Average CO2 levels rise past nasty threshold | NYT

The average carbon dioxide reading surpassed 400 parts per million at the research facility atop the Mauna Loa volcano on the island of Hawaii […] The best available evidence suggests the amount of the gas in the air has not been this high for at least three million years […]
http://www.nytimes.com/2013/05/11/science/earth/carbon-dioxide-level-passes-long-feared-milestone.html