Drawing down atmospheric CO2 – part 1

A few days ago, I was somewhat less than civil in response to a comment by Nic Lewis, which then lead to a typically juvenile post on Bishop Hill. It was, though, quite amusing to have Anthony Watts complaining about my lack of civility and suggesting that I could learn from Nic Lewis, who has published more climate papers than I have. It’s clear that Anthony Watts puts great stead in those who are both civil and well-published.

My frustration was largely because I had thought that Nic was simply being pedantic about my not qualifying something sufficiently. However, it seems that he was actually suggesting that not only could the equilibrium climate sensitivity (ECS) be low, but that – in fact – the carbon cycle could draw down CO2 rapidly enough, that only a small fraction would remain in the atmosphere for a very long time. Depending on how one defines “small fraction”, I think that this is not what is expected. I thought, therefore, that I would try to discuss our current understanding.

There is, however, rather a lot to this whole isssue, so my plan is to do it in two posts, this being the first. One of the first things to recognise is that what is of interest is not the residence time for an individual CO2 molecule, but the decay time for an enhancement in atmospheric CO2 concentration. In a sense, one can envisage the system as consisting of a number of CO2 reservoirs; the ocean, the biosphere, the atmosphere, and the planet’s lithosphere. There are continuous fluxes into and out of each reservoir. Some of the fluxes are large enough that an individual CO2 molecule will typically only spend a few years in the atmosphere, before being absorbed by the ocean, or the biosphere. However, this is considerably faster than the timescale over which an enhancement in atmospheric concentration would decay.

There are, also, essentially two carbon cycles; a fast carbon cycle, and a slow carbon cycle. The fast carbon cycle involves the ocean, the biosphere, and the atmosphere, and is associated with fluxes of 10s of GtC per year. The slow carbon cycle is associated with the sequestration of CO2 into the deep ocean, into rocks via weathering, and the emission of CO2 back into the atmosphere via volcanoes. It can involve fluxes of only about 0.1GtC per year, orders of magnitude smaller than those associated with the fast carbon cycle. However, it’s essentially the slow carbon cycle that sets the quasi-steady atmospheric CO2 concentration (i.e., the concentration that could be sustained for thousands of years with little change).

Prior to the industrial revolution, the atmospheric CO2 concentration was pretty steady at around 280ppm. The reason for this is that this is the concentration at which the rate of sequestration into the slow carbon sinks, matches the rate at which it is released through volcanic activity. Small perturbations away from this would change the sequestration rate, so that the atmospheric concentration would then return to about 280ppm. As long as these perturbations were small, this quasi-steady concentration could be maintained. However, what’s happened since the industrial revolution is that we’ve emitted a large amount of CO2 into the atmosphere (about as much as was in the atmosphere prior to the industrial revolution) at a rate far in excess of the rate at which it could then be sequestered back into the slow carbon sinks.

Given sufficient time, the CO2 that we’ve emitted will be sequestered into the slow carbon sinks and the concentration will return to pre-industrial levels. However, since it relies on the slow carbon cycle, this will take 10s of thousands of years, if not longer. We even have evidence for this kind of timescale. According to Archer et al. (2009)

Sediment cores from the deep ocean reveal a climate event 55 million years ago that appears to be analogous to the potential global warming climate event in the future. Isotopes of carbon preserved in CaCO3 shells reveal an abrupt release of carbon to the atmosphere-ocean system, which took about 150 thousand years to recover.

However, there is something I haven’t discussed. Even though it seems clear that it will take 10s of thousands of years for atmospheric CO2 to return to pre-industrial levels, what’s of interest to us is what fraction of our total emissions will remain in the atmosphere once the fast cycle has returned the ocean, biosphere, and atmosphere into a state of quasi-equilibrium. It’s this that I will try to discuss in another post.

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18 Responses to Drawing down atmospheric CO2 – part 1

  1. izen says:

    I think in part 2 you may need to address the argument that according to the Keeling curve about half the human addition of ~6Gt of fossil carbon to the carbon cycle(s) is sequestered in the fast carbon cycle, apparently permanently, within a year.

    Therefore (goes one brand of causal logic) if we half our emissions there would be no further increase in atmospheric CO2 levels as the fast cycle can sequester half our emissions.

    And if we stop emissions, or reduce to less than half, the carbon cycle will continue to remove the extra atmospheric carbon we have added at about the same 3Gt rate. Resulting in the return to the ‘proper’ level of CO2 in about the same time it took to reach the higher level.

  2. Joshua says:

    ==> “It’s clear that Anthony Watts puts great stead in those who are both civil and well-published.”

    If there’s one thing that’s obvious from reading WUWT it’s that Anthony, not to mention the typical commenter at his site, puts great stead in those who are both civil and well-published.

  3. Pierre-Normand Houle says:

    “And if we stop emissions, or reduce to less than half, the carbon cycle will continue to remove the extra atmospheric carbon we have added at about the same 3Gt rate. Resulting in the return to the ‘proper’ level of CO2 in about the same time it took to reach the higher level.”

    Yes, in the hypothetical scenario where we would immediately reduce anthropogenic emissions to zero, over the following few years the rate of decrease of atmospheric CO2 concentration would approximately mirror the previous rate of increase, assuming only the airborne fraction was close to 50%. However, while the partial pressure of atmospheric CO2 would decrease, the partial pressure in the ocean’s mixed layer will tend to remain much higher than they were when atmospheric concentration was originally at the same level. The removal of carbon from the mixed layer is dependent on slow biological processes and the slow mixing rate with deeper layers. Since the rate of (net) dissolution of atmospheric CO2 in the oceans depends on both the atmospheric and surface ocean partial pressures, it will soon cease to mirror the previous rate of increase. There likewise can be expected a smaller rate of intake by the terrestrial biosphere, and a small effect from the fast carbon cycle feedback response to increased temperatures.

  4. izen,

    I think in part 2 you may need to address the argument that according to the Keeling curve about half the human addition of ~6Gt of fossil carbon to the carbon cycle(s) is sequestered in the fast carbon cycle, apparently permanently, within a year.

    Yes, that’s coming in part 2, which I haven’t yet written.

  5. Tom Curtis says:

    anders, if you have not done so already, I highly recommend you read Solomon et al (2008) prior to part 2. One useful feature of that paper is it models CO2 concentration assuming a gradual increase followed by an abrupt cessation of emissions, rather than by using instantaneously introduced slugs of CO2 (as is more typical).

  6. Tom,
    Thanks, I’ll do that. I’ve been reading David Archer’s papers but – as you mention – they use an instantaneously introduced slug of CO2, rather than assuming a gradual increase.

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  8. mt says:

    I concur with P-N H above.

    Regarding this by ATTP:

    “One of the first things to recognise is that what is of interest is not the residence time for an individual CO2 molecule, but the decay time for an enhancement in atmospheric CO2 concentration. In a sense, one can envisage the system as consisting of a number of CO2 reservoirs; the ocean, the biosphere, the atmosphere, and the planet’s lithosphere. There are continuous fluxes into and out of each reservoir. Some of the fluxes are large enough that an individual CO2 molecule will typically only spend a few years in the atmosphere, before being absorbed by the ocean, or the biosphere. However, this is considerably faster than the timescale over which an enhancement in atmospheric concentration would decay.”

    I fully agree with this as well. I would like to call attention to it because it is widely misunderstood.

    Many people think the molecular residence time rather than the perturbation decay time is the time constant of the problem. If things ever get out of hand, then, they think it will take only a few years of adjusted behavior to bring everything back into order. Correcting this error requires careful explanation of the material ATTP summarizes so succinctly.

    I’m looking for a suitable analogy to make the point to a wider audience.

    I myself have long though of it as a series of basins in a fountain, with water flamboyantly but constantly spurting from one basin to the other. The total water in the fountain depends on the slow balance between evaporation and replenishment. This helps me think about it. But it’s too easy for someone to chip at this analogy – it works for the total, but for a single basin (corresponding to the atmosphere only) it works well only if certain assumptions (true of the carbon cycle) are implemented in the fountains.

    Another thought is comparing the situation of a person who has a real income to that of a person who transfers money back and forth between his checking account and his savings account. This captures the nature of the fallacy well enough but the sense of “having too much” is confused because, well, because you can’t be too rich.

    In short, I’m still looking for an accessible and appropriate analogy. Any suggestions?

  9. MT,
    I tend to think of basins of water too, but – as you say – it’s an analogy that people can chip away at. The issues seem to be a prevalent one when it comes to trying to distinguish between stocks and flows.

  10. izen says:

    The carbon cycle is like a Library.

    The books on the shelves and on loan to the readers represent the carbon locked into biological systems and the CO2 in the atmosphere respectively.

    There are always about the same proportion of books out on loan to the proportion in the library on the shelves, but which books are in, (sequestered) and which out on loan, (in the atmosphere) are changing every few weeks.

    If more books are added to the library collection, perhaps by readers donating old books they have dug up, then even if it is just 5% of the total number of books borrowed and returned in a year it will increase the number out on loan as well as on the shelves.

    Any persistent increase in the number of books the library has will persistently increase the number out on loan (as well as on the shelves). Only the slow process of discarding books when they get too damaged will reduce the amount in circulation. (the slow geological CO2 sequestration). So if the library gets fewer, or stops receiving, extra books it will still take a long time to reduce the number in circulation, and therefore the proportion out on loan (in the atmosphere).

  11. Poltsi says:

    There are at least a couple of studies which have looked into how long the added CO2 will affect earth’s climate and which are worth mentioning:

    Tyrrell et al. 2007

    Tzedakis et al. 2012

  12. dikranmarsupial says:

    MT I wrote a response to Prof. Essenhigh’s paper on the residence time argument that explains why it is incorrect (residence time is noth the same as adjustment time) using a few analogies and a *very* simple model that also explains why only a small proportion of atmospheric CO2 is of directly anthropogenic origin and why we see a rougly constant airborne fraction.

    Gavin C. Cawley, On the atmospheric residence time of anthropogenically sourced carbon dioxide, Energy & Fuels, volume 25, number 11, pages 5503–5513, September 2011.

    A pre-print is available here .

    I had hoped that this might help to stop this argument being promulgated (and hence time wasted discussing it), but apparently it has not.

    My MOOC lecture on the topic is here, which contains another analogy (team effort). Some people like analogies, but not others. They are useful here as they demonstrate that the definition of “causing X to rise” often used in relation to atmospheric CO2 would be obviously wrong in more familiar settings (which mean that some typically refuse to engage with analogies).

    Ferdinand Engelbeen has been tireless in trying to explain this on climate-skeptic websites (his patience is admirable), and his website has a lot of useful material on this topic.

  13. Willard says:

    Speaking of analogies, 450 ppm and economics, here’s an old story:

    Let’s say we’re talking about global populations of tuna, and that scientists are telling us that tuna are being caught at an unsustainable rate and that we need to cut the number of tuna we catch by 20% by 2020 in order to maintain a stable tuna population. Then Roger comes over and tells us that what we really ought to be looking at is not the number of tuna being caught every year but the consumption of tuna per capita in different countries around the world. Then Roger shows us graphs about rising populations in the developing world and the rising consumption of tuna per capita all over the world and tells us how difficult it will be to reverse this trend: how many more chickens we’d need to raise, etc. Finally, Roger comes to the seemingly inescapable conclusion that the number of tuna being caught every year is going to keep on rising. Anybody with half a brain can see that there is something missing from this story: What happens if there are biological limits to how many tuna we can catch? Anybody with a full brain should see that this analogy casts doubts on the value of Roger’s approach to climate change: What happens if there are physical limits in terms of the quantity of fossil fuels we can consume? What happens if there are biogeochemical limits in terms of the quantity of fossil fuels we can consume before blowing up the planet? This is not the time to pass judgment on these questions—for myself, I worry about the second question but not the first one—but it is the time to be concerned about the fact that these kinds of questions don’t even come up in Roger’s analysis.

    http://standupeconomist.com/thoughts-on-roger-pielke-jr/

    Numbers and the names in that analogy are replaceable.

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