The statistics of urban growth in the late 20th century surpass any other demographic indicators. The proportion of the world’s population that lives in cities rose from 29 percent in 1950 to 47 percent in 1998, and 55 percent is anticipated by 2015. Although two thirds of urban residents live in cities of less than a million people, megacities with a population of more than 10 million are on the increase. In 1975 there were five, by 1995 there were 15 and by 2015 there are expected to be 26.

Some modern megacities have an ancient history. Cairo, Istanbul (Constantinople) and Baghdad began the second millennium as they ended it – among the world’s top 20 cities. But as industrialization has moved around the globe the roll call amongst the top ten has changed. At the start of the 20th century, these were all in the wealthy and rapidly industrializing North. By 2015 Tokyo and New York alone will remain, to be joined by cities from the developing world, which has seen a sixfold increase in urban populations in just 50 years.

Cities grow around activities best carried out centrally, such as government, manufacturing, wholesaling and ports. They are encouraged by the development of new services such as banking and the accumulation of skilled labor, by the opportunities to socialize and enjoy recreation and cultural activities, and by the value of cities as centers for national and international communication.

Large numbers of people are flocking to cities in search of work as the mechanization of farming is reducing the demand for labor in the countryside. China has a “floating population” of 80 million rural people who have moved to the cities in recent years². Between a third and a fifth of the residents of its two largest cities, Shanghai and Beijing, are migrants³.

The globalization of industry and trade is further stimulating urbanization, and cities are undoubted economic powerhouses. The World Bank estimates that urban areas in the developing world account for between 65 and 80 percent of national GDP (roughly double what might be expected from their populations). Sao Paulo alone contributes 40 percent of Brazil’s GDP.

Cities represent, for many, the good life. On average, urban dwellers have higher incomes and live healthier, easier lives than their rural counterparts⁴. Surveys in 17 countries show that urban children under two have a 25 percent better chance of survival to adulthood than rural children⁵.

But the benefits are not universal. While on the whole urban populations have greater access to clean water and sanitation than their rural counterparts, between a quarter and a half of urban inhabitants in developing countries live in slums and squatter settlements with extremely limited services. Such overcrowding encourages epidemics of tuberculosis, diarrhea and other communicable diseases⁶. In Karachi, a city of 10 million people and growing by half a million a year, 40 percent of the population lives in squatter colonies and one in five babies do not reach their first birthday. Worldwide, more than a billion people live in urban areas where air pollution exceeds acceptable levels⁷. The death toll from lung disease associated with urban air pollution could be half a million a year in China alone. The notorious traffic congestion in Bangkok costs an estimated 2 percent of Thailand’s GDP. Cities can also be violent. The greatest causes of death among young people in Sao Paulo are traffic accidents and homicide.

Cities also have a large ecological “footprint”. They call on resources over a wide area to provide food and raw materials. Vancouver’s half a million people consume resources from an estimated 2 million hectares – 200 times the area of the city itself ⁸. London’s footprint is 120 times the size of the city, drawing on resources from the wheat prairies of Kansas, the tea gardens of Assam and the copper mines of Zambia among other places⁹. Locally, cities put huge strains on natural ecosystems, polluting rivers and coastal waters, consuming forests and water, degrading soils, disrupting drainage and stunting crops. Urban smog and acid deposition in China are estimated to be reducing crop yields by up to a third¹⁰.

Cities stop growing if and when the problems of congestion and pollution overwhelm the benefits, making the cities inefficient as well as unpleasant. Smoggy and congested Mexico City was once expected to grow to more than 30 million people by the year 2000, but was at around half that in 1996 and is not expected to be above 20 million in 2015¹¹. And even while cities are still growing rapidly in the developing world, the growth rates themselves are on a downward trend.

In the developed world, many cities are losing population as fertility rates fall below replacement levels and inhabitants leave for more attractive suburbs or rural areas. Good transportation systems and electronic communications encourage this. One result has been the formation of large low-density peri-urban zones, sometimes embracing several cities to create polycentric urban areas, such as the Japanese urban heartland between Tokyo and Osaka, the Rhine-Ruhr region of Europe and the east coast of the United States from Boston via New York to Washington DC.

The critical question for cities is whether the wealth they generate can justify their large ecological footprint, and whether development policies can reduce that footprint. A well-run urban sector can ensure national prosperity; a badly run sector can become a drag on the whole country. And cities do have potential advantages. Well-planned cities can utilize high population densities to minimize resource use and energy consumption – by developing mass transit systems to supplement car use, for instance¹². In developed and developing country alike, many cities include large areas of productive agricultural land amid the highways and high-rise. It is estimated that up to a fifth of the world’s food is grown in “urban” areas. Other cities, particularly in Western Europe, are investing large sums in recycling and composting as part of ambitious waste-management programs.