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Urbanization - the demographic shift from country to city - began with industrialization, and it has not let up. In 1900, fewer than 15 percent of the world’s population lived in cities. Fifty years later, that number had doubled to 30 percent of the world’s population of 750 million. By 2000, 2.9 billion people, or 47 percent of the world’s population, were living in urban areas, with the greatest growth occurring in Africa, Asia, and Latin America. In 2007, for the first time in history, the urban population exceeded 50 percent, and by 2050, according to World Health Organization (WHO), seven out of ten people will call urban areas home….

Contrary to common belief, fewer than 10 percent of urban dwellers are residents of megacities with populations of over ten million. A megacity consists of the city proper and its adjoining suburban centers. An example is the New York-Newark aggregation, which in 1950 was the world’s only megacity; by 2011, it was the sixth largest of 21 megacities. The population of Greater Tokyo, the world’s largest urban area and home to 36.7 million residents, is forecast to exceed 37 million by 2020.Megaciites Mumbai, Delhi, Dhaka, and Lagos, which do not yet appear on the Top Ten list, are steadily moving up the ladder. About half of the world’s urban dwellers live in cities of under half a million people, and these cities continue to outpace megacities in growth.

While the benefits of developed infrastructure, public transportation system, employment opportunities, better health care, and education, plus a wide range of services, make cities the place to live, work, and enjoy, they are plagued with enormous problems. As engines of growth, cities have also become engines of pollution, traffic congestion, waste production, and environmental destruction….

In the 1960s, the concept of urban ecology emerged from the growing awareness of cities’ impact on the environment. In 1975, the nonprofit organization Urban Ecology was founded in Berkeley, California, with the purpose of rebuilding cities in balance with nature. The 1992 Earth Summit in Rio de Janeiro established Agenda 21, a plan for the sustainable development of cities, and in 2002, 1,200 representatives (including 200 slum dwellers) from 80 countries participated in the first World Urban Forum, making urban ecology and sustainable eco-cities based on environment, economy, education, equity more than just a nice idea.