400 ppm
The carbon dioxide concentration over the Arctic reached 400 ppm [1]. The globally average carbon dioxide concentration will pass the 400 mark within few year. “This is the first time the entire Arctic is that high,” Pieter Tans said in the interview to the Washington Post [2], and called reaching this level "depressing".
It is not clear, however, whether 400 ppm is merely a psychological milestone, or a scientific one. The current understanding of carbon-climate-human system does not allow to make a definite conclusion about the most probable scenario of world development based on the fact of passing this impressive mark.
[1] NOAA: Carbon dioxide levels reach milestone at Arctic sites, http://researchmatters.noaa.gov/news/Pages/arcticCO2.aspx
[2] Seth Borenstein, When hitting 400 is not good: Levels …

