At Mauna Loa, Hawaii - the oldest and benchmark atmospheric measurement station in the world - since 1958, daily mean CO2 concentrations reached 400,03 ppm on May 9. The 400ppm threshold had already been broken first at stations in the Arctic in 2012, and at stations closer to the equator at the end of April 2013.

The upper safety limit for atmospheric CO2 is 350 ppm, according to the Inter-Governmental Panel on Climate Change. That's the limit necessary to keep global warming and climate change at bay, or at least within manageable levels. But the level of greenhouse gases concentration have stayed way above the limit since the mid-80s and, on average, have risen by 2ppm per year in the last 10 years.