Burgeoning ... not even national borders can slow the spread of Lagos in Nigeria. Photograph: David Levene  "Urbanisation is unstoppable, says UN," ran the headline in last week's Guardian. Do you detect a note of worry in the wording? It's almost as if the message were "Global warming is unstoppable" or "Kiss goodbye to your weekends in the country". But it might have been better written this way: "Urbanisation is the greatest design challenge we face in the 21st century."  The story was about UN-Habitat's latest report on the state of the world's cities, in which the agency predicts that some metropolises will join up like blobs of mercury to create "mega-regions". One of these is in west Africa, where the cities of Lagos, Ibadan, Lomé and Accra are threatening to merge. Which is fine, except that the amalgamation would sprawl across the national borders of Nigeria, Benin, Togo and Ghana.  For the first time in history, more people live in cities than in the countryside. By 2050, three quarters of us are expected to be urbanites. That's a lot of people heading for the bright lights. But here's the scary part: most of this growth is happening in places where millions of people already live in slums. Mumbai, Delhi, Karachi, Shanghai, São Paulo, Kinshasa: these are the fastest-growing cities in the world, most of them destined to have populations of more than 20 million by 2025. Between now and then, Lagos will have to make room for 67 new arrivals per hour. If we don't start designing for these new inhabitants now, then the potential for human misery is all the greater.  The cities we love most are slow burners, layered accretions of history – London, Paris or Rome, for instance. Yet here we are, in the position of having to manufacture new urban spaces, as though cities were just another type of product. The philosopher Henri Lefebvre anticipated this situation in the 1960s, when he argued that urban space was the new commodity. He saw that it wasn't just stocks and shares that were being speculated on but pieces of city. Lefebvre's theory was that industrialisation – the story of the last 150 years – was being replaced by urbanisation. Today, we talk about having moved from the industrial age to the information age. But this is also the urban age.  The commodification of urban space has come on in leaps and bounds since Lefebvre's day. The same deregulation that relieved the banks of any compunction to behave responsibly has also been changing the visible face of the city. The free-market agenda is what makes public spaces in so many cities nothing more than places to shop at chain stores or drink at Costa Coffee, often under the supervision of private security guards. The privatisation of public space, like that of public services, is one way that governments can avoid their democratic accountability.  Yet even Lefebvre didn't predict the extent to which cities would become products. These days a client can order a new metropolis simply by picking up the phone to a famous architect. Take Masdar, the eco-city in Abu Dhabi designed by Norman Foster, or the oft-cited eco-cities in China, such as Dongtan, designed by London-based engineers Arup (currently on hold), or Tianjin. With China expecting 300 million new urban dwellers in the next 20 years, it has no choice but to adopt the build-it-and-they-will-come approach.  Of course, city production has a history. There was Ebenezer Howard's Garden Cities movement at the turn of the last century, which resulted in new towns such as Letchworth; and New Urbanism in the US, which yielded places such as Seaside in Florida, used as the setting of The Truman Show. But these were escapist fantasies designed by people who hated cities. More relevant are those created as heroic emblems of political will, such as Brasilia in Brazil, Astana in Kazakhstan or (the example that trumps them all) Dubai. Some of the shine may have sloughed off Dubai since the recession, but there has never been such a glaring example of the city as a product ­– as a brand, even.  The brand consultant is the éminence grise of the modern city. Sometimes it's the mayor, and sometimes it falls to cult designers of Joy Division covers such as Peter Saville, who a few years ago was appointed creative director of Manchester. Or take the Canadian design guru Bruce Mau, who is called up with requests such as: "Can you provide a vision for the future of Mecca?" This has come to be known as "design thinking", and it is ever more important to cities in a competitive global environment where lucrative awards such as the Olympics, European Capital of Culture and World Design City are on offer. Cities, the engines of the world's wealth, are sometimes more important than the countries they are located in.  The question is this: how do we create cities that are not just containers for tightly-packed populations, but pleasant and equitable places to live? Someone once described the identical high-rises that ring so many capitals as the easyJet of urban living, because they offer everyone affordable access to the city; but they're not what you could call idealistic. The segregation and social polarisation of cities is getting so extreme that a violent future may be inevitable. The UN report has said as much. Now that city-making has become a priority, politicians need to have faith in designers. Because if there's one lesson to be learned from the last quarter of a century, it's that we need to shift our focus away from liberty and the free market, and move towards equality.
Justin McGuirk on design: Amid unprecedented levels of urbanisation, designers must be trusted to fashion cities that not only accommodate but also provide a pleasant environment