The Rise of the Mega-City

With few exceptions, human history can be viewed as an upward trend in population density. We evolved from nomadic “hunting and gathering” societies to tribal villages, then to small towns, and ultimately to the grand and glittering cities we see today throughout the industrialized world.

Consider how rapid that shift has been. According to a United Nations report, only 3 percent of the world’s population lived in cities in 1800. In 1900, 14 percent lived in cities, but only 12 cities had at least 1 million people. By 2.....