Humanity will make the historic transition from a rural to an urban species some time in the next year, according to the latest United Nations population figures. The shift will be led by Africa and Asia, which are expected to add 1,6-billion people to their cities over the next 25 years.

The speed and scale of inevitable global urbanisation is so great most countries will not be remotely prepared for the impact it will have, Thoraya Obaid, executive director of the UN Population Fund, says.

“In human history we have never seen urban growth like this. It is unprecedented.”

Obaid added: “In 2008, half of the world’s population will be in urban areas. The shift from rural to urban changes a balance that has lasted for millennia. Within one generation, five billion people, or 60% of humanity, will live in cities. The urban population of Africa and Asia is set to double in this time.”

She said that each week the numbers living in cities grew by nearly a million.

“Most cities [in developing countries] already have pressing concerns, including crime, lack of clean water and sanitation, and sprawling slums. But these problems pale in comparison with those that could be raised by future growth.

“If we do not plan ahead it will be a catastrophe. The changes are too fast to allow planners simply to react. If governments wait, it will be too late to [gain] advantages for the coming growth.”

According to the State of the World Population report, which Obaid launched on Wednesday in London, large-scale population growth will take place in the cities of Asia, Africa and Latin America.

It suggests the largest transition to cities will occur in Asia, where the number of urbanites will almost double to 2,6-billion in 2030. Africa is expected to add 440-million to its cities in the same period, and Latin America and the Caribbean nearly 200-million.

But urbanisation can be positive. “No country in the industrial age has ever achieved significant economic growth without urbanisation, said Obaid. “Cities concentrate poverty but they present poor people’s best hope of escaping it … The potential benefits of urbanisation, which include easier access to health centres and education, far outweigh the disadvantages.”

The report warns, however, that if unaddressed the growth of urbanisation will mean growth in slums and poverty, as well as a rise in attempted migration away from poor regions.

“Today one billion people live in slums, 90% of whom are in developing countries. The battle to cut extreme poverty … will be waged in the slums. To win it, politicians need to be proactive and start working with the urban poor. The only way to defeat urban poverty is head on,” said Obaid.

Climate is expected to increasingly shape and be shaped by cities. In a vicious circle, climate change will increase energy demand for air conditioning in cities, which will add to greenhouse gas emissions. It could also make some cities unlivable, adding to the “heat island” effect, which can lift temperatures in urban areas.

“Heat, pollution, smog and ground-level ozone [from cities] affect surrounding areas, reducing agricultural yields, increasing health risks and spawning tornadoes and thunderstorms. The impacts of climate change on urban water supplies are expected to be dramatic,” the report says. Cities like New Delhi, in the drier areas, will be particularly hard hit.

What is taking place today, says the UN, is a second great wave of global urbanisation. The first, in Europe, from 1750 to 1950, boosted the numbers living in cities to about 420-million, but the second is expected to increase urbanisation levels close to those found in Europe (72%) and the United States (81%) today.

However, developing countries are at a great disadvantage when they start to urbanise.

“Mortality has fallen rapidly in the last 50 years, achieving in one or two decades what developed countries accomplished in two centuries. The speed and scale of urbanisation today is far greater than in the past. In the first wave of urbanisation, overseas migrations [to the US or Australia] relieved the pressures on European cities.

“Many migrants settled in new agricultural lands. Restrictions on international migration today makes this almost impossible. They will also have to build faster than any rich country has ever done. It will require houses, power, water, sanitation and roads.”

The report also spells the end for growth of existing mega-cities. “Only Dhaka in Bangladesh, and Lagos in Nigeria, of the world’s 20 mega-cities, are expected to grow more than 3% a year in the next decade … most growth will be in smaller cities, of under 500 000 people. The good news is these cities are more flexible [in expansion]; the bad is they are under-served in housing, water, and waste disposal.”

Obaid said: “It concerns everyone, not just developing countries. If we plan ahead we will create conditions for a stable world. If we do not, and do not find education, jobs, and houses for people in cities, then these populations will become destructive, to themselves and others.”