Optimal scale of China's cities under the maximization of economic benefits and environmental benefits

The cities are the consumption concentration of energy, resources, and the concentration of CO₂ emissions. The cities' area only accounted for 2% in the world's surface; however, their population accounted for about 50% of the total population, and CO₂ emissions accounted for about 80% of the total emissions. The cities lie in a key position in solving the global climate change. China's urbanization level was just exceeding by 50%, which was in the intermediate stage of urbanization. The rapid development of the urbanization process and the expansion of city scale have brought economic growth and all kinds of environmental issues. Therefore, is there an optimal city scale which can make cities maintain economic growth and can also reduce or even avoid the environmental problems in the meantime? The question deserves deep research. Based on the background, the data from 1998 to 2014, and the goals of the cities' economic and environmental benefits, this paper builds the optimal scale model for the cities, and obtain two conclusions: (1) in a certain period and range, the cities have the optimal scale; (2) for the cities in China, the optimal scale is about 1.78 million people.