The urban population of Africa and Asia will double by 2030, according to the latest annual UN population report. This rapid growth in the world's cities will result in a dramatic increase in the number of teenagers living in extreme poverty, the UN Population Fund (UNFPA) warns, and will also lead to a surge in violent crime and HIV infection.
By 2030, 80 per cent of the world's urban population will live in the developing world and 60 per cent will be under 18. The majority of these teenagers will grow up in poverty. The picture is one of chaos and misery: a vast population of young people packed into squalid slums without opportunities, law or essential services.
The consequences could be catastrophic. Poor youngsters aged 15 to 24 are both the principal victims and the perpetrators of violent crime; about half of all new HIV infections occur in this age group. Unless something is done now, this "youth bulge" will signal a disastrous upsurge in violence and HIV infection. Significantly, the fastest-growing urban population is in sub-Saharan Africa, the epicentre of the Aids pandemic.
Africa's urban population will increase from 294 million to 742 million, Asia's from 1.36 billion to 2.64 billion and Latin America and the Caribbean from 394 million to 609 million. Urban populations of the developed world will show far slower growth: from 870 million to just over 1 billion.
The greatest growth will not be in the mega-cities (those with 10 million or more people such as Calcutta and São Paulo), but in settlements of 500,000 or less where there is room for expansion.
In sub-Saharan Africa, the driving factors of this increase will be the movement of people uprooted by drought, famine, ethnic conflicts and wars.
The authors do not believe this rapid growth of shanty towns and cities need necessarily spell disaster. They remain hopeful about the world's urban future, concluding that, if the right preparations are made now, the concentration of the world's population in cities can be positive.
Yet the catalogue of profoundly depressing statistics offers plenty to be concerned about.
There are already growing pressures on natural resources. In East Africa, the average time spent waiting for water increased from 28 minutes each day in 1967, to 92 minutes a day in 1997, raising concerns at how a region already suffering from water shortages will supply twice as many people just two decades from now.
"We are not on schedule to meet this challenge," says Stephen Turner, deputy director of Water Aid. "Sub-Saharan Africa is way off-track. The issue is not water scarcity, it is water equity. The hidden scandal is that governments and donors are not making this a priority."
Cities, the UN report suggests, can be the solution to some impending environmental problems. For example, they may be the most effective way of providing people with essential services, protecting the environment by containing human settlement to a smaller area.
But cities are environmentally friendly only if well planned, argue commentators such as climate expert, George Monbiot: "In urban sprawl you get the worst of all worlds - an urban population living with few natural resources to fall back on when times are hard but spread over a large area that is very difficult to supply with essential services."
Governments and local authorities have tended to worry about the civil unrest resulting from large populations of frustrated young people living in slum conditions, which is why current policies focus on reducing migration from rural areas and making it hard for new arrivals to settle in shanty towns.
But such policies are counter-productive, the report argues. Strategies to prevent people settling in cities merely encourage unplanned and unsafe housing. If policy-makers continue to resist the "inevitable" growth of cities, we will see more wealthy, fortress communities abutting slums that are rife with violence and disease.
The only option, the report's authors conclude, is to try to strengthen the positive aspects of urbanisation. This means co-operating with grass-roots organisations, empowering women, investing in education and improving access to contraceptives (all ways to reduce birth rates).
It also means, say the report's authors, accepting the inevitable and providing land for people to build new shanty towns but ensuring that they are equipped with roads, electricity, clean water, sewers and waste disposal.
"We are really at the crossroads," says George Martine, lead author of the report. "Unless we take action then chaos is what we have in store. Many of these ideas [such as the provision of sites and services for shanty towns] have been tried and later buried under political concerns. We need to correct the misconceptions that governments in the developing world have about urbanisation - we need a new mindset.
"Our message is one of hope, but it is also a call to action."
"State of World Population 2007: Unleashing the Potential of Urban Growth" is available from 27 June