May 9, 2013: NOAA confirms that CO2 hits 400 ppm

The earth’s atmosphere hasn’t held this much CO2 since the Pliocene Epoch, an ancient era that ended more than 2 million years before the first Homo sapiens appeared.  Global CO2 levels hovered between 170 ppm and 300 ppm for ages until the Industrial Revolution, when a steady ascent began.  The U.N. Intergovernmental Panel on Climate Change estimates 450 ppm is when the worst effects of climate change will begin.

http://www.mnn.com/earth-matters/climate-weather/blogs/earths-carbon-dioxide-levels-to-hit-400-ppm

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Graphic courtesy NOAA