Climate of the Past Does Antarctic glaciation cool the world ?

In this study, we compare the simulated climatic impact of adding an Antarctic ice sheet (AIS) to the “greenhouse world” of the Eocene and removing the AIS from the modern world. The modern global mean surface temperature anomaly ( 1T ) induced by Antarctic Glaciation depends on the background CO 2 levels and ranges from−1.22 to −0.18 K. The Eocene1T is nearly constant at ∼−0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as1T divided by the change in effective radiative forcing ( 1QAntarctica) which includes some fast feedbacks imposed by prescribing the glacial properties of Antarctica. The main difference between the modern and Eocene responses is that a negative cloud feedback warms much of the Earth’s surface as a large AIS is introduced in the Eocene, whereas this cloud feedback is weakly positive and acts in combination with positive sea-ice feedbacks to enhance cooling introduced by adding an ice sheet in the modern. Because of the importance of cloud feedbacks in determining the final temperature sensitivity of the AIS, our results are likely to be model dependent. Nevertheless, these model results suggest that the effective radiative forcing and feedbacks induced by the AIS did not significantly decrease global mean surface temperature across the Eocene–Oligocene transition (EOT −34.1 to 33.6 Ma) and that other factors like declining atmospheric CO2 are more important for cooling across the EOT. The results illustrate that the efficacy of AIS forcing in the Eocene is not necessarily close to one and is likely to be model and state dependent. This implies that using EOT paleoclimate proxy data by itself to estimate climate sensitivity for future climate prediction requires climate models and consequently these estimates will have large uncertainty, largely due to uncertainties in modelling low clouds. 1 AIS temperature sensitivity During the Eocene–Oligocene Transition (EOT) global climate deteriorated as the warm and ice-free conditions of the Eocene gave way to a colder, glaciated state in the early Oligocene ( Lear et al. , 2000; Zachos et al. , 2001; DeConto and Pollard, 2003; Macksensen and Ehrmann , 1992; Scher et al., 2011; Hambrey and Barrett , 1993). Evidence now exists that the cooling ( Liu et al., 2009; Eldrett et al. , 2009; Zanazzi et al. , 2007; Ivany et al. , 2000) and glaciation ( Lear et al., 2000; Edgar et al. , 2007; Coxall et al. , 2005; Miller et al., 2009; DeConto and Pollard , 2003; Zachos et al. , 2001) that occurred across the EOT was caused by a drop in CO 2 mixing ratios from∼ 1000 to∼ 600 ppm ( Pagani et al. , 2011; Pearson et al. , 2009). These values are in the likely range of values over the next century. The modern Earth system is currently in a glaciated state, but is showing signs of potentially losing glacier ice in the Arctic and Antarctic in the future (Joughin and Alley , 2011; Jacob et al. , 2012; Velicogna, 2009; Chen et al. , 2009; Pritchard et al. , 2009; Liston and Hiemstra, 2011). Thus a major, unanswered question in future climate change prediction is the degree to which melting of ice sheets in the future will contribute to substantially and irreversibly altering climate ( Solomon et al. , 2009). Past climate changes, such as the EOT may provide unique information to answer that question, but serious challenges remain. Indeed, with both greenhouse gas forcing and temperature change values in hand from EOT proxy records there is a temptation to estimate an Earth system sensitivity (ESS) parameter ( Lunt et al., 2010), i.e., a climate sensitivity parameter that includes the direct, fast feedback responses to radiative perturbation combined with the slower feedbacks, such as ice sheet growth, greenhouse gas and vegetation feedbacks ( Paleosens members , 2012; Royer et al. , 2012). One Published by Copernicus Publications on behalf of the European Geosciences Union. 174 A. Goldner et al.: Does Antarctic glaciation cool the world? approach is to use this Eocene ESS to draw a straightforward analogy to the future ( Hansen et al. , 1984, 2010; Hansen and Sato, 2012; Kiehl, 2011; Hay, 2011) avoiding the problems of accurately reproducing the individual processes and feedbacks which hamper modelling efforts ( Roe and Baker , 2007). We are motivated in this paper by the following concerns. On long time scales does a glaciated Antarctica cool global mean temperature? Does the Antarctic ice sheet (AIS) induce additional positive or negative climate feedbacks and if so what are the strength of these feedbacks? Are the results of this change dependent on background state? Recent estimates indicate that CO 2 levels over the EOT fell from 1000 to 600 ppm ( Pearson et al. , 2009; Pagani et al. , 2011, Myhre et al., 1998), causing a ∼ 2.1–2.5 W m−2 radiative forcing. To reconcile this forcing with the temperature shift at the EOT of∼ 3.5± 1.5 K (Liu et al., 2009) – assuming that this shift was entirely due to the fast feedbacks – would require a Charney-type temperature sensitivity of ∼ 1.5 K (W m−2)−1. By comparison a typical estimated modern value is 0.8 K (W m −2)−1 (Bitz et al., 2012; Kay et al., 2012a; Gettelman et al. , 2012). Thus while the shift in CO2 values over the EOT is more or less well established as the prime candidate for driving the cooling, this implies either a large value of fast sensitivity or substantial slow Earth system positive feedback that enhances the sensitivity. It is currently unknown, and indeed impossible to know directly from proxy data, what fraction of the cooling at the EOT was a direct climate response involving the fast, Charney-type feedbacks, such as shifts in clouds and sea ice ( Hansen et al. , 1981, 1997; DeConto et al. , 2007) and what fraction of the cooling involved the slower feedbacks, such as changes in the AIS ( Lunt et al., 2010), given that proxy records for the radiative forcing due to ice sheets do not exist. Here we will show, that adding or removing an ice sheet can have impacts that are strongly state dependent and are likely to be model dependent as well. This is a less straightforward problem than determining the forcing due to a doubling of CO2. We are focused in this paper on understanding what slow, Earth system feedbacks were operating across the EOT and their interactions with fast feedbacks, to help evaluate whether these feedbacks operate in the same way (in the model) as they do in the modern ( Haywood et al. , 2011). Specifically, we use the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM1.0) in slab ocean mode to investigate the impact of replacing the AIS with vegetation for the future case, and replacing vegetation with an ice sheet for the EOT cases. We ask the following questions: what is the climatic impact of adding or removing a large AIS? Does this response depend on the climate state, in particular is the response in the Eocene different than in the modern? What feedbacks are important for modulating this response? The remainder of paper will be focused on explaining and quantifying temperature change, effective radiative forcing, and the resulting climate sensitivity parameter induced by removing and adding the AIS and comparing this response in Eocene and modern contexts. This paper is broken into four sections. Section 2 describes the CESM1.0 modelling framework and how we constructed our Eocene and modern glaciated and unglaciated simulations. Then we present the climate sensitivity to Antarctic glaciation in modern and Eocene slab ocean simulations (Sect. 3.1) and describe the atmospheric response to Antarctic glaciation in the Eocene and modern slab ocean simulations (Sect. 3.2). Sections 4 and 5 include the discussion and conclusion, respectively.