Abstract
Abstract. Glacial cycles of the late Quaternary are controlled by the asymmetrically varying mass balance of continental ice sheets in the Northern Hemisphere. Surface mass balance is governed by processes of ablation and accumulation. Here two ablation schemes, the positive-degree-day (PDD) method and the surface energy balance (SEB) approach, are compared in transient simulations of the last glacial cycle with the Earth system model of intermediate complexity CLIMBER-2. The standard version of the CLIMBER-2 model incorporates the SEB approach and simulates ice volume variations in reasonable agreement with paleoclimate reconstructions during the entire last glacial cycle. Using results from the standard CLIMBER-2 model version, we simulated ablation with the PDD method in offline mode by applying different combinations of three empirical parameters of the PDD scheme. We found that none of the parameter combinations allow us to simulate a surface mass balance of the American and European ice sheets that is similar to that obtained with the standard SEB method. The use of constant values for the empirical PDD parameters led either to too much ablation during the first phase of the last glacial cycle or too little ablation during the final phase. We then substituted the standard SEB scheme in CLIMBER-2 with the PDD scheme and performed a suite of fully interactive (online) simulations of the last glacial cycle with different combinations of PDD parameters. The results of these simulations confirmed the results of the offline simulations: no combination of PDD parameters realistically simulates the evolution of the ice sheets during the entire glacial cycle. The use of constant parameter values in the online simulations leads either to a buildup of too much ice volume at the end of glacial cycle or too little ice volume at the beginning. Even when the model correctly simulates global ice volume at the last glacial maximum (21 ka), it is unable to simulate complete deglaciation during the Holocene. According to our simulations, the SEB approach proves superior for simulations of glacial cycles.
Highlights
Glacial–interglacial cycles of the Quaternary are characterized by large fluctuations in the continental ice mass in the Northern Hemisphere (NH)
Using results from an ensemble of transient simulations of the last glacial cycle performed with an Earth system model of intermediate complexity, we undertake a systematic comparison of ice sheet surface mass balance simulated using the surface energy balance (SEB) and PDD approaches for different ice sheets and during different periods of the last glacial cycle
The set-up of the Earth system model of intermediate complexity CLIMBER-2 (Petoukhov et al, 2000; Ganopolski et al, 2001) for simulations of glacial cycles and its performance are described in Calov et al (2005) and Ganopolski et al (2010)
Summary
Glacial–interglacial cycles of the Quaternary are characterized by large fluctuations in the continental ice mass in the Northern Hemisphere (NH). Numerical parameters for the PDD method can only be derived from observations over the existing ice sheets, primarily in Greenland, and it is unclear a priori how different such parameters should be when the PDD method is applied to completely different climate conditions and different geographical distributions of ice sheets during glacial times Another semi-empirical approach, the ITM (insolationtemperature-melt) scheme, does explicitly account for absorption of insolation and reveals reasonable agreement with the SEB method in the simulation of Greenland ice sheet surface mass balance for the Eemian interglacial (Robinson et al, 2011; Robinson and Goelzer, 2014). Using results from an ensemble of transient simulations of the last glacial cycle performed with an Earth system model of intermediate complexity, we undertake a systematic comparison of ice sheet surface mass balance simulated using the SEB and PDD approaches for different ice sheets and during different periods of the last glacial cycle
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