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For the past few months, the Keeling Curve, which measures atmospheric carbon dioxide concentrations at Mauna Loa Observatory, has been rising toward its annual late-May peak. Right now it's at 399.5, and any day now it could pass 400 ppm for the first time in modern history. So take a nice long look. This could be the last time in your life that you get to see carbon dioxide levels below 400 ppm on planet Earth.