We have spoken over the years -- here, here, and here -- about the centralizing and decentralizing forces that play out within our cities. Agglomeration economies, for example, are a centralizing force. There are real economic benefits to people and firms clustering together in cities.

However, there are also many decentralizing forces. Traffic congestion is one. And of course, the pandemic also proved to be a powerful one for many cities.

But the fact that we even have cities in the first place should tell you that the centralizing forces do tend to win out over the decentralizing ones. And a perfect example of this is Tokyo. Usually considered to be the largest metropolitan area in the world, Tokyo has about the population of Canada in one city region.

And here, the centralizing forces are so great -- even for families -- that the government actually pays people to relocate to places outside of Tokyo's 23 wards (and its immediately surrounding areas). Previously the maximum figure was ¥300,000 per child (~CA$3,056), but this has now been increased to ¥1 million per child (~CA$10,188).

A key driver of this is surely Japan's demographic problem (namely a shrinking and aging population base). But it doesn't change the fact that lots of people appear drawn to the world's largest city.

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.

Here is an excerpt from a Guardian article that was published last year (by Tim Burrows) about Grimsby, England:

In Grimsby’s 1930s heyday, fishermen used to head to Freeman Street as soon as they were off the trawler, straight to the Lincoln or the Corporation Arms to spend their bountiful earnings. A century previously, Grimsby had been a fairly sleepy fishing village, but by the 1890s it was on the way to becoming the biggest fishing port in the world. In the mid 20th-century, trawlers were bringing in 500 tonnes of fish a day.

Today, Grimsby still has a thriving indoor market (paid for by the EU and the Enrolled Freemen of Grimsby, an organisation that dates back to the 13th century), but the further north towards the docks you walk, the emptier and more dilapidated things get. A local businessman says sex workers wait around at night for lorries to take them to the deserted docks. “It’s a legacy of the old fishing days.”

There is scant legacy to be found elsewhere. After a long decline, the fishing industry died in the mid 1980s, its owners selling their trawlers to companies in Aberdeen or Japan. Unlike Hull across the river, currently basking in its year as Capital of Culture, Grimsby is the Humber city that never was.

More than 70% of people in Grimsby, England voted to leave the European Union in the 2016 "Brexit" referendum. It was one of the highest shares in the country. But with one of the highest unemployment rates in the country, that outcome is not all that surprising.

Supposedly, at its peak, there were eight onshore jobs for every one at sea in Grimsby. And like all thriving cities, there were economies of agglomeration, which resulted in things like the largest ice factory in the world. The fishing fleets needed crushed ice -- and lots of it.

The Grimsby story is, of course, not a unique one. You just have to replace fishing with some other industry. Many cities have managed to diversify their economies either out of necessity or because they saw the writing on the wall. But for others it has been a real struggle.

It's one of those things that is perhaps simple, but far from easy.