In 1700, fewer than 7 million people, or just 10 percent of the world’s total population, lived in cities, and only 5 cities had populations of several hundred thousand people or more. The world we live in today has been transformed by urbanization, the process whereby cities, towns, and suburbs grow as populations shift from rural to urban livelihoods. A little over half of the world’s population now lives in cities, and there are more than 400 cities of more than 1 million people and 28 cities of more than 10 million people.

The mechanization of food production has drastically reduced the need for rural labor while also increasing the food supply to the point where large non-farming populations can be supported. This has tended to push people out of rural areas and into cities, where the development of manufacturing and service economies has created many jobs. This process is called the push/pull phenomenon of urbanization. Numerous cities, especially in poorer parts of the world, have been unprepared for the massive inflow of rural migrants, many of whom now live in polluted slum areas plagued by natural and human-made hazards, poor housing, and inadequate access to food, clean water, education, and social services (Figure 1.24C). Often a substantial portion of the migrants’ cash income goes to support their still-rural families.

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The most rapidly growing cities are in developing countries in Asia, Africa, and Middle and South America. The settlement pattern of these cities bears witness to their rapid and often unplanned growth, fueled in part by the steady arrival of masses of poor rural people looking for work (see Figure 1.24B). Cities like Mumbai (in India), Cairo (in Egypt), Nairobi (in Kenya), and Rio de Janeiro (in Brazil) sprawl out from a small affluent core, often the oldest part, where there are upscale businesses, fine old buildings, banks, shopping centers, and residences for wealthy people. Surrounding these elite landscapes are sprawling mixed commercial, industrial, and middle-class residential areas, interspersed with pockets of extremely dense slums. Also known as barrios, favelas, hutments, shanty-towns, ghettos, and tent villages, these settlements provide housing for the poorest of the poor, who provide low-wage labor for the city. Housing is often self-built out of any materials the residents can find: cardboard, corrugated metal, masonry, scraps of wood and plastic. There are usually no building codes, no toilets with sewer connections, and there is little access to clean water. Electricity is often obtained from illegal and dangerous connections to nearby power lines. Schools are few and overcrowded, and transportation is provided only by informal, nonscheduled van-based services.

The UN estimates that currently more than a billion people live in urban slums, with that number to increase to 2 billion by 2030. Life in these areas can be insecure and chaotic as criminal gangs often assert control through violence and looting—all actions that are especially likely during periods of economic recession and political instability.