He’s talking about CO2, of course. What does it mean? Absolutely nothing, at least as far as humanity is concerned.
A lot of things humans do alter our immediate and even the global environ. According to CDIAC we’ve added about 3W/m2 in forcing from all greenhouse contributions combined, which the IPCC WGI suggests is almost half the total when accounting for black carbon (soot), deforestation, reduced albedo from ice, snow and clouds, land use change (cropping including irrigation), slightly more active sun and so on, yielding a grand total of about 7W/m2 over roughly two and one-half centuries (disregarding huge uncertainties).
Net global temperature change has been estimated at about +0.7 °C over that period, which sounds about right for the change in forcing given that “back radiation” (downwelling longwave radiation from the atmosphere and clouds) averages 320-340W/m2 and greenhouse effect is supposed to be 33 °C (33/330 = 0.1; 0.7/7 = 0.1).
Using the IPCC’s simplified formula to calculate increased forcing from increased atmospheric CO2 the ooh-scary 400ppm CO2 amounts to 5.35*LN(400/280) = 1.9W/m2 or 0.19 °C of already equilibrated warming.
Not much to write home about really but Lemonick managed to turn it into a column:
I’m not big on taking note of milestones. They’re artificial, and usually meaningless, but people get all worked up about them anyway. I don’t like to stay up late on New Year’s Eve, for example, because Dec. 31 is a purely arbitrary date. Nothing real actually begins the next day, but we all pretend otherwise. I have similar feelings about the first day of spring, the temperature reaching 100° as opposed to 99° and all sorts of other magic-sounding dates and numbers that don’t have any real significance.
But since no law says I have to be consistent, I’m going to take note of a milestone that happened some time in the past couple of months, and which was reported last week by NOAA. For the first time in recorded history, and almost certainly for much longer than that, the atmosphere’s concentration of carbon dioxide, or CO2, has nipped above 400 parts per million in at least one part of the world. Monitoring stations in Alaska, northern Europe, and Asia have all noted readings above that level during this past spring.