Exploring the Relationship between Urbanization and Eco-Environment Using Dynamic Coupling Coordination Degree Model: Case Study of Beijing–Tianjin–Hebei Urban Agglomeration, China

Abstract

The continuous and rapid development of global urbanization has brought great pressure to eco-environments. It is particularly serious in mega urban agglomerations, which determine the development process of urbanization in the world and affect the international competitiveness of countries. Taking the mega urban agglomerations with few research cases as the research area to explore the relationship between urbanization and eco-environment is vital to realize global sustainable development and optimize the development direction and trend of world urbanization. It is of great significance to assume the historical task of shifting the center of the world economy and the main position of the “Belt and Road” construction for China, and enhance its international competitiveness, as well as accelerate China’s high-quality development of new urbanization and the realization of ecological civilization. Previous studies mostly used the static coupling coordination degree (SCCD) model, which has limitations in describing complex interactions. This study used the dynamic coupling coordination degree (DCCD) model to analyze relationships in Beijing–Tianjin–Hebei urban agglomeration (BTH) from 2003 to 2019, which is one of China’s mega urban agglomerations. For the first time, we explain the progressiveness of the DCCD model from the construction concept, theoretically analyze the rising and falling laws of DCCD in break-in development stage, and propose the concepts of “benign transition” and “non-benign transition” when DCCD changes from break-in development to utmost development. Results show that BTH’s urbanization increased in fluctuation, with significant regional differences. The eco-environment was relatively good, but there are potential risks. The DCCD showed an S-shaped curve. Break-in development was the main type of DCCD. Moderate urbanization development and small degree of eco-environment sacrifice were necessary for “benign transition”. After the “benign transition” is realized, the high-level symbiosis of DCCD and the mutual promotion can be achieved through technical improvement. According to the identification results of the main controlling factors, the DCCD can be regulated by subsystems.

1. Introduction

The 2018 Revision of World Urbanization Prospects indicated that global population could add another 2.5 billion people to urban areas by 2050 [1]. Of this, China will add 255 million urban dwellers [1], accounting for 10.2% of the global growth. The development of global urbanization is advancing rapidly. The eco-environment is facing great pressure. It is very acute in China, which is urbanizing rapidly. Especially, BTH is facing more serious pressure on resources and eco-environment, as one of China’s mega urban agglomerations. Balancing the relationship between urbanization and eco-environment will determine the world urbanization development process, affect the international competitiveness of countries, and is vital to realizing the United Nations 2030 Agenda for Sustainable Development. It is of great significance to assume the historical task of shifting the center of gravity of the world economy and the main position of the “Belt and Road” construction for China, and enhance its international competitiveness as well as accelerate China’s high-quality development of new urbanization and the realization of ecological civilization. However, there are three shortcomings in the existing research on urbanization and eco-environment. (1) Insufficient attention has been paid to changes in the leading factors of urbanization and eco-environment. The analysis of the internal driving factors of CCD is not deep enough. (2) More studies use the SCCD model to quantify the relationship of urbanization and eco-environment [2,3,4,5]. In fact, their relationship is an open, unbalanced, dynamic fluctuation system with nonlinear interaction and self-organizing ability [6,7]. The interactions are extremely complex [8,9,10]. The SCCD model has some limitations in describing this relationship. (3) There are few case studies on the interaction between urbanization and eco-environment in mega urban agglomerations, and it is urgent to carry out research to meet the needs of the world and the countries.

Research on the relationship between urbanization and eco-environment has emerged since the 1990s. At present, a circular path of “empirical research–law summary–theory construction–empirical research” has been established [2,11]. The subjects involved include ecology, geography, management, economics, and environmental science. The research area is mainly ecologically fragile, urbanized, and metropolitan, covering multiple scales including world, national, provincial, urban, and regional [12]. And the research pays more attention to the coercion impact of urbanization on the eco-environment. The important achievements in theoretical research mainly include “Environmental Kuznets curve” [13,14,15], “Ecological Footprint Theory” [16,17,18,19], and “Eco-environment Pressure-State-Response model”. The empirical research includes the single-dimension research of eco-environment protection and governance in the process of urbanization, the coupling and coordination research of urbanization and eco-environment, the coupling law disclosure, the coordination relationship evaluation, the simulation analysis, and the prediction analysis. It is generally accepted that there is an extremely complex interaction and stress relationship between urbanization and eco-environment. The main performance is that urbanization has a coercive or promoting effect on the eco-environment through population growth, economic development, energy consumption, technological progress, urban management, and construction land expansion. And the eco-environment has a restrictive or carrying effect on urban development through resource carrying, environmental capacity, ecosystem services, environmental equity, and policy intervention [8,20]. That is, the development of urbanization is inevitably accompanied by the change of eco-environment, which is comprehensive and phased [7,21]. In the early stage of urbanization development, the comprehensive quality of eco-environment deteriorated, including the excessive consumption and destruction of atmosphere, water, land, organisms, resources, and energy. It even induced structural variation and functional loss of the entire ecosystem. In the middle and later stages of urbanization development, the quality of eco-environment improved. Meanwhile, the eco-environment, as the carrier and support of human activities, provides the material basis for the development of urbanization. Its material quantity and carrying capacity restrict the development of urbanization. The relationship between urbanization and eco-environment has received extensive attention.

The coupling coordination degree (CCD) model is the mainstream method to study the relationship between urbanization and eco-environment, accumulating many research results. Wu et al. found that the urbanization development in line with the eco-environment carrying capacity can avoid their mutual detriment, in their study on urbanization and eco-environment CCD [22]. Tian et al. found that there is an inverted U-shaped curve of CCD between urbanization and eco-environment [23]. Liu et al. found that urbanization hindered the further improvement in CCD [24]. There are many studies that have reached similar conclusions [14,25,26,27].

From 1978 to 2019, China’s urbanization rate has increased from 17.92% to 60.60% [28], and is expected to reach 76.0% by 2050 [29]. In the past few decades, China’s traditional urbanization development has successfully solved the problem of “fast or not fast” [30]. However, the attention given to eco-environment was not enough, and a series of eco-environment problems appeared, such as soil degradation and desertification [31,32], water and energy shortage [33,34,35], climate warming [36,37,38], loss of biological habitat [39,40,41], air pollution [42,43,44], and so on. In the new era, China’s urbanization development is in the transition period from the rapid growth stage to the quality improvement stage in the late stage, with the emphasis on high-quality development [30]. The high-quality development of China’s urbanization will not only determine the future of China, but also determine the development process of global urbanization. “Efficient, low-carbon, ecological, environmental protection, conservation, innovation, wisdom and safety” is the specific path to promote the high-quality development of new urbanization [30,45]. Accelerating the construction of urban agglomerations and improving their development quality is an important carrier for high-quality development of new urbanization [30,45,46]. The Central Urbanization Work Conference, the Party’s “19th National Congress” report meeting and so on have clear instructions on this. As a highly developed spatial integrated urban form, urban agglomeration represents the advanced stage of industrialization and urbanization. In China, the urbanization development in the past decades at the cost of eco-environment has made it a high incidence area of ecological and environmental problems. Among them, mega urban agglomerations are the representative. Their development presents unsustainable high-density agglomeration, high-speed expansion, high-intensity pollution, and high risk of resource and environmental protection threats [45]. Mega urban agglomerations are the strategic core area of national economic development and the main area of national new-type urbanization. They bear the historical responsibility of carrying the shift of the center of gravity of the world economy. As the hub for China to enter the world and the gateway for the world to enter China, it is profoundly affecting China’s international competitiveness and will determine the new pattern of world politics and economy in the 21st century. China’s mega urban agglomerations have low levels of development, low levels of resource and environmental protection, low regional economic aggregate, heavy environmental pollution, and prominent ecological problems, compared to the world’s most developed megacities. It is not yet able to take up the historic task of shifting the center of the world economy and building the main position of the “Belt and Road” [45]. There are few case studies on the interaction between urbanization and eco-environment in mega urban agglomerations, and it is urgent to carry out research to meet the needs of the world and the countries [45]. The BTH, the Yangtze River Delta urban agglomeration, and the Pearl River Delta urban agglomeration are the three major mega urban agglomerations in China. These regions have the most dynamic economy, the highest degree of openness, the strongest innovation capacity, and the largest number of foreign population, and their comprehensive development level is in an absolute advantage. Among them, the BTH is facing more serious resource and eco-environment stress pressure. Focusing on the BTH, in-depth analysis of the urbanization and eco-environment development and their CCD status is of great significance to assume the historical task of shifting the center of gravity of the world economy and the main position of the “Belt and Road” construction for China, and enhance its international competitiveness, as well as accelerate China’s high-quality development of new urbanization and the realization of ecological civilization.

This paper took BTH from 2003 to 2019 as the research area. Firstly, we constructed the evaluation index system of urbanization and eco-environment. Secondly, we used the DCCD model rather than the SCCD model to analyze their relationship. We analyzed the spatial–temporal changes of urbanization, eco-environment, and DCCD. We paid attention to the impact of subsystems on urbanization and eco-environment. We analyzed the characteristics changes of DCCD emphatically. Combined with the analysis of urbanization, eco-environment, and DCCD, we explored the possibility of realizing the benign interaction of urbanization and eco-environment by adjusting the subsystems. This study has some features. (1) We further developed the DCCD model on urbanization and eco-environment. (2) We analyzed the urbanization, eco-environment, and DCCD in depth from the subsystems level. (3) We clarified the existing problems of BTH. (4) We identified the weaknesses and strengths of the urbanization and eco-environment subsystems. (5) We made up for the deficiencies of existing research. We provide reference for the sustainable development of mega urban agglomerations and regional cities, and alleviate the contradiction between urbanization and eco-environment.

This paper verifies the scientificity and advancement of the DCCD model in the study of the relationship between urbanization and eco-environment from two dimensions of construction concept and empirical case. It further promotes the application of the DCCD model in the research field of the relationship between urbanization and eco-environment. Based on the evaluation results of the DCCD model, two new theoretical concepts of the relationship between urbanization and eco-environment are proposed: “benign transition” and “non-benign transition”. Meanwhile, this paper deeply explores the relationship between urbanization and eco-environment from the subsystems level. The key to realizing the mutually promoting relationship between urbanization and eco-environment is found. This study provides a more scientific and accurate research idea for the related research on the relationship between urbanization and eco-environment. The research results have important reference significance for the study of the relationship between urbanization and eco-environment, and have important practical significance for the realization of benign interaction between the two.

2. Methods

2.1. Study Area

BTH is located at the intersection of the longitudinal axis of the “coastal channel” and the “Beijing–Harbin and Beijing–Guangzhou Channel” in China’s “two horizontal and three vertical” urbanization strategy pattern. The geopolitical relationship between BTH and the two major newly industrialized countries, Japan and South Korea, and the strategic position of the core region of Northeast Asia formed there are very important. BTH is an important part of China’s economic core region and the core region leading China’s participation in global economic competition. Including Beijing, Tianjin, Shijiazhuang, Tangshan, Qinhuangdao, Handan, Xingtai, Baoding, Zhangjiakou, Chengde, Cangzhou, Langfang, and Hengshui, altogether there are 13 cities. The core areas are Beijing and Tianjin. As the capital of China, Beijing is one of the most dynamic and open regions in the world, as well as the political, cultural, international exchange, and technological innovation center of China. Tianjin is an international port city, an ecological city, and a northern economic center. The total area of BTH is 217,156 km². The terrain is high in the northwest and low in the southeast, and the main landform types include plateaus, mountains, and plains. The eco-environment in plateau, mountainous, and hilly areas is relatively fragile, and the plain area is one of China’s important grain-producing areas (Figure 1).

As one of the three major urban agglomerations leading the high-quality development in China, BTH shoulders the dual mission of balanced north–south and coordinated development within the region. Its historical development went through four stages [47]. (1) Since the Liao and Jin Dynasties, the Gyeonggi region with Beijing as the core developed as a whole. (2) Since modern times, the Gyeonggi region with Tianjin as the economic center of the north has developed as a whole. (3) After the founding of new China, the industrialization and administrative economy formed a pattern of regional competition. (4) Since 2014, BTH collaborative development has become a national strategy, opening up a new situation of regional cooperation and development. At present, the driving role of BTH in the hinterland of the north is limited, the development gap between the north and the south continues to widen, and the degree of collaborative development of BTH is still in the stage of further deepening. In terms of policy and system, from the 1980s to now, the cooperation policy of the three places of Beijing, Tianjin, and Hebei has gradually evolved into a formal system integration [48]. There are four stages. (1) The preliminary stage of cooperation led by the informal cooperation agreement between the governments of the three places (from 1980s to 2014). (2) The initial stage of coordinated development with formal systems such as the top-level planning of central ministries as the main, and local policies as the auxiliary (2014–2017). (3) In-depth coordinated development stage of continuous improvement in supporting policies at the executive level (2017–2022). (4) High-quality coordinated development phase of cross-regional cooperation supporting policies for advantageous driving forces (2022 to present). Judging from the historical development and institutional evolution of BTH, it is urgent for BTH to accelerate the benign interaction between urbanization and eco-environment at present, and it is also a key period of its development.

2.2. Construction of Index System and Data Sources

Different disciplines have different definitions of urbanization, but it is generally believed that the connotation of urbanization mainly includes population change, economic development, spatial expansion, and social culture [21]. Therefore, this paper divided the urbanization into population, economic, spatial, and social urbanization. The concept of eco-environment varies greatly in different scenarios. This paper studies the urban eco-environment; most of its problems are related to human activities. It mainly means the summation of natural factors that affect human development and survival, including water, land, gas, resources, energy, etc. This paper divided the eco-environment into resources, environment load and ecological civilization construction. Then, combined with the data availability, development status, and strategic positioning of BTH, we carried out the principal component and independence analysis of the indicators in existing research. We selected relatively independent indicators with rich connotation to form an indicator system for urbanization and eco-environment and used the entropy weight method to ascertain the index weight (

Table 1. Evaluation index system and weight of urbanization.

System	Subsystem	Weight	Indicators	Unit	Attribute	Weight
Urbanization	Population urbanization	0.2276	Natural population growth rate	‰	+	0.3669
			Proportion of employees in the secondary industry	%	+	0.3056
			Proportion of employees in the tertiary industry	%	+	0.3276
	Economic urbanization	0.2755	Per capita GDP	CNY	+	0.2294
			Proportion of the secondary industry output in GDP	%	+	0.2106
			Proportion of the tertiary industry output in GDP	%	+	0.2631
			Consumer price index (CPI)	%	+	0.2969
	Social urbanization	0.2886	Per capita total retail sales of consumer goods	CNY	+	0.2365
			Disposable income of urban residents Per capita	CNY	+	0.2511
			Buses per 10,000 people number	Vehicle	+	0.2553
			Practicing (assistant) doctors per 10,000 number	Person	+	0.257
	Spatial urbanization	0.2083	Built-up area per capita	m2	+	0.3341
			Road area per capita	m2	+	0.3435
			City population density	Person/km2	−	0.3224

and

Table 2. Evaluation index system and weight of eco-environment.

System	Subsystem	Weight	Indicators	Unit	Attribute	Weight
Eco-environment	Resources	0.3114	Coverage rate of green space in built-up area	%	+	0.363
			Per capita total grain output	kg	+	0.3441
			Park green area per capita	m2	+	0.2929
	Environment load	0.4074	Industrial wastewater discharge per capita	t	−	0.3298
			Industrial sulfur dioxide emissions per capita	t	−	0.3391
			Household electricity consumption per capita	KW·h	−	0.3312
	Ecological civilization construction	0.2812	Wastewater treatment rate	%	+	0.2619
			Comprehensive utilization rate of general industrial solid waste	%	+	0.2883
			Artificial afforestation area	ha	+	0.4497

).

The research data come from China Urban Construction Statistical Yearbook, China Cities Statistical Yearbook, China Price Statistics Yearbook, China Forestry Statistical Yearbook, China Statistical Yearbook and, China Regional Economic Statistics Yearbook, China Environment Statistical Yearbook, China Population and Employment Statistics Yearbook, and statistical bulletins of economic and social development of each city.

2.3. Construction of DCCD Model

Urbanization and eco-environment systems change as a nonlinear process [7,48,49]. Combined with Lyapunov’s first approximation theorem, to ensure the stability of nonlinear system development, the general function of the urbanization and eco-environment system change process can be approximately expressed as Equations (1) and (2) [48].

$$f\left(U\right)={\sum }_{i=1}^n{a_{i}x}_i, i=1, 2,3\cdots ,n$$

(1)

$$f\left(E\right)={\sum }_{j=1}^n{a_{j}x}_j, j=1, 2,3\cdots ,n$$

(2)

The relationship of urbanization and eco-environment is a complex interactive force. It is a complex system composed of the urbanization system $f\left(U\right)$ and the eco-environment system $f\left(E\right)$. Combined with the general system theory [50], the evolution equations of this system can be described as (3) and (4):

$$A=\frac{df\left(E\right)}{dt}={\alpha }_{1}f\left(E\right)+{\alpha }_{2}f\left(U\right), V_A=\frac{dA}{dt}$$

(3)

$$B=\frac{df\left(U\right)}{dt}={\beta }_{1}f\left(E\right)+{\beta }_{2}f\left(U\right), V_B=\frac{dB}{dt}$$

(4)

In Equations (3) and (4), $U$ and $E$ are the urbanization and eco-environment level.

Table 3. Classification standard of urbanization and eco-environment.

Value	0–0.5	0.5–0.6	0.6–0.7	>0.7
U/E type	Weak	Genera	Developing	Developed

shows the classification criteria of $U$ and $E$. $A$ and $B$ are the evolution states of urbanization and eco-environment. ${\alpha }_1$ and ${\alpha }_2$ are the influence coefficients of eco-environment and urbanization on the evolution state of the urbanization system. ${\beta }_1$ and ${\beta }_2$ are the influence coefficients of eco-environment and urbanization on the evolution state of the eco-environment system. $V_A$ and $V_B$ are the evolution speed of the two. The evolution speed of the composite system is the function of $V_A$ and $V_B$, so $V=f\left(V_A,V_B\right)$. Therefore, taking $V_A$ and $V_B$ as control variables, the DCCD is explored by analyzing the changes of $V$.

The evolution of the composite system conforms to the S-type development mechanism [49]. The dynamic coupling coordination relationship between urbanization and eco-environment changes periodically, and the change of $V$ is caused by $V_A$ and $V_B$. The evolution trajectories of $V_A$ and $V_{B }$ can be projected on a two-dimensional plane $\left(V_A,V_B\right)$ for analysis (Figure 2). The ellipse is the evolution trajectory of $V$, and the angle $\mathsf{\theta }$ between $V_A$ and $V_B$ can be expressed as Equation (5).

$$\mathsf{\theta }=\mathrm{arctg}\left(\frac{V_A}{V_B}\right)$$

(5)

$\mathsf{\theta }$ is the DCCD. According to the $\mathsf{\theta }$, we divide the evolution state and DCCD of the composite system into four types: low-level symbiosis; break-in development; utmost development; and high-level symbiosis.

(1)

Low-level symbiosis: $-90°<\mathsf{\theta }\le 0°$. The urbanization development is slow. The eco-environment does not restrict the urbanization. The urbanization influence on eco-environment is marginal.

(2)

Break-in development: $0°<\mathsf{\theta }\le 90°$. Along with the urbanization development rate increase, the stress on the eco-environment begins to appear. The eco-environment increasingly restricts the urbanization. The contradiction between the two has appeared, but it is not serious.

(3)

Utmost development: $90°<\mathsf{\theta }\le 180°$. Rapid urbanization has resulted in huge consumption of resources and eco-environment deterioration. The eco-environment obviously limits the urbanization. The contradiction between the two is prominent. This contradiction will be alleviated with the technical improvement.

(4)

High-level symbiosis: $-180°<\mathsf{\theta }\le -90°$. The relationship of urbanization and eco-environment gradually turns from mutual coercion to mutual promotion.

Overall, the DCCD model regards urbanization and eco-environment as a composite system. First, it fully considers the nonlinear change process of urbanization and eco-environment, and the stability of the nonlinear system movement process. It can highlight the complex development and interaction of them. Second, it fully considers that the evolution of urbanization and eco-environment are determined by their own and external influences. DCCD model results are more sensitive to the changes of various factors and their interactions. Third, according to the urbanization and eco-environment evolution speed, rather than their development status, it analyzes the composite system changes, and then studies the coupling coordination relationship of them. It can better reflect their dynamic relationship. Fourth, the composite system evolution meets the combined S-shaped development mechanism. The DCCD model assumes that the urbanization and eco-environment relationship presents periodic changes. The results can better reflect the phased and periodic characteristics of the DCCD. The DCCD model is a more advanced model that can consider the dynamic, stage, and periodic characteristics [51,52]. The development and application of the DCCD model bring the coupling relationship research to a new height.

2.4. Geographic Detector

Geographic detector is a tool used to detect spatial differentiation and reveal driving factors [53,54]. It is less restricted by data conditions, has no linear assumption, and has clear physical meaning. Therefore, it is widely used in many fields such as natural and social sciences. This paper analyzes the influence of urbanization and eco-environment subsystems on DCCD by using the geographic detector. The formula is as follows [54]:

$$\mathrm{q}=1-\frac{1}{n{\sigma }^2}{\displaystyle \sum _{i=1}^m{n}_i}{\sigma }_i^2$$

(6)

where $\mathrm{q}$ represents the influence of subsystems on DCCD, $\mathrm{q}\in [0,1]$, and the larger the value is, the greater the influence of the subsystem. $n$ is the sample number; ${\sigma }^2$ is the discrete variance of DCCD; and when ${\sigma }^2\ne 0$, the model holds.

3. Results

3.1. Analysis of Urbanization and Eco-Environment

From 2003 to 2019, the urbanization level of BTH showed a fluctuating upward trend, with significant regional differences. In 2003, the starting point of urbanization among cities was different (0.23–0.39). The higher the starting point, the smaller and more stable the urbanization growth. By 2019, there were only two cities with an urbanization level of developed type, seven cities with general type, and four cities with developing type. The development of the four subsystems was unbalanced. The leading factor of urbanization has changed to social urbanization; it started with population urbanization (Figure 3, Table 3).

Population urbanization and social urbanization development could be divided into two stages. Spatial urbanization development was different among cities. The economic urbanization development was consistent with the urbanization, and had little impact on the urbanization.

For population urbanization: The boundary of Shijiazhuang, Handan, Xingtai, Zhangjiakou, and Chengde was 2007, and the boundary of the other cities was 2010. In the first stage, population urbanization development occurred in advance, which drove the rapid development of urbanization. In the second stage, population urbanization gradually lagged, and the restriction effect on urbanization emerged.

For social urbanization: The boundary of Langfang was 2015, while the boundary of the other cities was 2012. In the first stage, social urbanization was seriously backward. It was the main factor hindering the high-quality urbanization development. However, the social urbanization had received much attention. Its growth rate has gradually accelerated. In the second stage, social urbanization level continued to increase, becoming the most important factor to promote the urbanization.

The spatial urbanization in Tianjin, Tangshan, Chengde, and Cangzhou was coordinated with urbanization. The spatial urbanization in other cities had stage characteristics, which could be divided into two stages. The boundary of Beijing and Hengshui was 2009, Handan and Xingtai was 2011, Langfang was 2016, and Shijiazhuang, Qinhuangdao, Baoding, and Zhangjiakou was 2014. In the first stage, spatial urbanization was developing in advance; it was the primary factor driving the urbanization development. In the second stage, with China paying more and more attention to the conservation and intensive and efficient use of land, the disorderly expansion of urban land was seriously curbed. The spatial urbanization of BTH has declined significantly. By 2019, the spatial urbanization in Beijing, Qinhuangdao, Handan, Baoding, Zhangjiakou, and Hengshui lagged significantly. The spatial urbanization in other cities was highly coordinated with urbanization.

From 2003 to 2019, the eco-environment level of BTH showed a fluctuating upward trend, with a large increase and fast growth rate. Especially since 2012, its rise was particularly obvious. By 2019, its eco-environment was generally good. There were 11 cities with an eco-environment level of developed type and 1 city with general type and developing type, respectively (Figure 4, Table 3).

The resources subsystem in Beijing fluctuated around the eco-environment, and had no obvious influence on eco-environment. The development of resource subsystem in Tianjin mostly lagged behind the eco-environment. However, Tianjin’s ecological civilization construction level was relatively high, which could fully eliminate the negative impact of environment load and resources subsystem. After 2016, its coordination with environment load, ecological civilization construction, and eco-environment was significantly improved. The resources subsystem in other cities experienced a “low–high–low” development process. Firstly, the development of resources subsystem was lower than that of environment load, ecological civilization construction, and eco-environment. It was the primary factor limiting the improvement in eco-environment. Secondly, the development of resources subsystem was significantly higher than that of environment load, ecological civilization construction, and eco-environment, which drove the rapid rise of eco-environment. Then, the development of resources subsystem in Tangshan, Xingtai, Zhangjiakou, Chengde, Cangzhou, Langfang, and Hengshui was relatively coordinated with eco-environment and other subsystems. The development of resources subsystem in Shijiazhuang, Qinhuangdao, Handan, and Baoding lagged behind seriously, with an obvious downward trend.

The environment load in Beijing was mostly higher than resources, ecological civilization construction, and eco-environment. Especially after 2012, the environment load increased significantly and the ecological civilization construction was weak. Beijing has become the city with the worst eco-environment among BTH. The environment load in Tianjin was slightly lower than the eco-environment, and its negative impact was small. The environment load in other cities changed from backward to advanced. Before 2016, the environment load obviously lagged behind resources, ecological civilization construction, and eco-environment. Its negative impact on eco-environment could be basically eliminated. After 2016, the environment load grew rapidly and obviously advanced. The eco-environment had a certain degree of decline.

The ecological civilization construction in Tianjin, Shijiazhuang, Tangshan, Qinhuangdao, Handan, Xingtai, and Baoding was higher than eco-environment, which had a positive impact on the eco-environment improvement. Ecological civilization construction lagged behind the eco-environment, and needs to be strengthened in other cities.

Further analysis showed that the development of resources, environment load, and ecological civilization construction in Tianjin was relatively balanced. Its coordination with the eco-environment was relatively high. In other cities, it is necessary to curb the rise of environment load and strengthen the ecological civilization construction so as to drive the development of resources subsystem and improve the resilience and quality of eco-environment.

3.2. Analysis of DCCD

We used OriginPro 9.1 to solve the urbanization and eco-environment curve fitting. The fitting results are shown in

Table 4. Curve fitting of urbanization and eco-environment.

City	Urbanization	R2	Eco-Environment	R2
Beijing	U = −2 × 10−5x4 + 0.0008x3 − 0.01x2 + 0.0514x + 0.3447	0.52	E = 7 × 10−6x5 − 0.0003x4 + 0.0042x3 − 0.0224x2 + 0.0586x + 0.3041	0.84
Tianjin	U = −0.0007x2 + 0.0391x + 0.2028	0.93	E = 0.0004x3 − 0.0102x2 + 0.0794x + 0.2229	0.89
Shijiazhuang	U = 0.2554x0.2903	0.71	E = 0.0005x2 + 0.0165x + 0.3139	0.82
Tangshan	U = 0.2013x0.3868	0.9	E = −0.0018x2 + 0.0577x + 0.279	0.77
Qinhuangdao	U = 7 × 10−5x4 − 0.0027x3 + 0.0322x2 − 0.1242x + 0.4973	0.63	E = 0.0021x2 − 0.018x + 0.4211	0.72
Handan	U = 6 × 10−6x5 − 0.0002x4 + 0.003x3 − 0.017x2 + 0.0579x + 0.3038	0.82	E = 7× 10−6x4 − 0.0004x3 + 0.0064x2 + 0.0103x + 0.1723	0.96
Xingtai	U = 0.2935x0.2458	0.76	E = 4 × 10−5x4 − 0.001x3 + 0.0098x2 − 0.0271x + 0.3716	0.94
Baoding	U = −0.0014x2 + 0.0441x + 0.1967	0.78	E = 0.0005x2 + 0.0229x + 0.2668	0.89
Zhangjiakou	U = −0.0003x2 + 0.0202x + 0.2956	0.73	E = 0.0019x2 − 0.0025x + 0.348	0.93
Chengde	U = −0.0002x2 + 0.0284x + 0.2229	0.89	E = 0.0021x2 − 0.0162x + 0.4559	0.81
Cangzhou	U = 0.2538x0.3173	0.85	E = 0.0004x3 − 0.0087x2 + 0.06x + 0.3742	0.72
Langfang	U = 0.2834x0.2728	0.83	E = 0.0005x3 − 0.0122x2 + 0.1001x + 0.288	0.69
Hengshui	U = 2 × 10−5x4 − 0.0004x3 + 0.0009x2 + 0.0349x + 0.2517	0.75	E = 0.0017x2 − 0.0004x + 0.3852	0.81

. Combining Equations (3)–(5) obtained the DCCD of urbanization and eco-environment (Figure 5). The results showed that there was a dynamic coupling and coordination relationship between urbanization and eco-environment, and the DCCD showed an S-shaped curve. Meanwhile, the DCCD in BTH was significantly different among cities.

The DCCD of urbanization and eco-environment in BTH has three types: low-level symbiosis; break-in development; and utmost development. There was no high-level symbiosis. The occurrence frequency of low-level symbiosis was low, only Qinhuangdao (2012–2018) and Baoding (2018–2019) had this type. Break-in development was the main type of DCCD in BTH. The utmost development was found in Beijing (2014–2019), Tianjin (2009–2012), Tangshan (2018–2019), Qinhuangdao (2005–2006), Chengde (2003–2005), and Cangzhou (2008–2010). By 2019, only Beijing and Tangshan were the utmost development type. Tianjin, Chengde, and Cangzhou were break-in development type, and the DCCD decreased gradually. The DCCD in Qinhuangdao experienced a changing process of utmost development, break-in development, low-level symbiosis, and break-in development. It showed that the utmost development of DCCD in BTH was relatively low in sustainability.

When the DCCD was transitioning from break-in development to utmost development, the urbanization and eco-environment of Beijing, Tianjin, Tangshan, Qinhuangdao, Chengde, and Cangzhou were in small-speed reverse development. That is, relatively stable development speed and small-degree urbanization or eco-environment sacrifice were necessary to achieve utmost development. The DCCD of Handan, Zhangjiakou, Langfang, and Hengshui approached 90° in 2017–2019, 2003 and 2010. However, the transition from break-in development to utmost development was not achieved. The main reason was the high rate of urbanization or eco-environment change (Figure 3 and Figure 4). This further supported the above research conclusions.

For the rise and fall of DCCD in the break-in development stage, urbanization and eco-environment were in a state of simultaneous rise or decline. On this premise, the development of urbanization and eco-environment can be divided into two situations. (1) The development rate of urbanization was lower than that of eco-environment. With the gradual increase in the extent that the urbanization development rate was lower than that of eco-environment, the DCCD decreased and approached 0°. On the contrary, the DCCD increased and approached 45°. (2) The development rate of urbanization was higher than that of eco-environment. With the gradual increase in the extent that the urbanization development rate was higher than that of eco-environment, the DCCD increased and approached 90°. On the contrary, the DCCD decreased and approached 45°.

Theoretical analysis showed the following: (1) When urbanization and eco-environment are both in a rising state and the DCCD approaches 90°, the DCCD will realize the transformation from break-in development to utmost development if urbanization development maintains a steady rising trend and eco-environment quality declines slightly. (2) When urbanization and eco-environment are both in a declining state and the DCCD approaches 90°, the DCCD will realize the transformation from break-in development to utmost development if urbanization keeps a steady downward trend and eco-environment quality rises slightly. This paper called the first state “benign transition”, and called the second state “non-benign transition”. In fact, the coordinated development of DCCD in BTH was a “benign transition”. That is, when the utmost development of DCCD was realized, its urbanization development was in a steady upward trend, and the eco-environment quality was slightly declining.

The above analysis results show that it is not advisable to blindly emphasize urbanization or eco-environment development. To realize the “benign transition” of DCCD from break-in development to utmost development, a moderate and steady advance development of urbanization and a small degree of eco-environment quality sacrifice are necessary. Therefore, determining how to grasp the degree of advance development of urbanization and sacrifice of eco-environment quality, and realize and maintain the relatively coordinated and stable development of urbanization and eco-environment are the key to realize the utmost development of DCCD. Only when the utmost development is realized is it possible to achieve the high-level symbiosis of DCCD through technological improvement and realize the mutual promotion of urbanization and eco-environment.

3.3. Identification of Main Driving Factors

This paper detected the driving force of urbanization and eco-environment subsystems on DCCD by geographic detector (

Table 5. Detection results of main control factors of DCCD.

City	Index	Population Urbanization	Economic Urbanization	Social Urbanization	Spatial Urbanization	Resources	Environment Load	Ecological Civilization Construction
Beijing	q	0.68	0.76	0.82	0.86	0.79	0.35	0.73
	p	0.02	0.01	0.01	0.00	0.02	0.49	0.02
Tianjin	q	0.49	0.64	0.90	0.48	0.43	0.48	0.61
	p	0.50	0.09	0.00	0.15	0.25	0.10	0.05
Shijiazhuang	q	0.13	0.75	0.79	0.10	0.46	0.79	0.58
	p	0.97	0.04	0.01	0.91	0.28	0.02	0.15
Tangshan	q	0.08	0.61	0.87	0.50	0.75	0.11	0.92
	p	0.92	0.04	0.00	0.14	0.17	0.88	0.00
Qinhuangdao	q	0.24	0.18	0.73	0.92	0.10	0.66	0.40
	p	0.75	0.75	0.01	0.00	0.93	0.06	0.48
Handan	q	0.27	0.56	0.88	0.87	0.73	0.67	0.80
	p	0.46	0.14	0.00	0.00	0.05	0.04	0.01
Xingtai	q	0.22	0.44	0.85	0.33	0.82	0.18	0.65
	p	0.61	0.31	0.00	0.36	0.00	0.76	0.07
Baoding	q	0.09	0.51	0.93	0.78	0.74	0.67	0.51
	p	0.93	0.18	0.00	0.01	0.02	0.16	0.23
Zhangjiakou	q	0.50	0.64	0.87	0.86	0.64	0.80	0.59
	p	0.21	0.08	0.00	0.01	0.04	0.01	0.26
Chengde	q	0.25	0.48	0.96	0.55	0.57	0.69	0.27
	p	0.84	0.46	0.00	0.29	0.14	0.05	0.71
Cangzhou	q	0.40	0.74	0.87	0.71	0.84	0.77	0.29
	p	0.69	0.01	0.00	0.01	0.00	0.02	0.78
Langfang	q	0.39	0.56	0.89	0.70	0.48	0.64	0.28
	p	0.50	0.11	0.00	0.03	0.22	0.15	0.86
Hengshui	q	0.11	0.55	0.88	0.68	0.69	0.63	0.30
	p	0.93	0.18	0.00	0.01	0.08	0.21	0.88

). In the calculation results, the factor detector module automatically outputs independent variable values $\mathrm{p}$ and $\mathrm{q}$, which are data randomness probability and factor influence value, respectively. This paper holds that when $\mathrm{p}\le 0.01$ and $\mathrm{q}\ge 0.5$, the factor is the main control factor of the DCCD.

Social urbanization was the common factor that affected the DCCD in BTH. The influence value of social urbanization was particularly prominent, ranking second in Beijing, Tangshan, and Qinhuangdao, and first in the other ten cities. This showed that the changes of DCCD were closely related to the social urbanization. The urbanization development of BTH has entered the social urbanization leading stage.

The influence scope of spatial urbanization was relatively wide. Seven cities’ main control factors included the spatial urbanization, namely, Beijing, Qinhuangdao, Handan, Baoding, Zhangjiakou, Cangzhou, and Hengshui. Cangzhou’s spatial urbanization development was relatively moderate from 2003 to 2019. When the spatial urbanization was in advance or lagged behind, its level could be adjusted in time, and the spatial urbanization was consistent with the urbanization development. Therefore, spatial urbanization was one of the main controlling factors of DCCD in Cangzhou. Spatial urbanization was the main control factor of DCCD in the other six cities, mainly because their spatial urbanization changed from significantly advanced to seriously lagged. The spatial urbanization of these cities had seriously restricted the improvement in urbanization. Ensuring the economical and intensive use of space, appropriately improving the spatial urbanization level was conducive to the optimization of DCCD. The economic urbanization was the main control factor of DCCD in Beijing and Cangzhou. This was because the economic urbanization of these two cities maintained a state of advanced development from 2003 to 2019. It was one of the main driving factors for the urbanization development. The DCCD of Beijing and Cangzhou was sensitive to the changes of economic urbanization. The influence of population urbanization on DCCD in BTH was not significant. This is mainly because BTH is an important part of China’s economic core region, its population urbanization has a high coordination with urbanization, and there is no long-term or serious advance or lag development.

Moreover, 61.54% of cities had no eco-environment subsystems as the main control factor. This indicated that urbanization played a dominant role in the coupling and coordination relationship between urbanization and eco-environment of BTH. Meanwhile, five cities in BTH had the eco-environment subsystem as the main control factor, and its influence value was relatively high $\left(\mathrm{q}>0.8\right)$. The ecological civilization construction was the main control factor of DCCD in Tangshan and Handan. The urbanization of Tangshan showed a continuous upward trend, and the eco-environment occasionally deteriorated under the influence of urbanization (Figure 3 and Figure 4). The good development of the ecological civilization construction had fully eliminated the negative influence of the environment load and enhanced the ability of Tangshan to quickly restore the eco-environment. The ecological civilization construction provided a guarantee for the continuous rise of DCCD. The urbanization of Handan developed slowly (Figure 3), and its DCCD changes were more dependent on the eco-environment. Handan’s environment load continued to rise, and the resources subsystem decreased significantly. Therefore, the ecological civilization construction level determined whether the eco-environment deteriorated or not, and had a great influence on the change trend of DCCD (Figure 4 and Figure 5). The resources subsystem was the main control factor of DCCD in Xingtai and Cangzhou. The reasons were as follows: Xingtai’s urbanization development showed a large fluctuation and rising trend, and Cangzhou’s urbanization development showed a rapid and continuous rising trend (Figure 3). These two cities’ urbanization developments brought great pressure to the eco-environment. The ecological civilization construction fluctuated greatly. The elimination of the negative impact of urbanization on eco-environment was more dependent on the resources subsystem. The environment load was the main control factor of the DCCD in Zhangjiakou. This was because its ecological civilization construction was relatively backward, the resources subsystem could not fully eliminate the negative impact of the environment load, and environment load was sensitive to the changes of urbanization. To realize the regulation and control of the DCCD, these five cities should pay attention to the changes of urbanization and eco-environment simultaneously.

Comprehensive analysis showed that the identification results of the main control factors could accurately reflect the driving force of subsystems on urbanization, eco-environment, and DCCD. When the imbalance between the development of urbanization and eco-environment appeared, the situation of each subsystem could be adjusted according to the identification results of the main controlling factors, to realize the optimization and adjustment of urbanization, eco-environment, and DCCD.

4. Discussion

The continuous and rapid development of global urbanization brought great pressure to the eco-environment. It is important to have a deep understanding of the relationship between urbanization and eco-environment, especially in mega urban agglomerations where the most representative and studied cases are extremely scarce. This will determine the process of urbanization in the world, affect the international competitiveness of countries, and hold the key to the realization of the United Nations 2030 Agenda for Sustainable Development. It is also an important part of the global countries achieving green, low-carbon, and sustainable development. With the rapid development of global urbanization, the contradiction between urbanization and eco-environment in various countries is becoming more prominent, and their interaction is becoming more complex. The SCCD model commonly used in previous studies and the research method that only analyzes the relationship between urbanization and eco-environment system cannot meet the profound needs of the research on the relationship between urbanization and eco-environment. Different from the previous studies that only analyzed urbanization and eco-environment changes, this paper deeply analyzed urbanization, eco-environment, and their subsystems changes, as well as the relationship between them. The DCCD model was used instead of the SCCD model to analyze the relationship between urbanization and eco-environment. Meanwhile, China’s urbanization is developing rapidly and is in an accelerated stage. BTH is one of the three major urban agglomerations and a representative of the rapid development of urbanization in China. It is also a region facing more serious resource and eco-environment stress in China’s mega urban agglomerations. The BTH in China was selected as the research area, and the research results are more representative and meaningful for reference. This paper can make up for the deficiencies of the existing research and provides a new research idea and methodological reference for studies on the relationship between urbanization and eco-environment.

However, in the research of urbanization and eco-environment coupling coordination relationship, the application of DCCD model was few, and the correlation analysis was not deep. The conclusions of this paper need more studies that are theoretical and cases from different countries and regions to support them. When the DCCD model is used to analyze the relationship between urbanization and eco-environment, we should take some measures: (1) Focus on strengthening the relevant theoretical research. (2) Expand the scope of the study area to clarify the similarities and differences of the DCCD changes in different development levels and different types of countries and cities. The practical implication of this study includes the following two aspect: theoretical and empirical aspects.

4.1. Practical Implication to Theoretical Research

This paper explained the progressiveness of the DCCD model from the construction concept and theoretically analyzed the rising and falling laws of DCCD in break-in development stage for the first time. This was confirmed in this study. Moreover, the changes of DCCD in break-in development stage of existing studies also conformed to this law [49,52,55]. We propose the concept of “benign transition” and “non-benign transition” when the DCCD changes from break-in development to utmost development for the first time. Meanwhile, it was found that moderate and steady advance development of urbanization and small degree eco-environment sacrifice are the key to achieve the “benign transition”. The theoretical analysis of this paper and the empirical analysis of existing research both show that the realization of “benign transition” is the premise of achieving the high-level symbiosis of DCCD through technological improvement, and is the key to realizing the mutual promotion between urbanization and eco-environment. Determining how to grasp the degree of urbanization advance development and eco-environment quality sacrifice, timely adjust their priority, and balance their relationship are important issues for sustainable development.

This finding is of great significance: (1) The changes of DCCD between urbanization and eco-environment may be predicted and regulated. (2) By regulating urbanization and eco-environment development, the key point of “benign transition” from break-in development to utmost development can be found. (3) When the DCCD passed the key point of “benign transition”, the technological improvement will become an important means of sustainable development, and the mutual promotion of urbanization and eco-environment is expected to be realized.

4.2. Practical Implication to Empirical Research

BTH’s urbanization development had entered the leading stage of social urbanization. Its social urbanization development was advanced. Urbanization development needs to take the social urbanization improvement as the breakthrough point in the short term to promote the development of other subsystems, balance the relationship among subsystems, reduce regional differences, and realize the improvement in urbanization level. In addition, Beijing’s urbanization level has changed from ranking second at the beginning of this study to ranking lowest at the end of this study. The unbalanced urbanization development between Beijing, Tianjin, and Hebei restricted the outward relaxation of Beijing’s noncapital functions. The improvement in urban agglomeration urbanization level is a long-term and complex systematic project, which must take urban agglomeration as the main form, complement the weak points, and promote coordination. It is necessary to draw on the development experience of cities with a high level of urbanization, such as Langfang (0.71), Cangzhou (0.70), and Tangshan (0.68), to promote high-quality development within each city. Efforts should be made to enhance industrial complementary and industrial chain extension between cities, seize the major strategic opportunities brought about by the new scientific and technological revolution for industrial transformations, solve the problem that the industrial structure of some cities is too heavy on industry, and promote the overall urbanization quality of urban agglomeration.

The DCCD of Beijing and Tangshan was of utmost development, and its development trend was decreasing and increasing, respectively. Beijing needs to curb the decline of DCCD through the diversion of noncapital functions. Tangshan should improve the utilization efficiency of resources through scientific and technological progress and maintain the rising trend of DCCD. The other 11 cities were in the critical period of DCCD’s transformation from break-in development to utmost development. DCCD was gradually rising in Qinhuangdao, Handan, and Hengshui, which was expected to achieve the “benign transition” by maintaining the steady development of urbanization and slightly sacrificing the eco-environment. In the other cities, the DCCD gradually decreased, and there was a risk of regression from break-in development to low-level symbiosis. Combined with the urbanization and eco-environment analysis, the eco-environment of these cities provided space for urbanization development. These cities should moderately increase the speed of urbanization and pay attention to the eco-environment carrying capacity improvement, maximize the protection of urbanization development, and turn the development of DCCD from declining to increasing.

The DCCD of Beijing, Tianjin, Tangshan, Qinhuangdao, Chengde, and Cangzhou was the utmost development type in 2014–2019, 2009–2012, 2018–2019, 2005–2006, 2003–2005, and 2008–2010, respectively. However, the transition from utmost development to high-level symbiosis has not been realized. The key to the transformation from utmost development to high-level symbiosis still lies in the promotion of scientific and technological level. In the long run, technical and policy support for the sustainable development of BTH is extremely important [56].

To achieve and maintain the utmost development type of DCCD faster and better, even the high-level symbiosis, it is necessary to conduct a comprehensive and accurate analysis of urbanization, eco-environment, their subsystems, and DCCD. On the premise of fully understanding the development status and changing trend, the relationship between urbanization and eco-environment can be optimized through the adjustment of subsystems. Simple spatial–temporal changes or single-angle analysis cannot guide the optimization of urbanization, eco-environment, and DCCD. For example, according to the spatial–temporal changes of urbanization and eco-environment, BTH had a good eco-environment quality, which can rapidly promote the urbanization development. However, the analysis of urbanization and eco-environment subsystems found that from 2003 to 2019, the ecological civilization construction level of Beijing, Zhangjiakou, Chengde, Cangzhou, Langfang, and Hengshui lagged behind the eco-environment. It needed to be strengthened. At the end of the study period, the development of resources subsystem in Beijing, Shijiazhuang, Qinhuangdao, Handan, and Baoding obviously lagged behind, and had a downward trend. Meanwhile, except for Tianjin, the environment load in BTH grew rapidly and was obviously advanced. In other words, all the cities except Tianjin had potential eco-environment risks. All these indicated that the further development of urbanization could not be separated from the continuous attention to eco-environment. It is necessary to analyze urbanization, eco-environment, and their relationship from the subsystems level.

5. Conclusions

This paper took the mega urban agglomeration with a few cases as the research area, and used the more advanced DCCD model in this research field to explore the relationship between urbanization and eco-environment. We verified the scientificity and advancement of the DCCD model in the study of the relationship between urbanization and eco-environment from the two dimensions of construction concept and empirical case. New concepts were proposed. Meanwhile, we further promoted the application of the DCCD model in the research field of the relationship between urbanization and eco-environment. We analyzed the urbanization, eco-environment, and DCCD from the subsystems level, providing a deeper analysis than before. We found the key to realizing the mutually promoting relationship between urbanization and eco-environment. We provide a more scientific and accurate research idea for the related research on the relationship between urbanization and eco-environment. This paper makes up for the deficiency of the existing research from the aspects of empirical research and theoretical research. Some important conclusions are drawn:

(1)

From 2003 to 2019, the urbanization level of BTH showed a fluctuating upward trend, with significant regional differences. The urbanization starting point among cities was different (0.23–0.39). The higher the starting point, the smaller and more stable the urbanization growth. The development of population, economic, social, and spatial urbanization subsystems was unbalanced. The leading factor of urbanization had changed from population to social urbanization.

(2)

From 2003 to 2019, the eco-environment level of BTH showed a fluctuating upward trend, with a large increase and fast growth rate. Its rise was particularly obvious, especially since the Chinese government in 2012 raised the construction of ecological civilization to an unprecedented strategic height. By 2019, the eco-environment of BTH was generally good. Tianjin’s resources, environment load, and ecological civilization construction development was balanced. Its subsystems’ coordination with the eco-environment were relatively high. All other cities need to curb the rise of environment load and strengthen their ecological civilization construction. Meanwhile, the impact of ecological civilization construction and environment load on resources and eco-environment had a relative lag.

(3)

There was a dynamic coupling and coordination relationship between urbanization and eco-environment in BTH. The DCCD showed an S-shaped curve. Break-in development was the main type of the DCCD in BTH. BTH’s urbanization development was in an accelerating stage, and the contradiction between urbanization and eco-environment was becoming increasingly serious. Moderate and steady advance development of urbanization and a small-degree eco-environment sacrifice are the key to achieve the “benign transition” of DCCD from break-in development to utmost development. Only when the utmost development is realized is it possible to achieve the high-level symbiosis through the technological improvement and realize the mutual promotion between urbanization and eco-environment.

(4)

Social urbanization was the common factor that affected the DCCD between urbanization and eco-environment in BTH, and its influence value was particularly prominent. Meanwhile, the urbanization, eco-environment, and DCCD could be regulated by monitoring and adjusting the subsystems.