evolution analysis [29][30][31].
In this paper, COG method was used to study the spatial correlation between economic development and environmental pollution in Guangdong province of China and analyze the internal driving force and evolution pattern, which is beneficial for finding positive measures for the coordination between economic development and pollution reduction. Promoting the coordinated and sustainable development of the environment and economy in Guangdong province will not only have important practical significance for itself but will also have important reference significance for China. Theoretically, this paper deepens the research about the relationship between economy and environment in Guangdong province and expands the theoretical framework of the evolution of regional gravity center. Practically, the exploration of the internal relationship between the economic development and the environmental pollution COGs contributes to the government management and achievements of both high economic quality and sustainable environmental development. Furthermore, its research methods and results support researches on economic development and environmental pollution in Guangdong province and China.

Concept of the Center of Gravity
The center of gravity (COG), also known as the "spatial mean," refers to the point at which the force moment of a certain attribute reaches an equilibrium on the spatial plane within a certain time, which is the extension of the sample mean in the two-dimensional space. Suppose a certain region is composed of n subregions and that the attribute value of each subregion is p i , for each subregion, the coordinate of the geographical COG is (X i , Y i ), and the COG is calculated as follows: If p i in the formula is the total gross domestic product (GDP) of subregion i, taking it as the weight, the economic scale COG is calculated after weighted average. When p i is taken as GDP, per capita GDP, GDP growth rate, volume of industrial wastewater, volume of industrial waste gas, or volume of solid waste, the corresponding COG coordinate will be calculated.
The moving direction of COG refers to the spatial plate deflection caused by the changes in the attribute values in the process of spatial evolution. It is calculated as follows: where s and k represent two different years; θrepresents the interannual moving angle (−180 • < θ< 180 • ); and 0 • represents the east, showing a positive value for counterclockwise rotation and a negative value for clockwise rotation. The spatial movement of COGs between different years is calculated as follows: where D s-k represents the distance of COG movement between two different years; C is a constant, which is the coefficient for the geographic coordinate unit (1 • ) to be converted into a plane distance (km); C*(X s − X k ) and C*(Y s − Y k ) represent the actual distances that the COGs move in longitude and latitude, respectively, from year k to year s.
From the above formula, the important factors affecting COGs are the geographical locations and certain attribute values of the subregions within the region. The change of COGs is mainly manifested in two aspects: migration direction and migration distance. The migration direction refers to the spatial reflection of the high-slope growth position of a certain attribute value. If COG moves in a certain direction, it indicates that the attribute value grows faster in this direction. The migration distance indicates the imbalance of some attribute values. If COG migration distance is large, the difference of the attribute values is large. To explore the spatial characteristics of economic development and environmental pollution in Guangdong province, this study classifies the economic development COGs and environmental pollution COGs. Economic development COGs include economic scale COG, economic quality COG, and economic growth COG. Environmental pollution COGs are divided into industrial wastewater COG, industrial waste gas COG, and solid waste COG. The classification is shown in Table 1.

Data Resources
The data mainly come from the Statistical Yearbook of Guangdong (2001-2018) and include the indicators of GDP, per capita GDP, GDP growth rate, industrial wastewater emissions, industrial waste gas emissions, and industrial solid waste emissions of 21 prefecture-level cities. The data on environmental pollution in 2017 are missing. All the spatial data come from the 1:4,000,000 database of the National Fundamental Geographic Information System (NFGIS), including the administrative maps and longitude and latitude coordinates of the cities. By taking area as the weight and substituting it into the COG formula, it can be calculated that the geographical center of Guangdong province is located at 113.75 east longitude and 23.0 north latitude in Xinhe Village, Wanjiang District, Dongguan city.

Hypotheses
Based on attributes such as economic development and environmental pollution, the evolution of the industrial structure and environmental regulations in Guangdong [26,27,31], this paper proposes several hypotheses on the evolution pattern and spatial correlation of the economic development and environmental pollution COGs.

Hypothesis 1.
The inverted U-shaped theory, which was put forward by an American economist Jeffery G. Williamson, points out that there is an inverted U-shaped relationship between development stages and spatial differences and that economic growth tends to be balanced in the long run [32]. The PRD has no doubt become the core area of development in Guangdong province, and it has disproportionate power over the Non-PRD in terms of industrial upgrading. However, due to the law of diminishing marginal returns and the gradual stabilization of industrial advantages in the Non-PRD, PRD may not have an advantage in the growth rate of various economic indicators. Therefore, this paper assumes that the economic development COGs are close to the PRD but tend to move away from the PRD with little range.

Hypothesis 2.
With the upgrading of the industrial structure and the industrial gradient transfer between regions, the environmental pollution level in regions dominated by tertiary industries will gradually decrease, and pollution problem in regions that are home to heavy chemical industrial enterprises may worsen [33]. Therefore, this paper assumes that the environmental pollution COGs are closely related to the spatial transfer of secondary industries, and the environmental impacts brought about by such transfer have a certain time lag.
Hypothesis 3. The environmental Kuznets curve (EKC) means that there is an inverted U-shaped relationship between environmental pollution and economic development [34]. The Porter hypothesis notes that effective environmental regulation can stimulate technological innovation and improve the productivity of enterprises, thus offsetting the increase in cost brought about by environmental protection, improving the profitability of enterprises, and in turn, achieving a win-win situation for economic development and environmental protection. Therefore, this paper assumes that the relationship between economic development and environmental pollution in Guangdong has passed the inflection point of the inverted U-shaped curve; there are negative correlations between the evolution paths of economic development and environmental pollution.

Evolution of Economic Development Centers of Gravity
By calculating the locations, directions, and moving distances of the economic growth, economic scale, and economic quality COGs in Guangdong from 2000 to 2017, we can draw the spatial evolution tracks as shown in Figure 1. Figure 1 shows that the economic development COG is in the south-central part of Guangdong. The economic scale ( Figure 1a) and economic quality (Figure 1b) COGs are relatively stable, with small changes and similar moving tracks. They both move steadily toward the northwest and then slightly back to the southeast, which shows that regions with larger economies and regions with higher per capita economic levels are basically the same. They both lie in the PRD, which does not have the incremental advantages of these two indicators. However, economic growth COG has changed contradictory, and the overall shift is mainly to eastern Guangdong (Figure 1c). The driving force is mainly related to Guangdong's promulgation of a series of plans, such as "Decision on Further Promoting the Revitalization and Development of the Northern, Eastern, and Western Guangdong." These plans entrust the industrial parks with the important task of leading the revitalization of industries in the Non-PRD.

Evolution of Environmental Pollution Centers of Gravity
By calculating the locations, directions, and moving distances of the industrial wastewater, industrial waste gas, and solid waste COGs in Guangdong from 2000 to 2016, we can draw the spatial evolution tracks as shown in Figure 2.   The moving path of the solid waste pollution COG spans several cities (Figure 2c), with a great range of movement. According to the emissions of pollutants of the cities in recent years, the solid waste discharge is basically zero except in western Guangdong; therefore, the solid waste pollution COG moves to the west. In recent years, the Non-PRD has taken on part of the secondary industries from the PRD, which has had a serious impact on the atmospheric and water environment.

Comprehensive Comparison
The results of placing the coordinates of economic development and environmental pollution COGs in the same coordinate system are shown in Figure 3.  Figure 3 shows that the economic development and environmental pollution COGs are quite close to each other, falling approximately on the border between Guangzhou and Dongguan, and close to the geographic COG. The changes in the economic development COGs are much smaller than those in the environmental pollution COGs. It indicates that the spatial solidification effect and inertia of the difference in economic development are stronger and that the time lag of the relative change in the difference of the environmental pollution discharge is shorter. Spatial transfers of secondary industries can drive the distribution of environmental pollution discharge within a short time; however, disrupting the existing economic spatial pattern is difficult.

Spatial Correlation Analysis Based on Longitude
The longitude evolution tracks of the economic development and environmental pollution centers of gravity are shown in Figure 4, and the correlation coefficients are shown in Table 2.   Figure 4 and Table 2 show that the economic scale and economic quality COGs basically overlap in longitude and change stably, with little change in the east-west direction. The longitude of the economic growth COG gradually increases, and the economic growth in eastern Guangdong indicates a slight advantage over that in western Guangdong. The evolution trends of industrial wastewater pollution and waste gas pollution COGs are relatively consistent in longitude with the evolution trend of the economic growth center of gravity, mostly falling in eastern Guangdong. The solid waste pollution COG changes significantly in longitude and mainly moves westward. At the 0.01 level, the economic scale COG is positively correlated with the economic quality COG. The industrial wastewater pollution and industrial waste gas pollution COGs are negatively correlated with the economic scale and economic quality COGs. The industrial wastewater pollution COG is positively correlated with the economic growth COG. The industrial waste gas pollution COG is positively correlated with the industrial wastewater pollution COG. At the 0.05 level, the economic quality COG is negatively correlated with the economic growth COG. This finding shows that the PRD, which has the highest production efficiency in China, is far ahead in terms of economic scale and development compared with the Non-PRD. The economic development potential in eastern Guangdong is increasing, but its environmental pollution is gradually becoming serious.

Spatial Correlation Analysis Based on Latitude
The latitude evolution tracks of the economic development and environmental pollution COGs are shown in Figure 5, and the correlation coefficients are shown in Table 3.  **: at the 0.01 level (double-tailed), the correlation is significant. *: at the 0.05 level (double-tailed), the correlation is significant. Figure 5 and Table 3 show that the economic scale and economic quality COGs basically overlap in latitude and are stable in the PRD all year round. Different from the change in the longitude, the changes in the economic growth, wastewater pollution, and waste gas pollution COGs on the latitude levels are relatively small. At the 0.01 level, the economic quality COG is positively correlated with the economic scale COG, and the solid waste pollution COG is positively correlated with the industrial waste pollution COG. At the 0.05 level, the economic growth COG is positively related to the economic scale COG. The spatial evolution of the economic development and environmental pollution COGs is relatively small in latitude, indicating that these centers of gravity mainly move in the east-west direction. In the north-south direction, the environmental pollution COGs are weakly correlated with the economic growth COG. It indicates that the acceleration of economic growth in the PRD has not caused significant environmental pollution, and its advantage in the economic growth rate is mainly associated with the rapid development of tertiary industries.

Discussion
The verifications of the hypotheses are based on the analysis of the evolution and spatial correlation of the economic development and environmental pollution COGs in Guangdong, and the results are as follows.
Hypothesis 1 holds: the ranges of movement of the economic scale and the economic quality COGs are proven to be small, and the overall moving directions are first toward the west and north and then slightly back toward the southeast. The evolution of the economic growth COG has no fixed direction. It can be seen that the economic aggregate and production efficiency of western and northern Guangdong have been improved, and the overall economic difference of Guangdong province has been converged. Hypothesis 2 holds: the foregoing proves that industrial wastewater and industrial waste gas COGs in Guangdong province have similar moving trajectory, which shows a trend of eastward movement. It basically corresponds to the spatial transfer path of Guangdong's industrial structure. As of 2017, 13 cities in Guangdong province are dominated by tertiary industries, including Guangzhou, Shenzhen, Zhuhai, and Dongguan, and the remaining cities (mainly in eastern Guangdong) are dominated by secondary industries. This is related to the "dual transfer" policy implemented by Guangdong province since 2008. Although the PRD vigorously promotes industrial transformation and upgrading, its environmental pollution is relatively reduced. However, the eastern Guangdong region has undertaken the industrial transfer from the PRD, which mainly includes labor-intensive industries such as textiles and garments, toys, ceramics, and leather. This has a negative impact on the water and atmospheric environment. Therefore, the industrial wastewater and industrial waste gas COGs move eastward.
Hypothesis 3 is not true: it is proved that the economic scale and economic quality COGs generally move to the northwest, whereas industrial wastewater and industrial waste gas COGs generally move to the east. The spatial evolution of the two shows deviation. This indicates that the improvement of economic scale and quality does not all depend on polluters in the central and western regions of Guangdong province. The inflection point of the EKC curve may have passed. However, facing the problem of increasing environmental pollution with rapid economic development, eastern Guangdong is still in the upward stage of EKC curve.
Compared with the latitude level, the longitude evolution of economic development and the environmental pollution COGs in Guangdong province is more obvious. It shows that industrial wastewater and industrial waste gas COGs are negatively correlated with economic scale and economic quality COGs, while the correlation between economic development COGs and environmental pollution COGs is not significant at the latitude level. The economic scale and economic quality COGs have been stable in the PRD, which tends to move northwest. The economic growth and the environmental pollution COGs mainly move east. It is mainly affected by the industrial structure, i.e., the industrial structure is more optimized and advanced, and the environmental quality is relatively better in the PRD. On the contrary, secondary industries are more dominant and the environmental quality is poor in eastern Guangdong. Therefore, optimization of the industrial structure is one of the key factors in the spatial evolution of the economic development and environmental pollution COGs. Based on the above findings, the logical path for the spatial evolution of economic development and environmental pollution in Guangdong is shown in Figure 6.
At present, Guangdong is in the stage of economic development driven by both secondary industries and tertiary industries. The PRD is in a postindustrial period currently with a large economic scale and high development quality after the continuous upgrading of its industrial structure. With the increase in production efficiency, the economy has changed from high-speed development to high-quality development. At the same time, the PRD is beginning to attach importance to environmental quality and protection. Environmental pollution has gradually decreased in the process of such economic development. The development status is shown in Cycle 2 . However, eastern Guangdong and other peripheral regions still regard economic growth as the top priority, where the industrial structure is starting to upgrade, but it is still at the stage of development of secondary industries. In the process of gradual expansion of economic scale and improvement of production efficiency, environmental pollution also increases. The development status is shown in Cycle 1 . However, the current development in eastern Guangdong is in line with the framework of existing experience, i.e., with the continuous improvement of production efficiency, the improvement of production technologies, and the implementation of appropriate environmental regulations, it will gradually but eventually transform from secondary industry-based development to tertiary industry-based development, from Cycle 1 to Cycle 2 . The spatial patterns of economic development and environmental pollution will then be more coordinated with fewer differences, which means that economic development and environmental quality are not antithetical. The potential for coordinated development is a significance aspect of "sustainable development" and the premise of this paper.

Conclusions
In this study, Guangdong province is used as a case study for the coordinated development of environment and economy. The results are of great significance for enriching theories of sustainable development, and the research methods and results also provide a reference for China.
The spatial evolution and correlation of the economic development and environmental pollution COGs of Guangdong are discussed, and their driving mechanism and logical path are determined. The results are as follows: (1) The space-time migration tracks of the economic development and environmental pollution COGs are closely related to policies. The PRD has an influence on the location of the economic development COGs. However, according to the growth rates of three economic indicators, the PRD does not have advantages. The economic scale and economic quality COGs generally show a trend of moving toward the northwest and then slightly back toward the southeast, which is related to the diminishing marginal returns of economic factors and the large absolute values in the PRD. In recent years, Guangdong has vigorously promoted the transformation and upgrading of the PRD, thus accelerating the pace of development through two-way causal interaction with the Non-PRD. The industrial wastewater and industrial waste gas COGs mainly move east. (2) There are spatial correlations between the economic development and environmental pollution COGs. The movements of the economic development and environmental pollution COGs are mainly shown in longitude. The industrial wastewater pollution and industrial waste gas pollution COGs are negatively related to the economic scale and economic quality COGs, whereas the industrial wastewater pollution COG is positively related to the economic growth COG. In recent years, eastern Guangdong has a strong economic growth momentum, where the industrial development has a great impact on the spatial pattern of environmental pollution. The changes of economic development COGs and environmental pollution COGs are different overall. The changes in the economic development COGs are relatively stable, moving to the northwest slowly. Meanwhile, the environmental pollution COGs have changed greatly, moving from west to east, which is also a new spatial pattern of economic development and environmental pollution in Guangdong. (3) The optimization and upgrading of the industrial structure in the PRD are helping to improve the environment. The industrial wastewater and waste gas pollution COGs generally move to the eastern Guangdong, which are dominated by secondary industries. The economic growth driven by secondary industries is positively related to environmental pollution, i.e., environmental pollution increases with increase in economic growth. This conclusion is consistent with relevant research findings. Liu used the panel data of Guangdong from 2003 to 2013 to study the trend in the changes in the economy and environmental pollution under the "Policy of Dual Transfer of Labor and Industry," confirming that eastern Guangdong has the strongest pollution transfer effect [33]. This paper also shows that the movements of the environmental pollution COGs toward the eastern Guangdong and the optimization and upgrading of the industrial structure in the PRD are helpful for improving the environment.
The possible innovation of this study is that the description of the shifting paths of the economic development and environmental pollution COGs and the analysis of the correlations between them have been established. The discussions on longitude and latitude levels can facilitate discussions about their spatial overlap and variation consistency and lead to in-depth analysis of the evolution of multifactor interactive coupling relationship patterns from the dynamic space-time perspective. On this basis, the logical paths of spatial evolution of the economic development and environmental pollution COGs are described. Cross-urban environmental management measures under the framework of sustainable development can be constructed in the PRD and the Non-PRD. All these measures can serve as a reference for the coordinated development of the economy and environment in China.
There are some limitations to this study. First, this paper uses the COG method to show the changes in spatial forces intuitively but weakens the analysis of economic development and environmental quality in terms of specific data. The method of evaluating the space moving track of these COGs can be further explained by the methods of space coupling calculation, gray correlation degree, etc. Second, for the convenience of data acquisition, the data used in this paper are for prefecture-level cities. If county-level statistical indicators are used, the analysis will be more detailed.

Suggestions
According to the above analysis results, environmental pollution COGs in Guangdong province have a negative correlation with economic scale and economic quality COGs, but positive correlation with economic growth COG. When the economic scale develops to a certain stage, optimization of the industrial structure and improvement of production efficiency can promote economic growth while alleviating environmental pollution. Therefore, it is imperative to balance the relationship between economic development and environmental governance.
First, the industrial structure is to be optimized and technological progresses be promoted. Underdeveloped regions should rationally develop secondary industries without destroying the environment or affecting economic development. Extensive production methods can be combined with new technologies, new models, and new industries. More developed regions should increase the effect of tertiary industries in driving economic growth, actively learn from the economic development experience of developed countries, and increase the potential for absorbing labor. Regional differentiation measures will make Guangdong's industrial structure more reasonable, leading to greener and healthier economic development.
Second, the environmental and economic pressures are to be alleviated. After the "double transfer" policy, the economic development and environmental protection between the transfer-in and transfer-out areas of Guangdong's industries cannot be coordinated with each other. Therefore, it is necessary to alleviate the environmental pressure of the transfer-in areas, unleash their growth potential, encourage low-energy, high-efficiency enterprises to settle in local areas, and help high-polluting enterprises to transform their production through policies such as tax cuts, increased investment, and sewage charges. Transfer-out areas should increase investment in ecological environmental governance, introduce more high-tech, technologically diffusive, and inducible industries, and minimize environmental pollution to the transfer-in areas.
Third, reasonable economic growth targets are to be set. With China's economic development entering a new normal stage, the government should abandon the "GDP only" performance appraisal mechanism. In the process of economic development in Guangdong, the industrial structure has gradually transformed into a labor-intensive and low-investment service industry. Slow economic growth is normal. Guangdong province should set reasonable economic growth targets and establish a green mechanism that links environmental protection and political performance, so as to achieve a win-win situation for environmental protection and economic development.