Sensitivity of the Eocene climate to CO<sub>2</sub> and orbital variability

Abstract

The early Eocene, from about 56 Ma, with high atmospheric CO 2 levels, offers an analogue for the response of the Earth's climate system to anthropogenic fossil fuel burning. In this study, we present an ensemble of 50 Earth system model runs with an early Eocene palaeogeography and variation in the forcing values of atmospheric CO 2 and the Earth's orbital parameters. Relationships between simple summary metrics of model outputs and the forcing parameters are identified by linear modelling, providing estimates of the relative magnitudes of the effects of atmospheric CO 2 and each of the orbital parameters on important climatic features, including tropical-polar temperature difference, ocean-land temperature contrast, Asian, African and South (S.) American monsoon rains, and climate sensitivity. Our results indicate that although CO 2 exerts a dominant control on most of the climatic features examined in this study, the orbital parameters also strongly influence important components of the ocean-atmosphere system in a greenhouse Earth. In our ensemble, atmospheric CO 2 spans the range 280-3000 ppm, and this variation accounts for over 90 % of the effects on mean air temperature, southern winter high-latitude oceanland temperature contrast and northern winter tropical-polar temperature difference. However, the variation of precession accounts for over 80 % of the influence of the forcing parameters on the Asian and African monsoon rainfall, and obliquity variation accounts for over 65 % of the effects on winter ocean-land temperature contrast in high northern latitudes and northern summer tropical-polar temperature difference.Our results indicate a bimodal climate sensitivity, with values of 4.36 and 2.54 • C, dependent on low or high states of atmospheric CO 2 concentration, respectively, with a threshold at approximately 1000 ppm in this model, and due to a saturated vegetation-albedo feedback. Our method gives a quantitative ranking of the influence of each of the forcing parameters on key climatic model outputs, with additional spatial information from singular value decomposition providing insights into likely physical mechanisms. The results demonstrate the importance of orbital variation as an agent of change in climates of the past, and we demonstrate that emulators derived from our modelling output can be used as rapid and efficient surrogates of the full complexity model to provide estimates of climate conditions from any set of forcing parameters.

Moderate levels of Eocene pCO2 indicated by Southern Hemisphere fossil plant stomata

“…Our results provide new SI data from the previously sparse Southern Hemisphere record, suggesting that the terrestrial pCO 2 proxy data may indeed be robust with an internally consistent global record of moderate Eocene pCO 2 levels. 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.…”

“…Climate modeling has been able to reconstruct this pattern with very high pCO 2 levels (up to ∼4500 ppm: Huber and Caballero, 2011), but such extremely elevated pCO 2 is not documented by proxy records. It is therefore assumed that Eocene cli-mate 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 pCO 2 , 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). A variety of geochemical and biological proxies as well as carbon cycle modeling have been used to estimate Eocene pCO 2 , but these estimates still differ hugely (with values ranging from hundreds to thousands of parts per million pCO 2 ); however, there is some convergence forming (Holdgate et al, 2009;Beerling and Royer, 2011;Foster et al, 2017).…”

PALEO-PGEM v1.0: a statistical emulator of Pliocene–Pleistocene climate

“…The moisture flux correction required in the Holden et al (2016) tuning was removed during a subsequent calibration (Holden et al, 2018). PLASIM-GENIE has been applied to studies on Eocene climate (Keery et al, 2018) and climatecarbon-cycle uncertainties under strong mitigation (Holden et al, 2018).…”

“…The emulated global temperature change over the last 800 000 years is plotted in Fig. 5, reflecting the familiar glacial cycles and compared to the observationally based global temperature reconstructions of Koehler et al (2010). Ten separate emulators were built (following the steps described in Sect.…”

“…An emulator was built 10 times, and the mean prediction time series of each emulator are plotted as grey lines, with the mean of these plotted as the single black line. The blue dotted line is the observationally based reconstruction ofKoehler et al (2010).than data-based estimates of ∼ 1 to 2 • C (Masson-Delmotte et al, 2013). Maximum warming in Marine Isotope Stage 11 is 0.1 ± 0.2 • C, peaking at 401 ka.…”