Chart of the Century: The Keeling Curve Edges Up to 400 ppm
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.