The atmospheric carbon dioxide (CO₂) concentration at Mauna Loa, Hawaii, where Dr C.D. Keeling started monitoring in 1958, recently exceeded 400 parts per million (ppm). The daily average recorded above 400 ppm for the first time on 9 May 2013, and the weekly average recorded 400.03 ppm in the fifth week of May (26 May–1 June). The monthly average CO₂ levels also exceeded 400 ppm at Ryori, Iwate Prefecture (February 2012), Yonagunijima  Island, Okinawa Prefecture (January 2013), and Minamitorishima Island, Tokyo (April 2013), where the Japanese Meteorological Agency is monitoring. Some experts suggest that in order to limit global warming to two degrees Celsius, it is necessary to stabilise the CO₂ level below 400 ppm, but this is no longer possible.

I used to analyse atmospheric concentration of greenhouse gasses (GHG) at the graduate school of Tohoku University. The main theme of the research was to analyse GHG concentrations above Siberia; around that time (1995), the yearly average CO₂ concentration at Mauna Loa was approximately 360 ppm. Each day, I analysed flask-filled air from various places with NDIR (Non-dispersive Infrared Analyzer). I remember reading the tick marks on recording papers and presuming 400 ppm would not be recorded until the distant future.

However, it is now a fact that the atmospheric CO₂ concentration is increasing rapidly. At Mauna Loa, the yearly average increase rate was 0.8 ppm/year in the 1960s, but it then accelerate, doubling the rate, at 2.0ppm/year, in the 2000s. Notably, the increased rate in 2012 was the largest ever, with 2.65 ppm/year. Given the yearly average concentration was 393.82 ppm in 2012, and has increased by about 2 ppm for the past 10 years, the yearly average concentration at Mauna Loa will exceed 400 ppm within two to three years.

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An excerpt from the UNFCCC Media Alert:

"With 400 ppm CO₂ in the atmosphere, we have crossed an historic threshold and entered a new danger zone. The world must wake up and take note of what this means for human security, human welfare and economic development. In the face of clear and present danger, we need a policy response which truly rises to the challenge. We still have a chance to stave off the worst effects of climate change, but this will require a greatly stepped-up response across all three central pillars of action: action by the international community, by government at all levels, and by business and finance."

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Source: Global CCS Institute, Insights Perspectives from around the world, 13 Nov 2013 | Yasuki Shirakawa