It's the tide of urbanization that has spread around the world. In the developed countries and Latin America it has nearly crested; more than 70 percent of people there live in urban areas. In much of Asia and Africa people are still surging into cities, in numbers swollen by the population boom. Most urbanites live in cities of less than half a million, but big cities have gotten bigger and more common. In the 19th century London was the only city of more than five million; now there are 54, most of them in Asia.
And here's one more change since then: Urbanization is now good news. Expert opinion has shifted profoundly in the past decade or two. Though slums as appalling as Victorian London's are now widespread, and the Victorian fear of cities lives on, cancer no longer seems the right metaphor. On the contrary: With Earth's population headed toward nine or ten billion, dense cities are looking more like a cure -- the best hope for lifting people out of poverty without wrecking the planet.
All images and captions courtesy of National Geographic. These and other images can be found in the December 2011 issue of National Geographic magazine, on newsstands now.
