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Climate of the Past An interactive open-access journal of the European Geosciences Union
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
11 Apr 2017
Review status
This discussion paper is under review for the journal Climate of the Past (CP).
Sensitivity of the Eocene Climate to CO2 and Orbital Variability
John S. Keery, Philip B. Holden, and Neil R. Edwards School of Environment, Earth & Ecosystem Sciences, The Open University, Milton Keynes, MK7 6AA, UK
Abstract. The early Eocene, from about 56 Ma, with high atmospheric CO2 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 CO2 and the Earth's orbital parameters. Two-dimensional model output fields are reduced to scalar values through simple summarising algorithms and by singular value decomposition. Relationships between these scalar results and the forcing parameters are identified by linear modelling, providing estimates of the relative magnitudes of the effects of atmospheric CO2 and each of the orbital parameters on important climatic features, including tropical-polar temperature difference, ocean-land temperature contrast, and Asian, African and S. American monsoon rains. Our results indicate that although CO2 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 CO2 spans the range 280–3000 ppm, and this variation accounts for over 95 % of the effects on mean air temperature, southern winter high-latitude ocean-land temperature contrast and northern winter tropical-polar temperature difference. However, the variation of precession accounts for over 75 % 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. 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 our singular value decomposition approach providing insights into likely physical mechanisms. The results demonstrate the importance of orbital variation as an agent of change in climates of the past.

Citation: Keery, J. S., Holden, P. B., and Edwards, N. R.: Sensitivity of the Eocene Climate to CO2 and Orbital Variability, Clim. Past Discuss.,, in review, 2017.
John S. Keery et al.
John S. Keery et al.
John S. Keery et al.


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Short summary
In the Eocene of ~ 55 million years ago, the Earth had high levels of atmospheric CO2, so studies of the Eocene can provide insights into the likely effects of present day fossil fuel burning. We ran a low-resolution but very fast climate model with 50 combinations of CO2 and orbital parameters, and an Eocene layout of the oceans and continents. Climatic effects of CO2 are dominant but precession and obliquity strongly influence monsoon rainfall and ocean-land temperature contrasts respectively.
In the Eocene of ~ 55 million years ago, the Earth had high levels of atmospheric CO2, so...