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

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.