Human populations are becoming more concentrated through urbanisation and it has been estimated that, worldwide, each week, 1 million people move from rural areas into cities. In developed countries, 80% of the population now live in cities, and this contrasts with only 30% in developing countries, but this latter proportion is increasing rapidly. In 1950, no Asian city had more than 8 million people. By 1970, the populations of Beijing and Shanghai reached this size and, by 1990, were joined by Kolkata, Mumbai, Seoul, Tianjin, Jakarta, Delhi and Manila. Cities in other parts of the tropics have shown similar rapid growth.

These large cities require enormous imports of food, water and other materials in order to support their large human populations. Wastes generated within cities have to be removed. These material imports and waste exports exert an ecological stress on the surrounding ecosystems that produce the imports and receive the waste products. Appropriation of these ecosystem goods and services extends the environmental impact of a city far beyond its urban boundary. The extent of this influence is the city’s ecological footprint.