This afternoon we went to the Mucem (Museum of Civilizations of Europe and the Mediterranean) here in Marseille. By far, my favorite exhibition was one that dealt with the urbanization of five cities surrounding the Mediterranean — namely Marseille, Venice, Istanbul, Cairo, and Alger. At one point, this was the center of the commercial world. Great empires were forged and great fortunes were made here. But things have changed a little since then. And in some cases, what we are now left with are scenes like the one you see above (photograph by Martin Parr). This is easily the greatest photo of Venice I have ever seen. Venice is a tiny city, but it receives tens of millions of tourists each year. This photo does an extraordinary job of capturing what that might feel like.

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.