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.