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Murry Salby finds CO2 rise is natural

What the science says...

Multiple lines of evidence make it very clear that the rise in atmospheric CO2 is due to human emissions.

Climate Myth...

Murry Salby finds CO2 rise is natural

"Salby’s argument is that the usual evidence given for the rise in CO2 being man-made is mistaken. It’s usually taken to be the fact that as carbon dioxide concentrations in the atmosphere increase, the 1 per cent of CO2 that’s the heavier carbon isotope ratio c13 declines in proportion. Plants, which produced our coal and oil, prefer the lighter c12 isotope. Hence, it must be our gasses that caused this relative decline. But that conclusion holds true only if there are no other sources of c12 increases which are not human caused. Salby says there are - the huge increases in carbon dioxide concentrations caused by such things as spells of warming and El Ninos, which cause concentration levels to increase independently of human emissions. He suggests that its warmth which tends to produce more CO2, rather than vice versa - which, incidentally is the story of the past recoveries from ice ages." (Andrew Bolt)

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.

Intermediate rebuttal written by dana1981

Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial



Last updated on 11 July 2015 by MichaelK. View Archives

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Comments 1 to 16:

  1. What is the basis for your statement that the natural flux of CO2 "balances out in the long term?" What do you mean by long term, and what evidence do you have for this assertion?

    In What the science says, you state that multiple lines of evidence make it very clear that the rise in co2 is due to human emissions. If you are basing this on analysis of c12/c13 isotope ratios! This is precisely one of the arguments that Salby takes apart. You do not address Salby's argument, but instead appear to appeal to authority.
  2. tompinlb @1:

    1) CO2 concentrations over the holocene show little variation prior to the industrial era showing that the net natural CO2 flux is close to zero. Even the small 0.003 ppmv flux over the 7000 years prior to the industrial revolution is probably due to land use changes, partly from the desertification of the Sahara, but primarily due to human agriculture, particularly the cultivation of rice. The supposition that natural fluxes should increase 150 fold (conservatively estimated) by stange coincidence at exactly the time when humans started burning fossil fuels at a rate approximately double that which is required to explain the increase in CO2 in the atmosphere strains credulity, both in requiring human emissions to magically not effect atmospheric concentrations, and by requiring the vastly larger natural emissions from an undiscoverable source to magically coincide with the curiously ineffective human emissions.

    So, to explain it for you, long term means over the period of the Holocene.

    2) Not only do you require your magical natural source to have a peculiar sensitivity to the onset of industrialization, it must also be an aquatic organic source. Organic because only organic sources have the right C12/C13 ratio. Aquatic because only ocean sources and fossil fuels would deplete C14 ratios (as occurred prior to the onset of atmospheric nuclear explosions), and also because land vegetation is known to be increasing in mass. This depletion of organic matter in the ocean has to march in lock step with industrial emissions which, of course mysteriously continue to vanish without a trace.
  3. tompinlb, did you read the article? Do you understand the mass balance argument? The natural world must achieve a rough equilibrium over geologic time scales, or else CO2 would disappear or supersaturate the atmosphere. The geologic record shows the development of the carbon equilibrium, and it has been in rough balance during most of the course of the latest interglacial. Suddenly, atmospheric carbon rockets upward. It's just coincident with massive human industrialization? Uhhhh . . . No. Humans have been dumping CO2 into the atmosphere faster than the historically developed (and developing) natural cycle can take it out. Eventually, there will be another point of rough equilibrium. Eventually. The isotope bit is just another line of evidence.
  4. tompinlb - also are you aware that a very rough "rock weathering thermostat" is postulated operating in geological time scales. CO2 absorption by ocean is limited by Ca ion release. As temperatures go up, so does weathering rate, eventually pulling CO2 from atmosphere and cooling temperatures. More detail here.
  5. @tompinlb That the rise in atmospheric CO2 is purely anthropogenic is easily established with a high degree of certainty. In order to put an end to this common climate myth, I wrote a paper on the subject, read about it here. I sent a pre-print of the paper to Prof. Salby, but I didn't receive a reply. Sadly Prof. Salby is simply incorrect on this one, and I strongly suspect that the journal paper he referred to in hs talk may have been withdrawn.

    I would be happy to go through the mass balance argument with you, it really is surprisingly very simple.
  6. @ Dikran Marsupial

    Re Q&A

    Audience question: "... more temperature -> more co2 -> more temperature? ..." (4'30")

    Murry Salby: "For reasons that I am not going to go into with this audience, there has to be a negative-feed back that bridles this positive feed-back and holds things in check."

    Oh yeah? Do tell more professor Salby! He didn't. There just "has to be".

    Am I offended because of the condescension? It might not be possible when speaking to lay people to explain everything in a short space of time, but it seems to be a crucial point that you can't hedge on if you're serieus.

    Pique or scepsis?
  7. You can see Prof. Salby's talk here and I would strongly recommend anyone interested in the topic to watch it. The key error is introduced in the slide at 21:08; more or less every scientific point prior to that appears essentially correct, but completely uncontraversial.

    @Sascha Tavere, the uptake of CO2 by the oceans is dependent on temperature, but also by the difference in partial pressure between the surface ocean and the atmosphere. The equilibrium warming due to CO2 increases only logarithmically, but the pressure relationship is IIRC linear, and opposes the change. I suspect this is the feedback that Salby had in mind, but I don't know why he didin't just say so.
  8. Beside the mass balance and stable isotope arguments, there is the fact that ocean pH is decreasing and pCO2 in the ocean is increasing in the surface mixed layer. If increasing temperature were causing the observed atmospheric CO2 increase by outgassing, you would see the opposite. Faced with that, he should have been, at the very least, motivated to try to find an alternative intrepretation for his observations. `
  9. Dikran,

    Actually I think he goes further wrong in the step before that, starting at 20:00. He says:
    … sensitivity… how much net emission of co2 is increased by an increase in temperature, or soil moisture.

    Because they are incoherent with human activity, they can only represent emission from natural sources.

    If you know that component from net emission, you can add it from one year, to the next year, and so forth. The total then represents the contribution to the observed CO2 from natural sources.
    His flaw is that he sees that changes in temperature correlate well with changes in CO2, as we all know they do and thoroughly expect (due to changes in solubility of the oceans).

    But he then conflates that well understood and even quantifiable mechanism with the idea that therefore any other mechanism (human emissions) is mere noise, and that therefore the obvious, visible effect must be the cause of the net increase.

    Interestingly, this presumes some magical source of carbon that he not only fails to identify, but never even attempts to mention or address.

    This is exactly equivalent to noting that clear skies and clouds cause the largest daily changes in temperature, and therefore the net increase in temperature as the seasons change from winter to summer must be entirely a result of the cumulative, net change from cloud cover but not from the change in the angle of incidence of sunlight over many months (because that net daily increment is small, and "is not coherent" with daily temperature changes).

    I hope in his paper he takes the time to do some real science, i.e. to compute the changes in partial pressure and solubility in the ocean and atmosphere for each year, to compute the expected change in CO2 from these levels, and then to make some effort to identify the source of the discrepancy.

    Gee, I wonder what it could be?
  10. Stephen,

    I think the single biggest nail in this whole coffin is that human emissions have extracted 337 Gt of carbon from the ground and turned it into CO2.

    So any "natural source" argument must both

    (a) come up with a comparable source of carbon (which would be utterly huge, and unheard of in the past 800,000 years where atmospheric levels have never topped 300 ppm)


    (b) a sink to absorb and completely hide the human emissions.

    As well as some rationale which explains why the sink works on human emissions but not natural emissions, so that the atmospheric and ocean CO2 increases are "entirely natural" while human emissions magically disappear into thin air and have no effect on anything.

    This, to me is complete and total denial in a nutshell. To make this sort of twisted, impossible, Alice-in-Wonderland argument work requires such a Lewis-Carrollian pretzel that it clearly demonstrates a mind in utter and complete denial of reality.
  11. @spherica estimating the sensitivity of natural changes in CO2 to changes in temperature is a worthwhile question, but the answer is only 8-9ppmv per degree centigrade.

    Salby is essentially making the same mistake as Spencer in noticing that there is a correlation between temperature and the annual change in atmospheric CO2. However if you differentiate a long term trend (to get the annual change) any linear increase becomes a constant, and the correllation only explains the variability around this offset, it doesn't explain the offset itself, and hence doesn't explain the long term increase in CO2.

    I really hope Prof. Salby withdraws the paper (I have sent him an email pointing out this error) his other work is apparently very good and it would be a pity for him to tarnish his reputation.
  12. 11, Dikran,

    Exactly. But what's even better in this case is that you've got simple, raw physics to fall back on (expected solubility change of CO2 by measured temperature). You can compute the variation you would expect in the atmosphere based purely on the ocean temperature and partial pressure of CO2. If you can find an adequate data source and guestimation, you can also adjust that changes in foliage/plant growth.

    Do that and you'd be left with this clear, unwavering linear trend.

    And absolutely no further information on where it came from.

    That is I guess my big problem with Salby... when he identifies the trend he assumes that it must be natural because *insert mumbled, incoherent reasoning here*. And yet what he says clearly says absolutely nothing about the cause of the increase, only the cause of the irregular variation.
  13. Just been pointed to this article that seems to support Salbys work. Do you have a rebuttal for its conclusions?


    [TD] Just more of the same for Humlum.  Search for "Humlum" on this page.  Also enter Humlum in the Search field at the top left of this page to find more.

    [TD] Sorry, I should caffeinate before typing in the morning.  Mark Richardson wrote a Skeptical Science post about this Humlum paper!

  14. Michaelf, Mark Richardson debunked Humlum's 2013 article in the latest issue of the same journal.


    [TD] Mark Richardson wrote a whole Skeptical Science post summarizing his comment.

  15. Michaelf, a debunking by Masters and Benestad was published in the July issue of the same journal.  The article is paywalled, but Masters explained the approach on his blog, and Benestad debunked previous, annoyingly similar "work" by Humlum on RealClimate.  Masters discusses their Masters and Benestad critique on his blog, including a discussion by he and Richardson about the complementarity of their critiques.


    [TD] Mark Richardson wrote a whole Skeptical Science post summarizing his comment.

  16. Thanks. Had not come across it before, read it, thought it was very similar to what Salby was trying to get published but couldn't, so was surprised to see those theories published. Probably because he left the question open about where the CO2 was coming from and did not answer the question of where our CO2 had gone. So before I had that argument I thought I would check over here. Glad I did :-) will go and do some reading.

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