Over the next two decades the number of city dwellers is expected to rise to 5 billion, putting pressure on transportation systems, energy and water use, and waste disposal. (Shankar S. Oldandsolo)
Smoke rises from the La Cygne Generating Station power plant in Kansas. Cities account for 70% of climate changing emissions from power plants, transportation and buildings.
Mexico City is among the most polluted cities in the world. While cities account for 65 percent of global GDP, they are faced with public health problems from dangerous pollution.
Brazil, with an 85% urban population, is making aggressive moves to implement public transport. This photo shows a typical traffic jam in Sao Paulo.