Following my recent post about the largest cities in the world (from 100 to 2015 CE), a number of you rightly pointed out that the data looked questionable. Where, for example, is Shanghai in this latest list of largest cities? So I think it's important that I do a follow-up post.

There are a number of nuances to consider when trying to measure urban populations. Perhaps the two most obvious are the geographic extent of each city (i.e. what urban boundary do you use) and the number of people living in informal settlements.

The UN recently estimated that there are some 1 billion people living in slums or informal settlements. That represents nearly a quarter of the world's urban population, which is a staggering number and a pressing global need. We desperately need more housing.

When it comes to measuring the size of an urban agglomeration, most of the studies that I have seen tend not to focus on municipal boundaries ("city propers") or metropolitan areas. The former is often based on arbitrary political boundaries and the latter often contains undeveloped rural land.

So for the purposes of this post, I'm going to go with Demographia's definition of "built-up urban area." They define this as being a continuously built-up area with one labor market and with no rural land. In their view, the world is either urban/built-up or rural. The built-up part is the lighted area that you would see on a nighttime satellite photo.

Given this definition, there are a number of interesting fringe cases. For example, contiguous/adjacent urban areas with more than one labor market get split up into multiple ones. This is the case in the US with the northeastern "megalopolis" that runs from Boston to Washington.

Conversely, if adjacent urban areas share a labor market and are linked together by similar commuting flows, then they get grouped into one urban area. This might be the case even if the area(s) straddle a national border. In this particular case, the free movement of people and goods would be another prerequisite.

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.

Following my recent post about the largest cities in the world (from 100 to 2015 CE), a number of you rightly pointed out that the data looked questionable. Where, for example, is Shanghai in this latest list of largest cities? So I think it's important that I do a follow-up post.

There are a number of nuances to consider when trying to measure urban populations. Perhaps the two most obvious are the geographic extent of each city (i.e. what urban boundary do you use) and the number of people living in informal settlements.

The UN recently estimated that there are some 1 billion people living in slums or informal settlements. That represents nearly a quarter of the world's urban population, which is a staggering number and a pressing global need. We desperately need more housing.

When it comes to measuring the size of an urban agglomeration, most of the studies that I have seen tend not to focus on municipal boundaries ("city propers") or metropolitan areas. The former is often based on arbitrary political boundaries and the latter often contains undeveloped rural land.

So for the purposes of this post, I'm going to go with Demographia's definition of "built-up urban area." They define this as being a continuously built-up area with one labor market and with no rural land. In their view, the world is either urban/built-up or rural. The built-up part is the lighted area that you would see on a nighttime satellite photo.

Given this definition, there are a number of interesting fringe cases. For example, contiguous/adjacent urban areas with more than one labor market get split up into multiple ones. This is the case in the US with the northeastern "megalopolis" that runs from Boston to Washington.

Conversely, if adjacent urban areas share a labor market and are linked together by similar commuting flows, then they get grouped into one urban area. This might be the case even if the area(s) straddle a national border. In this particular case, the free movement of people and goods would be another prerequisite.

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.