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The world is changing faster now than ever before. The dispossessed, and the ambitious are flooding into cities swollen out of all recognition. Poor cities are struggling to cope. Rich cities are reconfiguring themselves at breakneck speed. China has created an industrial power house from what were fishing villages in the 1970s. Lagos and Dhaka attract a thousand new arrivals every day. In Britain, central London's population has started to grow again after 50 years of decline.

We have more big cities now than at any time in our history. In 1900, only 16 had a population of one million; now it's more than 400. Not only are there more of them, they are larger than ever. In 1851, London had two million people. It was the largest city in the world by a long way, twice the size of Paris, its nearest rival.

That version of London would seem like a village now. By the official definition, London has getting on for eight million people, but in practical terms, it's a city of 18 million, straggling most of the way from Ipswich to Bournemouth in an unforgiving tide of business parks and designer outlets, gated housing and logistics depots. There might be fields between them, but they are linked in a single transport system and a single economy. Those villages in Suffolk that are close enough to a railway station to deliver you to Liverpool Street in under 90 minutes are effectively as much a part of London as Croydon or Ealing and they have the house prices to prove it. The other big conurbations - from Birmingham to Manchester and Glasgow, names for cities that spread far beyond the bounds of political city limits - can be understood in the same way.

Having invented the modern city, Britain promptly reeled back in horror at what it had done. To William Morris and John Ruskin, or the Salvation Army exploring the cholera-ridden back alleys of London's East End, the city was a hideous tumour sucking the life out of the countryside and creating in its place a vast, polluted landscape of squalor, disease and crime. In their eyes, the city was a place to be feared, controlled and, if possible, eliminated.

In William Blake's bitterly ironic words, Jerusalem had been overwhelmed by dark, satanic mills. Morris dreamt of a London abandoned by its population in favour of communal country life, leaving behind a dung heap in Parliament Square and empty streets enlivened by fluttering, worthless banknotes.

Such attitudes continue to shape thinking about the city and not only in Britain. In America, the Republicans have concluded that there are no votes to be had in cities. And wealthy suburbanites refuse to pay the property taxes that will support the downtown areas they fear and despise. Yet whether we like it or not, at some point in 2008, the city will have finally swallowed the world. The number of people living in cities is about to overtake those left behind in the fields. It's a statistic that seems to suggest some sort of fundamental species change, like the moment when mankind stopped being hunter gatherers and took up agriculture. It has been the trigger for a wave of task forces, academic disaster tourism and feverish speculation, from Forbes magazine and National Geographic to the United Nations' habitat programme.

When Forbes went to Lagos last year, drawn by what it called the 'Malthusian nightmare' of a city that had grown from 300,000 people in 1950 to 10 million today, in an unconscious echo of those Victorians horrified by the spectacle of the new industrial cities of the 19th century, it invited us to think of the future of Lagos in terms of the lawless chaos of Baghdad's Sadr City, multiplied by 100.

The future of the city has suddenly become the only subject in town. It ranges from tough topics such as managing water resources, economic policy, transport planning and law enforcement to what is usually presented as the fluffier end of the scale, such as making public spaces people want to spend time in. It's about racial tolerance and civilised airports, the colour of the buses and the cost of the fares on them. Unless you have some kind of framework to make sense of all that, the city can seem to be about so many diverse things that it is about everything and nothing.

And that is how I found myself swept up in Urban Age, a mobile think-tank set up by the London School of Economics Cities programme, with the Alfred Herrhausen Society, a well-funded charitable arm of Deutsche Bank.

The starting point of the Urban Age project, originated by Richard Sennett and Ricky Burdett of the LSE, was that a successful city has to be based on an understanding that it is shaped both by politics and by ideas about space and architecture. Their idea was to bring together a diverse selection of people, not only those who spend their time thinking about cities, but also those who have to try to do something about them.

There were a couple of mayors - of Washington and Bogotá - and a formidable American sociologist in the shape of Saskia Sassen. They were joined from time to time by Germany Chancellor Angela Merkel and Dutch architect Rem Koolhaas. On one occasion, Ian Blair, commissioner of the Metropolitan Police, turned up. There were Gerry Frug, a Harvard lawyer who writes constitutions for cities, a criminologist from the Sorbonne and an Austrian transport planner desperately worried about the impact of cars on the sustainability of cities. These are not the kind of people you usually find in the same room.

Over two years, the group toured six of the world's key cities - New York, London, Shanghai, Mexico City, Johannesburg and Berlin - in a series of conferences. At each stop, they met their local peers.

In New York I listened to Rem Koolhaas behaving badly and blaming our inability to face up to the realities of the contemporary city on our sentimental attachment to Jane Jacobs and the rose-tinted views on street life she expounds in her book The Death and Life of Great American Cities. Ever since the 60s, Jacobs ideas about protecting traditional neighbourhoods from planners trying to carve urban motorway through them have made her a heroine to generations of urban activists.

Later, I was in Johannesburg and saw Richard Sennett offend a roomful of South Africans as he attempted to put the ANC deputy mayor on the spot by asking her exactly what she intended to do about making life better in a city in which taking a journey on a suburban train is too dangerous for all but the desperate to contemplate. And in Berlin I asked Ian Blair how he defined the boundaries of London. His answer included Jamaica and Baghdad.

The results of all this have been boiled down to The Endless City, a 500-page doorstep of a book I edited with Ricky Burdett and which is published this week. It has a lot of messages about reducing the reliance of cities on the car, on high-density cities being more sociable places in which to live, as well as more sustainable environmentally, about the importance of a coherent form of city government. Though it doesn't shrink from the darker aspects of city life, it is also a powerful affirmation of the city as mankind's greatest single invention.

The nature of cities has already changed irrevocably and in The Endless City, there is plenty of evidence to show that they are changing us. In 1950, they were predominantly a Western phenomenon, with the developed world accounting for 60 per cent of the urban population. Now, 70 per cent of city dwellers are from the developing world. In China in 1970, one in five people lived in cities. In 30 years, that number has risen to two in five. The fastest-growing cities are all well outside the comfort zone of the Western world. Lagos, the fastest growing of them all, is adding 58 people every hour; Mumbai is growing by 42 every hour.

A score of cities including Los Angeles, Shanghai and Mexico City, which were still tiny in the 19th century, have all passed the once unimaginable 18 million mark. That puts them well ahead of all but eight of the 27 nations of the European Union. This is a dizzying rate of transformation and it's still accelerating. In 1900, 10 per cent of the world's population lived in cities; by 2050, it is going to be 75 per cent. And the biggest question is if such places can survive as coherent cities at all.

Cities bring out a lurking paranoia in some people. They see this explosive growth as a tide of slums engulfing the world. Certainly there is plenty to be worried about.

Half of the 12 million people in Mumbai live in illegal shacks, 200,000 of them on the pavement. Every day, at least two people are killed falling off overcrowded suburban trains. In Mexico City, fewer than four workers in 10 have formal jobs, public transport is largely in the form of mafia-controlled minibuses, and taxis. The last mayor's response was to build a second tier on the elevated motorway, to allow the rich to speed up their commuting time.

Johannesburg, with its horrifying levels of violent crime, has seen the affluent quit the city centre for fortified enclaves on its boundaries. As a result, South Africa is leading the world in developing new security techniques for gated housing, built appropriately enough in the style of Tuscan hill towns. Private security is also a divisive a topic in north London where I live where the clatter of police helicopters has become routine. My neighbourhood divides between those who want to install barriers and gates to cut us off from the world outside and those who see such measures as the ultimate negation of what life in a city should be. Despite our anxieties, London is a safe city by world standards. The murder rate is 2.1 for every 100,000 inhabitants. In Johannesburg, it is nine times that figure and you are eight times as likely to be killed in a car crash there.

Even in the traditionally more successful cities, there is plenty to fuel the anxieties of those who worry about such an uncontrollable surge of urban change. In the last 20 years, the percentage of people with manufacturing jobs in New York has fallen from 20 per cent to just 4 per cent. In London's central seven boroughs, more than 70 per cent of births last year were to mothers not born in Britain. In 1992, 38 per cent of newcomers to London were foreign-born. Five years later, it was 40 per cent and in 2001 it was 56 per cent.

Cities look different, too. Shanghai had just 121 buildings over eight storeys high in 1980. Twenty years later, it was 3,500, and just five years after that it was a staggering 10,000.

But for all their agonies, cities must also be counted as a positive force. They are an engine of growth, a machine for putting the rural poor onto the first rung of urban prosperity and freedom. Look at London, a city that existed for several centuries before anything approximating England had been thought of. It has a far stronger sense of itself and its identity than Britain as a whole or England. It has grown, layer on layer, for 2,000 years, sustaining generation after generation of newcomers.

You see their traces in Spitalfields, where a Huguenot chapel became, successively, a synagogue and a mosque, tracking the movement of waves of migrants from poverty to suburban comfort. It is a history of migration marked by place names like Lombard Street, Hindu shrines and mosques. It's a place without an apparent structure that has proved extraordinarily successful at growing and changing. Its old residential core, sheltering in the approaches to its fortress, has made the transition into the world's busiest banking centre.

Its market halls and power stations have become art galleries and piazzas. The simple terraced streets built for the clerks of the Great Western Railway in Southall have become home to the largest Sikh community outside India. The failed speculation of 19th-century housebuilders in Holland Park has provided the base for the international financiers. Hoxton's nonconformist chapels and Camden's wharves provide fertile territory for hipsters interested in tattoos, unnecessary facial hair and the internet.

And all of these worlds overlap in space and time. London is different for all its people. They make the most of the elements in it that have meaning for them and ignore the rest. A city is an a la carte menu. That is what makes it different from a village, which has little room for tolerance and difference. And a great city is one in which as many people as possible can make the widest of choices from its menu.

Ever since I began writing about cities, London's structure has fascinated me. It manages to be both monumental and intimate. I have lived in many cities: Edinburgh was spectacularly beautiful; Glasgow has the grim magnificence of its stone built core and its American style grid of streets. Milan has its glittering centre and its endless sprawl of factories and workshopts that make it a world capital of design. But it is London which has left me with a mental map to interpret every other city in which I have found myself. Which is the east end, which is south of the river? It is partly that phenomenon that made the experience of the Urban Age conferences so compelling. It was a chance to sample in quick succession a sequence of very different urban menus.

The South African leg of the programme took me to the top of the country's tallest skyscraper. Once it housed Johannesburg's only four-star hotel. It is shuttered now, trapped in the midst of a dystopia that could have been the product of JG Ballard's imagination. Nigerian squatters occupy brutalist concrete towers. Stalls selling bush meat and magic have taken over the streets. A particularly fearless band of yuppies is attempting to turn the Art Deco office blocks left behind by the gold rush years of the 1930s into city-centre lofts.

When the Urban Age got to Mexico, I found myself touring illegal squatter camps in Mexico City, in the company of armed guards courtesy of the Herrhausen Society, not so surprising, perhaps, since it was established by Deutsche Bank in memory of a chairman assassinated by terrorists. These settlements are called informal, yet they are planned with remarkable precision. A whole community from the rural south of the country arrives in a single co-ordinated movement to establish an instant suburb.

Mexico is a place in which globalisation works in unexpected ways. While America might be worrying about losing car plant jobs to Mexico, the street traders who crowd the heart of Mexico City are the tip of another global supply chain. The T-shirts and the plastic flip-flops they sell are from China.

In Berlin, Angela Merkel talked about the challenge that migration poses to the big cities, while Lord Foster discussed designing a symbol for the capital of a reunited Germany in the form of the Reichstag, and Berlin's mayor offered a model for the future of his city as 'poor but sexy'.

In Shanghai, the city had been hit by a rash of graffiti. There had been nothing like it on my first visit in the early 1990s, when the main road into the city was a two-lane blacktop. Now it's an eight-lane highway, amid a forest of skyscrapers. It turns out that the ubiquitous sequences of spray-painted numbers, with the occasional Chinese character thrown in, have nothing to do with tagging or politics - they are mobile phone numbers of migrants looking for work. In the turmoil that is modern China, it's the only way to show they are available for hire.

It is chastening, but valuable for a critic to be confronted with how little you really know. Before the Urban Age conference in Shanghai, I hadn't understood that the city has three million illegal immigrants from inland China with fewer rights than Mexicans in Los Angeles, or that the city had levels of inequality of an order close to Manhattan's. In the context of a state based on an ideology-free version of Marxism, it's hard to know if that is to be understood as a success or a failure. I knew that huge areas of the city centre had been bulldozed, but I had not understood that this relentless tide of construction had doubled the living space, which is still far from generous, of its most crowded inhabitants.

I knew that Johannesburg was a city shaped by apartheid, but I hadn't understood what it would mean to try to deal not just with social inequalities, but structural ones too. Johannesburg was built as a white city surrounded not by suburbs but by invisible black labour camps. It's not enough to open the city to the majority; the edges are still hugely dispossessed. I could not have imagined what it is like for the city's transport officials to work on its suburban rail system until I heard at first hand of the effects of a security-guard strike that involved scores of murders. Six dead bodies were found on one particularly bloody day.

The Urban Age did make me understand exactly how the Victorians felt about the city, and how threatening a place it seemed to them. It also forced me to think not just as a critic, but to look at things from the point of view of a politician. What is to be done about the city?

It's clear that every city is in desperate need of answers, and that events like the Urban Age act like hot spots for the transmission of avian flu. Ideas spread from them like epidemics. Culture-led renewal, congestion charging, elected mayors have all been strategies for the future of cities that have ricocheted around the world.

What may be surprising is that, for once, many of these ideas are coming from London. Ken Livingstone's electorate may be focusing on allegations of misdemeanours at City Hall, but in New York and Barcelona, they are taking his policies on planning, transport and housing with the utmost seriousness. Nobody knows what effect his insistence on making the builders of luxury flats include a percentage of affordable housing will have on land prices in the long term, but it looks like an experiment worth trying.

We do not belong to a generation that has the shared faith that the pioneer architectural modernists had when they chartered a liner to cruise the Mediterranean and drew up their vision of what the modern city ought to be, the Charter of Athens (1933). They divided their ideal city into functional zones, shaped by slabs arranged to maximise the sunlight falling on the ground between them.

Theirs was a generation that was freed from the luxury of self-doubt. Ours is not and that is why we struggle now when we try to think what cities should be. We have seen too many soured urban utopias that were invented by the architects on that liner, and propagated by a political system that measured success in the number of new buildings that it could deliver each month.

Politicians love cranes; they need solutions within the time frames of elections and cranes deliver them. But there are only a limited number of problems that are susceptible to this kind of time scale. The result is a constant cycle of demolition and reconstruction that is seen as the substitute for thinking about how to address the deeper issues of the city. Visions for cities tend to be the creation of the boosters rather than the theorists or the policy-makers. City builders have always had to be pathological optimists, if not out-and-out fantasists. They belong to a tradition that connects the map-makers who parcel up packages of swamp land to sell to gullible purchasers, and the show-apartment builders who sell off-plan to investors in Shanghai, who are banking on a rising market, making them a paper profit before they have even had to make good on their deposits.

Cities are made by an extraordinary mixture of do-gooders and bloody-minded obsessives, of cynical political operators and speculators. They are shaped by the unintended consequences of the greedy and the self-interested, the dedicated and the occasional visionary. The cities that work best are those that keep their options open, that allow the possibility of change.

The ones that are stuck, overwhelmed by rigid, state-owned social housing, or by economic systems that offer the poor no way out of the slums are in trouble. A successful city is one that makes room for surprises. A city that has been trapped by too much gentrification, or too many shopping malls, will have trouble generating the spark that is essential to making a city that works.

The pattern of the Victorian terraces of London has proved to be remarkably adaptable. A four-storey house 18ft wide can be used for almost anything and it supports a population dense enough for pedestrian life on the pavement that makes cafes and small shops flourish; a system-built tower block marooned in Tarmac is not so adaptable.

Similarly, giant out-of-town sheds, the predominant form of so many new cities now, are not designed for flexible use or even for the long term. They are built with a maximum of 20 years of life in mind and then trashed. Successful cities are the ones that allow people to be what they want; unsuccessful ones try to force them to be what others want them to be. A city of freeways like Houston or Los Angeles forces people to be car drivers or else traps them in poverty. A successful city has a public transport system that is easy to use; an unsuccessful city tries to ban cars.

Later this year the Urban Age programme moves to Sao Paolo, shifting its focus from the bigger picture to the fine grain of life in a city in which the rich commute by helicopter, the prisons are a state within a state, and in which the mayor is able to ban outdoor street advertising overnight in a bid to beautify his city.

A successful city has room for more than the obvious ideas about city life, because, in the end, a city is about the unexpected, it's about a life shared with strangers and open to new ideas. An unsuccessful city has closed its mind to the future.

Endless City, edited by Ricky Burdett and Deyan Sudjic, is published by Phaidon next week at £35.

guardian.co.uk © Guardian News and Media Limited 2008

What lies beneath - Sinai’s hidden texts

By Clive Cookson September 21, 2012 8:19 pm Financial Times

Spectral imaging reveals the hidden text on the medieval palimpsests. Copyright St Catherine’s monastery of the Sinai. Used with permission