In the carbon cycle of our natural, equilibrium atmosphere, each carbon atom in the atmosphere has a certain probability each year of being absorbed by the reservoir. We call this the probability of exchange in the atmosphere. According to our calculations, 37 Pg of carbon is absorbed by the reservoir each year. Meanwhile, the total mass of carbon in the atmosphere is 650 Pg. To the first approximation, the probability of exchange in the atmosphere is 5.7% per year.
Likewise, a carbon atom in the reservoir has a probability of being released into the atmosphere each year. We have 37 Pg of carbon emerging from the reservoir each year, and the reservoir contains 77,000 Pg of carbon, so the probability of exchange for the reservoir is 0.048% per year.
Almost all carbon in the atmosphere is bound up in CO2. In our previous post we showed that the reservoir is the deep ocean. When a carbon atom enters the deep ocean, it does so as part of a CO2 molecule. The CO2 molecule arrives by chance at the ocean surface, and by further chance it dissolves into the salty water. The CO2 molecule turns into some kind of carbonate ion. This ion mixes down through the top thousand meters of water until it reaches the deep ocean. The carbon atom is now part of the reservoir. The probability of this happening each year is 5.7% for each and every CO2 molecule in the atmosphere.
Likewise, the probability of any carbon atom in the reservoir emerging into the atmosphere as part of a new CO2 molecule each year is 0.048%. The molecule or ion containing the carbon atom mixes up through the top one thousand meters of the ocean, arrives by chance at the surface, and by further chance emerges from the surface as an atmospheric CO2 molecule. (See UPDATE below for discussion of ocean chemistry.)
The exchange of carbon between the atmosphere and the ocean is a first-order chemical process. During the process, each carbon atom is acting alone. It does not require the cooperation of any catalyst to permit it to be dissolved in saltwater or released from saltwater. If we were to double the number of CO2 molecules in the atmosphere, so that six hundred out of every million air molecules were CO2 instead of only three hundred, the probability of any one of them being absorbed by the reservoir each year would remain the same.
If the reservoir were something more complex than the ocean, such as a forest, we would be unable to assert that the probability of exchange was unaffected by the number of CO2 molecules in the atmosphere. A forest needs water and sunlight to convert CO2 into sugar and oxygen. If we double the number of CO2 molecules in the atmosphere, we might find that CO2 molecules are lining up inside forest leaves waiting for enough water and sunlight to arrive before they are turned into plant matter. But our reservoir is the ocean, and entering and leaving it is a statistical process in which each carbon atom acts in isolation.
So far, we have assumed that the atmosphere and reservoir are staying at the same temperature. They could be at different temperatures, but they are neither warming nor cooling. But we note that the probability of exchange is strongly affected by temperature. We have only to look at the decreasing solubility of CO2 in water with temperature, as presented here, to see that this strong effect must exist.
For now, we assume our natural, equilibrium atmosphere, and its carbon reservoir, are neither cooling nor warming. The probability of exchange in the atmosphere remains constant at 5.7% per year, even if we halve or double the atmosphere's CO2 concentration. The probability of exchange in the reservoir remains constant at 0.048% per year, even if the mass of carbon in the reservoir halves or doubles. The probability of exchange is independent of concentration.
This concludes our series of posts on carbon-14. In our upcoming posts, we will apply what carbon-14 has taught us about the Earth's carbon cycle to predict how human CO2 emissions will affect the CO2 concentration of the atmosphere.
UPDATE [08-NOV-16]: In many gas-liquid systems, changes in concentration or acidity can change the probability of emission for a dissolved gas molecule. This variation in probability is possible when the dissolved gas appears as several species in the liquid, some of which cannot emit a gas molecule, while others can. When the relative concentrations of these species changes, the probability of a dissolved gas molecule being emitted also changes. For example, if a gas dissolves into two species A and B in equal proportion, and A has a 10% per year probability of emission while B has a 0% probability, the average probability is 5% per year. If we add acid to the system and the ratio of the two becomes 75% A and 25% B, the probability of emission for each species remains the same, but the average probability rises to 7.5% per year. Our carbon-cycle model is based upon the assumption that the atmosphere-ocean system does not exhibit concentration-dependent nor acidity-dependent probability of emission. Let us justify our assumption with a brief discussion of ocean chemistry.
The top layer of the ocean is saturated with calcium carbonate (CaCO3). The CaCO3 co-exists with the carbonate ions created by CO2 dissolved in water. In Figure 5.6 of Carbonate Equilibria we see the pH of a liquid saturated with calcium carbonate is around 8.5 (log of the H+ concentration is −8.5) for CO2 partial pressure of 300 ppmv (log of CO2 partial pressure is −3.5). The pH of our contemporary ocean is around 8.2, while the pH of a system of only CO2 and water is around 5.8. Continuing with Figure 5.6, for CO2 partial pressures in the range 100 ppmv to 10,000 ppmv (log of CO2 partial pressure is −4 to −2) the carbon content of the solution is dominated by HCO3−. The concentration of HCO3− increases in proportion to the partial pressure of CO2 (its slope is 1.0 in the log-log plot). The concentration of HCO3− is 2.0 times that of Ca2+ throughout the range 100 ppmv to 10,000 ppmv (see Table 5.1 Case 2 for numerical values). As we increase the partial pressure of CO2, an equal number of of CaCO3 and CO2 molecules dissolve. Each CaCO3 molecule that dissolves adds one Ca2+ ion, one HCO3− ion, and one OH− ion to the solution. Each CO2 molecule that dissolves contributes one HCO3− ion and one H+ ion. The OH− and H+ ions combine to form H2O, leaving the other ions in solution. The HCO3− concentration, the dissolved CO2 concentration, and the dissolved CaCO3 concentration all increase in proportion to the partial pressure of CO2. As seawater changes temperature and pressure, the saturation concentration of CaCO3 changes, and CaCO3 can precipitate, as it does in the Persian Gulf, staining the water white.
When a gas and liquid are at equilibrium, there is as much gas entering the liquid per unit time as there is leaving it. Because gaseous CO2 has only one species, its probability of absorption into the ocean does not vary with its concentration. When we double the concentration of atmospheric CO2 from 300 ppmv to 600 ppmv, we double the rate at which it enters the ocean. When the ocean attains a new equilibrium with the 600-ppmv CO2 atmosphere, the rate at which CO2 is emitted by the ocean must be double the rate for 300 ppmv. At the same time, using the CaCO3-CO2-water system as our guide, we see that the concentration of dissolved CO2 will double for this doubling of atmospheric concentration. The doubling of dissolved concentration combined with the doubling of emission tells us that the probability of emission for CO2 molecules in the ocean is constant from 300 ppmv to 600 ppmv.
Another way to model the atmosphere-ocean system is to ignore the calcium carbonate and instead use a CO2-water system with added OH−, such as might come from mixing NaOH with the water. We add OH− in order to increase the pH of the system from 5.8, which applies to the CO2-water system alone, to 8.2, which applies to the ocean. This OH-CO2-water system exhibits more complex behavior in the range 100 ppmv to 1000 ppmv than the CaCO3-CO2-water system. The dissolved CO2 concentration does not increase in proportion to the atmospheric concentration. So far as we can tell, this OH-CO2-water model is what climate scientists are using when they conclude that the ocean will not absorb our CO2 emissions in proportion to atmospheric CO2 concentration. They express its non-linear behavior with a number they call the Revelle Factor. We do not understand why they prefer an OH-CO2-water model to the CaCO3-CO2-water model, nor have we seen in the climate science literature any plots like those of Figure 5.4 or Figure 5.6 for an OH-CO2-water system. The closest we have seen any promoter of the Revelle Factor come to plotting such graphs is here, but that author had no explanation for why they used the OH-CO2-water system instead of the CaCO3-CO2-water system.