Researchers have analyzed the fossil imprints of of raindrops, like the ones shown here, to study the atmosphere of the Earth, as it was 2.7 billion years ago. The rule at the top is 5 centimeters, or about 2 inches, long. W. Alterman/University of Pretoria hide caption

itoggle caption W. Alterman/University of Pretoria

A drilling rig sits on Oooguruk Island off the coast of Alaska's North Slope. The 6-acre island was built by Pioneer Natural Resources so it could drill for oil on the Arctic Ocean. Steve Quinn/AP hide caption

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F. Sherwood Rowland, pictured here in 1989, was one of three chemists who shared the 1995 Nobel Prize for chemistry for work on discovering chemicals that deplete the Earth's ozone layer. University of California/AP hide caption

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Last year's earthquake and tsunami crippled the Fukushima Dai-ichi nuclear power station. Foreign journalists saw cleanup and recovery work in process on Feb. 28. Yoshikazu Tsuno/AFP/Getty Images hide caption

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A worker is given a radiation screening as he enters the emergency operation center at Tokyo Electric Power Co.'s tsunami-crippled Fukushima Dai-ichi nuclear power plant on Feb. 20. AFP/Getty Images hide caption

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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

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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

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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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