A police officer guards Cambodia's famed temple of Angkor Wat. The powerful city-state collapsed in 1431 after suffering through two decades of droughts. Heng Sinith/AP hide caption

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Two Iberian lynxes at a nature reserve in northern Spain. (February 2006 file photo.) Victor Fraile /Reuters /Landov hide caption

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World War Z is just the latest pop-culture incarnation of the Zombie Apocalypse. Adam Frank says the zombies keep coming because they're trying to tell us something. MPC/Paramount Pictures hide caption

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The Capitol dome is seen behind the Capitol Power Plant, which provides power to buildings in the Capitol complex in Washington, D.C. Carolyn Kaster/AP hide caption

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Crew members unload a catch of sockeye salmon at Craig, Alaska, in 2005. Researchers say fish are being found in new areas because of changing ocean temperatures. Melissa Farlow/National Geographic/Getty Images hide caption

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Heron Island is located on the southern end of the Great Barrier Reef, about 25 miles off the northeast coast of Australia. Ted Mead/Getty Images hide caption

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Wild bees, such as this Andrena bee visiting highbush blueberry flowers, play a key role in boosting crop yields. Left photo by Rufus Isaac/AAAS; Right photo courtesy of Daniel M.N. Turner hide caption

itoggle caption Left photo by Rufus Isaac/AAAS; Right photo courtesy of Daniel M.N. Turner

A map depicts temperature changes over the past 20 years, compared to the average between 1901 and 1960. "The period from 2001 to 2011 was warmer than any previous decade in every region," according to the National Climate Assessment. NOAA NCDC/CICS-NC hide caption

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Wind generators at Pacific Hydro's Cape Bridgewater wind farm in Warrnambool, Australia, in May 2012. Each generator produces roughly enough energy to meet the annual needs of 12,000 households. Mark Dadswell/Getty Images hide caption

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