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.