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Murry Salby - Confused About The Carbon Cycle

Posted on 13 August 2011 by Rob Painting

Every year humans release about 30 billion tons of carbon dioxide (CO2) into the atmosphere through the burning of fossil fuels, such as coal and oil. This is causing the Earth to warm by disrupting the biological (fast) carbon cycle, and is therefore increasing the Greenhouse Effect. Although there are large annual fluctuations in carbon dioxide, as it is exchanged back-and-forth between the atmosphere, oceans, soils, and forests, just under half of human emissions (the airborne fraction) remain in the air because the oceans, soils and forests are unable to absorb all of it. As a result, carbon dioxide has been steadily accumulating in the atmosphere.

Figure 1 - Fraction of the total human emissions (fossil fuel burning & land use change) that remain in the: a) atmosphere, b) land vegetation and soil, c) the oceans. From Canadell (2007)

Murry Salby, a professor at Macquarie University in Sydney, Australia, has an upcoming paper that attempts to pin the current rise in carbon dioxide on rising temperatures. Having listened to a podcast of a talk Salby gave at the Sydney Institute earlier this week, he demonstrates a remarkably poor understanding of the carbon cycle, and his hypothesis seems to stem from this fundamental misunderstanding.

Salby's carbon cycle confusion

Professor Salby refers to a number of graphs in his talk, but I have been unable to track down copies of these, therefore we'll have to rely on what I'm able to glean from the podcast, and given it's length, I'll only address some of the more obvious mistakes. At the beginning of the talk Salby states:

"current CO2 values are 380pmmv"(parts per million by volume) 

Not an encouraging start that he cites the atmospheric CO2 concentration as it was in 2005, rather than the 393 parts per million by volume (ppmv) it currently is in 2011. Not a fatal flaw of course, but not encouraging either. 

"Net annual emission has an average increase of about 1.5ppmv per year. We're on the right planet. That's the annual average increase you just saw. But it varies between years, dramatically by over 100%. From nearly zero in some years to 3ppmv in others. Net global emission of CO2 changes independently of of the human contribution"

At this point the accentuation and drama in Salby's voice make it sound as though he has stumbled onto something momentous, something no one else has noticed before. On the face of it, it seems preposterous that the army of scientists that have worked on carbon cycling over the years could have missed something so glaringly obvious. No, of course they haven't.

As discussed in the first paragraph of this post (and evident in Figure 1), the natural flux of CO2 in and out of natural systems varies from year-to-year. This flux is 20-30 times larger than the annual contribution by humans, but this balances out in the long-term. This variability is driven largely by El Nino and La Nina in the tropical Pacific, which shifts rainfall patterns over much of the world and is associated with warming and cooling of equatorial waters in the Pacific. The change in seawater temperature, and episodic upwelling of carbon-rich deep water, significantly affects the uptake and outgassing of CO2 from the oceans, and of course rainfall variation greatly affects plant growth. 

The upshot is that land vegetation takes up more CO2 during La Nina, and expels more CO2 during El Nino. In the ocean, the opposite trend occurs - El Nino leads to more CO2 absorption, and La Nina is when the oceans give up more CO2 (Figure 2). 

Figure 2 - (a) time trend in the exchange of CO2 by land-based vegetation (& soil microbes) with the atmosphere. (b) same - but for exchange of CO2 by ocean with atmosphere. Red indicates El Nino and blue La Nina phase. See Keeling (1995).  

There is simply no reason why the annual fluctuation should match the human contribution. At least Salby doesn't explain why he expects this to be the case. 

Having now convinced himself that short-term net CO2 has nothing to do with the human contribution, Salby therefore deduces long-term net CO2 must also be unrelated to human emissions. He goes on to derive a formula for CO2 rise associated with temperature. Salby claims a good match back to 1960 but therefafter it deviates from actual CO2 measurements by 10ppmv. By 1880, prior to atmospheric CO2 sampling, he estimates atmospheric CO2 at 275ppmv with a whopping uncertainty of 220 to 330ppmv!

In order to explain the deviation between the surface temperature record and his calculated atmospheric CO2 level, Salby blames the surface temperature record as being unreliable. As for his calculated trend disagreeing with the ice core record for the year 1880 (i.e the CO2 in air, from that period, trapped in ice cores) he 'disses' the ice core record claiming it to be only a 'proxy'. Which is news, I'm sure, to respected ice core experts like Dr Richard Alley.

You will note that every time the data disagrees with Salby's 'model', he trusts his 'model' over the data. Which contravenes the 'skeptic lore' that models are worthless and must be bashed, and only data should be trusted.

Q&A time - try not to shoot yourself in the foot!

The question & answer session at the end of Salby's talk throws up a few more comments that just reinforce that he has strayed into a field of science which he just simply doesn't understand. Witness:

"I think it's a pitfall that people look at the ice proxy of CO2 and take it literally. It's not atmospheric CO2, and I don't believe it's CO2 that was even in the atmosphere when that piece of snow was layed down"

This is nonsense. Perhaps Professor Salby should have acquainted himself with glaciology research before making such comments, because CO2 from ancient air trapped in the ice cores is precisely what is measured, albeit with some uncertainty in dating some sections.

"CO2 after the turn of the (21st) century continued to increase, in fact if anything slightly faster, but global temperature didn't. If anything it decreased in the first decade of the 21st century. Now I'm confident the IPCC (Intergovernmental Panel on Climate Change) will come up with an explanation, in fact they've come up with several"

It's here we need to back the truck up a bit. Salby's entire premise is that CO2 in the air directly dependent upon temperature - increase temperature and you increase CO2. Yet here he argues that CO2 can increase without an accompanying increase in temperature. Which contradicts his 'model'. By this time Salby is too focused on 'dissing' the IPCC to notice his own incoherency, and none of the audience picks up on this either.  

Note that SkS recently discussed the 'noughties slow-down' in global temperature here and here.

If the curve fits 

Seasoned readers will notice similarities between this Salby claim and a Lon Hocker rebuttal here at SkS last year. But the whole premise seems to follow along the lines of other recent flawed works tendered by Roy Spencer and Craig Loehle & Nicola Scafetta. That is: find some tenuous statistical relationship between two sets of data, and use these to assert the mainstream scientific establishment is wrong. The fact that there is no physical basis for the statistical relationship, or it doesn't fit within the well-established scientific framework, or is contrary to numerous other sets of data, never seems to warrant attention by "skeptic" scientists. It should, because of the implications one can draw. 

So what does this work by Salby imply, if it were true? From what I can gather from Salby's podcast, a 0.8°C change in average surface temperature is supposed to lead to about 120ppmv change in CO2. Therefore we can work backward in time to estimate what he reckons atmospheric CO2 would be at the time of the last Ice Age (glacial maximum), a time when global temperatures were about 4-6°C cooler than now . Today atmospheric CO2 is about 393ppm, so with 4°C cooling you already have a negative value for CO2 when we re-trace our steps back to the last ice age. Therefore all plant-based life on Earth must have died (and all the animals that depended on them) according to Professor Salby. And the Earth froze solid too.

Figure 3 - the last Ice Age according to Murry Salby? Fictional image from  

Science - a description of reality, but YMMV

Without viewing Salby's calculations on the temperature/net global CO2 relationship, it's not possible to provide the 'killer blow' to his assertions; however, I don't believe that's necessary, considering the many flaws in Salby's work and fundamental reasoning.

The gradual increase in atmospheric CO2 is less than the total emissions of CO2 from human sources, so by elementary deduction, the excess must be going into the oceans, forests and soils, the other components of the fast carbon cycle.

A tell-tale signature of human fossil fuel emissions is the large fraction of CO2 being driven into the oceans. According to Henry's Law, we would expect the oceans to absorb more CO2 as the air above it becomes increasingly saturated with CO2. In other words the CO2 must be coming from a source external to the fast carbon cycle. This is supported by measurements showing that CO2 is accumulating in the ocean, and is reflected in the declining oceanic pH, showing the ocean is actually gaining CO2 over the long-term, not losing it, as Salby seems to believe.

We also know that the world's land vegetation has increased in mass - through re-growth in forests in the Northern Hemisphere, and CO2 fertilization of tropical forests. So that is gaining carbon too, and the areas affected are so large, we would expect them to have an effect on atmospheric CO2 levels at a global scale.  

There are a host of other problems with Salby's 'model', such as the ice core record, and where the warming came from in the first place, but there's no need to go into these details when the fundamental premise of Salby's argument is so clearly wrong.  

This blog post has been used as a rebuttal to the climate myth "Murry Salby finds CO2 rise is natural" which can be found at

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Comments 101 to 115 out of 115:

  1. Dale wrote: "In terms of the evidence presented in this thread, the cause is human emissions." Good, I'm glad you agree. So unless Salby can refute the data used here, his theory is dead in the water? "But I do proviso that statement with "in terms of the evidence in this thread". In his podcast Salby says the anthropogenic emissions data are the only reliable data we have. The Mauna Loa data are so solid even WUWT accepts them as accurate, and it is the atmospheric growth data that Salby uses (listen to his podcast). Those are the only datasets used here, and Salby endorses both of them! "For instance, how are human emissions calculated?" The anthropogenic emissions data would have to be an overestimate by a factor of two to change the result. The uncertainty of emissions data isn't anything like that large. The uncertainty is also likely to be assymetric with under-estimate more likely than an over-estimate, simply because energy usage is taxed, so energy companies have no benefit to be gained by over-reporting. "Which also brings questions on how the natural in and out figures are calculated. How accurate to reality are they?" This comment suggests that you still don't understand the mass balance argument as it does not assume any knowledge about the "in and out" figures for the natural environment. It is a method for calculating the difference between them without knowing their values.
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  2. Dale, you might not like it, but think how this series looks to others. When presented with a logic chain which contradicts the conclusion you support you do not say, 'hmmm... maybe the conclusion is wrong'. Instead, you decide to deny the validity of the starting data. The logic can't be refuted, so the only remaining way to 'defend' the conclusion is to assume a conspiracy to falsify the data. Yet, as Dikran has pointed out, Salby accepted the accuracy of the data. Thus, we can still clearly see that his analysis MUST be wrong... unless you can identify some flaw in the logic progression that Dikran walked you through (which was an excellent analysis BTW). Failing that you are essentially now arguing against Salby's assumptions (i.e. the data is correct) as the basis to 'defend' his conclusion (i.e. the increase in atmospheric CO2 levels is due to natural emissions). This is inherently self-contradictory. Your claim that we must wait for Salby to publish in order to see if there is a flaw in his position is disproven by the fact that you cannot maintain support for his conclusion without contradicting his assumptions. This is the kind of logical inconsistency / cognitive dissonance which SHOULD make people say, 'Hmmm... there seems to be a flaw in this position'. You haven't done that. Is it any wonder that this causes some people to suspect your integrity? Personally, I think it more likely that you are sticking to the position out of some sort of faith or emotional resistance to being incorrect. That is, you aren't so much lying/trolling US as you are doing so to yourself. However, the effect is the same... rather than making logical conclusions you are leaping to incredibly thin conspiracy theories (e.g. human emissions data accepted by all parties is overstated by more than 100%).
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  3. It might be worth waiting until you can see whats in the envelope before you comment.
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    Moderator Response: [Dikran Marsupial] Unless there is evidence that the carbon cycle is not essentially closed and large amounts of CO2 are dissapearing from the atmosphere without being taken up by the natural environment, Salby's conclusion in incorrect. Salby's paper is not the first along these lines, and sadly I suspect it won't be the last. The probability that Salby is as wrong as his predecessors is substantially greater than the probability that he has a proof that the carbon cycle is not a closed system.
  4. 103, hearlek,
    It might be worth waiting until you can see whats in the envelope before you comment.
    It might be worth it if "skeptical" climate scientists like Salby and Spencer engaged in science instead of grandstanding and PR. If they are going to engage in the latter, however, they are going to be soundly debunked at every turn where their presentations are a flawed. Flawed presentations will yield corrections and derision. Flawed papers will yield (as in the case of Spencer and Lindzen) correcting papers. Each response will be fairly quick, as is appropriate to each transgression. To imply that Salby's foolish presentation should be allowed to stand uncontested because he did so before releasing his paper is absurd.
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  5. Then again, maybe Hearlek is right. Salby may publish something very different after having the science of the presentation demolished in an unusually public (for him) way. To add to what Dikran said, if Salby has discovered any of these conditions, he should only submit to Nature or Science, as those discoveries would take many disciplines by surprise. I doubt if he has, though, because if he had, he would already have shared the information with his fellow climate scientists. He wouldn't wait for final publication. One wonders where Salby turned for feedback before he went public with the presentation.
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  6. I think you already dealt the killer blow. Salby states "CO2 after the turn of the (21st) century continued to increase, in fact if anything slightly faster, but global temperature didn't. If anything it decreased in the first decade of the 21st century. " He says what!!!! Salby's entire premise is that CO2 in the air is directly dependent upon temperature – if you increase temperature, you will increase CO2. Yet here he argues that CO2 can increase without being driven by an increase in temperature. Which contradicts his thesis.
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  7. Anybody have any news on Salby's paper yet? I emailed him to enquire where it will appear but got no reply.
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  8. Hi Dikran, That is weird, only yesterday I was wondering about the Salby paper. I thought that it should be coming out this week or next week.
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  9. Hi Albatross, the secrecy about the journal is interesting, I hope it isn't Remote Sensing, a third dodgy climate paper would be too much for any new journal! ;o)
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  10. Salby mentioned the Medieval Warm Period and the Little Ice age, neither of which were global events, and then claimed that CO2 followed temperature; these assertions have been refuted and debunked by scores of scientists. Murry Salby needs to update himself in the current literature. Note that our New Galileo here also ignored the fact that humans are currently putting 30,000,000,000 tons of carbon dioxide into the atmosphere EVERY YEAR, yet the non-human sources put in AND THEN REMOVE the same, or very similar, amounts of CO2 that they do every year. Just where does Mr. Salby believe our release of CO2 is going?
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  11. desertphile - a nitpick but evidence to date would say that LIA was indeed a global event, though less marked in SH than in NH.
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  12. Responding to civil engineer  from here.

    "I don't find the sks links that convincing" is not a useful comment. If you dont explain the problems you percieve with responses to Salby, then how are we to distinquish this from "I prefer a reassuring lie to an inconvenient truth"?.

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  13. Any claim is easily said.  Providing evidence to support the claim is more difficult. Civil engineer, how about exercising some civility and engineering a discussion around where the SkS links failed?  That way you can provide a valuable service to the entire community and become believable.  

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  14. Btw, the Salby paper appears to be in purgatory.  Everyone's been waiting now for about two years. Nothing yet.  

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  15. Well it would have to be published somewhere where reviewers would be unaware of the rebuttal to Humlum and where they were not aware of other methods of determining the contribution (eg d13 isotope balance, Henry's law etc). Good luck on that but then I'm amazed Humlum could find reviewers clueless enough to let his past.

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