400 PPM. Woo-hoo.
A curve shows the rising concentration of the greenhouse gas carbon dioxide in the atmosphere. PPM stands for parts per million of CO2 in air. Before the 1800s, the concentration did not exceed 280 parts per million for hundreds of thousands of years.
From Andy Revkin at the NYT:
For hundreds of thousands of years preceding the industrial revolution, the concentration of the greenhouse gas carbon dioxide in the atmosphere didn’t exceed 280 parts per million. Now it is poised to pass 400 parts per million, thanks to the burst of fossil fuel combustion and forest clearing that’s accompanied humanity’s recent growth spurt.
That shift was noticed thanks to decades of work by Charles David Keeling of the Scripps Institution of Oceanography and more recently his son Ralph. (Keeling’s work establishing a meticulous record of CO2 levels at the Mauna Loa Observatory in Hawaii was beautifully described in a 2010 feature by Justin Gillis.)
The Scripps Institution launched a Twitter account, @keeling_curve, to make it easy for people to track the gas’s path through 400 and beyond. Bryan Walsh of Time Magazine has a nice post up discussing what is and isn’t significant about that concentration and what comes next.
How did we get here?
The Earthist Chronicles 
Milestone or gravestone?
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