On the state-dependency of the equilibrium climate sensitivity during the last 5 million years

• Main Authors: Köhler, Peter, de Boer, B., von der Heydt, A. S., Stap, L. B., van de Wal, R. S. W.
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: COPERNICUS GESELLSCHAFT MBH 2015
• Subjects: Ice Sheet
• Online Access: https://epic.awi.de/id/eprint/38448/; https://epic.awi.de/id/eprint/38448/1/koehler2015cpd.pdf; http://www.clim-past-discuss.net/11/3019/2015/; https://hdl.handle.net/10013/epic.45796; https://hdl.handle.net/10013/epic.45796.d001

A still open question is how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – is depending on background climate. We here present paleo-data based evidence on the state-dependency of S, by using CO2 proxy data together with 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 was in similar approaches not accounted for due to previously more simplistic approximations of land-ice albedo radiative forcing being a linear function of sea level change. Important for the non-linearity between land-ice albedo and sea level is a latitudinal dependency in ice sheet area changes.In our setup, in which the radiative forcing of CO2 and of the land-ice albedo (LI) is combined, we find a state-dependency 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 prevents 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 2.82MyrBP) 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 wide 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 paleo data-based approaches to calculate climate sensitivity. However, the implications of this study for a ...