( CNN ) -- Just over 9 percent of the world 's urban population lives in a megacity today . By 2025 , the number of megacities with a population greater than 10 million will grow from 21 to 29 and account for 10.3 percent of the world 's urban population , according to the United Nations . Here we highlight some of the world 's megacities , such as the most populous : Tokyo in Japan , which , with 35.2 million inhabitants , has a larger population than Algeria , Canada or Uganda ; and the rapidly expanding Shanghai , China 's economic powerhouse , which is the world 's biggest metropolitan area , covering 2,914 square kilometers . Megacities can bring not just wealth and employment but also cultural benefits , says the United Nations . And some megacities are more efficient than their less populous counterparts . While New York houses 2.7 percent of the U.S. population , New Yorkers only account for one percent of the country 's CO2 emissions . However , the high density of people can generate problems , including health issues such as the rapid transmission of 'flu viruses and psychological problems caused by poverty and stress . But whatever we think of them , megacities are not only here to stay : They 're growing . New Delhi , for example , is projected to have 29.49 million inhabitants by 2025 , according to Demographia -- up from 20.99 million in 2010 .
By 2025 10.3 percent of the world 's urban population will live in megacities says the UN . The most populous megacity is Tokyo in Japan , with 35.2 million inhabitants . Comment: