The dominant issue remaining is whether human emissions of CO2, ACO2, are the primary or only cause for the increase in atmospheric concentrations of CO2, or whether natural emissions of CO2 are.
The issue is complex because there is a bulk and flux component to the role ACO2 may play in adding to CO2 levels.
The bulk component is the total amount of CO2 in the atmosphere. This can be expressed as an amount in gigatonnes or parts per million of volume. For instance the current amount of CO2 in the atmosphere is about 800 gigatonnes or 395 parts per million. Since 1900 the amount of CO2 in the atmosphere has increased by about 112 parts per million.
The flux component is the movement of CO2 and ACO2 into and out of the atmosphere over a year. The bulk amount of CO2 in the atmosphere will increase if the amount of CO2 moving into the atmosphere is greater than the amount leaving as measured by the annual flux. That difference is what causes the increase in the bulk CO2.
At the beginning of the AGW debate it was thought that ACO2 was causing all the increase in the bulk because of isotope tracking. There are 3 basic varieties of CO2 distinguished by their isotope and it was assumed that ACO2 could be separated from natural CO2 in the bulk because of differences in the relative isotope proportions. However similarities between the isotope of ACO2 and natural sources of CO2, particularly from vegetation and the differential operation of Henry's Law on the different isotopes [see comment 413] means the isotope distinction cannot be conclusively used to prove ACO2 is entirely responsible for the increase in the bulk.
However an argument against ACO2 being responsible for all the increase in the bulk can be found in Knoor’s 2009 paper.
Knoor 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. If the ACO2 AF is constant that must mean non-ACO2 or natural emissions of CO2 are contributing to the increase in the bulk CO2 concentration.
The reason for this is the principle of a constant in an increasing total: say ACO2 is 20% of all CO2 which is 100, so ACO2 is 20 and natural CO2 is 80; when all CO2 is 200 ACO2′s 20% will be 40 so natural CO2 will be 160, an increase of 80; at 300, ACO2 is 60, natural CO2 is 240 and so on; natural CO2 must be contributing to the increase in total CO2.
Knoor has been supported by the Gloor et al paper.
The other part of the issue of whether ACO2 is contributing to all of the increase in CO2 levels is based on the annual fluxes. The annual fluxes are shown by Figure 7.3 of AR4.
This shows that of the annual CO2 flux, ACO2 is 8Gt out of the total of 218.2Gt or 3.67%. US Department of Energy [DOE] figures put this % at 2.91% but for argument's sake it does not matter.
DOE, Table 3, shows that approximately 98.5% of the total flux is reabsorbed in sinks, predominantly natural although cropping would add a miniscule amount.
If one assumes that the same proportion of ACO2 of the total flux into the atmosphere is NOT reabsorbed but adds to the bulk atmospheric concentration [other methods are discussed here] the simple formula of how much ACO2 adds to the atmospheric increase would be annually:
3.67/100 X 1.5/100 = 0.000552
That is one ACO2 molecule has a 1 in 1811.594203 of still being in the atmosphere after 1 year.
After 2 years the probability would be 1 in 120772.9469 chance of remaining.
Clearly on this basis ACO2 would not be contributing to the increase in CO2.
But this is what caught Alan Jones out; it does so because it confuses the residence time of one molecule of ACO2 and the time required for the all the CO2 in the atmosphere to return to the level it was before the ACO2 emissions began adding to the atmospheric bulk.
Cawley asserts that the "one-box model of the carbon cycle used in ES09 [Essenhigh] directly gives rise to (i) a short residence time of ~4 years, (ii) a long adjustment time of ~74 years".
Effectively, this means that while one ACO2 molecule does not remain long [4 years] the effect of all the ACO2 on the atmospheric bulk is long-lasting [74 years] and therefore supports the idea that ACO2 is the main reason for the increase in CO2.
However, Essenhigh uses a "one box" model in which flux from the atmosphere has a proportional relationship with the concentration [bulk] of CO2. Essenhigh expresses this as:
F=k*C, where F is the flux from the atmosphere to the environment, C is the atmospheric concentration, and k is a proportionality constant.
Cawley changes this to F=k*C +F0, where F0 is a constant flux independent of atmospheric concentration.
This assumption by Cawley contradicts Henry's Law. Henry's Law says that the rate of diffusion or movement of gas from the atmosphere is dependent on temperature and the concentration of the gas; as the concentration changes so does the rate of movement. But Cawley’s constant, F0, would mean that when there is zero CO2 in the atmosphere (C=0) there would still be a finite flux (F0) of CO2 from the atmosphere! That is to say Cawley’s model allows for bulk CO2 to become negative.
Essenhigh's model is to be preferred and means that Knoor's bulk analysis is correct; that is, most if not all the increase in CO2 over the 20thC is not coming from ACO2.
How does this tally with the observed data and more importantly the official AGW position?
The official AGW position is that up to the 20thC emissions of CO2 were all natural and evenly matched with natural sinks. Sinks are absorbers of the atmospheric CO2. The main sinks are the ocean and vegetation. This ‘balance’ whereby emissions and sinks are about equal with no variation in atmospheric CO2 is shown by Figure 1.
The past record of CO2 is usually based on ice core records. There are some problems with the ice-cores as Drake and Jaworowski show. There is also some irony since the ice core record is rejected by AGW advocates when it contradicts AGW such as showing the Holocene temperature being higher than today.
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.
The second assumption by AGW is that the increase in the 20thC must be caused by ACO2 emissions because the atmospheric increase is always slightly less than the ACO2 emissions. The assumption here is that the sinks, which were previously in equilibrium with the natural CO2 emissions, have partially adjusted by expanding to absorb some but not all of the ACO2 emissions so the increase in the bulk must be due to the ACO2 which is left over.
If AGW were correct about the equilibrium between natural emissions of CO2 and sinks the atmospheric increase would mirror the ACO2 emissions; it doesn’t. That means that sinks and natural CO2 emissions vary and are not in equilibrium.
In fact Quirk shows the natural CO2 emissions and sinks are strongly correlated with changes in ENSO, the pattern of El Nino and La Nina episodes This confirms and possibly explains the past history of stomata based CO2 levels, and most importantly, in connection with Knoor and his finding that the airborne fraction of ACO2 has not changed in 150 years, shows that ACO2 is not the major contributor to rising CO2 levels.
The ramifications for AGW are profound; if human CO2 is not the main cause of the increase of atmospheric CO2 then AGW is wrong.