The Global Urbanist

Last month the UN Department of Economic and Social Affairs released a new revision of its World Urbanization Prospects, updated to 2009.

Measuring the size of cities is an overwhelmingly complex task considering that the peripheries of cities take diverse forms in different regions, and that urban areas can spill across administrative and statistical boundaries.

The UN tries to take the most organic definition of an urban area, by measuring the urban agglomeration — the contiguous built-up area of a city isolated from other cities and towns by agricultural or natural interstices.

The Global Urbanist defines a city in the same manner, although unlike the UN, we try to keep together urban areas that straddle international borders such as Juárez-El Paso, since the management of such areas are increasingly intertwined.

Twenty-one urban agglomerations now contain greater than 10 million people, and four — Tokyo, Delhi, São Paulo and Mumbai — are greater than 20 million people. They will be joined by Mexico City, New York, Shanghai, Kolkata and Dhaka in the next fifteen years, with Karachi soon to follow.

These meta-cities (as UN-HABITAT now likes to call them) are in a different league to the rest of the urban world, while Tokyo, at nearly 37 million people, is simply in a world of its own.

Below this, if one graphs the largest urban agglomerations, there are currently subtle steps down in population separating Paris (21st) and Seoul (22nd), Bogotá (31st) and Tianjin (32nd), and Hyderabad (40th) and Ho Chi Minh City (41st). These conveniently separate our list of the world's largest agglomerations for 2010 into the enormous and the merely huge.