For the first time ever, National Oceanic and Atmospheric Administation (NOAA) scientists measured the average concentration of atmospheric carbon dioxide at 400 parts per million in Mauna Loa, Hawaii.
"We have now crossed a dangerous line in the global build up of greenhouse gases. Greenhouse gas concentrations have moved from the pre-Industrial Revolution level that never exceeded 280 parts per million (ppm) to a new daily average of 400 ppm. Over a period of the last million years, CO2 never exceeded 280 ppm . The last time greenhouse gases reached 400 ppm was three million years ago. Put simply, humanity has now changed the chemistry of our atmosphere to replicate pre-historic levels -- a time when no humans existed." +Elizabeth May
Read more about it here: http://www.greenparty.ca/blogs/7/2013-05-22/400-ppm