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.
300 to 400 ppm is a 33% increase, not 25%
Re the first sentence: I thought the generally accepted pre-industrial level of CO₂ was about 280 ppm so a bit less, rather than a bit more, than 300 ppm. Or, are you thinking of another baseline?
Average ppmv for the past 3,000,000 years was 283.
Yes, with natural variation up and down of ten or so either way over the last thousands of years; 300 is a conservative estimate. 280 may well be a more accurate baseline, but it then includes (probably) natural variation. That's fine, but picking 300 as the baseline allows the statement that these 100 ppm are human release of fossil C to be fairly accurate.
Actually the 280/300 is taken during a period when at least a part of the earth is ice covered. It would be interesting to know what the readings where during the last (Wisconsin, Illinoian) interglacial. Also interesting to know what the reading might have been during the time of the roman empire. Its clear if you go back before the Pleistocene the readings were significantly higher than 280.
The average of the last 3 million years includes a lot of periods of serious continental glaciers.
This 400 ppm milestone underscores our duty to pay more attention to first world problems that mildly inconvenience certain women here.
The biosphere breathes in and out on a yearly basis- green plants absorb CO2 in the northern hemisphere in the global summer, Then give some back during winter. I believe the overall increase in atmospheric CO2 is something like 2 ppm per year now, correct? So living humans will probably never again in our lifetimes ever see a CO2 reading of, for example, 395 ppm. before we can reverse the rise of atmospheric CO2, we must first stop the Rise. As regards coastal sea level prospects for the 22nd century, this is terrifying.