SCIENCE of
CLIMATE CHANGE

International Journal of Science and Philosophy

What controls the atmospheric CO2 level?

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Abstract

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.

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Description

Authors

  • Hermann Harde and Murry L. Salby
  • Helmut-Schmidt-University, Hamburg, Germany
  • Ex Macquarie University, Sydney, Australia

Abstract

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.

Introduction

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) [1], 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.

 

1 review for What controls the atmospheric CO2 level?

  1. david.andrews (verified owner)

    Comments on Harde and Salby’s “What Controls the Atmospheric CO2 Level”.
    I agree wholeheartedly with the strategy of using 14C to study atmospheric carbon. But this paper is filled with mistakes invalidating its conclusions. It is an attempt to respond to the observation that I made of a common error in two previous papers by Harde and papers by Essenhigh and Berry. See this paper’s Ref 30: (D. E. Andrews, 2020: Correcting an Error in Some Interpretations of Atmospheric 14C Data, Earth Sciences, 9(4), pp 126-129.) This is reachable with the following link: https://www.sciencepublishinggroup.com/journal/paperinfo?journalid=161&doi=10.11648/j.earth.20200904.12
    The three authors all misinterpreted the variable “Delta14C” as measuring carbon 14 concentration, say in moles of 14C per standard liter of gas. In fact, Delta14C measures the deviation of the ratio of 14C to total carbon from a standard in parts per thousand. Unsurprisingly, they all subsequently reached a common erroneous conclusion that carbon is removed from the atmosphere on a time scale of years to a couple of decades.
    The attempts made in this paper to recover from Harde’s original error are inadequate, as outlined below:
    1. Harde and Salby still have the misconception that only some of the archived Delta14C data need to be converted if concentration is of interest. In principle it all must be, though sometimes the adjustment is small. But this is not the main problem with their analysis.
    2. The model of Harde and Salby explicitly assumes that the processes involved in removing CO2 from the atmosphere operate in parallel and depend only on the atmospheric carbon concentration. They are correct that were that the case, a single time constant should describe the removal. They reference the earlier papers by Harde to support this, the papers that misinterpreted Delta14C. In those papers, the single time constant model was supposedly validated by the apparent single time constant exponential decay of what they thought was carbon 14 concentration after the bomb tests. But they were looking at Delta14C, not carbon 14 concentration!!! The true evolution of carbon 14 concentration is nothing like a single exponential decay. Putting aside the math, does it make sense to conjecture that the uptake of CO2 by the oceans only depends on the atmospheric CO2 concentration, and not on the increasing CO2 content of the ocean? It does not. Does it make sense to conjecture that the uptake of CO2 by plants is proportional only to the atmospheric CO2 concentration? Biological activity is a lot more complicated than that. In summary, the assumptions of their model are shaky at best, and the validation fails.
    3. The various graphs of the “bomb pulse” presented in this paper all start in 1959. This is an odd choice because the pulse began several years earlier from a baseline Delta14C of around -20. The authors statement that the concentration was constant at their starting point is false. I presume this choice was made in an attempt to de-emphasize the approximately 30% increase in atmospheric carbon 14 now, compared to before the testing. For some honest graphs, see my paper.
    4. Harde and Salby do recognize that the large carbon 14 concentration baseline shift from before the nuclear testing requires some explanation. They mostly blame increased cosmic rays and sketch a line on a graph of “anomalous neutron flux” that does not come close to fitting the data. The specific activity of atmospheric carbon 14 over the millennia have been carefully studied, as it is essential information for the carbon 14 dating community. See for example Cheng, H. et al. Science 362, 1293–1297 (2018). Indeed atmospheric Delta14C has sometimes changed with time, but prior to the nuclear testing, it had not been as high as 200 for over 10,000 years.
    In summary, 14C analysis is indeed a valuable tool for understanding the dynamics of carbon transfer. We are now sixty years removed from the nuclear testing, and much of the carbon 14 produced then is still with us. We can conclude that much of the CO2 emissions from a gas guzzling ’59 Chevy are still with us too, and that our current emissions will outlive our grandchildren. The human contribution to atmospheric carbon increases is incontrovertible. We should be analyzing the consequences of that increase rather than debating its reality.

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