Imagine a seething tangle of humanity so dense in population it puts jam-packed Manila in the shade. One whose architectural wonders are universally hailed and yet 95% of its citizens live in such cramped high-rise quarters that there is little direct sunlight or ventilation. Where human waste contaminates the water supply and soil. Where noise is so severe and traffic so congested that daytime deliveries are banned. Where periodic epidemics wipe out swathes of the urban population. Where the threat of food riots is so pressing that the wealthy elite, living in their hillside villas, are forced to dole out free supplies of daily bread to the squalid masses below.

Yes, life in Ancient Rome at the beginning of the Christian era offers a depressingly familiar story of urban misery and inequality. A multi-ethnic melting pot of one-million-plus citizens and slaves, all walled into six square miles, this was the world’s original ’megacity’ in terms of inordinate size and influence. If Rome today were to boast the same share of the global population as it did back in 100 AD, it would eclipse every known municipality on earth. But not for much longer.

Although it would take several centuries before another city would match Ancient Rome’s benchmark – Baghdad was reputed to have been a million-strong by 930, as apparently was Cambodia’s Angkor in the 12th century and Peking by the early 1800s – comparable city empires are now erupting with a ferocity that has no historical parallels.

It took London 130 years to swell from one million to eight million people. But in Lagos, where around 300,000 Nigerians lived in 1950, the population has since mushroomed to at least 11 million – and could well double again in the next decade. Britain, the first nation to become primarily city-dwellers as industrialisation took hold in the Victorian Age, still only has nine urban centres of more than 500,000 people; China has 236, says the UN, a number that has quadrupled since 1980 – and another 100 will arrive by 2025. If all roads once led to Rome, they are now heading very much away from it.

The UN this year officially designated 21 urban agglomerations as megacities, those defined as having at least 10 million inhabitants. Together, they house 9% of the global urban population, according to its 2009 Revision of World Urbanisation Prospects. The UN’s belief is that these alpha conurbations will be joined in short order by seven other megacities, including Guangzhou, Jakarta, Lima, Bogotá and Kinshasa, as cities absorb much of the coming population swarm. Forged from the forces of globalisation, this super league of heaving hubs are the world’s economic command centres, exerting a greater gravitational pull on business dealings than even their own national governments.

“If the last century was known as the age of globalisation, then this century will be the age of cities,” says Andrew Tan, director of Singapore’s Centre for Liveable Cities that organised this year’s World Cities Summit. “Cities throughout history have been cradles of civilisation, engines of growth, as well as patrons of the arts and culture. It is small wonder that cities also account for a disproportionate share of national wealth.”

If anything, the UN forecasts underplay the true level of urban escalation, as metropolitan areas are so difficult to define. Chongqing, the south-western Chinese city on the upper reaches of the Yangtze River, is also seen hitting that 10 million megacity threshold by 2020. But now that its government has annexed the surrounding territory, Chongqing is already in fact the world’s most populous municipality. With some 33 million inhabitants, this little-known urban nexus boasts almost as many people as Canada.

The UN also short-changes that other economic boiler-room, India. On track to overtake China’s population by 2050, the sub-continent that once helped fuel Britain’s industrial explosion is now experiencing its own rapid urban growth. The country has 25 of the world’s 100 fastest-growing urban areas, according to City Mayors, the international think tank on urban affairs; China has eight.

But even these super-sized cities won’t be enough to accommodate India’s dramatic demographic shifts. According to a recent McKinsey Global Institute report, India’s Urban Awakening, 590 million Indians will live in cities by 2030 – a massive influx from the rural hinterland that will require capital investment of €950bn and up to 900 million square metres of additional commercial and residential space just to keep up. That’s the equivalent, says McKinsey, of “a new Chicago very year”.

Although McKinsey criticises India for its negligent urban management, efforts are being made to siphon that impending pressure away from the already over-stressed metropolitan areas. Five brand-new megacities have just been earmarked for greenfield sites in the Mumbai-Delhi industrial belt. Each is due to house up to 10 million people by 2015 with the largest, Dholera, taking up 360km2 of Gujurat state. That’s more land than is occupied by Navi Mumbai, currently the largest planned city in the world.

For all the mind-boggling dimensions of these Asian, African and Latin growth spurts, it is still hard to see past the associated Dickensian litany of despair. Particularly when it comes to chronic cities such as Mumbai, where the slum population alone outnumbers that of Norway; or pollutant-spewing Beijing where annual per capita CO2 emissions are two and half times that of Belgium.

“Megacities are feared. Their seemingly relentless growth, their burgeoning size, their envelopment of surrounding communities, their massive slums, all seem to combine to present an image of an intimidating, inhuman scale that cannot be managed,” Paul Taylor told a German conference on governing megacities, speaking on behalf of UN-Habitat, the UN agency for human settlements. “But despite offering a Blade Runner-like nightmare of dystopia, megacities are also magnets of hope, creativity and growth.”

In fact, as many urban thinkers now point out, it is their anarchic mess that makes megacities such hubs of innovation in arts, manufacture and finance in the first place. In their zeal to cleanse cities, evicting the poor and replacing ramshackle neighbourhoods with Sudoku-grids of skyscrapers, city planners run the risk of draining that essential vitality and killing the entrepreneurial dynamism. As Richard Florida, the influential thinker on the knowledge economy, puts it: “People with abundant creative energy don’t want to be safely tucked away somewhere. They want accidents to happen, look for the rough edges and seek the authentic.”

Similarly, renowned Dutch architect and urban theorist Rem Koolhaas talks admiringly of the “improvisational urbanism” he sees in chaotic megacities such as Lagos. In successive years, the Pritzker Prize-winner has taken a class of Harvard graduate design students to observe the Nigerian city in all its delirium and constant flux. Instead of despairing at the daily gridlock, he marvels at the way such two-hour traffic jams create an impromptu market as sellers swarm around the static vehicles. This is the city where wheelbarrows are rolled out of a construction site at night to serve as rented beds for the homeless – at 20 cents a time. “Anguish over the city’s shortcomings obscures the reasons for the continued, exuberant existence of Lagos and other megacities like it. These shortcomings have generated ingenious, critical alternative systems,” enthuses Koolhaas in a documentary that followed his Nigerian explorations. “Lagos is not catching up with us. Rather, we may be catching up with Lagos….”

Such perpetual inventiveness is certainly not limited to Africa, nor is it limited to small-scale interventions at the community level. Many are the result of inspired leadership at City Hall level, often under the direction of maverick mayors who borrow freely from their idiosyncratic peers in other countries. It took just 10 years and two charismatic mayors –

Antanas Mockus and Enrique Peñalosa – to transform Bogotá, Colombia, from the one of the most violent corners of the planet into peaceful model city. Critical to their success was a bike path network and the TransMilenio bus rapid transit system that was modelled directly on that of Curitiba, Brazil. The combination of cycle-ways, parks and efficient buses led to a drastic reduction in both commuting times and crime.

Tokyo’s tart-tongued governor, Shintaro Ishihara, may draw media attention for his offensive remarks and displays of Japanese nationalism, but he is also willful enough to want to turn his sprawling metropolis into the world’s greenest megacity. While Japan’s federal leaders continue to haggle over a national carbon emission strategy – more than a decade after hosting the climate conference that led to the Kyoto Protocol – Tokyo is going full-bore into large-scale recycling, rainwater reuse, waste-heat recapture and landscaping. Ishihara’s environmental bent echoes that of Mexico City’s mayor Marcelo Ebrard, not to mention the 800 cities and towns across the US that have gone against national law and enacted more stringent eco-policies.

Equally transformative has been the combined work of Bayani Fernando and his wife Maria Lourdes Fernando, both of them mayors of Marikina, one of the cities that make up Metropolitan Manila. What was once a poor municipality plagued by violent crime and with only one paved road has become a “little Singapore” that is celebrated as one of the healthiest, liveable cities within Asia-Pacifi c. “Our 220ha river used to be a haven for squatter colonies that conveniently served as a giant garbage receptacle,” recalled Maria Lourdes, speaking this summer at the World Bank’s Innovative Cities dialogue series to help urban policymakers share innovations. In Marikina’s case, the solution hinged on cleaning rivers, building thoroughfares and waterfront parks, and creating a “two jobs per family” training programme to upgrade citizens’ skills at designated productivity centres. Once known only as the shoe-making capital of The Philippines, Marikina has moved up the global value chain.

Not all ideas translate well to other megacities. New York’s Michael Bloomberg, for example, was unable to implement the congestion zone charge pioneered by London’s former mayor Ken Livingstone. But such initiatives do demonstrate how effective a combination of good governance, decisiveness and unorthodox thinking can be in tackling even the largest obstacles. Unsurprisingly, many of these activist mayors enjoy approval ratings that are the envy of their paralysed presidents and prime ministers.

The savviest among them also realise that global engagement is vital if their city is to prosper in a world where talent is free to find its most desirable home. London's mayor, Boris Johnson, for one, knows that the city cannot rest on its laurels as “the economic powerhouse of the UK contributing around £90bn to the British economy.” Johnson remains a staunch supporter of big-spending initiatives that keep London in the global eye, such the £9bn (€11bn) Olympics, the £12bn (€15bn) Crossrail project, and two recently unveiled cycle lanes costing £43m (€52m), in spite of public spending cuts mandated by his own party-in-power.

Ensuring London’s global relevance becomes more urgent in light of the reordering of the world’s economic map around enormous urban corridors. As cities push beyond their original boundaries some are merging into “mega-regions” whose vast expanses are challenging the power hierarchies.

“The world’s 40 largest mega-regions account for two-thirds of all the global economic activity and 85% of the world’s technological innovation while housing just 18% of its people,” says Richard Florida, who together with a team, recently used a mix of nighttime satellite imaging and socio-economic data to delineate numerous contiguous powerhouses, several of which are being accelerated through high-speed rail links.

There are at least 10 in the US, led by the Boston–Washington corridor, which generates $2.5tn (€1.9tn) in economic activity, enough to make it the fourth largest economy ahead of both France and the UK. Other endless clusters are scattered throughout the world (see page
36), and include China’s Hong Kong-Shenhzen-Guangzhou region, which is home to around 120 million people.

As Europe reorganises itself along two corridors that could compete for brain-power and cultural cachet – the Euro-Lowlands uniting Benelux with Germany and France, and the Euro-Sunbelt from Barcelona to Marseille – London, as an island-bound financial centre, needs to make sure it doesn’t cut adrift

UN-Habitat noted these urban corridors in its recent State of the World’s Cities 2010/2011 report as a generally positive development. “They, rather than countries, are now driving wealth,” said the report’s co-author Eduardo Lopez Moreno. The flipside is that they can also lead to “unbalanced regional development” as inter-linked cities draw on country resources to strengthen their ties to one another. “These urban configurations are becoming the new engines of global and regional economies, even as they create further paths of regional divisions,” Lopez Moreno added.

Although technology and globalised commerce was supposed to level the economic playing-field for all, it turns out the world is not getting flatter after all.