Carbon dioxide level breaks 3-million-year record

By Stephanie PappasLiveScienceThe proportion of carbon dioxide in the atmosphere broke 400 parts per million Thursday, according to one of the best climate records available.NBC News.com contributing writer John Roach takes an in-depth look at this greenhouse gas problemThe Keeling Curve, a daily record of atmospheric carbon dioxide, has been running continuously since March 1958, when a carbon di....