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]