A power-generating windmill turbine is seen in front of the Arc de Triomphe on the Champs Elysees avenue in Paris ahead of the COP21 World Climate Summit, which begins Monday. Christian Hartmann/Reuters /Landov hide caption

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Delegates from about 170 countries gathered in Kyoto in December 1997 during the United Nations Framework Convention on Climate Change. This year in Paris, the stakes are even higher, negotiators say. AFP/Getty Images hide caption

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Oranges ripen in a grove in Plant City, Fla. Citrus greening, a disease spread by a tiny insect that ruins oranges and eventually kills the trees, has put the future of the state's $10 billion citrus industry in doubt. Chris O'Meara/AP hide caption

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This year's flu vaccine protects against a few different virus strains, including the H1N1 seen here. The fuzzy outer layer is made of proteins that allow the virus to attach to human cells. NIAID/Flickr hide caption

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An illustration shows what a helicopter drone would look like on the surface of Mars. NASA/JPL-Caltech hide caption

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