In this photo released by the U.S. Marines and taken in December 2010, Lance Cpl. Dakota Hicks, from Laharpe, Ill., connects a radio battery to a portable solar panel communication system in Sangin District, in Afghanistan.The U.S. military is trying to wean itself off reliance on fossil fuels by employing solar energy and biofuels, among other measures. Gunnery Sgt. William Price Small/AP hide caption

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A group of harbor porpoises in San Francisco Bay, photographed from the Golden Gate Bridge. Harbor porpoises haven't been seen in the bay since the 1930s. Researchers believe World War II activity may have contributed to their disappearance. The Navy strung a seven-mile underwater net across the mouth of the bay to keep out enemy submarines. William Keener/Golden Gate Cetacean Research hide caption

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Air travel contributes only 2 to 4 percent of carbon dioxide emissions worldwide. A new ruling says airlines flying into and out of European airports will have to pay a price for the carbon dioxide they emit from burning jet fuel. Above, a plane takes off from the Geneva airport last year. Fabrice Coffrini/AFP/Getty Images hide caption

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