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Climate Tipping Point? Concentration of Carbon Dioxide Tops 400 ppm for First Time in Human History

The 400 ppm threshold is widely recognized as a dangerous level that could drastically worsen human-caused global warming.

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MICHAEL MANN: There are valid points of view across the spectrum. Thank you.

AMY GOODMAN: I thank you so much for being with us, leading climate scientist, distinguished professor of meteorology, speaking to us from Penn State University. His book,  The Hockey Stick and the Climate Wars.


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