Convergence, that is the concentration of human populations into small areas, has been common throughout history. However, convergence has now reached historically unprecedented levels. Historically, convergence has tended to increase the long-term risk to humanity caused by many factors, including the associated environmental damage. Large cities appear to be at greater risk of unforeseen disasters, and may even collapse when some maximum sustainable size is exceeded. Based on a review of published papers, we discuss the historical and future safety of China’s major cities and civilization in the context of the increasing environmental risks created by convergence. This may help the public, scientists, and politicians to better understand the gap between social institutions and the scientific knowledge and technology that sustains them. It may also help to improve the sustainability of large cities and the civilizations that depend on them. Widespread unsustainable development currently jeopardizes large cities, especially when their managers fail to take appropriate measures to protect the environment that sustains the city. To achieve safe and sustainable urban development, China’s government must find ways to improve social and cultural institutions that promote environmental conservation, and it must encourage socioeconomic development in under-developed rural regions to reduce some of the pressures leading to urbanization.