Population, Urbanization, and the Environment: Growing Cities Stress Their Natural Surrounding, but They Can Also Help Protect Them
Jiang, Leiwen, Young, Malea Hoepf, Hardee, Karen, World Watch
People have been moving from the countryside to the city for at least 9,000 years, but this key population trend has now become one of the most visible and profound forces on Earth: 2008 is the first year in which more than half of us have become city dwellers. The process of becoming a mainly urban species has accelerated during the past century and has now concentrated nearly three-and-a-half billion people on less than 3 percent of the planet's land surface. These monumental agglomerations of people, buildings, factories, roads, and vehicles--along with their associated social systems--have manifold and powerful environmental impacts, as well as effects on fertility and population growth rates, that we are only beginning to understand.
The urbanization trend is global, but rates of urbanization have varied significantly by country and region. The world's more developed countries (as classified by the United Nations) were predominantly urban by the 1950s, but the group of less developed countries is not projected to reach this point until 2019, with some important regional and country variations (see Figure 1).
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Between 2007 and 2050, the UN projects that global population will increase by 2.5 billion (from 6.7 to 9.2 billion), while the global urban population will nearly double (from 3.3 billion to 6.4 billion), absorbing all increased population growth as well as inflows from rural areas. This enormous increase in the global urban population will be greater than the current populations of China, India, the United States, Indonesia, and Mexico combined. Further, this increase will be concentrated in the developing world, particularly in Asia and Africa. While developing countries will continue to be predominantly rural for some years to come, they are already home to over 70 percent of the global urban population, a proportion that will increase in the coming decades to over 80 percent by 2050. In contrast, the urban population of the highly urbanized developed countries represents less than 30 percent of global total, and its share will drop to less than 20 percent by 2050 (see Figure 2).
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The world's urban dwellers live in many different types of cities. Megacities--those with over 10 million inhabitants, such as New York, Shanghai, Tokyo, Bombay, and Mexico City--make up 9 percent of the global urban population. However, 52 percent of the world's urban population lives in cities with fewer than 500,000 inhabitants (see Table 1), although the bottom threshold for an urban designation varies by country.
Table 1. City Size and Percentage of Global Urban Population City size Percent global urban population Under 500,000 52% 500,000-1,000,000 10% 1-5,000,000 23% 5-10,000,000 7% Over 10,000,000 9% Source: UN Population Division
It is commonly perceived that urbanization and urban growth are driven by waves of international and rural-urban migration. However, natural increase (the growth in population resulting from a higher rate of births than deaths) is responsible for more than half of urban growth, with the reclassification of rural areas to urban ones contributing to urbanization as well. This natural increase is significant even though fertility rates are nearly always lower in urban than rural areas, influenced by the higher costs of childrearing in urban areas and the lesser need for children's household labor (e.g., for tending to fields or livestock). The trend toward lower fertility rates is further accelerated by increased economic opportunities for women, increased access to education, and greater access to family planning and reproductive health services. For example, urban fertility rates in Kenya, Tanzania, and Uganda are around 40 percent lower than rural rates, and well below the national averages (see Figure 3). …