Having got into the CO2 pit with the good Diplom Beck (aka Anon), Eli was idling his time away sucking Saturday beers and reading the CDIAC CO2 mixing ratio records. (Of course E has better things to do, why do you think he was looking at CDIAC data?) While there is much there with which to bedevil the good Diplom, a more interesting thought occurred.
If one looks at the Mauna Loa CO2 record (taken with a continuous not a cryogenic method btw DB)
the seasonal variation is easy to see. More interesting is to compare the Mauna Loa seasonal variation with that observed at Barrow Alaska
and Bering Head, New Zealand
The decrease in seasonal variation as one moves north to south is obvious. In the northern hemisphere there is a great deal of land that blooms in the spring and summer. The large increase in vegetation which contributes to photosynthesis (and the warmer temperatures) noticably reduce the CO2 mixing ratios. The effect is most extreme in the far north. On the other hand, there is much less land and change in temperature during the year in the southern hemisphere, so the variation of CO2 mising ratios with the change of seasons is small.
Eli Rabett's long, and perky ears (it is Saturday) leaped as he thought, what would happen if we plotted the difference between the CO2 maxima (before summer warming and greening) and the minima. Would this not be a marker of how much EXTRA CO2 is being absorbed by the biosphere? So ignoring the calls of his loved ones to, well do what loved ones do, such is the call of science, he did for Barrow
and for Mauna Loa
Apologies for these not being on the same scales, but wth, the point is that we DO observe a small increase in the difference between the maximum and minimum observed CO2 during the year.
In the Mauna Loa (MLO) record, while the mixing ratio was going from ~315 ppm in 1958 to ~380 today, the seasonal difference increased about 1 ppm, from about 5 ppm to 6. In the shorter Barrow record, the CO2 mixing ratio went from ~333 ppm in 1974 to ~380 ppm today, but the seasonal difference only increased about 3 ppm from ~14 ppm to ~17 ppm.
In percentage terms the ratio of increase in the seasonal difference to the total increase is 1.5% for Mauna Loa and 6.4% at Barrow. So, those are the numbers, and we are left with the question of what they mean. Obviously, I would like to know if this has been considered in the past, but my first cut at it is at least provocative.
It is taken as a given in carbon cycle circles that about half of the anthropically emitted CO2 is taken up by the biosphere and the oceans and the other half has resulted in the observed increase in atmospheric CO2. A favorite denialist claim is that plants will grow bigger, faster better with increased CO2. In some cases it is claimed that this will completely compensate for human emissions.
However, as we have seen here, if this was the case, one would assume that the seasonal differences would be much larger. That leaves us with the following choices:
- Northern Hemisphere land vegetation accounts for relatively little photosynthesis, and this analysis sets some strong limits on it. This appears to conflict with studies that show a great deal of CO2 is taken up by Northern Hemisphere forests.
- Photosynthesis in the Northern Hemisphere evergreen forests accounts for most of the photosynthesis and does not depend strongly on temperature and time of year. Well there go the tree rings.
- There is a balance between increased photosynthesis in the summer and increased CO2 outgassing from the warmer oceans (and decay [added 10/8]). On net today photosynthesis at warmer temperature wins, but remember that vapor pressure increases exponentially with temperature.
And so to bed:)