400 ppm

Of all the events that occurred during my blogging hiatus, the one that struck me as the most significant, even including the massive flooding of my hometown, was the Earth experiencing for the first time in millions of years an atmosphere containing 400 ppm carbon dioxide, as measured at the Mauna Loa Observatory in Hawaii on May 10th.

As important as the Calgary flood was, it was a local event. Reaching an atmospheric CO2 of 400 ppm was a global event, important to every member of our species and to many other species as well.

It is an arbitrary number of course, 400 ppm is hardly worse than 399 ppm, but round figures catch the eye. This round figure tells us we are going in the wrong direction and fast, from 315 ppm to 400 in little over half a century. We are on the fast track to hit 450 ppm, the UN's official red line, by mid-century.

To quote leading climate scientist James Hansen, "If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted … carbon dioxide will need to be reduced … to at most 350 ppm." Climate scientists such as Hansen are the people who know what they are talking about on the issue of global warming, the ones we ought to be paying attention to. So, yes, 400 ppm, as arbitrary as it is, demands our attention. It speaks to the survival of civilization as we know it, and indeed to life on Earth itself. Significant indeed.