The Guardian recently published this reminder that the real population growth in the world today is happening in Asia and Africa. The article is called, “The 100 million city: is 21st century urbanisation out of control?” Much of the data is from this 2016 paper by Daniel Hoornweg and Kevin Pope, which projected the populations of the world’s biggest cities by 2100.

The standout example is that of Lagos, Nigeria, which went from under 200,000 people in the 1960s to an estimated 20 million people today. Though, I would imagine that the ubiquity of informal settlements makes it difficult to come up with an accurate number.

Still, it is one of the world’s top 10 largest cities and, by 2100, it may be the largest city in the world. The Guardian described the population as young, fertile, and increasingly urban. The median age in Nigeria is 18 and the fertility rate for the content is 4.4 births per woman.

I am mentioning all of this today because I think it grants some perspective. This is an immense city building challenge, not only because of the unprecedented growth rate, but also because it remains largely poor. Lagos, a city, may add more than 2.2x the population of Canada, a country, during the balance of this century.

One of the reasons I’m so fascinated by cities is that it’s becoming increasingly more important to get them right. From about 1831 to 1925, London was the largest city in the world. Its population went from somewhere around 1.5 to 2 million people to nearly 7.5 million. London surpassed Beijing as the largest city and was then surpassed by New York.

Today our largest cities are significantly bigger. Tokyo has almost 40 million people and London doesn’t even make the top 10. But there’s also a broader shift taking place. According to a new report by the United Nations, most of the world’s largest cities will be in Africa and Asia by 2030. Here’s a chart from Quartz:

And the reason for this shift is because Asia and Africa are newly urbanizing, whereas the rest of the world has already urbanized. In North America, over 80% of people already live in cities.

But even though Asia and Africa are following a trend that has already taken place in the rest of the world, it doesn’t mean we should assume we know what we’re doing. Having spent time in cities like Dhaka, I can attest to the many challenges that these mega-cities are facing and will continue to face as people flood in from the rural areas looking for economic opportunities.

So while it’s important that we talk about strategies for reviving cities like Detroit – which has a population somewhere around 700,000 - 800,000 people – we should also keep in mind that we have some significant challenges ahead of us in terms of creating a sustainable urban planning agenda for the world.

More and more people are moving to cities. This much we know. But in our fast paced world, census data is becoming increasingly limited in its ability to tell us exactly where and how people are moving.

Thankfully we now have Facebook. 

I just discovered a fascinating study conducted by the Facebook Data Science Team called “Coordinated Migration." What they did was anonymously analyze every Facebook user that has inputted both a hometown and a current city to see where people of a particular hometown are most likely to live, today.

But more specifically, the study is looking at coordinated flows, rather than just total flows to a particular city. A “coordinated migration” is defined as an instance where “a significant proportion of the population of a city has migrated, as a group, to a different city.”

What the study found (perhaps not surprisingly) was that the vast majority of coordinated migrations are happening in countries that are in the midst of rapid urbanization. It’s a case of people moving within the country to its largest city.

Here are the top coordinated migration destinations:

If you’re interested in the nitty gritty of how they actually computed the coordinated migrations, check out the original post by the Facebook Data Science Team.