Journal cover Journal topic
Climate of the Past An interactive open-access journal of the European Geosciences Union
Clim. Past, 11, 1801-1823, 2015
http://www.clim-past.net/11/1801/2015/
doi:10.5194/cp-11-1801-2015
© Author(s) 2015. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
21 Dec 2015
On the state dependency of the equilibrium climate sensitivity during the last 5 million years
P. Köhler1, B. de Boer2,3,a, A. S. von der Heydt3, L. B. Stap3, and R. S. W. van de Wal3 1Alfred-Wegener-Institut Helmholtz-Zentrum für Polar-und Meeresforschung (AWI), P.O. Box 12 01 61, 27515 Bremerhaven, Germany
2Department of Earth Sciences, Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, the Netherlands
3Institute for Marine and Atmospheric Research Utrecht (IMAU), Utrecht University, Princetonplein 5, 3584 CC Utrecht, the Netherlands
anow at: School of Earth and Environment, University of Leeds, Leeds, UK
Abstract. It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of ice-sheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S[CO2,LI], for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevent a well-supported calculation for S[CO2,LI], but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 Myr BP), the specific equilibrium climate sensitivity, S[CO2,LI], was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.

Citation: Köhler, P., de Boer, B., von der Heydt, A. S., Stap, L. B., and van de Wal, R. S. W.: On the state dependency of the equilibrium climate sensitivity during the last 5 million years, Clim. Past, 11, 1801-1823, doi:10.5194/cp-11-1801-2015, 2015.
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Short summary
We find that the specific equilibrium climate sensitivity due to radiative forcing of CO2 and land ice albedo has been state-dependent for the last 2.1Myr (most of the Pleistocene). Its value is ~45% larger during intermediate glaciated climates and interglacial periods than during Pleistocene full glacial conditions. The state dependency is mainly caused by a latitudinal dependency in ice sheet area changes. Due to uncertainties in CO2, firm conclusions for the Pliocene are not yet possible.
We find that the specific equilibrium climate sensitivity due to radiative forcing of CO2 and...
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