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Climate of the Past An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/cp-2020-29
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/cp-2020-29
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  09 Mar 2020

09 Mar 2020

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A revised version of this preprint was accepted for the journal CP and is expected to appear here in due course.

The middle-to-late Eocene greenhouse climate, modelled using the CESM 1.0.5

Michiel Baatsen1, Anna S. von der Heydt1, Matthew Huber2, Michael A. Kliphuis1, Peter K. Bijl3, Appy Sluijs3, and Henk A. Dijkstra1 Michiel Baatsen et al.
  • 1Institute for Marine and Atmospheric Research, Department of Physics, Utrecht University, Princetonplein 5, 3584CC Utrecht, the Netherlands
  • 2Purdue University, 610 Purdue Mall, West Lafayette, IN, 47906 USA
  • 3Department of Earth Sciences, Marine Palynology and Paleoceanography, Laboratory of Palaeobotany and Palynology, Utrecht University, Princetonlaan 8a, 3584 CB Utrecht, the Netherlands

Abstract. While the early Eocene has been considered in many modelling studies, detailed simulations of the middle and late Eocene climate are currently scarce. To get a better understanding of both Antarctic glaciation at the Eocene-Oligocene transition (~34 Ma) and late middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Results of higher (CMIP5-like) resolution coupled climate simulations are represented here using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with the same general boundary conditions but a different radiative forcing, using a new detailed 38 Ma geography reconstruction.

Under 4× pre-industrial concentrations (PIC) of both CO2 (i.e. 1120 ppm) and CH4 (~2700 ppb), equilibrium sea surface temperatures correspond well to the available late middle Eocene (42–38 Ma; ~Bartonian) proxies. Being generally cooler, the simulated climate under 2× PIC forcing is a good analog for that of the late Eocene (38–34 Ma; ~Priabonian). Terrestrial temperature proxies, although their geographical coverage is sparse, also indicate that the results presented here are realistic.

The reconstructed 38 Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. The collective effects of geography, vegetation and ice accounts for a global mean 5–7 °C difference between pre-industrial and 38 Ma Eocene boundary conditions, with important contributions from cloud and water vapour feedbacks. These simulations effectively show that a realistic middle-to-late Eocene climate can be reconstructed without the need for greenhouse gas concentrations much higher than proxy estimates (i.e. ~500–1200 ppm CO2).

Equilibrium climate sensitivity is reduced (0.62 °C/W m2; 3.2 °C warming between 38 Ma 2× PIC and 4× PIC) compared to that of the present-day climate (0.79 °C/W m2; 3.1 °C per CO2 doubling). Despite very limited sea ice and snow cover in both 38Ma cases, the model still shows a factor ~2 polar amplification in response to a further increase of atmospheric greenhouse gas concentrations. High latitudes in the modelled Eocene climate are mainly kept warm by an altered radiative balance in combination with global changes in geography and the absence of polar ice sheets compared to the pre-industrial reference.

Michiel Baatsen et al.

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Michiel Baatsen et al.

Michiel Baatsen et al.

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Latest update: 28 Nov 2020
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Short summary
Warm climates of the deep past have proven to be challenging to reconstruct with the same numerical models used for future predictions. We present results of CESM simulations for the middle-to-late Eocene (~38 Ma), in which we managed to match the available indications of temperature well. With these results we can now look into regional features, the response to external changes and ultimately better understand the climate when it is in such a warm state.
Warm climates of the deep past have proven to be challenging to reconstruct with the same...
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