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

Abstract

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 CO 2 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 CO 2 radiative forcing depends on the CO 2 data set used. This nonlinearity 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 icesheet 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 CO 2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S [CO 2 ,LI] , for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S [CO 2 ,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 CO 2 data uncertainties prevent a well-supported calculation for S [CO 2 ,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 [CO 2 ,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 CO 2 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.

The influence of ice sheets on temperature during the past 38 million years inferred from a one-dimensional ice sheet–climate model

“…Therefore, decreased sensitivity at higher CO 2 is not only determined by reduced ice volume variability. This finding may be compared to the hybrid data-model results for climate sensitivity of Köhler et al (2015) and to the modelled climate sensitivity of Friedrich et al (2016). Köhler et al (2015) investigated the relation between the radiative forcing of proxy data CO 2 (Petit et al, 1999;Siegenthaler et al, 2005;Lüthi et al, 2008;Hönisch et al, 2009), which is linearly related to logarithmic CO 2 (Myhre et al, 1998), and Figure 8.…”

“…This finding may be compared to the hybrid data-model results for climate sensitivity of Köhler et al (2015) and to the modelled climate sensitivity of Friedrich et al (2016). Köhler et al (2015) investigated the relation between the radiative forcing of proxy data CO 2 (Petit et al, 1999;Siegenthaler et al, 2005;Lüthi et al, 2008;Hönisch et al, 2009), which is linearly related to logarithmic CO 2 (Myhre et al, 1998), and Figure 8. Relation between the logarithm of CO 2 divided by the PI value of 280 ppm and global temperature anomalies with respect to PI (of the reference run) for the reference simulation (red dots), the simulation with uncoupled ice (blue dots), and the simulation with only surface height (orange dots) or albedo (black dots) coupled.…”

“…Deep sea temperature records can be reconstructed based on the Mg/Ca proxy (Lear et al, 2000;Sosdian and Rosenthal, 2009;Elderfield et al, 2012), but a global average deep sea temperature is hard to obtain. Sea level records are also available but are subject to the same problem of inferring a global mean (Miller et al, 2005;Kominz et al, 2008;Rohling et al, 2014). Studies using sea level records face the additional challenge of converting local sea level to ice volume, which is not straightforward mainly because of dynamic topography (Mitrovica and Milne, 2003;Kendall et al, 2005).…”

Technical Note: Calculating state dependent equilibrium climate sensitivity from palaeodata

“…However, results point more and more in the direction, that climate sensitivity is state dependent, implying that for the ∆T g -∆R [X] -scatter plot non-linear regressions (for example higher order polynomials) are describing the data more appropriate than a simple linear fit (von der Heydt et al, 2014;Köhler et al, 2015). In such cases the quantification of S [X] becomes more intricate.…”

“…Since then climate sensitivity from palaeodata has continued to be analysed by some regression analysis in the scatter plot of ∆T g versus ∆R [X] (e.g. von der Heydt et al, 2014;Martínez-Botí et al, 2015;Köhler et al, 2015).…”