From PennFuture (http://www.pennfuture.org):
The world’s air has reached what scientists call a troubling new milestone for carbon dioxide, the main global warming pollutant. Monitoring stations across the Arctic this spring are measuring more than 400 parts per million of the heat-trapping gas in the atmosphere. The number isn’t quite a surprise, because it’s been rising at an accelerating pace. Years ago, it passed the 350 ppm mark that many scientists say is the highest safe level for carbon dioxide. It now stands globally at 395. [AP Story by Seth Borenstein]
It’s been at least 800,000 years — probably more — since Earth saw carbon dioxide levels in the 400s….