The world is fast approaching the point where the majority of the human population will be found in urban areas.
The projection is that in 50 years' time, two-thirds of humanity will live in cities.
Six experts outline their vision of the urban world in 2050.
Hank Dittmar is an American transport expert and head of the Prince's Foundation for the Built Environment, which was set up by Prince Charles to promote traditional building design.
In 2050, I would hope to see cities that restored a more intimate relationship with the countryside around them both in the use of local materials in building construction and local traditions of architecture.
We should be moving towards cities that are based around walking than around the motor car, and around living in a way that relies on the sort of energy budget and food budget that's available to us close by.
One way to move in that direction is to start to think about some timeless patterns of how cities ought to be. And that means thinking about cities as being typological [examples of different types of buildings, streets, squares and spaces] rather than a series of one-off sculptural objects that generate the "wow" factor.
We need to think of ways of having our workplace close to where we live. If we do that, we reduce transport intensity and make it easier for people to be close to their families. You will then see polycentric cities emerging rather than mono-centric cities, where everyone leaves their home to go to work.
The thing that I worry about most for the future is if energy becomes unaffordable and more scarce. We could move into a situation where those who can afford energy sort of withdraw and continue to use it and those who don't move into more deprivation.
This could lead to further destabilisation. A lot of the cities where people are urbanising are being fed by petroleum-based agriculture and petroleum-based economies - and that's mighty scary.
Michael Dear is Professor of Geography at the University of Southern California. He is also author of The Postmodern Urban Condition.
One of the effects of global capitalism is the creation of an increasingly polarised world. On the one hand, you have what my colleague Mike Davis calls a planet of slums, and on the other hand, you have what we call cities of gold.
While these terms can apply on a global scale, they can also apply to cities, such as Los Angeles and Mexico city, where there is a lot of slum and a lot of gold.
What this polarisation within cities creates is what I call post-modern urbanisation and I think we're going to see a lot more of it by 2050.
We don't build a city and towns with city centres any more you add city centres afterwards as an aesthetic afterthought or as a consumption opportunity
Basically, in conventional cities - modernist cities - the norm has been for the centre to organise the hinterland.
However, in post-modern urbanism, this has been reversed - the hinterland organises what's left of the core. Look at Southern California, the Pearl River Delta, or Barcelona, there is a huge decentralised spread of urban development and no real single core to speak of.
LA, for example, has 20 or 30 downtowns - there isn't the conventional pattern of people travelling into the city and out of the city in the morning. People cross the city in a wide variety of ways and this means a lot more choices - a lot more dispersed patterns of behaviour. It also means a lack of central authority in organising a city region or its government.
We don't build a city and towns with city centres any more you add city centres afterwards as an aesthetic afterthought or as a consumption opportunity. We simply have a collage or pastiche of almost random urban spread which ultimately collides and creates cities and then we start adding the trappings of conventional cities.
So, you have an extraordinary fragmented urban region which extends in the case of LA over 14,000 square miles.
This offers up opportunities for intense local autonomies - on the one hand you have the rich seceding but on the other, you have poorer people claiming their spaces.
Local autonomies develop in a metropolis. In our region that tends to be Hispanics, and that's one of the most important demographic trends that you can imagine.
Nigel Thrift is vice-chancellor of the university of Warwick and one of the leading human geographers and social scientists.
In the developed world my guess is by 2050 energy sustainability will have become a big deal and the result of that will be that the kind of sprawl that we have seen in the United States in particular will actually be halted, on the grounds of energy costs.
In Europe, things ought to be better because on the whole, European cities are much more compact and should be able to last out some of problems that the larger sprawling cities in the developed world will have. Even if you look at London - it doesn't spread over a vast area.
I think the issue then becomes whether the more severe forecasts on global climate change do start to bite and if they did then some cities especially coastal cities like London would start to have problems in terms of flooding and so on - indeed that has already been forecast.
What I would really like to see is some kind of a Marshall Fund, but for cities around the world
I don't think that this means doom or anything of the kind. But it will involve some quite substantial government action at some point to start thinking about the way that cities ought to be and at the moment it seems to me that that thinking is only being half done.
There are some important urbanists, like Richard Rogers, who are quite right when they say we need to do more thinking in this area about the form of cities in the future and how they link up. Some types of transport will turn out to be really problematic. We may have seen the heydays of certain kinds of air travel over the next 10 to 15 years.
With regards to other parts of the world, the future looks patchy. In parts of Asia and Africa, you can see examples of countries and cities that will be able to weather the worst - as well as some of the worst weather. On the other hand, there are some cities that are highly vulnerable - what is needed is worldwide action to prevent some of the problems there.
What I would really like to see is some kind of a Marshall Fund, but for cities around the world.
Stephen Graham is a professor of human geography at the University of Durham. He is editor of The Cybercities Reader.
There was a lot of hype in the last 30 or 40 years somehow implying that the more important your technology and more important your information technology, the less and less you need to move around, the less and less that you need to meet face-to-face with people and the less and less you need to rely on the city.
There was an assumption with the shift towards broadband, virtual reality would somehow allow people to withdraw. The evidence seems to go completely against that expectation.
This may seem paradoxical but the evidence indicates that the more economies, social interactions and cultures rely on advanced technologies, the more cities seem to grow.
I think there is a radical democratisation going on based on much lower cost access and based on things like wireless technologies which are much cheaper to lay out across cities
I think there is a demand to be face-to-face with people no matter how capable the technology - and, of course, by 2050, we will have had fairly radical technical shifts. Those processes of change seem to go together rather than in opposition.
In India, China and Africa, the picture has been very polarised. Only a small number of elite people have been connected to the new technologies. However, I think there is a radical democratisation going on based on much lower cost access and based on things like wireless technologies which are much cheaper to lay out across cities.
It is also based on all kinds of interesting entrepreneurship where people in informal settlements or squatter settlements set up little internet spaces. These bring whole communities that have an awful amount of energy and into the digital age.
I think there are signs for optimism based on this extraordinary rapid democratisation.
Walden Bello, Executive Director of the Bangkok-based research and policy institute Focus on the Global South and professor of sociology at the University of the Philippines.
An urban nightmare in less than 50 years' time is certainly what will engulf us if current trends continue.
In the South, urban populations are growing at twice the rate of national populations. People continue to be expelled from the countryside in large numbers, and a key reason for this is that agriculture has simply been made unattractive
by the lack of agrarian reform
- the dumping of cheap subsidised agricultural products from the North courtesy of the World Trade Organization's Agreement on Agriculture
- decades of city-biased and industry-first economic development policies that consistently pushed down the price of grain and other farm products.
At the same time, the capacity of industry and manufacturing to absorb the influx from the countryside is being eroded by de-industrialisation.
Local manufacturers are being driven out of business by radically lowered tariffs on foreign products under economic programmes imposed by the International Monetary Fund and WTO, and foreign investors are closing up shop and moving to China to take advantage of dirt-cheap wages.
One of the results of this migration-without-absorption is the mushrooming of vast shantytowns populated by what some have called a "subproletariat."
The urban poor living in such settlements under terrible conditions of squalor, crime, and insecurity now make up 30-40%t of the population of cities such as Manila, Jakarta, Mexico City, and Lagos.
With their budgets gutted by austerity programs pushed by the IMF and World Bank and unable or unwilling to tax the rich, city governments cannot provide basic services needed by this swelling urban mass such as water, electricity, and infrastructure.
Northern cities have their equivalent of these third world shantytowns: inner city ghettoes, overcrowded housing projects, and suburban slums where racial minorities and immigrants and their children cluster, unable to find jobs or able to find only low-paying unskilled jobs unwanted by the dominant society.
The capital of the empire is becoming a paradigm for the rest of the urban America: Washington, DC, is a predominantly black city dominated by white minority that works in the city by day but lives in the suburbs of Virginia and Maryland by night.
Assaulted by climate change, massive air pollution, and biologically dead rivers, the cities of the South are becoming environmental disaster areas.
Manila, Shenyang, Mexico City are the rule. In the North, the gains registered in restoring the environments in some cities in the last few years are now threatened by the combination of climate change, tight city budgets imposed by fiscal conservatives, and influential pro-development lobbies.
The urban landscape depicted by Paul Theroux in his classic 1986 novel O-Zone, where the rich live in artificial "green" enclaves protected by private corporate armies from the environmentally devastated areas surrounding them that are populated by the rest, will soon move from fiction to fact.
These trends can be reversed, but only by moves that would truly be revolutionary, among them a rigorous regime of very deep cuts in greenhouse gas emissions; an end to the poverty and inequality creating programs of the World Bank, IMF, and World Trade Organization; a new economic relationship based on justice and equity between the North and the South that would involve strict controls on the operations of transnational corporations.
If the 20th century is any indication, sceptics say, such deep-seated changes can only come after terrible wars and social turmoil.
But perhaps the increasingly common realisation among the rich and well-off that their privileges can no longer be purchased at the expense of the misery of the many and the destruction of the planet might just be the spectre that can bring about a relatively peaceful transition this time around.
Saskia Sassen is a leading theorist of globalisation and its impact on cities. She is the author of the newly published: Territory, Authority and Rights - from Medieval to Global Assemblages.
The urban landscape, no matter where we are, will look different from what it does today. This will certainly be the case in the big cities that are also powerful economic centres.
The way we experience the city today in Europe will be very rare in the future. European cities will feel more like cities of the global south.
I think we are moving in that direction and that means there will be a lot of innovation - it will be a bit more of a free-for-all and we will invent new political forms of membership
Europe will see a lot more immigration and more big cities - and they will have a sense of the frontier town. The city will be a frontier space.
We will have dreadful situations in some of these cities because there will be an awful lot of dispossessed people and a lot of struggle. The centre will not hold, necessarily.
We are just at the beginning of the future - but we can't quite see it. We are entering a phase in which the political will be profoundly changed - in the same way that when citizenship and secular statehood was implemented and there were no more divine monarchs.
What we are going to see is the reinvention of the notion of political. The notion of rights will become rights to the city and that will mean rights to things like housing and rights to water.
Here, in London, for example, you have a sense that things are really governed - in New York less so, in Mexico City even less so and in Sao Paulo, even less so.
I think we are moving in that direction and that means there will be a lot of innovation - it will be a bit more of a free-for-all and we will invent new political forms of membership, which will enable people who are truly marginal to claim their rights to the city.
Interviews by Kathryn Westcott