There has been an interesting discussion concerning the airborne CO2 fraction at WUWT recently which is still ongoing. Our friend ‘Bart’ found an close correlation between the surface temperature of the southern hemisphere and the rate of change of the airborne co2 level, and posits the hypothesis that the source of the increase is due to natural co2 sources dependent for their rate of emission on temperature, rather than human emission of the trace gas through the combustion of fossil fuels. I found that the correlation is even tighter with the UAH global lower troposphere data, see fig 1 below.
This is clearly a very good fit, notwithstanding a couple of aberrations around 1992 and 2010, which we’ll discuss below.
Something worth noting immediately is that whereas the actual changes in levels of CO2 in the atmosphere always lag behind changes in temperature by nine months or so at the annual scale, the rate of change of co2 levels sometimes lags and sometimes leads the changes in lower tropospheric temperature. There is not a simple relationship here.
what clues might we gain by looking at the two aberrations in the relationship visible around 1992 and 2010?
These were both El Niño years, but we see no similar divergences during the big El Niños of 1988 or 1998. So should we discount this phenomena as the possible cause? Not yet, because El Niño and its aftermath varies in terms of its effect on ocean circulation. Bob Tisdale may want to weigh in on this question. Bob noticed step changes in global surface temperature occuring following the El Niños of 1988 and 98, but not in 1992. The picture is likely complicated by the eruption of Pinatubo, but it should be noted that LT and co2 ROC go their separate ways for a while well before Pinatubo erupted in 1991 (june 14) and before Dubbi erupted in Eritrea 20 years later to the day.
What else might be involved? The solar cycle was just past maximum (and spiking) when Pinatubo erupted, and just past minimum when dubbi erupted, so in both cases the changes in solar activity levels had recently reversed. Still nothing very slam dunk here though. Another possibility is that the very large source of co2 outgassing, the volcanic soils, got less sunshine on them during those periods, or that major blooms of plankton occurred, absorbing more co2 than usual. We might just have to accept that we don’t know enough about the carbon cycle yet to nail this one, but I hope further ideas will arise in comments to help us piece this together.
Despite those uncertainties, I think the correlation Bart has found and the recent discovery of the much larger than previously estimated volcagenic sources have moved this debate along, and a serious reassessment of the arguments supporting the idea that human emissions are largely responsible for the rise in co2 seen since records began at Mauna Loa is in order. The closeness of the correlation doesn’t appear to leave much room for sources which are not temperature dependent.
Another conversation which can be had here is the question of how well the rate of co2 change serves as a proxy for checking the veracity of various global temperature indices… To have a play with this, try this URL and then change the dataset used, along with the offset if required to get the best fit. No sneaky detrending allowed. :)
UPDATE: Contributor Les Johnson has kindly sent me his spreadsheets from a similar study he did in 2010. You can download them below.