Proxy evidence for state-dependence of climate sensitivity in the Eocene greenhouse

Abstract

Despite recent advances, the link between the evolution of atmospheric CO2 and climate during the Eocene greenhouse remains uncertain. In particular, modelling studies suggest that in order to achieve the global warmth that characterised the early Eocene, warmer climates must be more sensitive to CO2 forcing than colder climates. Here, we test this assertion in the geological record by combining a new high-resolution boron isotope-based CO2 record with novel estimates of Global Mean Temperature. We find that Equilibrium Climate Sensitivity (ECS) was indeed higher during the warmest intervals of the Eocene, agreeing well with recent model simulations, and declined through the Eocene as global climate cooled. These observations indicate that the canonical IPCC range of ECS (1.5 to 4.5 °C per doubling) is unlikely to be appropriate for high-CO2 warm climates of the past, and the state dependency of ECS may play an increasingly important role in determining the state of future climate as the Earth continues to warm.

DeepMIP: model intercomparison of early Eocene climatic optimum (EECO) large-scale climate features and comparison with proxy data

“…The results from the last 100 years were used in the study. Note that the NorESM simulations were carried out with the Baatsen et al (2016) paleogeography (based on a paleomagnetic reference frame), not the Herold et al (2014) paleogeography (based on a mantle reference frame), in contrast to the other simulations described in this paper.…”

The Monterey Event and the Paleocene‐Eocene Thermal Maximum

“…The Early Eocene Climatic Optimum (EECO; 53-50 Ma) stands out as an interval of extreme Cenozoic warmth and is followed by a long-term cooling and eventual glaciation of Antarctica (~34 Ma) (Zachos et al, 2008;Westerhold et al, 2020). Atmospheric CO 2 records that broadly track trends of Eocene ocean temperature (Anagnostou et al, 2016(Anagnostou et al, , 2020Cramwinckel et al, 2018) and episodes of NAIP volcanic activity (56-50 Ma) could also be an important contributor to sustain EECO warmth. Storey et al (2007) correlated terrestrial and marine ash deposits that provided an essential chron ostratigraphic framework to estimate magmatic produc tion rates over the late Paleocene and early Eocene epochs.…”

Geological Society of London Scientific Statement: what the geological record tells us about our present and future climate

“…2. (a) The relationship between atmospheric CO 2 concentration and global mean surface temperature (GMST) for five time intervals where both variables have been recently well constrained by geological data (Anagnostou et al 2020;de la Vega et al 2020;Inglis et al 2020;Sherwood et al 2020;Tierney et al 2020). Error bars reflect 68% confidence intervals and in some cases are smaller than the symbol.…”