Atmospheric CO2 lifetime

The Hockey Schtick appears to be excited about a new paper that finds that the lifetime of CO2 in atmosphere is only 5.4 years. This appears to actually be an abstract for a conference presentation, rather than an actual paper.

The abstract says

The isotope ratio C13/C12 of atmospheric CO2 has been measured over the last decades using mass spectrometry. From these data the fraction of fossil CO2 in atmospheric CO2 is straightforwardly calculated: 5.9%(1981) and 8.5%(2002). These results indicate that the amount of past fossil fuel and biogenic CO2 remaining in the atmosphere, though increasing with anthropogenic emissions, did not exceed in 2002 66 GtC, corresponding to a concentration of 31 ppm, that is 3 times less than the CO2 increase (88 ppm, 24 %) occurred in the last century.

So, the abstract appears to be correctly associating the change in C13/C12 ratio with anthropogenic emissions and uses this ratio to claim that the amount of atmospheric CO2 that is of anthropogenic origin is 31 ppm. The atmospheric CO2 concentration has increased by 88 ppm in the last century so, according to the abstract, only about a third of this is anthropogenic. Hmmm, strange! Not what most others seem to think. The general view is that all of this increase is essentially anthropogenic.

Let’s go a step further before trying to address what seems to be a slightly odd result. The abstract goes on to say

This low concentration (31 ppm) of anthropogenic CO2 in the atmosphere is consistent with a lifetime of t(1/2) = 5.4 years,

Now, I’m not an expert at this but I think they may have just explained why it appears that only 31ppm of the 88ppm increase this century appears to be anthropogenic. It is indeed correct that the C12/C13 ratio in fossil fuels is higher than in the atmosphere (plants “prefer” using C12 than C13) hence the change in the C12/C13 ratio is an indication that the rise in atmospheric CO2 is anthropogenic.

However, as the abstract itself indicates, an individual CO2 molecule does indeed only remain in the atmosphere for about 5 years before being used by a plant or absorbed in the ocean. The atmosphere doesn’t, however, simply lose a CO2 molecule; it is typically replaced by a CO2 molecule released from the biosphere or from the oceans. So, 5.4 years is not the time required for the excess atmospheric CO2 to be reduced by half, it is simply the typical lifetime of an individual CO2 molecule. Now, there is a higher C13/C12 ratio in the oceans than there is in fossil fuels, so we wouldn’t expect the C12/C13 ratio to remain precisely consistent with all of the increase being from fossil fuels (or, rather, a CO2 molecule using C12 absorbed by the oceans can be replaced by a CO2 molecule using C13).

So, we would only expect the change in C13/C12 from 1981 to 2002 (as observed by the authors of the abstract) to be entirely consistent with all of the CO2 increase being anthropogenic, if an anthropogenic CO2 molecule never left the atmosphere. But, as the abstract itself says, it typically only remains in the atmosphere for about 5 years. As I said, I’m not an expert at this, so maybe I’ve misunderstood something here, but it does seem that they’ve done a rather simplistic calculation that essentially ignores the carbon cycle, and also confuses the residence time of an individual CO2 molecule (about 5 years) with the time it would take for the increase in atmospheric CO2 to reduce by half (hundreds of years – see Skeptical Science for more).

When I first started reading this I thought that maybe the authors really were just trying to determine the typical residence time of individual CO2 molecules and that their abstract had been misinterpreted by the Hockey Schtick, but the abstract ends by saying

On these assumptions are based both the theory of Anthropogenic Global Warming and the climate models.

So, they do seem to be claiming that their results bring into question the fundamental tenets of anthropogenic global warming and climate change. When I read these types of things I often wonder what went through the authors’s minds when they did the work and wrote the abstract. Are they really experts in this field? Did they not consider that maybe they’ve made some fundamental mistake (as all good scientists should do)? Have they checked everything properly and spoken to others to see if maybe they have mis-understood something (as I clearly think they have)? Are they intentionally trying to mislead people? Of course, there’s always a possibility that I may have mis-understood something and maybe I have got some detail wrong. However, I doubt that what seems to be the conclusion of the study presented in this abstract (that most of the increase in atmospheric CO2 this century is not anthropogenic) is correct.

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15 Responses to Atmospheric CO2 lifetime

  1. I’ve just realised that this paper was presented at a conference in 2009. Has probably been effectively rebutted elsewhere but, I guess, it does no harm to do it again. Wonder why the Hockey Schtick decided to highlight it now.

  2. I’ve also just noticed that the Hockey Schtick post goes on to talk about the work of Gosta Pettersson. It also confuses the residence time of individual CO2 molecules with the time is would take for the increase in atmospheric CO2 to reduce. I discussed it in an earlier post.

  3. Again, the deniers are screwing with data and playing what amounts to rhetorical slight of hand. This particular false argument crops up again and again. The problem, for deniers, is that the molecule is just swapping places with another one in the ocean or biosphere. It’s the large pool of carbon we keep pumping into the carbon cycle that matters.

    If the crappy sleight of hand argument were true, CO2 wouldn’t have risen so rapidly. For all practical purposes, because of ocean-atmosphere-biosphere CO2 swapping, most of our emissions will remain in the atmosphere for decades, with a substantial portion remaining for centuries and millennia. Even worse, the added heat we cause reduces the depth of key carbon sinks resulting in EArth creating its own increasing carbon emission over time.

    In this case, the deniers have trotted out an old and very nearly dead horse to beat yet again.

  4. Rachel says:

    I’m always curious about who writes papers like this too, although a paper presented at a conference is not necessarily a peer-reviewed paper is it? I can’t find this paper in a library journal search.

    The lead author is possibly this guy – http://www.pi.ipcf.cnr.it/users/luciano-lepori – who appears to work at an Italian institute for chemical and physical processes. He’s got a number of publications – http://65.54.113.26/Author/11197284/luciano-lepori – but they don’t look to me to be in the field of atmospheric CO2.

  5. Rachel says:

    If there’s very little science to back up your point of view – as is the case with those critical of climate science – then you need to recycle old papers over and over again to make it look like the science is progressing. This is probably why the Hockey Schtick has taken something presented at a conference years ago. These are the dregs and that’s all they’ve got.

  6. It also might be because Murry Salby has supposedly responded to his critics and this, according to the Hockey Schtick, corroborates Murry Salby’s work.

  7. I agree and I find these arguments more and more surprising as time goes on. It seems clear that the evidence that the rise in atmospheric CO2 might be natural is vanishingly small. Also, as I wrote about a few days ago, the chance that the climate sensitivity is less than 2oC is also vanishingly small (and – as you’ve pointed out – if you include fast- and slow-feedbacks it ‘s almost certainly quite a bit bigger than 2oC). If those who were sceptical were genuinely interested in the science then they should, in my view, at least accept these scientific arguments – or at least acknowledge that the evidence for these arguments is strong. One could then discuss what to do and whether this is significant or not (I think it is, obviously), but still arguing about aspects of the science that are reasonable settled is very odd.

    As I learn more and more about the debate, I start to realise why some are less civil than I’ve tried to be.

  8. I too wonder what goes through people’s minds when they do these type of things. Is ideology winning over scientific honesty? Do they really think that as a non-expert scientist they’ve suddenly seen something obvious that experts have missed? Why don’t they talk to experts?

    I’ve done part of this, in a sense, myself. There have been a few occasions when I’ve considered that maybe people have been missing something obvious and that I may have solved some kind of interesting problem. I’ve yet to be right though 🙂

  9. Marco says:

    It’s the latter, most likely. These people have a method, they tried it on something “relevant”, and then think they found something “new”. The reason they never published is probably because someone pointed out that they had just reconfirmed (as in: this has been done many, many times before) the exchange time of CO2, not what the IPCC considers the half-life of the ‘excess’ CO2, which is what is of importance to AGW.

    I hardly dare say this, but once again we have chemical engineers with large claims about the climate scientists getting it all wrong…and failing miserably.

  10. I agree, it probably is the latter and they may well realise, now, that they’ve made a mistake. I tried to write a bit about the involvement of other disciplines a few days ago, but it is interesting how many non-experts engage directly in climate science. There are engineers and economists who seem more than willing to write papers/abstracts about climate science but often don’t seem to include an experienced climate scientists in their collaboration. I find that a bit strange and is not something I’ve seen before.

  11. They seem to be physical chemists. For the typical laboratory work they do, their thinking is mostly right. I guess there you normally do not have additional reservoirs (biosphere, ocean) that are even larger than the one you are measuring (atmosphere).

    In the web of science I could not find any works by these authors on climate. Thus it is likely an honest mistake and they seem to have noticed this, otherwise they would have published it by now.

  12. You may well be right and, to be honest, if you do make such a mistake in a conference presentation the norm would be to either take it no further or correct it before trying to publish. You wouldn’t normally be expected to provide some kind of correction as it isn’t a peer-reviewed article. So the authors haven’t really done anything wrong. Maybe they have indeed (as I suggest they should have) spoken to climate scientists at the conference and realised their mistake. Good on them, if so.

    In this case, of course, pseudo-skeptics can continue pointing to this abstract as proof of something, even though it is not. Not much one can do about that other than trying to point out that it is indeed wrong.

  13. Some people are unlucky enough to get a paper making this same mistake published. For example Essenhigh (2009), who had Crawley (2011; http://pubs.acs.org/doi/full/10.1021/ef200914u) explain where the mistakes were. Crawley has a very nice analogy to explain why the turnover of CO2 and the life-span of a pulse of CO2 are not linked.

  14. Thanks, Richard. Abstract looks interesting, but can’t seem to access the paper from home. Will have another go tomorrow when I’m back in the office.

  15. I started out being civil. Now, I have a lot of trouble with it.

    Imagine a navy vessel that is under enemy fire. A large wave of missiles approaches. Radar can give a rough estimate of the threat — bad to very bad — but can’t give the exact number. One of the gunnery officers can’t believe it’s real and decides not to respond at all. Another sees the threat and freaks out so much that he becomes despondent, muttering ‘we’re doomed.’

    You try to toggle the missile defense weapons, but as you do the despondent gunnery officer and the in denial gunnery officer leap to stop you.

    Sometimes that’s where I feel we are, but in a kind of horrible slow motion, stop frame. And that’s probably why I’m less civil than I often should be.

    Best to you and keep up the great work. My hopes for you to retain a level head 😉

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