A test field of sorghum outside Manhattan, Kan., planted by Kansas State University. Dan Charles/NPR hide caption

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The Larsen B ice shelf on the Antarctic Peninsula, which is among the places where such ice has been breaking off. Mariano Caravaca /Reuters/Landov hide caption

itoggle caption Mariano Caravaca /Reuters/Landov

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

itoggle caption Victor Fraile /Reuters /Landov

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

itoggle caption Carolyn Kaster/AP

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

itoggle caption Melissa Farlow/National Geographic/Getty Images

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

itoggle caption Ted Mead/Getty Images