Before the release of vast amounts of CO2 into the atmosphere mainly through the burning of fossil fuels, the atmospheric concentration of this gas was about 300 ppm or a bit more. Soon, that number will be 400 ppm. How soon? Let’s see … it is now Tuesday at about 7pm. Maybe mid morning tomorrow? Maybe early next week? In fact, there have been one or two readings over the last few weeks that have registered above the 400 ppm mark.

So, this is important because humans have officially increased the concentration of this key greenhouse gas by a third. That’s a big deal.

Having said that, I’d like to be the first person to say the following because you’ll be hearing this form climate science denialists sooner or later anyway.

Aside from the long term trend of increasing CO2, there is an annual variation as well. Over the course of the year, CO2 moves in and out of the atmosphere on a fairly regular schedule. Surely, you’ve seen the famous Mauna Loa graph, this one cribbed from Wikipedia:

There is a lot more land in the Northern Hemisphere that goes through a dramatic cycle in plant activity, with most plants playing (or even being) dead over the winter and springing to life in the Spring. The Southern Hemisphere has much less land. So a small amount of CO2 moves into the atmosphere over the Northern Hemisphere winter and into spring, and then moves back into newly grown plant tissue during the northern growing season.

So, right now, CO2 should be at a short term peak. The range of this variation is around 8 ppm, so if we hit, say, 401 ppm next week, expect that value to go back below 400 ppm in a few weeks. In other words, we can and should note that we are probably hitting the 400 ppm barrier, but then later when we drop slightly below, temporarily, 400ppm, the climate science denialists will be all over that claiming that there is no global warming. Cuz they’re morons.

In a few years … certainly by the end of the present decade …. the low values will be over 400 ppm unless something dramatic happens.