http://www.huffingtonpost.com/al-gore/carbon-dioxide-400-parts-per-million_b_3253361.html?utm_hp_ref=green
http://www.guardian.co.uk/environment/2013/may/14/record-400ppm-co2-carbon-emissions
http://www.latimes.com/news/science/sciencenow/la-sci-sn-carbon-dioxide-400-20130513,0,7196126.story
Or very nearly, they have reached this milestone.
http://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth's_atmosphere
The National Geographic noted that the level of carbon dioxide in the atmosphere is this high "for the first time in 55 years of measurement—and probably more than 3 million years of Earth history".[14]
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Edited by Miiji: 5/16/2013 9:56:38 AMAs long as we have enough trees and plants... We can still breathe. Worst case, the planet becomes inhabitable and by that time we have an intergalactic war over "paradise" level planets. However, most of us will be likely dead by that time, so...