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Increasing levels of urbanization are caused by natural growth of the urban population and migration of the rural population towards cities. Over the past halfcentury, a great rural-to-urban population shift has occurred and the process of urbanization (the concentration of people and activities into areas classified as urban) is set to continue well into the 21st century. Driving forces include the opportunities and services offered in urban areas - especially jobs and education - while in some parts of the world, notably Africa, conflict, land degradation and exhaustion of natural resources are also important (UNEP 2000).

Cities play a major role not only as providers of employment, shelter and services but also as centres of culture, learning and technological development, portals to the rest of the world, industrial centres for the processing of agricultural produce and manufacturing, and places to generate income. There is a strong positive link between national levels of human development and urbanization levels (UNCHS 2001b). However, the implications of rapid urban growth include increasing unemployment, environmental degradation, lack of urban services, overburdening of existing infrastructure and lack of access to land, finance and adequate shelter (UNCHS 2001b). Managing the urban environment sustainably will therefore become one of the major challenges for the future.

Levels of urbanization are closely correlated with national income - the more developed countries are already mostly urbanized - and in almost every country, urban areas account for a disproportionate share of the gross national product (GNP). Bangkok, for example, produces 40 per cent of Thailand's output, whereas only 12 per cent of its population lives in this city (UNCHS 2001b). Worldwide, cities produce on average 60 per cent of a country's GNP.

The rapid increase of the world's urban population coupled with the slowing of rural population growth has led to a major redistribution of the population over the past 30 years. By 2007, one-half of the world's population will live in urban areas compared to little more than one-third in 1972, and the period 1950 to 2050 will see a shift from a 65 per cent rural population to 65 per cent urban (United Nations Population Division 2001a). By 2002, some 70 per cent of the world's urban population will be living in Africa, Asia or Latin America (UNCHS 2001a).

The most striking current changes are the levels of urbanization in less developed nations: rising from about 27 per cent in 1975 to 40 per cent in 2000 - an increase of more than 1 200 million people (United Nations Population Division 2001b). Furthermore, there is every indication that the trend will continue for the next 30 years, adding 2 000 million people to the urban population of the presently less-developed nations. Within these global averages, there are complex regional differences in urban growth and change. The annual percentage change in the urban population by region shows a general slowing in the rate of urbanization for all regions except North America - see figure below (United Nations Population Division 2001b).

Distribution of global population (%) by size of settlement, 1975 and 2000
  rural areas <1 million 1-5 million >5 million
  1975 2000 1975 2000 1975 2000 1975 2000
world 62.1 53.0 25.1 28.5 8.0 11.6 4.8 6.9
developed regions 30.0 24.0 46.8 48.1 13.9 18.5 9.3 9.5
developing regions 73.2 60.1 17.6 23.7 6.0 10.0 3.2 6.3
Source: United Nations Population Division 2001a

There has been a dramatic increase in the number and size of megacities (cities with more than 10 million inhabitants) and urban agglomerations in the second half of the 20th century, as well as a change in the geographical distribution of these cities: in 1900, nine of the ten largest cities were in North America and Europe, whereas today only three (Los Angeles, New York and Tokyo) are located in the developed world. However, most of the world's urban population still lives in small and medium-sized cities (see table above) which, in most countries, are now growing faster than the very large cities (United Nations Population Division 2001b).

Urban population (% of regional totals) by region

Nearly half the world population now lives in urban areas. Africa, and Asia and the Pacific, are the world's least urbanized regions, North America, Europe, and Latin America the most urbanized

Source: compiled from United Nations Population Division 2001b

Annual percentage increase in the urban population

While all regions are still urbanizing, the rate at which most are doing so is falling, although rates are changing little in Africa and are actually increasing in North America

Source: compiled from United Nations Population Division 2001b