Cenozoic Climate and CO2 Proxy Reconstructions

Cenozoic CO2 and Temperature

A new paper was published this month that I think will produce some exciting new insights for those interested in historical geology and paleoclimate studies. The paper is a product of the Cenozoic Carbon dioxide Proxy Integration Project (CenCO2PIP) Consortium, and it looks to reconstruct the proxy evidence for CO2 levels during the Cenozoic (the last 66 million years). The Cenozoic began after the asteroid impact (and/or volcanism) at the Cretaceous-Tertiary (K-T) boundary that caused the mass-extinction that included the extinction of non-avian dinosaurs.

The value of this kind of work will have significant benefits for scientists as they seek to constrain estimates for long-term climate sensitivity (ESS). We can think of "sensitivity" on roughly three time scales: 

1. TCR: On a near-immediate time scale, GMST increases with increasing CO2 in what is called transient climatic response (TCR), which generally speaking tells you how quickly temperature rises in response to forcings before equilibrium is reached. Most estimates for TCR are in the neighborhood of 2 C for 2xCO2.
2. ECS: On the time scale of decades, scientists quantify an equilibrium increase in GMST in response to doubling CO2, which we call Equilibrium Climate Sensitivity (ECS). This seeks to quantify the equilibrium changes in temperature in including short-term feedbacks (water vapor, clouds, sea ice, etc.). Most estimates for ECS are in the neighborhood of 3 C for 2xCO2.
3. ESS: On time scale of hundreds to thousands of years, long-term feedbacks kick in that can cause a higher increase increase in temperature, including the poleward movement of boreal forests, and ice-albedo feedbacks from shrinking ice sheets and glaciers. This is called Earth System Sensitivity (ESS).

As a general rule of thumb, scientists have estimated that ECS is about 50% more than TCR and ESS is about 50%-100% more than ECS. So an ECS of ~3 C for 2xCO2 would suggest an TCR of ~2 C and an ESS of ~4.5 - 6 C. Over time scales of thousands of years, the earth's climate system turns out to be very sensitive to external forcings, provided that the Earth system has time to reach full equilibrium with those forcings. And since increases in CO2 stay in the atmosphere for a very long time [1], these long-term feedbacks are important to consider.

A few years ago, Carolyn Snyder published a paper[2] that came to a significantly higher estimate for ESS. Her conclusion:

A comparison of the new temperature reconstruction with radiative forcing from greenhouse gases estimates an Earth system sensitivity of 9 degrees Celsius (range 7 to 13 degrees Celsius, 95 per cent credible interval) change in global average surface temperature per doubling of atmospheric carbon dioxide over millennium timescales. This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation and atmospheric dust continue to respond to global warming.

Snyder's estimate for ESS of 9°C (95% CI: 7-13 C) was about double the above rule of thumb. The response to this estimate for ESS was somewhat controversial. Most believed her reconstruction of 2 million years of climate history was a big step forward in our understanding of Quaternary climate variability. However, Gavin Schmidt wrote at RealClimate that he believed the ESS estimate and implication that long-term committed warming to today's CO2 levels was 3-7 ºC was "simply wrong." The problem with Snyder's estimate was that it was depended on a regression between GHGs and GMST, particularly over glacial cycles. The glacial cycles of the Quaternary ice age are generally triggered by Milankovitch cycles with CO2 functioning as a feedback that amplifies warming. Further amplifications occur due to ice-albedo as large ice sheets in North America and Europe expand and retreat. Most of those ice sheets are gone with Greenland and Antarctica being the last remaining ice sheets. It's not accurate to assume that ESS will remain the same value as the ice-albedo feedback from shrinking ice sheets becomes smaller - you can't melt ice that's already melted, so ice-albedo feedbacks should decrease. Schmidt claimed that better estimates for ESS are in the range of 4.5ºC to 6ºC for 2xCO2.

CenCO2PIP Consortium's reconstruction [3] covers a much longer period of geologic history, and during most of this time the Earth's climate was warmer than today (The Antarctic ice sheet didn't exist for the first half of the Cenozoic), and forcings associated with Milankovitch cycles average out on time scales of tens of millions of years. Thus, CenCO2PIP assumed that global temperature variability was controlled by CO2 and the linear increase in solar luminosity. The latter is important because the Sun is getting brighter with time due to nuclear reactions within the Sun. Accounting for this, this paper estimated ESS to be between 5°C and 8°C for 2xCO2, essentially splitting the difference between Schmidt and Snyder on ESS. They summarize their findings as

the Cenozoic compilation confirms a strong link between CO2 and GMST across timescales from 500 kyr to tens of Myr, with ESS[CO2] generally within the range of 5-8°C – patterns consistent with most prior work..., and considerably higher than the present-day ECS of ~3°C.

The Slopes of 5°C and 8°C ESS Values Shown on Graph

This result seems to agree with a forthcoming paper by Emily Judd and Jessica Tierney that calculates the slope oft he regression line between log CO2 and GMST to be 8.2 ± 0.4 °C with an r^2 of 0.94. I take this value to be an upper bound estimate for ESS in another post.

The paper also finds that ESS values were higher when ice volumes were larger, particularly in the last 10 million years, meaning that the ice-albedo feedback significantly increases ESS, but as ice volume shrinks, this feedback will amplify warming less.

From paleo-co2.org

The CenCO2PIP Consortium maintains a website at paleo-co2.org that provides more details about temperature and CO2 proxies during the Cenozoic, with descriptions for the various types of proxies used and graphs with individual CO2 proxy results with uncertainties. This site allows you to see the proxy evidence as it now stands, including some proxies for which uncertainties have not yet been fully quantified. 

The importance of this kind of work is hard to underestimate when considering how our actions are impacting global temperatures. Our recent geologic history (last 125K years) is dominated by temperatures much colder than the Holocene, and temperatures are rapidly increasing beyond what has been seen in the Pleistocene. To constrain sensitivity and estimates for what future climate states may look like, we need to investigate times when the globe was warmer than today, and the Cenozoic is the most data-rich era we have with several analogues to current warming (though none as rapid). The following are the results of a previous study from Tierney et al 2020[4] covering the last 100 million years.

Tierney et al 2020

The big takeaway for me with these studies is that as a general rule of thumb it's probably best to go with ESS ≈ 2xECS.  I expect to be checking in on this site from time to time for further learning about how these CO2 proxies get quantified. It's also an important reminder that our actions now have long term consequences lasting thousands of years that will continue to be amplified by the climate system. Without carbon removal, GMST will stay elevated significantly above preindustrial levels for a very long time.

References:

[1] Solomon S., G. K. Plattner, R. Knutti, and P. Friedlingstein, Irreversible climate change due to carbon dioxide emissions, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.0812721106, 2009.

[2] Snyder, C. W. (2016). Evolution of global temperature over the past two million years. Nature, 538(7624), 226–228. doi:10.1038/nature19798

[3] 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

[4] Jessica E. Tierney et al. ,Past climates inform our future.Science370,eaay3701(2020).DOI:10.1126/science.aay3701