- Hermann Harde and Murry L. Salby
- Helmut-Schmidt-University, Hamburg, Germany
- Ex Macquarie University, Sydney, Australia
The evolution of nuclear-perturbed 14CO2 is used to determine the removal time of atmospheric CO2. The exponential decline of anomalous 14CO2 establishes that absorption of CO2 is determined, not by extraneous reservoirs of carbon, but autonomously by the atmosphere. Specifically, the rate at which CO2 is absorbed from the atmosphere is directly proportional to the instantaneous abundance of CO2 in the atmosphere.
It operates with a single time scale, which reflects the collective absorption by all sinks of CO2 at the Earth’s surface. The long-term decline of anomalous 14CO2 reveals an effective absorption time of about 10 years. The accompanying removal of atmospheric CO2 is much faster than has been presumed to interpret observed changes. Jointly with the Conservation Law governing atmospheric CO2, that absorption time is shown to reproduce the observed evolution of CO2, inclusive of its annual cycle.
The latter treatment provides an upper bound on the absorption time, independent of but consistent with the value revealed by the decline of anomalous 14CO2. Together, the two determinations of absorption provide an upper bound on the anthropogenic perturbation of atmospheric CO2.
A central question in the climate science of today is: How much does anthropogenic emission of CO2 contribute to rising atmospheric CO2 and, thereby, to global warming? The answer to this question requires a quantitative understanding of CO2 exchange between the atmosphere and the Earth’s surface, which removes CO2 from the atmosphere.
A popular metric of such exchange is the residence time of CO2, which characterizes how long CO2 remains in the atmosphere before being absorbed at the Earth’s surface. In its Fifth Assessment Report (AR5-Ch. 6) , the UN’s Intergovernmental Panel on Climate Change (IPCC) defines multiple residence times, as well as adjustment times. They represent exchanges between extraneous carbon reservoirs at or beneath the Earth’s surface. Unlike the atmosphere, those global reservoirs are virtually unobserved, leaving their exchanges largely a matter of speculation. Such time scales are relevant to the storage and sequestration capacity of those reservoirs. However, they are of no direct relevance to CO2 in the atmosphere – because its abundance is dictated solely by transfers into and out of the atmosphere at the Earth’s surface. What transpires to carbon outside of the atmosphere is immaterial.
Residence time is, in fact, incidental to the physics that controls atmospheric CO2. Because CO2 is conserved in the atmosphere, its abundance is determined entirely by emission and absorption of CO2 at the Earth’s surface. Residence time does not determine absorption of CO2; it is determined by it.