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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 12 Apr 2018

Research article | 12 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Climate of the Past (CP).

Equilibrium state and sensitivity of the simulated middle-to-late Eocene climate

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
  • 3Marine Palynology and Paleoceanography, Laboratory of Palaeobotany and Palynology, Department of Earth Sciences, 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 understand Antarctic glaciation at the Eocene-Oligocene Transition (~34Ma) as well as middle Eocene warmth, it is vital to have an adequate reconstruction of the middle-to-late Eocene climate. Here, we present a set of high resolution coupled climate simulations using the Community Earth System Model (CESM) version 1. Two middle-to-late Eocene cases are considered with new detailed 38Ma geographical boundary conditions with a different radiative forcing. With 4× pre-industrial concentrations of CO2 (i.e. 1120ppm) and CH4 (~2700ppb), the equilibrium sea surface temperatures correspond well to available late middle Eocene (42–38Ma) proxies. Being generally cooler, the simulated climate with 2× pre-industrial values is a good analog for that of the late Eocene (38–34Ma). Deep water formation occurs in the South Pacific Ocean, while the North Atlantic is strongly stratified and virtually stagnant. A shallow and weak circumpolar current is present in the Southern Ocean with only minor effects on southward oceanic heat transport within wind-driven gyres. Terrestrial temperature proxies, although limited in coverage, also indicate that the results presented here are realistic. The reconstructed 38Ma climate has a reduced equator-to-pole temperature gradient and a more symmetric meridional heat distribution compared to the pre-industrial reference. Climate sensitivity is similar (~0.7°C/Wm2) to that of the present-day climate (~0.8°C/Wm2; 3°C per CO2 doubling), with significant polar amplification despite very limited sea ice and snow cover. High latitudes are mainly kept warm by albedo and cloud feedbacks in combination with global changes in geography and the absence of polar ice sheets. The integrated effect of geography, vegetation and ice accounts for a 6–7°C offset between pre-industrial and 38Ma Eocene boundary conditions. These 38Ma 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. The general circulation and radiative budget allow for mild high-latitude regions and little to no snow and ice cover, without making equatorial regions extremely warm.

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Michiel Baatsen et al.
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Michiel Baatsen et al.
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Publications Copernicus
Short summary
The Eocene marks a period where the climate was in a hothouse state, without any continental-scale ice sheets. Such climates have proven difficult to reproduce in models, especially their low temperature difference between equator and poles. Here, we present high resolution CESM simulations using a new geographic reconstruction of the middle-to-late Eocene. The results provide new insights into a period for which knowledge is limited, leading up to a transition into the present icehouse state.
The Eocene marks a period where the climate was in a hothouse state, without any...