Urban land expansion in China's six megacities from 1978 to 2015

Megacities pose both challenges and opportunities for the transition towards sustainability, and understanding the evolution of urbanization in megacities has profound implications for human societies in an increasingly urbanized world. Here, we mapped and quantified spatiotemporal dynamics of urban expansion in China's six megacities (i.e., Beijing, Chongqing, Guangzhou, Shanghai, Shenzhen and Tianjin) from 1978 to 2015, integrating remote sensing and GIS technology combined with landscape metrics and urban growth type analysis. The results show that six Chinese megacities have all undergone extensive physical expansion over the past four decades, and the magnitude of urban expansion is ranked in the order of Shenzhen, Guangzhou, Chongqing, Shanghai, Tianjin and Beijing, with annual growth rates of 11.02%, 8.07%, 5.80%, 5.37%, 4.56% and 3.46%, respectively. The megacities with smaller initial urban areas were associated with higher urban expansion rates. Differences in the direction, extent and location of expansion for each megacity related largely to the topography, policies and urban master planning. Temporal dynamics of urban growth and landscape metrics suggested that the urbanization processes of Beijing, Shanghai, Shenzhen and Tianjin were basically consistent with urban growth theory, while those of Chongqing and Guangzhou did not match the theory well. Temporal coevolution of the urban area with urban population implied efficiency of urban land use in Shenzhen and Beijing, which are the first special economic zone and the capital of China, respectively. The efficiency of wealth creation in the urbanized area base was observed for all Chinese megacities, signifying the effectiveness of urban expansion as a vehicle to promote economic growth. We face the challenge of managing trade-offs between the benefits and costs of urban agglomeration.