Guest writes "Urbanization is one of the dominant demographic trends of our time. In 1900, 150 million people lived in cities. By 2000, it was 2.9 billion people, a 19-fold increase. By 2007 more than half of us will live in cities—making us, for the first time, an urban species.
Cities require a concentration of food, water, energy, and materials that nature cannot provide. Concentrating these masses of materials and then dispersing them in the form of garbage, sewage, and as pollutants in air and water is challenging city managers everywhere.
Earth Policy Institute"
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