An activist dressed as a polar bear holds a sign urging the U.S. to stick to the safe limit of 350 parts per million CO2 in the atmosphere, at the UN Climate Summit in Copenhagen, Friday Dec. 11, 2009. The largest and most important UN climate change conference is underway in Copenhagen, aiming to secure an agreement on how to protect the world from calamitous global warming. (AP Photo/Heribert Proepper)

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