National Geographic : 2011 Mar
URBAN SUPERSPRAWL Mexico City, Mexico PABLO LÓPEZ LUZ Some 20 million people live in Mexico City, the world's fifth largest metropolitan area. In 1800 the urban fraction of the global population was 3 percent. Today it is 50 percent and rising. In crowded shantytowns, the need for clean water and sanitation is urgent. But urbanization has an upside: Per capita, cities use less energy and pol- lute less than rural areas.