Photo via kerrykriger.com

The past six months have been packed with mind-boggling geopolitical events, and marked by wars, revolutions and a general sense of political upheaval. But according to columnist Doug Saunders, the “Arab Spring” is just a single, comparatively-tiny aspect of a larger global trend, an event that could safely be called the most important of modern times: the unprecedented shift of rural populations to cities. For centuries, villagers and subsistence farmers have moved to the city in search of economic opportunities and relief from the crushing pressures of rural life. This historical trend is set to accelerate over the next few decades:

Never in human history have so many people changed their locations and lifestyles so quickly. Each month, there are 5 million new city dwellers created through migration or birth in Africa, Asia and the Middle East. China alone has an estimated 200 million “floating” citizens with one foot in a village and the other in a city. If current trends continue as expected, between 2000 and 2030, the urban population of Asia and Africa will double, adding as many city dwellers in one generation as these continents have accumulated during their entire histories. Between now and 2050, the world’s cities will add another 3.1 billion people.

Three point one billion people. That’s equivalent to 387.5 New Yorks, or 10.3 USAs, or 2.8 Chinas, or, you know, roughly half the world’s current population. Saunders thinks it’s important that cities figure out just how to absorb these millions of new inhabitants. Countries with unsound urban policies could shunt rural migrants to the margins of their cities, creating powder kegs of violence and political instability. On the other hand, countries successful in coping with the collapse of their urban-rural cleavages (the most important political cleavage in history, arguably) have reaped incredible benefits from urbanization, including “the end of starvation as a mass phenomenon, a vast rise in living standards and the end of uncontrollable population growth.”

But how? What exactly is sound policy in an era of explosive urban growth? At least on the transportation front, that would seem to mean adding as much capacity as possible in as many different parts of a city as possible. Delhi, whose metro is less than a decade old but covers virtually every corner of India’s rapidly-expanding capitol, is a prime example of this. So is Mexico City, whose relatively-recent light rail and electrified bus lines attempt to serve the more peripheral (and usually poorer) parts of the city that its metro doesn’t reach. On the opposite side of the spectrum is a city like Istanbul, which attracts migrants from all over the Middle East and Eastern Europe and seems to sprawl into infinity. There’s much of Istanbul that the city’s Metro and tram lines skip; although authorities are looking to a trans-Bosporus tunnel as a solution to the city’s transit woes.

Transit can be instrumental in creating a sense of social cohesion, regardless of where it goes—just look at New York City, where people of every racial and economic category ride the city’s subways and commuter trains. But in a period of barely-controllable urbanization, the accusation that transit only serves the needs of certain communities could have some very serious consequences.