Some of these new entrants to the club of these megacities will be vibrant economic and cultural centers that raise standards of living and quality of life. Others will struggle with congestion, pollution and poverty, much as developed cities did in their infancy.

Still, there's little doubt that the massive global migration to urban centers is irreversible and unstoppable.

"It's as much to do with the lack of opportunity in rural areas as opposed to this great magnet of cities," said Graeme Harrison, an economist who tracks the growth of megacities at Oxford Economics. "Because everything else—such as unemployment and high cost—would normally act against people migrating to cities."

A hundred years ago just 2 out of 10 people on Earth lived in cities. Today more than half the world's population lives in cities, generating some 80 percent of global economic output. By the middle of this century, 7 out of 10 people will be urban dwellers, according to Hot Spots 2025, a report prepared by the Economist Intelligence Unit for Citibank.

To be sure, any forecast 25 years into the future is nothing more than a rough guess. Population trends will shift based on a city's ability to cope with the last wave of migration—or the political backlash aimed at curbing it. Continued economic growth depends on wider forces at work in a given country or the industries that cluster in an urban area.

A major challenge these cities will pose for society will be the strain on resources. Globally, cities today represent 2 percent of the Earth's surface but use 75 percent of its resources, according to Euramet. As the urbanization trend accelerates, so, too, will issues of sustainability and infrastructure to support city dwellers.

(Read more: America's cities on the edge)