The empirical evidence for an increase in atmospheric CO2 over the 20thC is fairly uncontroversial. The increase in CO2 levels began about 1850 when the Little Ice Age ended. The acceleration in CO2 levels began about 1900 when they were 280 parts per million [ppm] and have continued until they are now 394 ppm.
Until the 20thC records of atmospheric CO2 levels show a basically constant CO2 with variation as little as 10ppm over centuries. These past records are based on ice cores. The theory is that natural emissions of CO2 and sinks, or places where the CO2 is absorbed, were in equilibrium. This all changed when human emissions of CO2, ACO2, began.
The official viewpoint about why CO2 levels have increased over the 20thC is put forward by the EPA. The EPA makes a comparison with a bath-tub. They say prior to the increase in ACO2, natural emissions of CO2 were like water from the "spigot" going into the bath-tub and natural sinks were like the drain. The level of CO2 never changed because nature, like the bath-tub, was in balance. ACO2 upset this balance by being like a glass of water which was poured into the bath-tub and is responsible for the increase in CO2.
There are a number of problems with this official view. The first is the assumption that past CO2 levels were as unvarying as the ice cores suggest. The problems with the ice cores is they are subject to close-off fractionation, or leakage of gases from the gas trapped in the ice, because of ice pressure; this process is related to the kinetic diameter of the gas molecules not the collision diameter; something which confuses even experts who gave ice cores a clean bill of health as a proxy for past CO2 levels.
The result of close-off fractionation is that ice cores may underestimate past levels and fluctuations of atmospheric CO2, as Drake, Jaworowsky, Glassmanand Segalstadhave proposed.
Arguably the most reliable history of past CO2 levels is from plant stomata records as Figure 12 shows. Figure 12 indicates past levels of CO2 were almost at today's levels as recently as 1550. In addition chemical analyses of CO2 levels in Figure 13 show CO2 levels higher than today in the early 1800's.
If the stomata and chemical analysis records are right this would mean that the natural emissions and sinks, the spigot and drain of the system are not always in equilibrium. In turn this would throw doubt on the assumption that there was equilibrium between natural emissions and sinks at the beginning of the 20thC before ACO2 allegedly caused the increase in CO2.
Further doubt on this assumption of a natural equilibrium is contained in 2 recent papers.
The first is by Tom Quirk. Figure 1A from Quirk shows the ACO2 emissions with the increase in atmospheric CO2:
If the assumption about the equilibrium between natural emissions and CO2 and sinks was correct the increase in atmospheric CO2 would correlate with the ACO2 emissions since, according to the EPA, the increase in the CO2 is entirely due to that extra glass of ACO2 being added to the 'bath-tub' or atmosphere. However there is no such correlation; and since we know what the ACO2 emissions are the lack of correlation between ACO2 and the increase in CO2 must mean there is no equilibrium between the natural emissions and sinks.
Quirk also shows the natural CO2 emissions and sinks are strongly correlated with changes in ENSO, the pattern of El Nino and La Nina episodes. There have been stronger warm ENSO periods since the LIA. Warmer ENSO periods would lead to higher sea surface temperatures. According to Henrys Law with warmer sea surface temperatures more natural CO2 would be emitted from the oceans into the atmosphere. This is one possible reason why natural CO2 has been contributing to the increase in atmospheric CO2.
Another reason is contained in the 2nd paper by Knorr which supported by the Gloor et al paper.
Knorr found the airborne fraction [AF] of ACO2 has not changed in 150 years. The AF is the % of ACO2 emissions which remain in the atmosphere contributing to the increase in atmospheric CO2 levels. The constant AF found by Knorr was about 40%.
Figure 1 from Knorr illustrates this:
Knorr describes Figure 1 as "Observed atmospheric CO2 increase derived from direct measurements, taking the average of Mauna Loa (Hawaii) and the South Pole (thin solid line), and two ice cores: Law Dome (dashed thin line) and Siple (dotted thin line). This is compared to total anthropogenic emissions (thick solid line) and 46% of total emissions (thick dashed line)".
The 46% in fact was an estimate by Dr Knorr [as indicated in private correspondence] and should be 40% as the calculated amount of the constant AF. The graph shows that general atmospheric CO2 is increasing by much less than the amount of the ACO2 emissions. It is this which underpins the argument that ACO2 is responsible for all the increase in atmospheric CO2. That is, given the assumption that natural emissions and sinks were in equilibrium before ACO2 started to increase, the official position is now that the sinks have increased to absorb some of the increase in ACO2 but not all and that remainder of non-absorbed ACO2 is what is causing the increase in atmospheric CO2.
Quirk and the stomata history has already shown that the assumption that natural CO2 emissions and sinks were in equilibrium is problematic. What the constant AF shows is that the increase in ACO2 and natural sinks does not explain the increase in atmospheric CO2. The AF does this because it is the ONLY amount of ACO2 available to increase the atmospheric CO2. The expanding sinks are absorbing about 60% of the increase in ACO2 but as Knorr's figure 1 plainly shows atmospheric CO2 increase is expanding more than the remaining 40% of ACO2 left. This is shown by all the peaks above the thick dashed line. If the AF were graphed at its correct 40% it would be slightly lower than shown in Knorr's graph. This is plainly shown in this graph based on the Mauna Loa measurements from 1959:
Graph by Ian Hill
Clearly the AF is predominantly below the yearly increase in atmospheric CO2. This is reflected in a comparison between the slopes of the increase atmospheric CO2 which is 0.0575 and the AF which is 40% of the increase in ACO2, or 0.0447. That is, the AF is increasing LESS than the increase in atmospheric CO2. Even in those years when the increase falls below the AF it cannot be said that natural CO2 emissions are not contributing because of the lack of equilibrium between CO2 emissions and sinks.
The effect can be compared with the principle of a constant in an increasing total: say ACO2 is 40% of all CO2 [as per the constant ACO2 AF] which is 100, so ACO2 is 40 and natural CO2 is 60; when all CO2 is 200 ACO2′s 40% will be 80 so natural CO2 will be 120, an increase of 60; at 300, ACO2 is 120, natural CO2 is 180 and so on; natural CO2 MUST be contributing to the increase in total CO2.
Man made global warming [AGW] is based on human emissions of CO2 disrupting the climate. The basic science supporting AGW has been shown to be defective. If it turns out that human emissions of CO2 are not the primary reason for the increase in atmospheric CO2 then even further doubt is cast on the theory of AGW.