The Phanerozoic CO2 and GMST Relationship

Along with the publication of Emily Judd's reconstruction[1] of global temperatures, she and her team compiled an up-to-date reconstruction of CO2 concentrations from proxy data. I shared in a previous post how this new reconstructions shows a CO2 and GMST as well-correlated on geologic time scales, with GMST increasing by ~8 C for 2xCO2. Her CO2 reconstruction is another step forward for paleoclimate because it makes use of a growing database of proxy evidence, much of which is being compiled at the Paleo CO2 project.


For the Paleozoic and Mesozoic, Judd's reconstruction followed Foster et al 2017[2] pretty closely, especially during the Paleozoic, but she also included additional data from Witkowski et al 2018[4][3]. Differences between Judd's and Foster's reconstruction had to do with different ways of site and time averaging and updates from Witkowski's study. During the Cenozoic, she followed more recent work by Rae et al 2021[5] which agrees closely with the CenCO2PIP project[6]. She plotted Foster and the CenCO2PIP reconstructions along with her PhanDA reconstruction so that you can see divergences above.

Judd also produced graphs showing the correlation between CO2 and GMST during the Phanerozoic, including the Paleozoic and Cenozoic. These graphs are tremendously helpful, and I included them in my previous post on Judd's paper. However, I'm already seeing posts on Facebook and X that are working really hard to misrepresent Judd's work. Some are trying to say that temperatures are cold now (compared to the rest of the Phanerozoic) and so AGW can't be a problem. Others are saying that Judd's work is the result of data manipulation. Of course, no evidence has emerged for the latter claim, but the former claim may perhaps helpfully addressed by making graphs that correctly illustrate the content produced in this paper. 

So I decided to make some graphs to be able to share in these discussions, but also so that others that find themselves in these discussions may have them readily available. A few notes should be made about these. To illustrate the log relationship between CO2 and GMST, I sometimes showed CO2 plotted on a logarithmic scale, noted by Ln(rCO2) where rCO2 is the CO2 (ppm) at some time in the past divided by 276 ppm (the paper's preindustrial level). I showed changes in radiative forcing by RF = 5.35*ln(rCO2) in W/m^2. In graphs with CO2 in the x-axis, I plotted the correlation between CO2 and GMST, the calculated slopes of the regression line differed slightly from Judd's, since she applied York regressions to account for errors in the predictor and predictand. I can't duplicate this, so my graphs show Judd's calculated slopes with 95% CIs and r^2 values (instead of r values). When plotting CO2 as ppm, I used multiples of 400 ppm to make it easier to compare CO2 levels in the past to multiples of near present CO2 concentrations.

Phanerozoic GMST and CO2

The Phanerozoic covers all of Geologic history since the Cambrian explosion, though proxy evidence is limited until the Ordovician, so Judd's reconstruction actually only covers the last 485 million years. The following graphs show GMST, CO2, and both together in one graph.

Since the relationship between CO2 and temperature is logarithmic, plotting CO2 in ppm can distort the perception of correlation, so below I plotted CO2 as Ln(rCO2) so the correlation will be more apparent. As you can see the only noticeable period of divergence is during the Cretaceous hothouse, where temperatures spiked without an corresponding increase in CO2. This could be do to lack of necessary data for CO2 or GMST, but it may also be that there are other forcings involved during this time. Overall, however, the GMST correlates with CO2 at about 8 C for 2xCO2 on geologic time scales.

Here I plotted both CO2 and solar forcings, and I plotted the overall trend in CO2 forcings with time. As you can see, there's an overall downward trend in CO2 forcings over geologic history that is compensated by a slightly larger increase in solar forcings.
Judd also includes geographic forcings, largely based on the location of continents and the fraction of the globe covered by subaerial land vs ocean. I haven't found the data for this yet, but I find it interesting that the relationship between GMST and CO2 is stronger than the relationship between GMST and CO2+Solar+Geography. This suggests to me that there are more forcings that have not yet been quantified, like perhaps the impact of methane.

Cenozoic GMST and CO2

The Cenozoic covers the geologic history of the Phanerozoic from the time of the extinction of non-avian Dinosaurs to the present. During this time, the continents are closer to their current positions, biological process are more similar to today, and we have the most reliable data. The Cenozoic should have the fewest issues with the limitations of data, and correspondingly, we see a stronger correlation between CO2 and GMST, confirming that overall, 2xCO2 correlates with ~8 C warming on geologic time scales. I decided to plot this correlation in two ways. The data is the same in both graphs, but the top graph shows CO2 plotted on a log scale, which makes the relationship between CO2 and log(CO2) linear. The second graph shows CO2 plotted in a standard scale, and so the relationship shows up as a log curve. 

If you find any of these graphs and information useful, feel free to use at your convenience. It would be great if you could link to this post so others can see this and for further context, but that's entirely up to you.




References

[1] Emily J. Judd et al., A 485-million-year history of Earth’s surface temperature. Science 385,eadk3705 (2024).DOI:10.1126/science.adk3705

[2] G. L. Foster, D. L. Royer, D. J. Lunt, Future climate forcing potentially without precedent in the last 420 million years. Nat. Commun. 8, 14845 (2017). doi:10.1038/ncomms14845

[3] S. K. Lauvset, R. M. Key, A. Olsen, S. van Heuven, A. Velo, X. Lin, C. Schirnick, A. Kozyr, T. Tanhua, M. Hoppema, S. Jutterström, R. Steinfeldt, E. Jeansson, M. Ishii, F. F. Perez, T. Suzuki, S. Watelet, A new global interior ocean mapped climatology: The 1° × 1° GLODAP version 2. Earth Syst. Sci. Data 8, 325–340 (2016). doi:10.5194/essd-8-325-2016
129. 

[4] C. R. Witkowski, J. W. H. Weijers, B. Blais, S. Schouten, J. S. Sinninghe Damsté, Molecular fossils from phytoplankton reveal secular Pco2 trend over the Phanerozoic. Sci. Adv. 4, eaat4556 (2018). doi:10.1126/sciadv.aat4556

[5] James W.B. Rae, Yi Ge Zhang, Xiaoqing Liu, Gavin L. Foster, Heather M. Stoll, Ross D.M. Whiteford. "Atmospheric CO2 over the Past 66 Million Years from Marine Archives." Annual Review of Earth and Planetary Sciences 2021 49:1, 609-641.
https://www.annualreviews.org/doi/full/10.1146/annurev-earth-082420-063026

[6] The Cenozoic CO2 Proxy Integration Project (CenCO2PIP) Consortium, Toward a Cenozoic history of atmospheric CO2. Science 382,eadi5177(2023). DOI:10.1126/science.adi5177. Accepted version online at: https://oro.open.ac.uk/94676/1/Accepted_manuscript_combinepdf.pdf



Comments

  1. FizzySeptember 29, 2024 at 12:59 PM

    Thanks for these clear and useful summaries of Judd et al.

    ReplyDelete

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