Guest geology by David Middleton
We are often told that the warmth of the Early Paleogene was driven by CO2; and that the cool-down from the Late Paleogene, into the Neogene and Quaternary Periods was driven by a draw-down of atmospheric CO2. The notion of a CO2-driven climate has apparently become a paradigm.
This paradigm didn’t exist in the 1970’s .
Suggestion that changing carbon dioxide content of the atmosphere could be a major factor in climate change dates from 1861, when it was proposed by British physicist John Tyndall.
Unfortunately we cannot estimate accurately changes of past CO2 content of either atmosphere or oceans, nor is there any firm quantitative basis for estimating the the magnitude of drop in carbon dioxide content necessary to trigger glaciation. Moreover the entire concept of an atmospheric greenhouse effect is controversial, for the rate of ocean-atmosphere equalization is uncertain. Dott, Robert H. & Roger L. Batten. Evolution of the Earth. McGraw-Hill, Inc. Second Edition 1976. p. 441.
Why geology is supposed to avoid paradigms
When I was studying geology, way back when The Ice Age Cometh in the 1970’s, we were taught to avoid getting hooked on paradigms or “ruling theories”. Geology, as a science, has very few unique solutions. This is why we were were taught to embrace Chamberlin’s Method of Multiple Working Hypotheses. I have to assume that either this is no longer the case or that homage must be paid to the current paradigm in order to get published.
Hat tip to Brian Pratt for sending me this paper…
Moderate levels of Eocene pCO2 indicated by Southern Hemisphere fossil plant stomata
Margret Steinthorsdottir, Vivi Vajda, Mike Pole, and Guy Holdgate
Reducing the uncertainty in predictions of future climate change is one of today’s greatest scientific challenges, with many significant problems unsolved, including the relationship between pCO2 and global temperature. To better constrain these forecasts, it is meaningful to study past time intervals of global warmth, such as the Eocene (56.0–33.9 Ma), serving as climatic analogues for the future. Here we reconstructed pCO2 using the stomatal densities of a large fossil Lauraceae (laurel) leaf database from ten sites across the Eocene of Australia and New Zealand. We show that mostly moderate pCO2 levels of ∼450–600 ppm prevailed throughout the Eocene, levels that are considerably lower than the pCO2 forcing currently needed to recreate Eocene temperatures in climate models. Our data record significantly lower pCO2 than inferred from marine isotopes, but concur with previously published Northern Hemisphere Eocene stomatal proxy pCO2. We argue that the now globally consistent stomatal proxy pCO2 record for the Eocene is robust and that climate sensitivity was elevated and/or that additional climate forcings operated more powerfully than previously assumed.
The anthropogenic rise in CO2 concentrations (pCO2) is predicted to result in a global average temperature increase of up to 4 °C by the year 2100 (IPCC, 2014), with severe socioeconomic and ecosystem impacts predicted. However, the exact relationship between pCO2 and temperature—or climate sensitivity (the equilibrium response in mean global surface temperatures to a doubling of pCO2, generally reported as ∼3 °C)—is still not well understood…
The Eocene epoch was such a time interval, with average global temperatures 4–15 °C higher than at present (Zachos et al., 2001; Huber and Caballero, 2011; Anagnostou et al., 2016; Cramwinckel et al., 2018). In the earliest Eocene (ca. 55.5 Ma), there was a transient episode of extremely elevated temperatures—the Paleocene-Eocene Thermal Maximum, or PETM (McInerney and Wing, 2011). Later, after the peak warmth of the Early Eocene Climatic Optimum (EECO, ca. 52–50 Ma), a gradual cooling began, briefly interrupted by a major warming reversal at ca. 40 Ma, called the Middle Eocene Climatic Optimum (MECO) (Zachos et al., 2001; Cramwinckel et al., 2018). The Eocene climate still constitutes one of the greatest unsolved problems in paleoclimate research. Temperatures were globally much higher than today, with a significantly weaker equator-to-pole temperature gradient and a muted seasonal cycle compared to today, referred to as the “Eocene equable climate problem” (Sloan and Barron, 1990; Greenwood and Wing, 1995; Greenwood et al., 2003a). Climate modeling has been able to reconstruct this pattern with very high pCO2 levels (up to ∼4500 ppm: Huber and Caballero, 2011), but such extremely elevated pCO2 is not documented by proxy records. It is therefore assumed that Eocene climate sensitivity—often defined as Earth system sensitivity for longer time scales, including both “fast” and “slow” feedbacks (Lunt et al., 2010)—was elevated compared to present, and/or that other mechanisms, in addition to the dominant forcing of pCO2, were in operation (Caballero and Huber, 2013; Anagnostou et al., 2016; Zeebe et al., 2016; Carlson and Caballero, 2016; Cramwinckel et al., 2018; Keery et al., 2018).
Comparison to Existing pCO2 Records and Implications
The most striking feature of the Eocene stomatal proxy record is that some of the highest pCO2 is indicated in the early middle Eocene (until ca. 46–44 Ma), well beyond the end of the EECO…
Although it is premature to make strong statements, this would imply that Earth system sensitivity was likely in the range of ∼4–8 °C during the Eocene, significantly elevated compared to the “modern” climate sensitivity of ∼3 °C (Lunt et al., 2010; Royer et al., 2012; Maxbauer et al., 2014; Wolfe et al., 2017; Keery et al., 2018; Schneider et al., 2019). However, the various feedback mechanisms affecting Earth system sensitivity in an ice-free world are still poorly understood. Steinthorsdottir, M., Vajda, V., Pole, M., and Holdgate, G., 2019, Moderate levels of Eocene pCO2 indicated by Southern Hemisphere fossil plant stomata: Geology, v. 47, p. 914–918, https://doi.org/10.1130/G46274.1
In summary, we find pCO2 of ∼450–600 ppm recorded by Southern Hemisphere fossil plants throughout the Eocene—significantly less than the forcing required by modeling, suggesting that climate sensitivity was elevated and/or that other climate forcings were stronger than previously assumed.
- The Eocene was, on average, 4–15 °C warmer than today.
- Atmospheric CO2 was very likely in the 450-600 ppm range.
- Modern climate models would require 4,500 ppm CO2 to simulate the Eocene temperature range;
- And/or a climate sensitivity of 4-8 °C per doubling;
- And/or “that other climate forcings were stronger than previously assumed”.
They totally missed the most obvious reason why just about every effort to gin up a paleo example of CO2-driven climate change falls apart: Atmospheric CO2 is not a primary driver of climate change over geologic time. This wouldn’t mean that it isn’t a greenhouse gas or that it has no effect on temperature. It would simply mean that it was a relatively minor climate driver, like volcanic eruptions.
At some point over the past 30 years or so, the assumption that CO2 drives modern climate change has become a paradigm. And I think we have seen a rare failure in the application of the geologic principle of Uniformitarianism.
Uniformitarianism is often incorrectly cited as the reason geologists were slow to accept plate tectonics, the impact theory of the K-Pg extinction and why the hypotheses for a Younger Dryas impact and abiotic oil are generally unaccepted. However, Uniformitarianism may be why a CO2-driven climate paradigm appears to have come into wide acceptance, at least in academia.
The past history of our globe must be explained by what can be seen to be happening now. No powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle.James Hutton, 1785
Geologists are taught that the processes we observe today are the same processes that formed the ancient rock formations that comprise the geologic history of the Earth. An example would be oolitic limestone. By observing where and how modern oolitic carbonate sediments are formed and deposited, we can deduce the past depositional environments of oolitic limestone formations.
“The present is the key to the past” is valid axiom… Unless the present is fundamentally misunderstood.
Here is figure 2 from Steinthorsdottir et al., 2019 (S19):
Did you notice something odd? The moderate CO2 concentrations actually increase from the warmer Early Eocene into the cooler Middle to Late Eocene.
I added S19’s Eocene stomata CO2 estimates to my compilation of Cenozoic Era estimates and temperatures (note that my plot has older toward the left).
A note regarding the δ18O temperature reconstruction: The conversion of δ18O to temperature is based on an ice-free model, more suited to the Paleocene and Eocene, than later epochs. However the relative changes in temperature would be in the same direction.
It is evident in Figure 3a that only the foram δ11B reconstruction yields exceptionally high pCO2 concentrations during the Paleogene. δ11B is a proxy for pH, which is related to pCO2, although not necessarily a good proxy for pCO2 itself. The alkenone δ13C and stomata reconstructions all indicate moderate pCO2 concentrations during the Paleogene and Neogene. Clearly, there was no significant coupling of temperature and CO2 over first 65,999,850 years of the Cenozoic Era.
It is very tempting to assume that past CO2 concentrations can be used to directly calculate pre-industrial seawater pH and vice-versa. The following graph is a skeptic favorite:
This graph didn’t sit well with the CO2-Driven Climate Paradigm crowd, so they  decided to “fix” the temperatures by adjusting them to pH values calculated from CO2. We can see that this yields a much better correlation between CO2 and temperature.
One slight problem…
This fairly decent correlation yields an equilibrium climate sensitivity (ECS), inclusive of all feedback, of only 1.28 °C per doubling of atmospheric CO2 over the past ~540 million years. This would mean that the transient climate response (TCR), the one that actually affects us, is only about 0.85 °C per doubling of atmospheric CO2, very much inline with the low end of recent low sensitivities calculated from satellite-era instrumental observations.
 Middleton, David H. “A Clean Kill of the Carbon Dioxide-Driven Climate Change Hypothesis?” WUWT. 25 September 2019.
 Middleton, David H. “Middle Miocene Volcanism, Carbon Dioxide and Climate Change”. WUWT. 3 June 2019.
 Dott, Robert H. & Roger L. Batten. Evolution of the Earth. McGraw-Hill, Inc. Second Edition 1976. p. 441.
 Steinthorsdottir, M., Vajda, V., Pole, M., and Holdgate, G., 2019, “Moderate levels of Eocene pCO2 indicated by Southern Hemisphere fossil plant stomata”: Geology, v. 47, p. 914–918, https://doi.org/10.1130/G46274.1
 Royer, D. L., R. A. Berner, I. P. Montanez, N. J. Tabor and D. J. Beerling. “CO2 as a primary driver of Phanerozoic climate”. GSA Today, Vol. 14, No. 3. (2004), pp. 4-10
Berner, R.A. and Z. Kothavala, 2001. “GEOCARB III: A Revised Model of Atmospheric CO2 over Phanerozoic Time”, American Journal of Science, v.301, pp.182-204, February 2001.
Pagani, Mark, Michael Arthur & Katherine Freeman. (1999). “Miocene evolution of atmospheric carbon dioxide”. Paleoceanography. 14. 273-292. 10.1029/1999PA900006.
Pearson, P. N. and Palmer, M. R.: Atmospheric carbon dioxide concentrations over the past 60 million years, Nature, 406, 695–699,https://doi.org/10.1038/35021000, 2000.
Royer, et al., 2001. Paleobotanical Evidence for Near Present-Day Levels of Atmospheric CO2 During Part of the Tertiary. Science 22 June 2001: 2310-2313. DOI:10.112
Tripati, A.K., C.D. Roberts, and R.A. Eagle. 2009. “Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years”. Science, Vol. 326, pp. 1394 1397, 4 December 2009. DOI: 10.1126/science.1178296
Zachos, J. C., Pagani, M., Sloan, L. C., Thomas, E. & Billups, K. “Trends, rhythms, and aberrations in global climate 65 Ma to present”. Science 292, 686–-693 (2001).