Altering the upper atmosphere could block enough sunlight to offset the warming effects of climate change and protect food crops. But what are the risks? iStockphoto hide caption

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In February 1960, college students (from left) Joseph McNeil, Franklin McCain, Billy Smith and Clarence Henderson began a sit in protest at the whites-only lunch counter at a Woolworth's in Greensboro, N.C. Jack Moebes/CORBIS hide caption

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Ground beef used to be a cheap, go-to dinner meat, but no longer. iStockphoto.com hide caption

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Rachel Zayas, a registered nurse, sets up the shift board for the night shift at the Cleveland Clinic. Chuck Crow/The Plain Dealer /Landov hide caption

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