Reuters

The United Nations projects a huge global urban population boom by 2050, but in some places more than others.

"Yeah, yeah," you're probably saying. "More than half the world's population is urban. We get it."

That milestone was reached years ago and is not likely news to anyone reading this website. But the story's not over. The world is undergoing a sustained urbanization process that's pulling more people into city centers and turning more places from rural outposts into denser urban organisms. A new report [PDF] from the United Nations projects that the world's urban population – roughly 3.6 billion in 2011 – will grow by about 72 percent between now and 2050, bringing the urban population up to 6.3 billion. That's about the same amount of people on the entire planet in 2002.

What's really important about these projections though is that next few decades of urban population growth will be highly uneven. The vast majority of that growth will occur in less developed regions. The UN expects the urban population in the developing world to nearly double, from 2.7 billion in 2011 to 5.1 billion in 2050. As the graph below shows, this increase is far more dramatic than the slow and steady urban population growth expected in more developed regions, which are expected to make up just about 1.1 billion of the urban population in 2050.

Currently, 78 percent of people in more developed regions live in urban areas, compared to 47 percent of people in less developed regions. By 2050, these numbers are expected to reach 86 percent in more developed regions and 64 percent in less developed regions. The entire world will be 67 percent urbanized by mid-century.

This population growth will have an enormous impact on the world's cities. In 2011, 23 cities had populations of at least 10 million. That number is expected to increase to 37 by 2025. These urban mega-agglomerations will account for 13.6 percent of the global population, an increase from their 9.9 percent share today.

In 2011, more than half of the world's urban population lived in cities with fewer than half a million people. By 2025, the UN expects that only 42.5 percent will be living in cities of that size. It's hard to think of cities of 500,000 small, but the UN's numbers indicate that they will be, comparatively, in 2025.

The most substantial growth is expected to occur in Africa. In 2011, urban residents in Africa made up about 11 percent of the world's urbanized people. By 2050, they will represent 20 percent. Between 2011 and 2030, Africa's urban population is expected to grow at an annual rate of 3.09, the highest in the world.

Asia, where urbanization has been happening at great speeds, will see its growth rate settle down. Home to 52 percent of the world's urban population in 2011, Asia is expected to maintain about that same proportion in 2050, with 53 percent.

With rising urban populations, rural populations will begin to fall. The UN projects that the world's rural population will peak at 3.4 billion in 2021 and then gradually decline to about 3.05 by 2050. Even though it may have already fallen into decline in developed regions, rural areas are still a huge part of less developed regions and will continue to be for many years. By 2050, 60 percent of the world's rural population is projected to live in Asia and 30 percent in Africa.

While the urban areas of North America and Europe are expected to simply grow on top of themselves by natural increase, the cities of Africa, and to a lesser extent Asia, will be emerging mainly due to places and populations transitioning from rural to urban. How this process unfolds will be something to watch.

Top image: A construction site is seen in Cairo, February 28, 2012. Reuters/Asmaa Waguih

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