Abstract
Abstract. 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 (ΔT) induced by Antarctic Glaciation depends on the background CO2 levels and ranges from −1.22 to −0.18 K. The Eocene ΔT is nearly constant at ~−0.25 K. We calculate an climate sensitivity parameter S[Antarctica] which we define as ΔT divided by the change in effective radiative forcing (ΔQAntarctica) 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.
Highlights
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)
When considering the sensitivity to both components of ice sheet growth, which we have done at a range of CO2 values, we find that the Eocene has a T(α+oro) = −0.17 to -0.29 K while the modern has a T(α+oro) = −0.18 to −1.22 K (Table 1, α + oro cases)
No climate modelling study has separated the Antarctic ice sheet (AIS) component in terms of S[Antarctica] for these time periods, and we hope the results can be used to compare against proxy data derived climate sensitivity estimates
Summary
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 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 CO2 mixing ratios from ∼ 1000 to ∼ 600 ppm (Pagani et al, 2011; Pearson et al, 2009). 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)
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