Any day now, the concentration of the greenhouse gas carbon dioxide sampled from the air wafting above a barren lava field in Hawaii could be above 400 parts per million (ppm), a level not seen since the Pliocene, between 3.2 and 5 million years ago.
Carbon dioxide levels were around 280 ppm when the Industrial Revolution got under way in the 18th century and humans started pumping greenhouse gases into the atmosphere from burning fossil fuels. Levels have continued to accelerate higher since then.
Hitting the 400 ppm milestone is symbolic to science and policy discussions about efforts to control global climate change, according Ralph Keeling, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif.
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