I was quite intrigued by a recent Watts Up With That (WUWT) post called the bombtest curve and its implications for atmospheric carbon dioxide residency time. It includes the following figure which shows the change in carbon-14 to carbon-12 ratio with time (black data points with red curve) and an estimate of how it should have changed based on the residency time estimated using what is called the Bern model.
If I understand the figure correctly, the y-axis is the percentage increase in carbon-14 to carbon-12 over what it was prior to the start of nuclear testing in the 1950s. Above ground nuclear tests in the 1950s and 1960s essentially doubled the carbon-14 concentration in the atmosphere.
What this WUWT post seems to be claiming is that the model for how CO2 concentration should change with time (assuming we were to stop adding more) has a residency time that is much greater than is estimated based on the change in the carbon-14 to carbon-12 ratio since nuclear testing stopped. At first I thought that maybe the author of this post had a point, but I think there are two fundamental problems with his analysis. One is that our use of fossil fuels has increased the concentration of carbon-12 by about 25% since about 1960 (there is no carbon-14 in fossil fuels because it decays with a half-life of 5700 years). This alone – if I’ve done my calculation correctly – would change the carbon-14 to carbon-12 ratio by about 40%.
The other problem is that there are, in a sense, two residence times for CO2. One is the amount of time an individual CO2 molecule spends in the atmosphere before being absorbed by the oceans or used by a plant. However, in this case, it will be replaced by a different CO2 molecule and so the net atmospheric concentration doesn’t change. This residence time is a few years. The other timescale is the time it would take for the atmospheric concentrations to drop by some factor. This timescale is much longer than the residence time of an individual molecule – hundreds of years. Because there is very little carbon-14 in the carbon cycle, when a carbon dioxide molecule comprising carbon-14 is absorbed by the ocean or by a plant, it is more likely to be replaced by a molecule containing carbon-12 than one containing carbon-14.
So, as far as I can tell, the decay of the carbon-14 to carbon-12 ratio in the figure above is more representative of the residence time of an individual molecule than representative of the decay timescale of the atmospheric CO2 concentration. The mismatch between what is observed and what is predicted by the Bern model seems, therefore, to be entirely consistent with how one would expect the process to work. Admittedly, I’m no expert, so happy to be corrected by those who know more.