The ecological boundary gap is gradually tightening in China's megacities: Taking Beijing as a case

Planetary boundaries define the safe operating space of human beings relative to the Earth's system, which is of great significance in helping human beings predict environmental safety limits. However, ecological boundaries have not been presented or downscaled to regional levels. Moreover, a method has not been developed to quantitatively measure the gap between the value of the services provided by the ecosystem and the size of the urban population and economy that the ecosystem can carry. Herein, we propose the concept and calculation model of the “ecological boundary” to quantitatively assess the ecosystem security gap size transgressed by humans. Ecological boundaries are defined as the maximum population and economic scale that a certain area can support under a certain ecologically productive area. The purpose of this paper is to investigate the ecological boundary in megacities, and Beijing is considered as a specific case. The results show that the consumption of natural resources has transgressed its ecological boundary in Beijing. The direct consequence is that the ecological well-being per capita continues to decrease. Fortunately, with decreases in the ecological footprint and land use/land cover change dynamic degree, the ecological boundary gap is gradually tightening. Moreover, the role of ecological boundaries as early warning signals of undesirable urban ecological environmental changes is discussed, the significance of the monetization of ecological boundaries is described, the factors underlying the ecological boundary gap in the process of accelerating urbanization in China are analyzed, and policy recommendations for resolving the threat to ecological security boundaries by megacities are presented. The primary purpose of our study is provide policymakers with information on the gap between the current well-being of humankind and critical capacity thresholds, which can help determine whether human activities have fallen into an unsustainable state that may result in undesirable eco-environmental changes that could have detrimental or even disastrous consequences for the population of a region.