Now that there are more people living in urban areas than rural ones, cities must cope with issues such as unemployment, environmental degradation, lack of services and inadequate housing. BBC World Service's Metropolis season examines urbanisation in the 21st century

The world's population has grown from three billion to over six billion people in just 40 years, and many experts are worried that the Earth's "carrying capacity" is already overstrained. Although the rate of population growth began to decline about 30 years ago, annual additions to the human population are still close to their highest level - around 77 million being added each year, 200,000 a day.

This is because much of the world's population, especially in the south, is young and therefore has the potential to reproduce. It is thought that the total world population will eventually stabilise at between eight and eleven billion people; this will place a huge additional burden on our planet.

This population growth is fuelling rapid urbanisation, especially in the developing world. Currently, the rate of urbanisation averages 0.8%; this varies from about 1.6% across Africa to about 0.3% in highly industrialised countries. Estimates suggest that by 2030, 60% of the world's population - nearly five billion people - will live in towns and cities.

Rise of the mega city

Significantly, most of today's global population growth is taking place in urban centres and - in a disproportionate number of cases - in the poorest parts of the world. "If current trends continue," said Joe Fontana, Canada's minister of labour and housing, ahead of the forum, "poor countries will have to build the equivalent of a city of more than a million people each week for the next 45 years just to absorb the increased numbers."

Much of the world's population, especially in the south, is young and therefore has the potential to reproduce

Over the past 20 years, Earth has witnessed the rise of the mega-city - urban centres with at least ten million inhabitants - in developing countries. Housing and basic services have struggled to keep pace. Today, the list of the world's 30 largest cities is changing as those in developed countries are knocked from the top spots by the rapidly expanding cities in developing ones.

Between 1980 and 2000, Lagos, Dhaka, Cairo, Tianjin, Hyderabad and Lahore were among the cities from developing countries that joined the list. Meanwhile, Milan, Essen and London (16th in 1980) disappeared, and New York, Osaka and Paris have slipped significantly - without having experienced any fall in their actual size. Tokyo, however, has held on to the number one spot as the world's largest human agglomeration throughout.

The housing crisis

According to a UN report, within 30 years one in every three people will live in a slum unless governments control urban growth. Almost one-sixth of the world's population - about 940 million people - already lives in squalid, unhealthy areas without water, sanitation, public services or security. It is estimated that the number of slum dwellers could rise to two billion by 2020.

Polution and environmental degradation

The rapid growth of cities over the past 30 years has made them major contributors to local, regional and global environmental problems. At present, most cities burn their waste, thus contributing further to pollution levels. Air pollution is high in many parts of the world, and the majority of cities continue to grow without taking appropriate environmental measures to prevent it. Trends suggest that transport in developing countries is a key factor, often responsible for 70-80% of local air pollution. However, in industrialised developing countries such as China, which is dependent on coal, industry is the major cause.

As air and water pollution are no respecters of national boundaries, a global response is needed to reverse environmental degradation and ensure our cities remain healthy places for their inhabitants.

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