By Mark Kinver
Science and nature reporter, BBC News
The world is fast approaching the point where the majority of the human population will be found in urban areas.
Somewhere, sometime in 2007, someone migrating from their rural home to begin a new life in a town or city will tip the global rural/urban balance, the UN estimates.
Expanding slums are a characteristic of 21st Century urbanisation
Throughout history, the world has experienced urbanisation but the huge rise in the number of people making their homes in towns and cities is a recent phenomenon.
In 1950, less than one-in-three people lived in urban areas. The world had just two so-called "megacities" with populations in excess of 10 million: New York and Tokyo. Today, there are at least 20.
Greater Tokyo, the world's biggest city, has expanded from 13 million residents in 1950, to today's figure of 35 million.
The United Nations estimates that about 180,000 people are being added to the urban population every day. This means the world's urban infrastructure has to absorb the equivalent of the population of two Toykos each year.
North America and Europe's urban areas already account for about 70-80% of the regions' populations, and these are expected to stabilise at these levels.
Developing nations are shouldering the vast majority of this burden, leaving them struggling to cope with the huge influx of people into urban areas. Some cities' populations are 40 times larger than what they were in 1950.
In the traditional model of urbanisation, which North America and Europe experienced during the Victorian era, people were pushed away from the countryside by the mechanisation of agriculture, and pulled towards urban areas by the offer of jobs and wages.
Sub-Saharan Africa, which has the world's highest rate of urban migration, is not following this pattern.
The size of its cities bears no resemblance to their economic wealth and are experiencing what the UN's human settlements agency, UN-Habitat, calls "premature urbanisation".
The agricultural sector is not flourishing and urban areas are not generating economic growth but failing crops, natural disasters and conflicts are forcing people to flood into towns and cities.
Currently, about 36% of Africa's population lives in urban areas but the continent is experiencing urbanisation rates twice as high as those seen during the West's industrial revolution. It is predicted that Africa will be an urban continent by 2030.
Because the urban areas are economically stagnant or in recession, local authorities do not have the money or expertise to provide services such as access to water, housing, education and healthcare.
As a result, 70% of Africa's urban population find themselves living in slums.
Africa is not alone. An estimated one billion people in Latin America, Asia, as well as Africa, live in slums or informal settlements that are not legally recognized.
Without any intervention, this number could double by 2020.
In Asia, China's urbanisation has followed the traditional drivers experienced by the West. Its industrial revolution is the most rapid the world has seen, and the Chinese government says it has helped lift more than 200 million people out of poverty.
Millions of people migrated from rural to urban areas to fill the jobs generated by the economic explosion.
Not everyone sees it that way. Anti-poverty campaigners say many workers receive low wages and live in poor conditions. An estimated 200,000 people each year move to slums on the southern outskirts of the capital, Beijing.
Although China's large-scale poverty reduction strategy could act as a framework for others to adopt, not all regions have the export markets and trade links that South East Asia enjoys.
The urbanisation of poverty is blamed on failed policies
UN-Habitat says the "urbanisation of poverty" has been overlooked. Traditionally, Western aid agencies have focused their efforts on the impact of floods, droughts and conflicts affecting rural dwellers.
In an effort to focus attention on the problem, the UN Millennium Declaration set the target of significantly improving the quality of life for 100 million slum dwellers by 2020.
The UK government's Commission for Africa said that the international community had to work together to tackle the urban poverty gripping the continent.
The commission's concluding report warned: "These slums are filled with an increasingly youthful population, unemployed and disaffected. Africa's cities are becoming a powder keg of potential instability and discontent."
More than 10,000 delegates are expected to attend the third World Urban Forum, being held later this month in Vancouver, Canada.
The two-yearly meeting, organised by UN-Habitat, is viewed as a chance to share experiences and knowledge, and aims to forge partnerships that will help deliver the goal of balancing urbanisation with a city's ability to absorb new inhabitants.
The delegates are aware of the growing sense of urgency of the challenge ahead because the next time they gather, they are likely to meet in an urban world.