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This article from Scientific American is a thorough answer to your question, written by (and based on a study conducted by) Mark Z. Jacobson, a professor of civil & environmental engineering at Stanford University and director of the Atmosphere/Energy Program there and Mark A. Delucchi, a research scientist at the Institute of Transportation Studies at the University of California, Davis: http://www.scientificamerican.com/article.cfm?id=a-path-to-sustainable-energy-by-2030
Also read 'Plan B' by Lester Brown.