"Carbon dioxide above 400 parts per million was first seen in the Arctic last year, and had also spiked above that level in hourly readings at Mauna Loa. But the average reading for an entire day surpassed that level at Mauna Loa for the first time in the 24 hours that ended at 8 p.m. Eastern Daylight Time on Thursday. The two monitoring programs use slightly different protocols; NOAA reported an average for the period of 400.03 parts per million, while Scripps reported 400.08." "Indirect measurements suggest that the last time the carbon dioxide level was this high was at least three million years ago, during an epoch called the Pliocene. Geological research shows that the climate then was far warmer than today, the world’s ice caps were smaller, and the sea level might have been as much as 60 or 80 feet higher. Experts fear that humanity may be precipitating a return to such conditions — except this time, billions of people are in harm’s way."
Source: Justin Gillis, "Heat-trapping Gas Passes Milestone, Raising Fears," New York Times. New York Times, 10 May 2013. Web. 30 May 2013. http://www.nytimes.com/2013/05/11/science/earth/carbon-dioxide-level-pas... [verified 4/17/14]