Climate Central: “Last year, atmospheric carbon dioxide briefly crossed 400 parts per million for the first time in human history. However, it didn’t cross that threshold until mid-May. This year’s first 400 ppm reading came a full two months earlier this past week and the seeming inexorable upward march is likely to race past another milestone next month.”
“[Ralph] Keeling runs a carbon dioxide monitoring program for Scripps Institute of Oceanography.” The Keeling Curve shows monthly carbon dioxide concentrations at Mauna Loa Observatory.
“‘It’s just a matter of time before it stays over 400 forever,’ said Ralph Keeling in a blog post.”