Predicted increase in population by 2025: 3.44 million
China's largest and most densely populated city is fortuitously situated on the East China Sea, with direct access to the ocean. As such, major manufacturing operations keep Shanghai operating as the busiest port in the world. But unlike several of the cities on this list, Shanghai has been able to combat many of the problems that come with industrialization as the government has relocated many factories to the city's outskirts. Shanghai is a well-connected, global city, but that has come at a cost. China's one-child rule has kept the birth rate low, but an influx of migrants, who give up farming for better opportunities in the city, continue to bump the population upward. And in coming years, Shanghai will face a previously unfathomable problem: an aging population. More than 22% of the population is currently over 60, which has led the government to launch a campaign to attract younger citizens as the city could face an economic crisis with too few workers to keep the city afloat. Another cause for concern is that dirty fuels are still the norm in Shanghai, and with millions needing to heat their homes and power an ever growing automobile sector, pollution is on the rise.