A Survey of Scientific Literature on CO2 and Paleoclimate

Reconstructed Temperatures (Top), Modeled CO2 (Bottom) and Glaciation (Blue)

My interest in climate science came from my background in geology, which lead to an interest in paleoclimates, which then lead to better understanding more recent climate changes. I thought it might be interesting to share some of the more recent studies examining the correlation between CO2 and global temperature over the course of the 21st century so far. The tl;dr is simply this: there is a robust correlation between modeled and proxy evidence for CO2 and both reconstructed temperatures and evidence for glaciation across the Phanerozoic.

In 2001, Robert Berner constructed a climate model to “predict” CO2 levels throughout the Phanerozoic.[1] In that paper, he also provided the proxy data for CO2 during the last 420 years or so. He was able to demonstrate that lower levels of CO2 correspond to periods of glaciation, and high CO2 levels correspond to periods with no glaciation.

GEOCARBIII Climate Model CO2 Reconstrution

In 2004, Royer, Berner and others conducted a study examining the role of CO2 as a driver of Phanerozoic climate. In particular, they were responding to a study that claimed that the cosmic ray flux modeled by Shaviv and Veizer (2003) correlated better with temperatures than CO2. In Royer’s study, they examined Veizer’s reconstruction of Phanerozoic SSTs that he inferred from shallow marine carbonate δ18O, and he corrected for the influence of seawater pH in Veizer’s temperature reconstruction. The resulting temperature record correlates better with CO2 than with cosmic ray flux. But in this study we continue to see a strong correlation between temperatures and glaciation with CO2 over the phanerozoic.[2]

In 2006, Berner published an update to his GEOCARB model. The GEOCARBSULF model shows both fluctuations in CO2 and O2 throughout the Phanerozoic. The fascinating portion of this study was showing the decrease in CO2 concentrations as trees spread throughout the continents during the Devonian and Carboniferous. “They brought about large increases in the rates of chemical weathering of silicates and rates of burial of organic matter resulting in a dramatic rise of O2 and drop in CO2 during the mid-to-late Paleozoic. These trends ended abruptly at the Permian-Triassic boundary as a result (and probably contributing cause) of the massive biological extinction at that time.”[3] Royer has a treatment of the same subject here.[4]

Also in 2006, Royer published a study that compared 490 proxy records for CO2 since the Ordovician with globally cool events. He finds that CO2 levels need to be below 500 ppm for the initiation of continental glaciation, with “cool events” all occurring below 1000 ppm CO2. He finds the correlation to be “pervasive” and “tight” at 10 million year time scales to million year time scales. His study also included a correction for increases in solar luminosity over the Phanerozoic. With the sun brighter now than it was hundreds of millions of years ago, the CO2 threshold for glaciation is lower than it was in the distant geologic past.[5]

Global Temperatures with CO2 Forcings Corrected for Solar Luminosity

In 2007, Royer and Berner published another study with what I consider a novel and fascinating way of estimating climate sensitivity over the course of the Phanerozoic. Taking the fact that temperatures increase with ln (CO2), they entered different values for sensitivity (dT2x) to see which values would produce results that would best correspond with known CO2 levels. Of course, they had to account for other factors affecting the long-term carbon cycle. “Such factors include solar evolution, changes in palaeogeography, palaeolithology, palaeohydrology, global degassing, organic and carbonate burial rates, and land plant population.” This didn’t require that they use independent estimates for temperatures. This study found that “the best fit between the standard version of the model and proxies occurs for dT(2x) = 2.8.”[6] During a presentation in 2009 on this same subject, Royer wrote, “Over the past 420 Myr, the modal climate sensitivity is ~3 °C; a sensitivity of < 1.5 °C is highly improbable, while sensitivities of 6+ °C cannot be excluded. Deep-time climate sensitivity thus matches the present-day, despite the two approaches capturing fundamentally different carbon cycle processes; a climate sensitivity of around 3 °C appears to be a robust feature of the Earth system, independent of temporal scaling. The geologic record generally supports a positive link between CO2 and temperature, and we should expect a climate sensitivity of 3 °C or more in the near future.” [7]

A Climate Sensitivity of ~2.8 Best Reconstructs the CO2 Proxy Record

In 2014, Franks published a study that further constrained CO2 concentrations from the Devonian onward, and shows that the flourishing of trees in the Devonian permanently altered the concentrations of CO2 in the atmosphere. “Our results highlight a fundamental transition in Earth's atmosphere following the evolution of forests in the mid Devonian (~390 Myr ago) [Stein et al., 2012]. Up until this point, according to both the GEOCARBSULFvolc and fossil models, ca exceeded 1000 ppm. However, for the remainder of the Phanerozoic ca was less than 1000 ppm, consistent with the emergence of global forests that captured and sequestered vast amounts of carbon from the atmosphere [Berner, 2003].”[8] Since temperatures since the Devonian have exceeded ~6 C warmer than preindustrial levels while CO2 has likely not exceeded 4x preindustrial levels, this raises the possibility, at least, that sensitivity may be higher than 3 C.

And lastly, Foster 2017 further examined the last 420 million years of geologic history. The CO2 timeline is expressed both in terms of absolute concentrations and radiative forcing, accounting for both CO2 and the increase of solar luminosity over time. The resulting timeline was then compared to the “representative concentration pathways” for future CO2 emissions. Under RCP 8.5, “fossil fuel emissions suggest that atmospheric CO2 could peak in 2,250 AD at ∼2,000 p.p.m.” This would produce CO2 levels similar to the latter half of the Triassic (220-200 million years ago). However, given the increase in solar output, the radiative forcing this would produce would be similar to the early Eocene, likely exceeding the geologic record for 99.9% of the last 420 Myrs. Even under RCP 4.5, though, we would still expect to see an increase in radiative forcing of +5 W/m2.[9]

CO2 Proxy Evidence (top) corrected for Solar Evolution (bottom) with RCP Projections

I'm often told that climate science today focuses on the short time-frame of the last 150 years, and that a greater appreciation of climate on geologic timescales would cause me to adopt a larger perspective on climate, and I would no longer be concerned about the 1.2 C warming over the last 120 years or so. It's been much warmer in the past, and life has thrived.  It is of course true that a greater appreciation of paleoclimatology would give me a larger perspective on current climate change. It is also true that life has thrived when temperatures have been much warmer.  But this does not alleviate my concern one bit. When temperatures have been much warmer, CO2 levels have been much higher, and the geologic evidence supports the estimate that climate sensitivity is within the range estimated by the IPCC, about ~3 C. If increasing CO2 levels in the past also resulted in warmer global temperatures, we would expect the same thing to happen as a result of the human addition of CO2 to the atmosphere. The last time CO2 was 400 ppm was about 3 million years ago, when global temperatures were 2-3 C warmer, sea levels were ~15 m higher, and Greenland supported only an ephemeral ice sheet. Trees grew in West Antarctica about 300 miles from the South Pole.[12][13][14][15][16] The larger perspective that geology gives us is that this is the climate our current CO2 levels will likely attain when equilibrium with 400 ppm CO2 is reached. Of course, it will take hundreds to thousands of years before this is reached (it takes a long time for glacial ice to melt), but we still continue to add CO2 to the atmosphere. Human activity is pushing climate out of anything the globe has seen during Quaternary ice age. That's what the larger perspective from geology tells us.

References:

[1] Robert A. Berner, “Geocarb III: A Revised Model of Atmospheric CO2 Over Phanerozoic Time,” American Journal of Science 300 (2001):182-204.
http://earth.geology.yale.edu/~ajs/2001/Feb/qn020100182.pdf

[2] Dana L. Royer, “CO2 as a primary driver of Phanerozoic climate” GSA Today 14.3 (2004): 4-10.
doi: 10.1130/1052-5173(2004)014<4:CAAPDO>2.0.CO;2.
https://www.geosociety.org/gsatoday/archive/14/3/pdf/i1052-5173-14-3-4.pdf

[3] Robert A. Berner, “GEOCARBSULF: A combined model for Phanerozoic atmospheric O2 and CO2” Geochimica et Cosmochimica Acta 70 (2006): 5653–5664.
http://www.image.ucar.edu/idag/Papers/Berner_phanozericO2.pdf

[4] D. L. Royer, “Atmospheric CO2 and O2 During the Phanerozoic: Tools, Patterns, and Impacts,” in Farquhar, J., editor, The Atmosphere-History: Oxford, Elsevier, Treatise on Geochemistry (Second Edition), v. 6, p. 251–267.
https://web.archive.org/web/20181222151221/http://droyer.web.wesleyan.edu:80/Royer_2014_Treatise.pdf

[5] D. L. Royer, “CO2-forced climate thresholds during the Phanerozoic “ Geochimica et Cosmochimica Acta 70 (2006) 5665–5675.
http://www.eeenergia.org/wp-content/uploads/2018/02/CO2-forced-climate-thresholds-during-the-Phanerozoic-DRoyer.pdf

[6] Royer and Berner (2007), “Climate sensitivity constrained by CO2 concentrations over the past 420 million years” Nature 446(7135):530-2. DOI: 10.1038/nature05699
https://www.researchgate.net/publication/6416974_Climate_sensitivity_constrained_by_CO2_concentrations_over_the_past_420_million_years

[7] Dana L. Royer, “Climate Sensitivity during the Phanerozoic: Lessons for the Future,” Search and Discovery Article #110115 (2009).
http://www.searchanddiscovery.com/documents/2009/110115royer/ndx_royer.pdf

[8] Franks, P. J. et al. “New constraints on atmospheric CO2 concentration for the Phanerozoic.” Geophysical Research Letters 41 (2014): 4685–4694.
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2014GL060457

[9] Gavin L. Foster, Dana L. Royer & Daniel J. Lunt, “Future climate forcing potentially without precedent in the last 420 million years,” Nature Communications 8.14845 (2017).
https://www.nature.com/articles/ncomms14845

[10] Dana L. Royer, Yannick Donnadieu, Jeffrey Park, Jennifer Kowalczyk and Yves Goddéris. Error analysis of CO2 and O2 estimates from the long-term geochemical model GEOCARBSULF. American Journal of Science November 2014, 314 (9) 1259-1283; DOI: https://doi.org/10.2475/09.2014.01

[11] Witkowski CR, Weijers JWH, Blais B, Schouten S, Sinninghe Damsté JS. Molecular fossils from phytoplankton reveal secular Pco2 trend over the Phanerozoic. Sci Adv. 2018 Nov 28;4(11):eaat4556. doi: 10.1126/sciadv.aat4556. PMID: 30498776; PMCID: PMC6261654.

[12] M. Robinson, “Pliocene Role in Assessing Future Climate Impacts” Eos 89, No. 49 (2008). https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2008EO490001

[13] Julie Brigham-Grette, “Pliocene Warmth, Polar Amplification, and Stepped Pleistocene Cooling Recorded in NE Arctic Russia” Science 340 (June 21, 2013).
http://science.sciencemag.org/content/340/6139/1421

[14] Stephanie Paige Ogburn, “Ice-Free Arctic in Pliocene, Last Time CO2 Levels above 400 PPM: Sediment cores from an undisturbed Siberian lake reveal a warmer, wetter Arctic.” SA
https://www.scientificamerican.com/article/ice-free-arctic-in-pliocene-last-time-co2-levels-above-400ppm/

[15] Rhian L.Rees-Owen. “The last forests on Antarctica: Reconstructing flora and temperature from the Neogene Sirius Group, Transantarctic Mountains” Organic Geochemistry Volume 118, April 2018, Pages 4-14
https://www.sciencedirect.com/science/article/pii/S014663801730219X

[16] Damian Carrington, “Last time CO2 levels were this high, there were trees at the South Pole” The Guardian
https://www.theguardian.com/science/2019/apr/03/south-pole-tree-fossils-indicate-impact-of-climate-change?CMP=share_btn_fb&fbclid=IwAR1YJaoKxGipuDDcAmTk0xx0OebTkn-GuyJ0TUD1rNS39ANvNhSRlp_GShw