May 13, 2013

On May 9, the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, surpassed 400 parts per million for the first time since measurements began in 1958. It marks an important milestone because Mauna Loa, as the oldest continuous carbon dioxide (CO2) measurement station in the world, is the primary global benchmark site for monitoring the increase of this potent heat-trapping gas.