Humankind is fast becoming an urban species, according to the United Nations. Whereas only 14 percent of the world's population lived in cities in 1900, that number climbed to 47 percent by the end of the 20th century. Tokyo, Bombay, and New York count among the world's 20 “megacities,” metropolises with populations exceeding 10 million. In the United States, more than 80 percent of the population resides in urban areas.