Megacities and the Future of Urbanisation

For almost the whole of recorded history, country life was the norm and the city the exception. The reckoning came in 2007, when official tallies showed that more than half of the world''s population was, for the first time, living in towns and cities rather than on the land. That share has gone on rising, and demographers now expect close to seven in ten people to be urban dwellers by the middle of this century. The most dramatic emblem of the shift is the megacity, an urban area conventionally defined as one with more than ten million inhabitants.

Such giants were once a rarity. In 1950 the world held just two of them, New York and Tokyo. The figure today exceeds thirty, and most are clustered not in the wealthy economies of Europe and North America but across Asia, Africa, and Latin America. Lagos, Dhaka, and Karachi are swelling faster than their roads, pipes, and power grids can cope with, taking in hundreds of thousands of new residents annually. What fuels the surge is not births within the city but migration from rural areas, as families give up on farming in pursuit of work, schooling, and the prospect of something better.

To economists, this clustering of people has long looked like a gift rather than a burden. Dense cities, in their account, are engines of productivity: knowledge travels faster, employers and job-seekers find one another more readily, and the per-head cost of running transport networks or sewers drops once people live cheek by jowl. A villager who relocates to a city tends, on average, to earn considerably more, and the tax revenues thrown off by urban economies frequently flow back to subsidise the rural districts those migrants left.

Step into the streets, though, and the picture turns murkier than the theory promises. In city after fast-growing city, housing supply has lagged hopelessly behind demand, pushing a large proportion of arrivals into informal settlements without clean water, dependable electricity, or proper sanitation. In some places these districts shelter more than half of a city''s residents. Critics make a pointed case: the economic gains of urbanisation, real though they are, have landed unevenly, enriching a fortunate minority while consigning others to lives scarcely better than the ones they escaped.

Then there is the environmental reckoning, which is formidable. Cities sit on a small fraction of the Earth''s land surface and yet consume the bulk of its energy and account for a comparable share of its greenhouse-gas emissions. The cruel irony is that as coastal megacities sprawl outward, they grow ever more vulnerable to the climate hazards their own expansion has helped to set in motion: flooding, violent storms, encroaching seas. Whether they cope may turn less on how rich they are than on how far ahead their planners are willing to think.

Is the megacity, then, the answer or the affliction? No tidy verdict suggests itself. A well-run city can open doors that no village could ever match; a badly run one can pen millions inside poverty and squalor for a generation. The consensus among observers is that the verdict rests not on urban growth as such, which looks unstoppable, but on the wisdom with which it is steered.