Clim. Past Discuss., 9, 5701-5745, 2013
© Author(s) 2013. This work is distributed
under the Creative Commons Attribution 3.0 License.
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This discussion paper has been under review for the journal Climate of the Past (CP). Please refer to the corresponding final paper in CP.
Uncertainties in the modelled CO2 threshold for Antarctic glaciation
E. Gasson1,2, D. J. Lunt3, R. DeConto2, A. Goldner4, M. Heinemann5, M. Huber4,*, A. N. LeGrande6, D. Pollard7, N. Sagoo3, M. Siddall1, and A. Winguth8
1Department of Earth Sciences, University of Bristol, UK
2Climate System Research Center, University of Massachusetts, Amherst, USA
3School of Geographical Sciences, University of Bristol, UK
4Earth, Atmospheric, and Planetary Sciences, Purdue University, USA
5International Pacific Research Center, University of Hawai'i, USA
6NASA/Goddard Institute for Space Studies, USA
7Earth and Environmental Systems Institute, Pennsylvania State University, USA
8Department of Earth and Environmental Sciences, University of Texas, USA
*now at: Department of Earth Sciences, University of New Hampshire, USA

Abstract. A frequently cited atmospheric CO2 threshold for the onset of Antarctic glaciation of ~ 780 ppmv is based on a study using an ice sheet model and the GENESIS climate model. Proxy records suggest that atmospheric CO2 concentrations passed through this threshold across the Eocene–Oligocene transition ~ 34 Ma. However, atmospheric CO2 concentrations may have been close to this threshold earlier than this transition, which is used by some to suggest the possibility of Antarctic ice sheets during the Eocene. Here we investigate the climate model dependency of the threshold for Antarctic glaciation by performing offline ice sheet model simulations using the climate from a number of different climate models (HadCM3L, CCSM3, CESM1.0, GENESIS, FAMOUS, ECHAM5 and GISS_ER). These climate simulations are sourced from a number of independent studies, as such the boundary conditions, which are poorly constrained during the Eocene, are not identical between simulations. The results of this study suggest that the atmospheric CO2 threshold for Antarctic glaciation is highly dependent on the climate model used and the climate model configuration. A large discrepancy between the climate model and ice sheet model grids for some simulations leads to a strong sensitivity to the lapse rate parameter. However, with the exception of HadCM3L and its reduced complexity version FAMOUS, the simulations suggest the growth of an intermediate sized ice sheet (> 25 m sea level equivalent) for atmospheric CO2 concentrations in the range of 560–920 ppmv, which is consistent with previous studies.

Citation: Gasson, E., Lunt, D. J., DeConto, R., Goldner, A., Heinemann, M., Huber, M., LeGrande, A. N., Pollard, D., Sagoo, N., Siddall, M., and Winguth, A.: Uncertainties in the modelled CO2 threshold for Antarctic glaciation, Clim. Past Discuss., 9, 5701-5745, doi:10.5194/cpd-9-5701-2013, 2013.
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