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.

With these definitions out of the way, below is another stab at sharing an accurate list of the world's largest megacities or built-up urban areas. This is one is by Demographia and there are a number of key changes compared to the last one I shared. Shanghai now features in the top 10. But Lagos drops down to number 20, which remains a bit of a question mark for me.

For a copy of Demographia's full report, click here. It looks at all urban areas with a population greater than 500,000 people (total is 985). Of course, if any of you have any other data sources that you think I should take a look at, feel free to share them in the comment section below.

This UN report (2018) on urbanization trends is a fascinating way to understand how our world is growing and changing. So today's post is about some of my takeaways. If you have others, feel free to add them to the comment section below.

But first, some definitions.

The UN report considers 3 ways to measure the size of a city, all of which we have used before on this blog. The first is the "city proper." That is the current administrative boundary of a city. The second is the "urban agglomeration" area, which is a city's contiguous built-up area. And the third is the "metropolitan area," which is the approximate area of economic and social interconnectedness.

Above is what these 3 boundaries might look like for Toronto (which is the example they use in their report). About the only one that isn't debatable is the "city proper" boundary; but it really doesn't capture the full extent of a city. Wherever possible, the UN report relies on the city's urban agglomeration area. They also define a "megacity" as a city of over 10 million people.

The largest city in the world is currently Tokyo. However, from 2018 to 2030 it is expected to decline by almost 900,000 people. Whereas, the city in 2nd position -- Delhi -- is expected to add more than 10 million inhabitants during this same time period. By 2030, these are expected to be the largest cities in the world:

Most current megacities are located in what the UN refers to as the "Global South." And 9 out of the 10 cities projected to become megacities by 2030 are located in developing countries. The one exception is London. Though all regions in the world are becoming more urban, the real population growth is happening in Asia and Africa.

Most cities -- 59% of cities with 500,000 or more people -- are at risk of at least one natural disaster. And 3 megacities -- namely Manila, Osaka, and Tokyo -- are high risk for 3 or more types of natural disaster.

Going through the report's data charts, it's also interesting to note that Toronto is not projected to become a megacity by 2030. However, the Toronto area already represents over 20% of Canada's entire urban population.

In the United States, Chicago's urban agglomeration is projected to continuing growing and does come close to megacity status by 2030. The Miami region is similarly expected to grow and is actually right on top of Toronto in terms of population. But the fastest growing regions are, of course, expected to be the city's that can more easily sprawl (Las Vegas, Phoenix, and so on).

Bogotá, Colombia is already a megacity and is expected to add almost 2 million people by 2030. It currently represents about 26.5% of the country's entire urban population. São Paulo remains one of the top 10 largest cities in the world and is similarly projected to add over 2 million people in the same time period, but to a much larger base.

In Europe, it's London, Paris, and Moscow, with the latter two already in possession of megacity status.

Now quantity isn't everything. Despite not ranking in the top 10 in terms of population, both New York and London are widely considered to be the world's preeminent global cities. At the same time, we do know that the size of a city does create certain socioeconomic benefits. Urban agglomerations create agglomeration economies.

If you'd like to download a copy of the World's Cities in 2018 (United Nations), click here.

Charts/Maps: United Nations

Last week the Government of Canada filed a 2,100-page submission with the United Nation's Commission on the Limits of the Continental Shelf. Under UN Convention on the Law of the Sea (UNCOLS), states with coastal territory have the exclusive rights to about 370 kilometers beyond their shores in order to conduct economic activity. This includes the exclusive rights to any resources. However, states may also make claims to further extensions underneath the water if they can substantiate them through scientific research. Last week's submission attempts to do exactly that for an additional 1.2 million square kilometers of sea bed.

Here is a map from High North News:

The challenge with all of this is that Norway, Denmark, and Russia all have their own continental shelf claims, and there's geographic overlap. (The US has not yet ratified their UNCOLS agreement.) So it is unlikely for this to be resolved anytime soon, though all states seem willing to work with the UN. This is a relatively new debate because the North Pole and Arctic Ocean were previously considered neutral territory. But climate change is opening up new economic opportunities (i.e. there's a lot less ice). That's worrisome in its own right.

Click here for the full press release from the Government of Canada.