From Motherjones.com:
Last week in Washington, DC, leading climate scientist Michael Mann of the University of Pennsylvania sat down with Climate Desk Live to talk about the significance of an planetary milestone—we've reached 400 parts per million of carbon dioxide in the atmosphere. As Mann explained, humans are altering the content of the atmosphere at an alarming rate—one perhaps never seen before in the history of Earth itself.
http://www.motherjones.com/blue-marble/2013/05/michael-mann-hockey-stick-climate-desk-live