In this Sunday, Dec. 2, 2012 photo, a flock of Geese fly past the smokestacks at the Jeffrey Energy Center coal power plant as the suns sets near Emmett, Kan. Worldwide levels of the chief greenhouse

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In this Sunday, Dec. 2, 2012 photo, a flock of Geese fly past the smokestacks at the Jeffrey Energy Center coal power plant as the suns sets near Emmett, Kan. Worldwide levels of the chief greenhouse gas that causes global warming have hit a milestone, reaching an amount never before encountered by humans, federal scientists said Friday, May 10, 2013. Carbon dioxide was measured at 400 parts per million at the oldest monitoring station in Hawaii which sets the global benchmark. The last time the worldwide carbon level was probably that high was about 2 million years ago, said Pieter Tans of the National Oceanic and Atmospheric Administration. (AP Photo/Charlie Riedel) less 

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Associated Press
Sun, May 12, 2013 1:57 AM EDT