Prachi Vidwans explains why we should worry about them:

If it seems like conflict over slums is mounting, that’s because it is: The urbanization of the world is accelerating. In 1950, just 29 percent of the world’s population lived in cities; back then, that was roughly 742 million people. Today, more than half of the world’s people — more than 3.5 billion — are citydwellers. That may sound like a dramatic shift, but you ain’t seen nothing yet. Roughly 70 million people move into cities every year, and the vast majority of them usually end up in illegal or informal urban settlements. According to U.N. estimates, by 2050, a third of the world’s population will live not just in cities, but in slums.

The growth of slums is a bit like climate change: We know it’s happening. We know it’s important. But no one, so far, seems to have much of a response. Policymakers tend to view slums as a necessary evil, a problem best contained through coercion or ad hoc responses. Experts point out, however, that there is a rational way to deal with the coming surge of urbanization: Plan for it. If cities are prepared to anticipate and acknowledge the inevitable influx of urban migrants, slums might not be slums.