So where are we, exactly, with regard to carbon dioxide?
According to The New York Times, citing a report from the the World Meteorological Organization, global atmospheric CO2 concentrations, measured as monthly averages, met and surpassed 400 parts per million (or ppm) this last spring.
As a reference point, in 1750, they are estimated to have stood at 278ppm. That’s a 43% increase since George Washington was a young man.
NB: If you distrust estimates, and the methods used to produce them, and you want to see the graph of continuously observed CO2 measurements taken at Hawaii’s Mauna Loa Observatory starting in 1958, google the Keeling Curve.
Is 400ppm important, scientifically? No. Meaning it’s not appreciably worse than 399ppm, or better than 401ppm. It’s a round number, a multiple not just of 10, but of 100, and so it’s something of a psychological milestone. But I think it does serve to symbolize the apparent inexorability of the graph’s upward climb; a cold, discomfiting rebuke… proof that the steps we’ve taken to date to reverse this process have been negligible in their effect.