Watt about the CO2 residence time?

Watts Up With That (WUWT) has a recent post by Christopher Monckton called On CO2 residence times – the chicken or the egg. Ths post discusses the bombtest curve shown in the figure below. This curve shows the decay of atmospheric 14C after the end of nuclear testing. It shows a half life of around 10 years and that most is gone within 50 years.

Bombtest curve (credit : European records by Gösta Pettersson.)

Bombtest curve (credit : European records by Gösta Pettersson.)


The conclusion that is drawn (based on some earlier work by Gosta Petterson) is that this implies that the enhanced atmospheric CO2 concentration (resulting from anthropogenic emissions) should disappear within a few decades rather than over hundreds of years, as claimed by the IPCC. I’ve already discussed this in an earlier post, so won’t go into too much detail here. The basic point is, though, that what Gosta is confusing is the residence time of an individual molecule (which is indeed only a few years) with the time it would take for an enhancement to decay [Amendment : Not only is my interpretation here a bit simplistic, but I believe that Gosta Petterson has also acknowledged an error in his first calculation. See the comments by Lars Karlsson for details]. An individual molecule will only stay in the atmosphere for a few years before being absorbed by the biosphere or the oceans. It is, however, typically replaced by another molecule from the biosphere and oceans and so, on short timescales, this carbon cycle doesn’t change the atmospheric concentration.

Christopher Monckton points out that

Mr. Born, in comments on my last posting, says the residence time of CO2 has no bearing on its atmospheric concentration: “It’s not an issue of which carbon isotopes we’re talking about. The issue is the difference between CO2 concentration and residence time in the atmosphere of a typical CO2 molecule, of whatever isotope. The bomb tests, which tagged some CO2 molecules, showed us the latter, and I have no reason to believe that the residence time of any other isotope would be much different.”

So, I think Mr Born is pointing out that the bombtest curve is telling us something about the residence time of individual molecules, not the timescale over which a concentration enhancement would decay.

Christopher then goes through various things in great detail that I won’t dissect here as I don’t have the time or the energy. The conclusion of his post begins with

It is because the consequences of this research are so potentially important that I have set out an account of the issue here at some length. It is not for a fumblesome layman such as me to say whether Professor Pettersson and Professor Salby (the latter supported by Professor Lindzen) are right.

It’s quite good to show some circumspection. However, it is a little odd that someone who is so certain about much of the science associated with global warming/climate change is now suddenly uncertain about this, especially as it’s not a particularly tricky aspect of this topic. Christopher could try reading this, or this, or this, or this, or this.Christopher then finishes his post by saying

Or is Mr. Born right?

If I’ve understood what Mr Born is saying, then the answer to this question is Yes.

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25 Responses to Watt about the CO2 residence time?

  1. chris says:

    Yes, it’s a rather dismal “argument” and difficult to understand how anyone can advance this other than as an attempt to mislead on something that is very straightforward.

    Simply put the bulk distribution of a molecule that freely partitions between two compartments (say the oceans and atmosphere in the case of CO2) is set by the physical properties of the system (e.g. the solubility of CO2 in water and its temperature dependence and the dissociation of CO2 into its acid and ionized species, the partition coefficients in relation to Henry’s law, the rate constants for mixing of CO2 and dissociated species within the oceans etc.). A slug of trace labelled version of the partitioning molecule (e.g. [14C]CO2) added to one compartment (e.g. to the atmosphere as a result of atomic bomb tests) will come to equilibrium with respect to the bulk CO2 partitioning, and so its atmospheric concentration will undergo an exponential decrease as observed, since as CO2 exchanges across the ocean/atmosphere there will be a net flow of [14C] CO2 into the oceans (i.e. the likelihood of a [14C] CO2 molecule being replaced with a [12C] CO2 molecule as it exchanges across the atmosphere/ocean interface is extremely high). This tells us something about the kinetics of molecular exchange across the compartment interface (ocean/atmosphere) but doesn’t directly inform about the equilibrium partitioning until the [14C] pool equilibrates fully with the bulk CO2.

    This is a very common phenomenon widely used for example in biophysics/biomolecular medicine. So to measure K+ conductance in a nerve cell with a high intracellular potassium (K) concentration of around 150 mM and a low extracellular K+ of ~ 5 mM: add a slug of the K40 radioactive potassium isotope to the external solution. Its concentration will decay (likely exponentially) as the K40 equilbrates with the bulk [K] inside and outside the cell. Is the actual concentration of K inside and outside the cell changing? Nope.

  2. Lars Karlsson says:

    Actually, Gösta Pettersson’s mistake is that he doesn’t take into account that fossil fuels have added a considerable amount of C12 but no C14, which has decreased the C14/C12 ratio.

    This error has been pointed out to Pettersson, and he has done the honourable thing and admitted the error (in Swedish). In particular, notice the corrected bomb curve (red) in figure 1 in my link. Unfortunately, the error continues to exist and be propagated independently of Pettersson.

  3. Lars, your link seems to go back to this post 🙂

    Presumably that can’t be his only error. There must be two factors that then reduce the C14 concentration. One is that fossil fuels increase the amount of C12, but add no C14. The other, as Chris points out above, is that when a C14 molecule is absorbed by the biosphere or oceans it is much more likely to be replaced by a C12 (or C13) molecule than by a C14 molecule. Also, it’s not clear that the figure in my post depends on C12. Isn’t it just enhancement of C14 over some baseline (or is it the enhancement in concentration – in which case you have a point)?

  4. Lars Karlsson says:

    Δ14C is the change in the 14C/12C ratio, and is defined as:

    Δ14C = ([14C/12C]sample / [14C/12C]standardsample – 1) x 1000

    NOAA has more details.

  5. Lars, thanks. Okay, so ignoring the C12 does indeed make a difference. So, then I’ve become confused (not difficult). Presumably Chris’s point (and I think what I said in my earlier post) are still issues. When a C14 molecule is absorbed by the biosphere and oceans, it’s unlikely to be replaced by another C14 molecule. Hence, the decay of C14 should more properly represent the lifetime of an individual molecule, rather than the decay time of a concentration enhancement. However, correcting for increasing C12 (as in Figure 1 of your link) does seem to give a decay time that is longer than I was expecting (the residence time of an individual molecule is something like 4 years – I think). Am I missing obvious here, or is it just more complicated than I first thought?

  6. Tom Curtis says:

    Wotts, the other factor you may be forgetting is the ongoing generation of C14 by cosmic rays, and at a much lower rate, by the nuclear industry. All the factors are discussed in Graven et al (2011), and their relationship is displayed in Fig 3. The contribution to the trend is displayed in Fig 4. As can be seen in Fig 4, ocean absorption of C14 and anthropogenic emissions of C12 have about equal influence on the trend in the early 90s, but ocean absorption has since fallen to about 1/7th of the influence of anthropogenic emissions on the trend. Release of C14 that had been absorbed in the biosphere at higher concentration levels is a third factor on the other side of the equation.

    All in all, the current decrease in C14 ratios is a subtle thing, which is why I prefer pointing to the decrease prior to nuclear testing, when anthropogenic emissions was the only significant perturbation from equilibrium.

  7. chris says:

    Yes Lars’s point is also pertinent. So simply by diluting out the [14C]CO2 by the greatly enhanced fossil fuel emissions which is [12C]CO2, the [14C]/[12C] CO2 ratio will decrease. In effect this will be an instantaneous change with respect to any increment in the atmospheric [CO2] ratio, and since atmospheric CO2 is incrementing continuously, this will result in a downward drift in the [14C]/[12C] CO2 ratio.

    However the fall of [14C]/[12C] CO2 ratio resulting from equilibration with the bulk [CO2] amongst the atmosphere/ocean/biosphere compartments will be go much further (it will tend towards some “baseline” level that probably won’t be far from zero).

    Of course the “baseline” of the (atomic-test-free) [14C]/[CO2] ratio will always be a little higher than that resulting from equilbrated partitioning since [14]C CO2 is continuously being created in the upper atmosphere as a result of the effects of cosmic rays on N2 (otherwise we wouldn’t have a [14C]-dating technique!), and man-made sources of [14C] (nuclear reactors and sneaky bomb tests that aren’t supposed to happening etc.!).

  8. Lars Karlsson says:

    There were already 14C in the atmosphere, and in the biosphere and ocean surface water before the bomb tests. There was also an exchange of 14C between these reservoirs, but when they were in equilibrium the net exchange was 0, And although in this exchange a 14C is most likely replaced by a 12C, there is also a small chance that a 12C is replaced by a C14. The bomb tests doubled 14C in the atmosphere, creating a disequilibrium, but the system eventually moves to a new equilibrium..

    So the bomb test curve is not about individual molecules.

  9. Lars, yes I was thinking about this a little more after my last comment. I can see that it’s not just about individual molecules. Presumably, however, because an absorbed C14 could be replaced by C12 or (sometimes) by C14, the decay is also not representative of the decay of a general enhancement of atmospheric CO2. Presumably, that’s why it’s somewhere between what we’d expect for the lifetime of an individual molecule and what we’d expect based on the Bern model?

  10. Tom, thanks. That’s a useful paper. I’m glad I now said If I understand what Mr Born is saying at the end of my post, as it seems that maybe I didn’t 🙂

  11. chris says:

    Lars I think we are pretty much all in agreement. However there might be some confusion of interpretation about your last sentence “So the bomb test curve is not about individual molecules.”

    The dilution effect on [14C]/[12C] CO2 ratio isn’t about individual molecules. It’s about bulk changes in the [12C] CO2 content resulting from fossil fuel CO2 emissions.

    However the decrease of [14]C/[12C] resulting from the atomic test [14C] CO2 “slug” coming to equilibration with the bulk CO2] by partitioning amongst the atmosphere/ocean/biosphere compartments is really about “individual molecules”, since this occurs without any necessary change in the absolute or relative distribution of bulk CO2 amongst these compartments.

    The contribution of each of these (dilution and redistribution) to the “bomb test curve” is presumably known/estimated.

  12. Figure 4 in the paper linked to by Tom seems quite relevant. It seems to show that the trend due to ocean exchanges has decreased since the early 90s and that the decay is now dominated by fossil fuels effects.

  13. chris says:

    Yes that would make sense Wotts on the simple grounds that the decay of the [14C]/[12C] ratio due to repartitioning amongst the compartments for CO2 is exponential, and since this is seemingly quite fast (on the decadal timescale) we’re well into the tail of the exponential. On the other hand changes in [14C]/[12C] CO2 due to dilution from fossil fuel emissions is linear.

    of course looking at a paper that addresses these things rigorously with direct measurements and simulation by measuring rate constant and so on is always preferable to qualitative arguments! On the other, other hand these things are very easy to understand in a qualitative sense and supposed informed commentators shouldn’t make obviously fallacious “arguments” about them! 🙂

  14. Lars Karlsson says:

    The people who developed the Bern model and other models of the carbon cycle obviously knew about 14C from bomb tests and used this information when developing the models. It would be very strange if the 14C curve contradicted those models.

  15. Lars, indeed that would make sense. Maybe I should add a bit more. It seems clear that Christopher Monckton is arguing that the bombtest curve implies that enhanced atmospheric CO2 concentrations might decay much faster than we think (based on, for example, the Bern model). This interpretation seems clearly wrong. It seemed that Gosta Petterson was making a similar argument in an earlier WUWT post, but at least seems to have corrected that in some sense. The main point though, that I think we all agree on, is that the bombtest curve does not imply what Christopher Monckton is suggesting that it does.

  16. johnrussell40 says:

    The way I’ve always looked at residency is probably, not being a scientist, a little simplistic. Clearly the vast majority of CO2 entering the atmosphere each year is part of the natural carbon cycle, which totally swamps the anthropogenic component by 25:1 (hence the denial meme). However because the natural carbon cycle has evolved over millennia to create a planetary equilibrium, the small anthropogenic component added annually since the start of the industrial revolution has been building up to take atmospheric concentrations from the historical level of ~270ppm, up to 400ppm recorded today. Consequently if we stopped putting the human-created component into the atmosphere this instant, it would take at least as long for the atmospheric CO2 concentration to return to its former equilibrium level — ie., 150+ years.

    Whatever the origin, whatever markers it carries, a CO2 molecule is handled in exactly the same way by the natural carbon cycle. What the natural carbon cycle cannot easily do is adapt to an increased flow of CO2 molecules to move to a new (warmer) equilibrium.

    The small stream of individual CO2 molecules of anthropogenic origin, once mixed with the 25 times bigger natural carbon cycle, is totally swamped and on average the residency time will be only a few years. However the speed of turnover (ie residency time) of individual CO2 molecules of any origin has no connection with the residency time of the human-caused atmospheric build-up of CO2.

    I’d be grateful if a scientist frequenting this thread could tell me if my basic understanding is correct, and especially my wording, as it’s the way I explain it to lay people who are even less knowledgeable than I.

  17. Tom Curtis says:

    johnrussel, not too bad. The situation is more complicated, however, in that there are more than two reservoirs involved, and hence more than one equilibrium relationship. Keeping it simple we can model the system as Atmosphere plus Shallow Ocean plus Deep Ocean plus Solid Earth. The equilibrium relationship whose time scale is shown by the draw down of C14 is that between atmosphere and Shallow Ocean, and equilibrium between them is in fact established within just a few years of perturbation. Establishing equilibrium between the Shallow Ocean and Deep Ocean depends on mixing by the thermohaline circulation, and takes hundreds of years. Finally, there is an equilibrium relationship between the three surface reservoirs and the solid Earth, with CO2 entering the surface reservoirs by volcanism, and being drawn down by weathering.

    That last is the real problem. Volcanism adds CO2 to the surface reservoirs at about 100th the rate of anthropogenic CO2 emissions. That means, if we could switch the volcanoes of, weathering would get rid of excess CO2 at one hundredth of the rate at which it has accumulated, ie, about 10,000 years. Unfortunately we cannot switch of volcanoes, so we have to wait for chemical weathering to accelerate, which it does with warmer weather.

    The net effect of all this is that the Surface Ocean (plus the biosphere) gets rid of half of our emissions more or less immediately. Over a few hundred years the deep ocean will approximately halve that again; but the remainder will hang on effectively for ever in human terms. Longer than since the invention of agriculture. Of course, unless we pursue some idiotic BAU scenario that will not be a major problem. A long term 360 ppmv from a 450 ppmv peak is not going to have overwhelming warmth, and because it is long term, our civilization can merely adapt to the new conditions.

  18. John Russell (Twitter@JohnRussell40) says:

    Thanks, Tom. Yes, it is quite a complex system, isn’t it? It’s amazing that Monkton clearly doesn’t understand the first thing about it but has the audacity to believe he’s ahead of the scientists! Dunning-Kruger in extremis, I presume?

  19. John, yes it is very complex. I was on the verge of responding to your earlier comment and then thought “I’m sure someone who knows more than me will give a much better answer”. Thankfully, Tom did exactly that 🙂

  20. This figure [http://cdiac.ornl.gov/oceans/Bomb_Pulse.html] of post-bomb radiocarbon curves in marine molluscs is interesting. It shows an initial rise in 14C, followed by a plateau or decline, presumably as 14C is exported to the deep ocean or sediment.

  21. John Mashey says:

    John Russell:
    Dunning-Kruger is pervasive, but it is possible that the Viscount’s problems go much deeper. Wikipedia entry is OK, I happen to know more. He has written insulting letters to US Senators, suggesting they apologize or “resign the offices they pollute.” His House of Lords claims are well known. He has frequently threatened lawsuits, claimed investigations in progress or written letters to university officials demanding apologies or resignations from faculty who dated to debunk his claims. (I know 3 such personally.). He has claimed to have cures for many diseases, including the Graves’ Disease with which he is afflicted.
    Google: Graves’ disease delusions
    Read especially of delusions of grandeur, persecution or reference

    I know people who encountered Monckton at Harrow and Cambridge…
    “The child is father to the man.”

    I’m no clinical psychologist, but gave consulted one I know on this. People can look at Lord Monckton’s behavior and compare with Graves, and see what they think.
    By the way, Mr Klaus-Martin Schulte in 2007 wrote a paper, introduced by Monckton as a London researcher, eventually published in Energy and Environment, supposedly refuting Oreskes’ 2004 essay on consensus. The paper was as bad as BennyPeiser’s earlier attempt, unsurprisingly. Then employed by the NHS and Kings’s College London, Schulte was Monckton’s endocrinologist.

    All this goes far beyond the usual D-K.

  22. Tom Curtis says:

    richard telford, the accompanying paper details factors that influence the concentration of C14 in seawater, and they are to varied, and complex for me to pretend to interpret. Look under discussion. Export to the deep ocean and sediments may be a factor, but it is only two among many.

  23. Eli Rabett says:

    The rapid increase in 14C would, within a decade or so, relax into the biosphere and the upper ocean reservoirs, producing something like what is seen above with the 14 C partitioned roughly equally between the three rapidly exchanging reservoirs, the atmosphere, the biosphere and the upper ocean.

    However, the level in all three reservoirs would only decay slowly (centuries) by interchange with the lower ocean that is about ten times larger than the other three. That leveling, would, in turn decay by incorporation of the 14C into the lithosphere at ocean rifts over millenia.

    So the question to ask about the figure is where was the 14C fraction before WWII and what was the effect of the increase in 12C. Upsalla Initiative has a showing the absolute amount of 14C in the atmosphere from 1960, not exactly what is needed, but a good indicator that fits well with Eli’s argument above.

  24. Eli, I think that sounds consistent with what Lars was suggesting. I’d assumed that they injection of C14 could be seen as simply a bunch of tracer particles, but some more thought has made me realise that that was incorrect. If I understand what you and Lars are suggesting, then we’d expect it to settle to some new equilibrium, with the different reservoirs, relatively quickly (decade or so).

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