Since the emergence of the first concentrated human habitats some 10,000 years ago, urbanization has increased vertiginously. Even if not everyone agrees on what exactly constitutes a city or an urban area, most people agree that rapidly increasing urbanization is a new and seemingly uncontrollable phenomenon. At the beginning of the twentieth century, only about 5 percent of the world population lived in urban areas. Today, the figure is 40 percent and is projected to increase to 60 percent in the next 20 years. In the United States, the percentage will be even higher. If current trends continue, by 2030 all of the world’s population growth will be in urban areas. Over the next 30 years, urban population will increase from 2.9 billion to 4.9 billion people, concentrated mostly in developing nations. The greatest population growth will occur in Asia, but Africa will have a higher rate of growth. The number of cities with 5 million inhabitants will increase from 41 to 59, and the number of cities with 10 million inhabitants (called megacities) will increase from 19 to 23, mostly in the developing world (Brennan-Galvin, 2000).
Urbanization is the most powerful and most visible anthropogenic force on Earth, affecting the surface of the Earth, the atmosphere, and the seas. The expanding surface area occupied by cities and the resources required to supply their needs are absorbing or transforming, directly or indirectly, increasing amounts of forests and arable land. Because cities are virtually devoid of oxygen-generating vegetation, they exacerbate the problems of atmospheric pollution. The surface “footprint” of a typical city consists predominately of buildings and concrete or asphalt, all of which repel water and can lead to deprivation and even subsidence