The number of people who live in a city, as we may have mentioned before, is surprisingly hard to work out. That's partly because populations shift and change, unnoticed by demographers.
Mostly, though, it's because you quickly run into the fact that different authorities define their cities in entirely incompatible ways, rendering any attempt to compare them a bit like comparing apples with trucks. What you really need is a single source that follows a single set of rules: the results may not be perfect, but at least they'll be consistent.
That’s where Demographia comes in. The St. Louis-based consultancy was founded by the urban planner Wendell Cox (who, unusually for someone in that line of work, likes to write articles for conservative publications about the benefits of private car ownership). It also has a website that is, with the best will in the world, not massively easy on the eye.
But every year Demographia publishes something rather good: the World Urban Areas Report, a sort of bumper book of city population stats. You can check out the 11th edition here.
Here’s how it defines a city:
An urban area ("built-up urban area," urbanized area or urban agglomeration) is a continuously built up land mass of urban development that is within a labor market (metropolitan area or metropolitan region). An urban area contains no rural land... [It] is best thought of as the “urban footprint” – the lighted area that can be observed from an airplane (or satellite) on a clear night.
This year's edition of the World Urban Areas Report covers more than a thousand of the largest cities in the world, from Tokyo (pop. 37,000,000), right down to tiddlers like 850th-ranked Leicester (Pop. 534,000): all 1,009 cities, in fact, with a population of half a million of more.
Here are the top 10. See if you can spot any pattern.
Seeing it yet? Need a clue?
Okay, here's the same chart again. This time, we’ve coloured the cities by continent.
Asia's dominance continues as you move down the league tables, too. Here's the top 20:
The world's largest continent, in fact, is home to 33 of the world's 50 largest cities, and 50 of the top 100. After that we got bored and stopped counting.
This isn't quite as extreme a result as it may appear: Asia, after all, is home to just shy of 60 per cent of the world's population. So the way in which the 2.1bn people who live in cities of half a million souls or more are distributed around the globe is largely a function of where people actually live. Look:
Comparing those two graphs, you can see that the Americas and Australia are unusually urbanised, Africa unusually under-urbanised, and Europe and Asia are pretty close to what you'd expect.
There are two other striking things about these league tables.
The first is the dominance of China, which takes 12 places in the top 50, and 22 of the top 100.
Again, this shouldn't be surprising – it's the world's most populous country, home to nearly a fifth of humanity – but nonetheless, it's striking how many of these megacities you almost certainly haven't heard of.
Where power lies
The other striking thing here is how far down the league tables many of the world's most powerful cities fall. Here's where some of the places that commonly make the “world city” league tables fall:
Size, it seems, isn't everything.
Here, for completism's sake, is a chart showing the figures for all the city population charts above. You can find the full data set on Demographia's website, here.