In 1950, there were two cities with populations of more than 10 million in the world. According to estimates, by 2030, there will be 53.

These megacities are also merging into megaregions. The Pearl River Delta (Hong Kong-Shenzhen-Guangzhou region) in China has a total population of 120 million people. And cities such as Tokyo, Istanbul, Sao Paulo, Delhi and New York City are the anchors for megaregions with populations in the tens of millions.

This growth brings challenges like urban sprawl. Outlying areas are cheaper, so the people who live there tend to be on lower incomes. But limited transport can make it hard for them to access jobs in the city centre – adding to income inequality. More people also mean more vehicles and pollution, which makes cities less liveable.