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 2000, 47 percent of the world’s population was urban dwellers, and there were 411 cities with populations of 1 million or more.

As these metropolises have grown in size and importance, more and more people have been drawn to them for several reasons: abundant employment opportunities, diverse cultural experiences, and a higher standard of living.

As a result of this mass migration, for the first time in human history, more people live in cities than in the countryside.

What does it mean for the global quality of life? Is there an upper limit to the growth of cities, and when might we reach it? Are people better off living in the new “mega-cities” than they were living in more rural settings? And, are these population concentrations making us more vulnerable to disasters?

These questions are becoming more and more pressing as the urban populations soar. So, let’s try to answer them and then look at some future possibilities.

To start with, consider the fact that almost all of the world’s net population growth over the next few decades will take place in cities. According to an article in The Guardian, by 2030 the world will have to support 2.2 billion more people than it does today. And 2.1 billion of those people will live in cities.

But, it’s not simply a matter of global population growth. It’s also a matter of existing people who have been spread out across countries like India, China, and Brazil, coming together in just a few enormous cities. An average of 67 million people per year, or 1.3 million a week, are pouring into the world’s urban areas, most of them through migration.

As highlighted in an article in Singapore’s Straits Times, virtually all of this new growth is taking place in the world’s less...