Climate Central: The last atmospheric monitoring station on Earth that had not detected carbon dioxide levels above 400 ppm has now done so. Three years ago the first such reading was made by a station on Mauna Loa in Hawaii; last year the global average cleared the same level. But the station located at the South Pole did not do so until 23 May 2016. The last time CO2 levels were that high at the South Pole was 4 million years ago. The 400 ppm level is a symbolic one, but the spread of CO2 globally is not, with particularly high concentrations in the populated Northern Hemisphere spreading to Earth's most remote regions.
