The importance of snow albedo for ice sheet evolution over the last glacial cycle

Matteo Willeit,Andrey Ganopolski

doi:10.5194/cp-14-697-2018

Abstract

Abstract. The surface energy and mass balance of ice sheets strongly depends on the amount of solar radiation absorbed at the surface, which is mainly controlled by the albedo of snow and ice. Here, using an Earth system model of intermediate complexity, we explore the role played by surface albedo for the simulation of glacial cycles. We show that the evolution of the Northern Hemisphere ice sheets over the last glacial cycle is very sensitive to the representation of snow albedo in the model. It is well known that the albedo of snow depends strongly on snow grain size and the content of light-absorbing impurities. Excluding either the snow aging effect or the dust darkening effect on snow albedo leads to an excessive ice build-up during glacial times and consequently to a failure in simulating deglaciation. While the effect of snow grain growth on snow albedo is well constrained, the albedo reduction due to the presence of dust in snow is much more uncertain because the light-absorbing properties of dust vary widely as a function of dust mineral composition. We also show that assuming slightly different optical properties of dust leads to very different ice sheet and climate evolutions in the model. Conversely, ice sheet evolution is less sensitive to the choice of ice albedo in the model. We conclude that a proper representation of snow albedo is a fundamental prerequisite for a successful simulation of glacial cycles.

Highlights

The net surface mass balance of ice sheets is equal to the difference between accumulation, which is controlled by the hydrological cycle, and of ablation, which is determined by the surface energy balance
Since ice sheets are mostly covered by snow, the albedo of snow plays a crucial role for the surface energy and mass balance of ice sheets
The global temperature decreases by ∼ 6 ◦C from the Eemian interglacial (126 ka) to the Last Glacial Maximum (LGM, 21 ka) (Fig. 2a)

Summary

Introduction

The net surface mass balance of ice sheets is equal to the difference between accumulation, which is controlled by the hydrological cycle, and of ablation, which is determined by the surface energy balance. While the amount of radiation reaching the surface is mainly determined by the insolation at the top of the atmosphere and cloud cover, the fraction of radiation absorbed at the surface is controlled by its albedo. Since ice sheets are mostly covered by snow, the albedo of snow plays a crucial role for the surface energy and mass balance of ice sheets. The importance of snow and ice albedo parameterizations for ice sheet modelling has been known for long time. The role of snow albedo parameterization on modelling of glacial cycles is much less understood. Most previous simulations of glacial cycle(s), e.g. Bonelli et al (2009), Tarasov and Richard Peltier (2002), Zweck and Huybrechts (2005), Charbit et al (2007), Abe-Ouchi et al (2007), Lunt et al (2008), Gregoire et al (2012), Liakka et al (2016), and many others, employed the so-called positive degree day (PDD) scheme, which does not account explicitly for snow and ice albedos

Methods

Results

Conclusion