THE WORLD HAS reached a point of hyper-urbanization: 2007 marks the first year when more than half the global population is "urban," not "rural." Indeed, this is the era of the "mega-city" -- metropolises of 10 million-plus. In 1950, only Tokyo and New York met that threshold. Today there are 20 mega-cities, including Mexico City, Karachi, Manila, Dhaka, Lagos, Jakarta and Chongqing.
This type of drastic population shift isn't without precedent. During the Industrial Revolution, concentrations of people in U.S. and European cities were part and parcel of a factory economy. But that economic and technological progress came with a price -- decades of fetid slums, horrific child mortality, raging epidemic disease. This time around, with cities 10 times bigger and demand for workers uncertain, the costs could be exponentially larger.
In general, an optimist might cheer urbanization as a sign of modernization; Residents of developed countries are much more likely to live in cities than their counterparts in still-developing nations (74% vs. 43%). The city, after all, is the hub of culture, a magnet that draws artists, writers, musicians -- the place where creative spirits create. Great cities have ballet troupes, opera companies, orchestras. The city is, likewise, the hub of industry, generating the bulk of most countries' gross domestic product. Most important, the city is the hub of ideas. The mingling of people spurs the intellectual innovation that fuels thriving societies, at least in the developed world.
But urbanization historically also has spawned an impoverished underclass of the marginally employed, or unemployed, living in a cruel despair. Think of Charles Dickens' London: Scrooge wanted to diminish the "surplus population." Or remember Karl Marx's ruminations on the "lumpen proletariat," doomed to subsistence.
Cholera, typhoid, influenza -- all cut a swath through 19th and early 20th century urban populations. Yet in time those horrors abated as infrastructure -- clean water, enclosed sewers, labor laws, public education, medical advances -- was created. In time, the 19th century cities morphed into exciting places. Today, Dickens or Marx could contentedly sip cappuccino in Florence, take in the opera at London's Covent Garden or peruse the museums of Paris.
Cities in the United States and Europe still have dense clusters of the poor, to be sure. They live in cramped housing with few amenities, but they no longer starve or die from cholera. Immigrants, in particular, who crowd -- legally and not -- into these developed cities believe that however desperate their straits, their children will fare better.
The newly ascendant mega-cities in the developing world, though, can dishearten even the most persistent optimist. They are relentless agglomerations of people, drawn not so much by the promise of prosperity as by the hope of survival.
It is internal migrant populations that are pouring into most of these exploding urban areas. In China, for instance, 150 million people have left their rural homes in the last 10 years, leaving a dearth of workers in the agricultural sector. Political and war refugees, too, flow in steadily. A fortunate few may realize a steady income, maybe even own property, but most live in slums whose filthy water, political chaos and nonexistent municipal infrastructure would startle Dickens and Marx.
The United Nations estimates that, today, 2.8 billion people live on less than $2 a day. And it is this huge, desperate underclass that is filling these mega-cities. Children are more likely to roam in gangs than attend school. Cholera and typhoid -- diseases listed as "rare" in Western textbooks -- are endemic. Often there is no geographic core, just as there is no governmental core to oversee the chaos. Parts of these cities are modern, with the familiar skyscrapers, highways and BlackBerry-toting workers. Yet they are surrounded by rings of shocking poverty where millions live in paper-covered hovels.
Without some concerted action from nations and international institutions, these mega-cities will grow larger and more desperate. Philanthropy helps, but these developing countries need public policies that promote property ownership, increase access to credit and enhance government transparency.
There is no quick panacea to improving the lot of billions of people; it took more than 50 years to address the slums of the 19th century. But there is an urgency to today's task. The slum dwellers of Lagos and Manila and Karachi are part of the global economy, bound to the rest of the world. Their misery will spill beyond their borders, and if that happens, our urban age risks becoming a global nightmare.