From left, enginers Eric Nicosia, Amin Ahmadi and Gavin Boogs work to solve an issue with part of a wind turbine at the Gamesa Corp. factory in Langhorne, Pa., on Feb. 10. Maggie Starbard/NPR hide caption

itoggle caption Maggie Starbard/NPR

This map shows what the Earth's landmass looked like in the Precambrian Era, about 738 million years ago. Chris Scotese/University of Texas at Arlington hide caption

itoggle caption Chris Scotese/University of Texas at Arlington

Natural gas is much cleaner than coal. But some energy analysts say an overabundance of the fuel could depress development in even cleaner energy sources like wind and solar power. Above, a rig in Washington, Pa., drills into shale rock to extract natural gas. Keith Srakocic/AP hide caption

itoggle caption Keith Srakocic/AP

Delegates worked into the early hours of Sunday morning on the final day of the climate talks in Durban, South Africa. Rajesh Jantilal/AFP/Getty Images hide caption

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U.S. envoy Todd Stern delivers a speech on Thursday in Durban, South Africa, during the U.N. Climate Change Conference. Stephane De Sakutin/AFP/Getty Images hide caption

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Key provisions of the Kyoto Protocol expire in December of 2012, and experts say there's no real global framework in place to replace the treaty that was supposed to be the first step toward ambitious actions on climate change. Above, a coal-fired power plant in eastern China. China is now the leading carbon dioxide emitter in the world. AFP/Getty Images hide caption

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The U.S. is second only to China in emitting gases that cause global warming. Above, the smoke stacks at American Electric Power's Mountaineer power plant in West Virginia. Saul Loeb/AFP/Getty Images hide caption

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A U.N. climate panel says that we can expect more extreme weather conditions as a result of climate change. Above, people run from a high wave on Nov. 8 in Nice, France, where heavy rain and flooding forced hundreds to evacuate. Vallery Hache/AFP/Getty Images hide caption

itoggle caption Vallery Hache/AFP/Getty Images