For the first time since the measuring of the atmospheric carbon dioxide concentration began in 1958, the Keeling curve surpassed the 400 ppm CO2 milestone on May 9, 2013.
The CO2 censor reading at the Mauna Loa Observatory on May 09, 2013: 400.03 ppm
Source: National Oceanic and Atmospheric Administration. (2013). Carbon Dioxide at NOAA’s Mauna Loa Observatory reaches new milestone: Tops 400 ppm. Silver Spring, MD: NOAA Research. Retrieved May 10, 2013 from http://researchmatters.noaa.gov/news/Pages/CarbonDioxideatMaunaLoareaches400ppm.aspx