Deglacial ice sheet meltdown: orbital pacemaking and CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; effects

M Heinemann,A Timmermann,A Abe-Ouchi,F Saito,O Elison Timm

doi:10.5194/cp-10-1567-2014

Abstract

Abstract. One hundred thousand years of ice sheet buildup came to a rapid end ∼25–10 thousand years before present (ka BP), when ice sheets receded quickly and multi-proxy reconstructed global mean surface temperatures rose by ∼3–5 °C. It still remains unresolved whether insolation changes due to variations of earth's tilt and orbit were sufficient to terminate glacial conditions. Using a coupled three-dimensional climate–ice sheet model, we simulate the climate and Northern Hemisphere ice sheet evolution from 78 ka BP to 0 ka BP in good agreement with sea level and ice topography reconstructions. Based on this simulation and a series of deglacial sensitivity experiments with individually varying orbital parameters and prescribed CO2, we find that enhanced calving led to a slowdown of ice sheet growth as early as ∼8 ka prior to the Last Glacial Maximum (LGM). The glacial termination was then initiated by enhanced ablation due to increasing obliquity and precession, in agreement with the Milankovitch theory. However, our results also support the notion that the ∼100 ppmv rise of atmospheric CO2 after ∼18 ka BP was a key contributor to the deglaciation. Without it, the present-day ice volume would be comparable to that of the LGM and global mean temperatures would be about 3 °C lower than today. We further demonstrate that neither orbital forcing nor rising CO2 concentrations alone were sufficient to complete the deglaciation.

Highlights

The last glacial termination (∼ 25–10 ka BP) is a welldocumented period of global climate reorganization and ice sheet retreat based on extensive paleo-data coverage (Waelbroeck et al, 2002; Peltier and Fairbanks, 2006; Yokoyama and Esat, 2011; Clark et al, 2012; Shakun et al, 2012; Masson-Delmotte et al, 2013)
The numerical model used in this study is based on the Ice sheet model for Integrated Earth system Studies IcIES (Saito and Abe-Ouchi, 2004; Abe-Ouchi et al, 2007), bidirectionally coupled to the atmosphere–ocean–sea ice–land components of the intermediate complexity model LOVECLIM (Driesschaert et al, 2007) with LGM (Last Glacial Maximum) boundary conditions (Roche et al, 2007)
The surface mass balance is approximated by a positive degree day (PDD) scheme based on Reeh (1991), with PDD factors for water equivalent snow and ice melting of 3 mm day−1 K−1 and 8 mm day−1 K−1, respectively

Summary

Introduction

The last glacial termination (∼ 25–10 ka BP) is a welldocumented period of global climate reorganization and ice sheet retreat based on extensive paleo-data coverage (Waelbroeck et al, 2002; Peltier and Fairbanks, 2006; Yokoyama and Esat, 2011; Clark et al, 2012; Shakun et al, 2012; Masson-Delmotte et al, 2013). It provides an optimal test bed to study the combined effects of orbital forcing (Milankovitch, 1941; Berger, 1978) and greenhouse gas (GHG) feedbacks on the ice sheet–climate system. A number of recent transient modeling studies (Timm and Timmermann, 2007; Timmermann et al, 2009; Ganopolski and Roche, 2009; Roche et al, 2011; Smith and Gregory, 2012; He et al, 2013) quantified the effects of varying orbital parameters, GHG forcing, and ice sheet changes on the evolution of the atmosphere–ocean system during the last deglaciation. The ice sheet evolution in these numerical studies was prescribed rather than interactively computed.

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Results

Conclusion