Population, Urbanization, and the Environment

Leiwen Jiang, Malea Hoepf Young, and Karen Hardee

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.

Between 2007 and 2050, the UN projects that global populationwill increase by 2.5 billion (from 6.7 to 9.2 billion), while the global urban populationwill nearly double (from3.3 billion to 6.4 billion), absorbing all increased population growth aswell as inflows fromrural 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.