ESDD Earth System Dynamics Discussions ESDD Earth Syst. Dynam. Discuss. 2190-4995 Göttingen, Germany 10.5194/esdd-2-211-2011 Geologic constraints on earth system sensitivity to CO<sub>2</sub> during the Cretaceous and early Paleogene Royer D. L. 1 Pagani M. 2 Beerling D. J. 3 Department of Earth and Environmental Sciences and College of the Environment, Wesleyan University, Middletown, Connecticut, 06459, USA Department of Geology and Geophysics, Yale University, New Haven, Connecticut, 06520, USA Department of Animal and Plant Sciences, University of Sheffield, Sheffield, S10 2TN, UK 03 03 2011 2 1 211 240 Copyright: © 2011 D. L. Royer et al. 2011 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://esd.copernicus.org/preprints/2/211/2011/esdd-2-211-2011.html The full text article is available as a PDF file from https://esd.copernicus.org/preprints/2/211/2011/esdd-2-211-2011.pdf

Earth system sensitivity (ESS) is the long-term (>10<sup>3</sup> yr) equilibrium temperature response to doubled CO<sub>2</sub>. ESS has climate policy implications because global temperatures are not expected to decline appreciably for at least 10<sup>3</sup> yr, even if anthropogenic greenhouse-gas emissions drop to zero. We report quantitative ESS estimates of 3 °C or higher for much of the Cretaceous and early Paleogene based on paleo-reconstructions of CO<sub>2</sub> and temperature. These estimates are generally higher than climate sensitivities simulated from global climate models for the same ancient periods (~3 °C). We conclude that climate models do not capture the full suite of positive climate feedbacks during greenhouse worlds. These absent feedbacks are probably related to clouds, trace greenhouse gases, seasonal snow cover, and/or vegetation, especially in polar regions. Continued warming in the coming decades as anthropogenic greenhouse gases accumulate in the atmosphere ensures that characterizing and quantifying these positive climate feedbacks will become a scientific challenge of increasing priority.

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