On May 9 CO2 reached 400 ppm at the National Oceanic and Atmospheric Administration’s (NOAA) monitoring centre at Muana Loa and at at the Scripps Institution of Oceanography in San Diego, California. This is what’s been happening over the last 130 years in broad terms:

It seems many news organisations, for example the BBC, and some scientists are stressing that the last time concentrations were so high was 3 to 5 million years ago. In the linked article Michael Mann, director of the Earth System Science Center at Penn State gives a different view:

Mann said the last time scientists are confident that CO2 was sustained at the current levels was more than 10 million years ago, during the middle of the Miocene Period. Continue reading CO2 hits 400 ppm →