The term 'demographics' refers to the distribution of populations over geographic areas. The phenomenon of urban growth during the late 20th century is the most significant demographic trend of our time. For the first time in history, humans are predominantly urban dwellers.

In 1950, 29 percent of the world's population lived in cities. In 1998, the urban population jumped to 47 percent. The trend of urbanization is continuing, with a 61-percent urban population anticipated by the year 2030. Worldwide, 95 percent of population growth between 1996 and 2030 will occur in urban areas.

The extent of urbanization varies around the globe. In 2000, North America, with 310 million, was the most urbanized, at 77 percent. Asia, with a total population of 3.7 billion, was only 38 percent urban, as was Africa, with a population of 784 million. The Pacific Islands, with 30 million, were 70 percent urban. Europe, with 729 million, and Latin America and the Caribbean, with 519 million, were each 75 percent urban.

City living can offer advantages over rural life. Cities have long been regarded as sources of economic opportunity by displaced rural people. Urban dwellers have higher incomes, on the average, and live easier lives than do those living in rural areas. Urban children under the age of two years have a 25 percent better chance of surviving to adulthood than do rural children.

The benefits of city living are not shared equally among all urban dwellers. Although on the whole, urban populations have better access to clean water and sanitation than do rural dwellers, between a quarter and half of urban residents in developing countries live in slums with extremely limited services and widespread medical problems. In Karachi, Pakistan, a city of 10 million, 4 million of whom live in squatter colonies, 20 percent of all infants die before their first birthday. More than one billion people worldwide live in cities with unacceptable air quality. In China, lung disease associated with urban air pollution causes half a million deaths each year.

Urbanization can have both positive and negative effects on the environment. On the plus side, as the human population shifts from rural to predominantly urban areas, there is less pressure to clear land from forests and other natural habitats. Population growth may slow down, as urban women tend to have fewer children than rural women.

Negative environmental effects of urbanization include increased energy use and carbon dioxide output. Urban residents burn fossil fuels rather than fuel wood, and their food and other necessities must be transported from longer distances. In rapidly growing cities, supplies of clean water, electricity, roads, and sewage treatment facilities often cannot keep pace with population increases. Pollution can cause loss of biodiversity in the surrounding areas.

In cities with sufficient per-capita incomes, political pressure eventually causes environmental standards to be imposed, leading to improved air and water quality. In general, cities tend to stop growing when pollution and congestion problems become excessive. As an example, Mexico City, which was once predicted to grow to 30 million people by the year 2000, is now not even expected to reach 20 million by 2015.