Megacities shift the world's centre of gravity

In 2008, for the first time in history, half the world's population lived in cites. In 2011, China passed the same milestone. According to a study by McKinsey, the consultancy, one-fifth of the planet resides in just 600 cities that generate up to 60 per cent of the world's output. By the middle of this century, many projections indicate that three-quarters of humanity will be urban.

Many of these people are moving to second-tier cities. China has more than 100 cities - nearly 200 by some definitions - with 1m people or more. But some people are drawn to the megacities, those "cities on steroids", in the phrase of Edward Glaeser, an economics professor at Harvard, that have a population of more than 10m. That creates potentially enormous problems of housing, transport, job creation and the provision of health and security.

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Many of the world's biggest cities are in unfortunate locations, vulnerable to flooding, typhoons and earthquakes. Much of Bangkok was under water in 2011, as was Manila in 2012. Tokyo, the largest conurbation in the world, with 36m people, was rattled by the huge earthquake of March 2011. Seismologists say the big one cannot be far off.

Many megacities, most of them in poor countries, are under severe man-made strain too. Most have grown precipitously as rapid industrialisation has pushed people from the countryside and drawn them to cities that offer the prospect, if not always the reality, of a better standard of living. While they struggle to improve their lives, many newcomers live in squalid conditions without proper housing or sanitation. In cities such as Mumbai, Mexico City, Lagos and Dhaka, vast slums have become an integral part of the urban fabric.

In 1975, according to National Geographic, the world had just three megacities: New York, Mexico City and Tokyo. Today, there are at least 20 megacities, though precise comparisons are difficult because of the different ways in which city boundaries are measured.

As the picture is moving so rapidly, it is worth looking forward. According to a study by McKinsey, by 2025, 14 of the world's megacities will be in Asia. Only four of them - New York, London, Los Angeles and Paris - will be in North America or Europe, with the remaining seven in Africa and Latin America. As the consultancy says, the world's centre of gravity is moving "south and decisively east". The top seven cities by population - Tokyo, Mumbai, Shanghai, Beijing, Delhi, Calcutta and Dhaka - will all be in Asia.

Besides being big, these seven cities have more differences than similarities. Tokyo is prosperous and well planned, with a world-class transportation system (and hence very little congestion) and good air quality. Beijing and Shanghai are also planned and modern. Shanghai is a vertical city with more skyscrapers than New York and with a subway system among the longest in the world. Even so, both suffer from terrible air quality and congestion. Housing is expensive. Nevertheless, unlike the unplanned, largely low-rise cities of the Indian subcontinent, there are few slums. Crime is also relatively low.

"Megacities combine the�best and worst of urban density at mammoth scale," says Glaesar, author of Triumph of the City. "In the developed world, megacities look pretty good. There is a lot to be said for New York, London and Paris." But in the developing world, he says, the downsides of density are more apparent. "In large part this is because these big cities need plenty of management, and governments in the developing world are pretty imperfect."

The world - outside small city states such as Singapore - is arranged politically into nation states rather than cities. Many city governments are weak, without the authority to raise taxes or set their own spending priorities. Nevertheless, there are things city leaders can achieve.

Under mayor Rudolph Giuliani, New York turned around its crime problem. Shintaro Ishihara, the four-term rightwing governor of Tokyo, introduced a mandatory carbon trading scheme. Bo Xilai, the disgraced party secretary of Chongqing, changed the nature of that megacity (official population 30m) by cracking down on organised crime, building public housing and planting trees. His challenge to the authority of the Communist party, plus his undoubted excesses, led to his downfall.

In some ways, mayors and governors should be as important as presidents and prime ministers. Sometimes there is a connection. In South Korea, being mayor of Seoul was a staging post to the presidency for Lee Myung-bak. The recently elected mayor of Jakarta, Joko Widodo, is already being talked about as a possible candidate for the presidency of Indonesia. A political outsider, he swept aside the incumbent based on his record of clean, fair and efficient government in the medium-sized city of Solo.

Robert Zoellick, former World Bank president, has argued that cities should work together more closely to disseminate best practice. In 2011, he told a conference in S�o Paulo that cities should co-operate to counter climate change since urban populations were responsible for 80 per cent of greenhouse gas emissions. "When the world's largest cities pledge to work together on energy efficiency, clean energy programmes, adaptation and mitigation strategies, they can be a powerful force for change," he said.

Harvard's Mr Glaesar says that cities in general and megacities in particular get a bad rap. Properly managed, he argues, they are the most creative, dynamic and interesting places on earth.

"These megacities are a big part of�humanity's future," he says. "The prospect should be both exhilarating and terrifying - and a call to action for better urban policies."