Urban areas expanded by at least 22,400 square miles between 1970 and 2000 and are on track to grow by 590,000 square miles by 2030, according to a study by researchers from Yale, Arizona State, Texas A&M and Stanford. As 1.47 billion more people demand space in urban areas, cities probably will extend into "biologically diverse" regions, including "forests, biological hotspots, savannas, coastlines -- sensitive and vulnerable places," said lead author Karen Seto, a professor at the the Yale School of Forestry & Environmental Studies. China, India and Africa have the highest rates of urban growth.

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