CO2 thermometer

Climate Change: News and Comments

The Carbon Dioxide Thermometer

There’s an idea that keeps turning up like a bad penny, and I’ve had a part in it. But as I’ll explain, there’s a problem with it. The proposition is that the increase in the carbon dioxide concentration in the air has little to do with human emissions and a lot to do with prevailing temperatures, perhaps especially at the sea surface.

If CO2 follows temperature rather than the other way around, then changes in CO2 become a measure of temperature, as in a thermometer.

The latest manifestation is on the Watts Up website from Lon Hocker at

Let me mention three previous appearances of this idea:

Cynthia Kuo et al., “Coherence established between atmospheric carbon dioxide and global temperature”, Nature 343, pp. 709-14, 1990, which concluded: “Changes in carbon dioxide content lag those in temperature by five months.”

Nigel Calder, “The carbon dioxide thermometer”, Energy & Environment,10, pp. 1-18, 1999

Jarl Ahlbeck, “The carbon dioxide thermometer”, 2001, see

Since my own offering of 11 years ago, I’ve kept checking to my own satisfaction that what I suggested still works into the 21st Century. (The new story from Lon Hocker supports this.) The CO2-temperature link, with cause and effect swapped around, also looks arguable on geological timescales. Where the idea runs into difficulties is in the Holocene, since the end of the last ice age.

There have been repeated ups and downs of temperature, like those between the Medieval Warm Period, the Little Ice Age and the Modern Warm Period, without the strong variations in CO2 that you’d expect from this hypothesis. I’m not talking about the CO2 results from ice layers, which are suspect because CO2 is soluble in ice and variations are smoothed out. No, I mean results from leaf stomata, by Rike Wagner-Cremer and her colleagues at Utrecht, which are much more trustworthy. They’ve been a little disappointing for the hypothesis.

Here for example is the abstract of one that group’s papers:

T. B. van Hoof et al., “A role for atmospheric CO2 in preindustrial climate forcing,” PNAS, October 14, 2008; 105(41): pp. 15815 – 15818.

Complementary to measurements in Antarctic ice cores, stomatal frequency analysis of leaves of land plants preserved in peat and lake deposits can provide a proxy record of preindustrial atmospheric CO2 concentration. CO2 trends based on leaf remains of Quercus robur (English oak) from the Netherlands support the presence of significant CO2 variability during the first half of the last millennium. The amplitude of the reconstructed multidecadal fluctuations, up to 34 parts per million by volume, considerably exceeds maximum shifts measured in Antarctic ice. Inferred changes in CO2 radiative forcing are of a magnitude similar to variations ascribed to other mechanisms, particularly solar irradiance and volcanic activity, and may therefore call into question the concept of the Intergovernmental Panel on Climate Change, which assumes an insignificant role of CO2 as a preindustrial climate forcing factor. The stomata-based CO2 trends correlate with coeval sea-surface temperature trends in the North Atlantic Ocean, suggesting the possibility of an oceanic source/sink mechanism for the recorded CO2 changes.

At first sight, these researchers are singing much the same song as the CO2 thermometer-makers. So what’s the problem? Simply that the CO2 fluctuations reported in this and other papers are small compared with those of the past century. The period of which van Hoof et al. write includes the Medieval Warm Period when temperatures were at least as high as today. But their CO2 never got above 319 ppmv, compared with 392 at Mauna Loa for May 2010.

The implication is that the CO2 thermometer is not the whole story. Man-made emissions must have contributed to the increase of the past 100 years. How significant that has been as a driver of the temperature increase is another question entirely.


13 Responses to CO2 thermometer

  1. Bob Campbell says:

    Dear Nigel,
    The link to WUWT and Lon Hocker now works.
    I have seen other accounts where the temp/co2 lag was getting out to 800 years.
    Would you like a pointer?
    Bob Campbell.

    • calderup says:

      Thanks a lot, Bob.
      The 800-year lag, as in the ice-age recoveries, is handy if you’re arguing with Al Gore, but what with melting ice sheets and changes in the deep ocean circulation, it’s a pretty tricky “thermometer” to try to understand.

  2. John Finn says:


    Re: 800 year lag

    I understand that this lag exists at the ‘peaks’ and ‘troughs’ but is this because the CO2 gradient is shallower than the temperature gradient or is it because the CO2 response actually kicks in ~800 years later. It’s difficult to tell on most plots due to the resolution.

    Just to clarify.

    Intuitively, I would expect a CO2 response to a major temperature change to occur within a few decades to a century, i.e. not ‘wait’ 800 years before an initial response is evident.

    • calderup says:

      As I see it, John, you can have responses within days or hours from the CO2 in the surface layer of the ocean, emitted or absorbed depending on temperature. Storms agitating the sea speed the process up. But then you have to average the changes around the globe, which takes months.
      By then the temperature may have changed again and the system may never quite reach equiblibrium.
      That doesn’t seem to matter in the year-to-year CO2 Thermometer, but with glacial-interglacial transitions the quest for equilibrium becomes much more important, and you need changes in the ocean circulation to dig out CO2 “buried” in the ocean abyss.

  3. John Finn says:


    Great, thanks – and very quick.

    What prompted my question was that a number of people seem to be suggesting that the MWP is responsible for the current CO2 rise (the reasoning being the MWP was ~800 years earlier). I thought this unlikely but couldn’t think of a convincing reason particularly as few of them seem to accept the ice core data. I’m not sure if this is reasonable but, from your reply, I’ve deduced the following.

    Although the MWP and LIA are on longer timescales than, say, ENSO events which cause many of the short-term (annual) fluctuations, they are still subject to temperature/equilibrium constraints, e.g. the warmer MWP was followed by the colder LIA.

  4. Ben says:

    I have to agree. Looking at ice cores, CO2 as a thermometer doesn’t appear to work on human time scales. I cannot see any reason to doubt that the majority of the recent atmospheric CO2 increase is due to human fossil fuel consumption. There is a direct, simple mechanism (CXHY+O2 -> X CO2 etc) which is of the correct order of magnitude. Everything fits with the observations.

    I don’t see why people try to keep saying that the CO2 increase is non-anthropogenic. It undermines science just as much as the “birthers” undermine people who oppose Obama’s policies.

  5. Bart says:

    An obvious correlation between modern measurements of temperature and CO2 on the one hand, versus reliance on unverifiable theories on how processed ice cores from a handful of specific locations can retain and divulge a faithful record of global CO2 concentration over centuries on the other. Hmm… which to choose?

    • John Finn says:

      Why can’t the ice core records and observations be correct. They are not inconsistent. There has always been year-to-year variability in the CO2 record which is clearly temperature dependant, i.e. when it’s warmer atmospheric CO2 levels rise faster than when it’s colder. There’s no mystery here. But, in addition to the yearly fluctuations, there is a longer term (50 years at least) underlying trend which has seen CO2 levels increase by ~70ppm. The ~0.5 deg rise in temperature isn’t responsible for the increase in CO2.

      In a nutshell:

      Fossil fuel burning causes the rise; temperatures determine how much it rises in the short term.

  6. Bart says:

    Ben says:
    11/06/2010 at 16:45

    “There is a direct, simple mechanism (CXHY+O2 -> X CO2 etc) which is of the correct order of magnitude. Everything fits with the observations.”

    Consistency with observations is not proof. And, simple accounting such as you suggest is not appropriate for a system regulated by feedback. See my comment here.

  7. Pascvaks says:

    It appears appropriate to beg the question – might other forms of anthropogenic ‘carbon’ pollution explain the difference in CO2 levels? (ie: carbon monoxide, ozone, and various other carbon and non-carbon waste gases that were not present prior to the Industrial Revolution in such quantity as we see today, but which react to increase CO2)

  8. […] paper corroborates prior work by Salby, Humlum et al, Frölicher et al, Cho et al, Calder et al, Francey et al, Ahlbeck, Pettersson, and others demonstrating that man-made CO2 is not the […]

  9. […] by the IPCC. The paper corroborates prior work by Salby, Humlum et al, Frölicher et al, Cho et al, Calder et al, Francey etl, Ahlbeck, Pettersson, Segalstad, and otherswhich has demonstrated that man-made CO2 is […]

  10. […] IPCC. The paper corroborates prior work by Salby, Humlum et al, Frölicher et al, Cho et al,Calder et al, Francey etl, Ahlbeck, Pettersson, Segalstad, and others which has demonstrated that […]

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