Advertisement'Dangerous Territory': Carbon Dioxide Levels Reach MilestoneMay 10, 2013Richard HarrisfacebookEmailCarbon dioxide readings at the Mauna Loa Observatory in Hawaii have reached what atmospheric scientist Ralph Keeling calls a "psychological threshold" of 400 parts per million. The amount of carbon dioxide in the atmosphere has been steadily increasing since near-constant measurements began at the observatory in 1958. (National Geographic/Getty Images)