Almost 3 000 million people, about half of the world's population, live in urban areas and about 160 000 more join them every day. Cities affect far more than the areas they occupy: their 'ecological footprints' can be enormous because of their huge demands for energy, food and other resources, and the regional and global impacts of their wastes and emissions to soil, air and water. London's ecological footprint, for example, considering only its consumption of food and forest products, and the area needed to assimilate its emissions of carbon dioxide, is calculated to be 125 times greater than the area of the city itself (IIED 1995).
Urbanization has been one of the most striking developments of the 20th century. In Africa, for example, only 5 per cent of the population lived in urban areas at the beginning of the century, about 20 per cent in the 1960s and about 35 per cent in 1995. Africa's annual urban growth rate is now the highest in the world, at more than 4 per cent (see graph). The urban population of the Asia-Pacific region, now about 35 per cent of the total population, grew by 3.2 per cent a year between 1990 and 1995, compared with 0.8 per cent a year for the rural population (United Nations Population Division 1997). About 70 per cent of the population of North America, Europe and Latin America now lives in cities, and worldwide 326 cities have populations of more than 1 million, compared with 270 in 1990 (WRI, UNEP, UNDP and WB 1996). In Western Europe (EEA 1998) and North America (WRI, UNEP, UNDP and WB 1996), in contrast with most other regions, there is a move out of large cities into suburbs and smaller urban centres.
Most of the growth in the world's population is taking place in developing countries and most of the projected increase of 1 000 million people between 1999 and around 2010 is likely to be absorbed by cities in these countries - cities already faced with enormous backlogs in housing and infrastructure development, and struggling with increasingly overcrowded transportation systems, insufficient water supplies, deteriorating sanitation and environmental pollution. In spite of this, people continue to migrate to cities in the hope of a better life, often as a result of the devastation of rural economies by land degradation.
At least 600 million urban dwellers in Africa, Asia and Latin America live in squatter settlements and shanty towns, in housing of such poor quality and with such inadequate provision for water, sanitation, drainage and garbage removal that their lives and health are under continuous threat (UNCHS 1996). The number of people living in such conditions is likely to expand very rapidly; while large cities in some developing countries have been growing at rates of up to 10 per cent per annum, slums and squatter settlements in some of them are growing twice as fast. An increasing number of the urban poor, probably more than 100 million, are homeless, a serious problem in developed as well as in developing countries (UNCHS 1996).
Among the most serious environmental problems in cities are air and water pollution, solid waste accumulation and disposal (including toxic and hazardous wastes), and noise. Many cities are also at risk from natural hazards or hazards whose origin may be natural but where human actions have significantly increased the level of risk.
Most urban air pollution comes from the combustion of fossil fuels, in motor vehicles and for industrial processes, heating and electricity generation, but some also comes from incinerators, petrochemical plants and refineries, metal smelters and the chemicals industry. Some primary pollutants can combine to form even more damaging secondary pollutants. For example, ozone and other photochemical oxidants are formed when hydrocarbons react with nitrogen oxides and oxygen in the presence of sunlight. Tropospheric ozone is one of the major components of urban smog, a growing problem in cities around the world.
While urban air pollution is coming under control in some countries, the situation is deteriorating rapidly in many heavily industrialized cities in developing countries. In China, for example, smoke and small particles from burning coal cause more than 50 000 premature deaths and 400 000 new cases of chronic bronchitis a year in 11 of its largest cities (World Bank 1997), and private car circulation has been restricted in some cities in South America and Europe in attempts to reduce harmful levels of air pollution. Worldwide, more than 1 000 million urban residents are exposed to health-threatening levels of air pollution (Schwele 1995).
Many cities are facing serious shortages of safe water as a result of over-exploitation of resources and pollution. Bangkok's water table, for example, has fallen 25 metres since the late 1950s and saltwater has penetrated its wells (WWF 1990). Daily demand for water in Beijing increased almost 100 times between 1950 and 1980 (WRI, UNEP and UNDP 1992). Urban demand for water in Latin America is likely to rise fivefold during the next four decades (WRI, UNEP and UNDP 1994).
Recent trends in urbanization reflect economic and political changes. Within the context of the structural changes in the world economy, some regions and cities have proved more flexible than nations in adapting to changing economic conditions. Well-managed urbanization can lead to improvements in the living standards of the world's population. However, the transition to an urbanized world has profound implications for the state of the world environment.