The retreat of a tidewater glacier: observations and model calculations on Hansbreen, Spitsbergen

Abstract

Based on observations and model calculations, the retreat over the last two decades of Hansbreen, a tidewater glacier in southern Spitsbergen, Svalbard, is investigated. The observations of the calving-front position between 1982 and 1998 show an abrupt retreat in 1990, which is suggested to be related to a depression in the glacier bed. The observed seasonal variations of the front position are mainly due to variations of the calving rate. The observations of Hansbreen further indicate that during periods of slow front-position changes, melting at the water-line may play an important role in triggering the process of calving.The evolution of Hansbreen between 1982 and 1998 is simulated with a numerical model for the dynamics of tidewater glaciers. Using a flotation criterion for calving in which for each time-step the part of the glacier terminus which is below a critical height above buoyancy is removed, we are able to reproduce the observed rapid retreat of Hansbreen through the depression in the glacier bed. From the observations and model calculations, we conclude that the rapid retreat is mainly an effect of basal topography in the terminus region and not a direct response to a change in mass balance.

Recent Deceleration of the Ice Elevation Change of Ecology Glacier (King George Island, Antarctica)

“…Locations where the Ecology Glacier ice front position was relatively stable and the retreat rate was low coincide with a presence of the bedrock outcrops (Figure 10), indicating that the glacier retreat may be governed by the presence of pinning points [58]. Despite shallow depths of the proglacial lagoon, it seems that the water depth controls termini retreat rate through enhanced calving.…”

Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications

“…Maximum rates, although high, are still several times below the typically observed calving rates, suggesting that basal melt must rather act through changing the shape of the front thereby influencing the stress field, such as through undercutting, and consequently affecting the calving rates (Rignot et al 2010;Motyka et al 2002;Vieli et al 2002). This provides a major challenge for forcing ice flow models with ocean data as it is in this case unclear how to link ocean melt rates to the process of calving.…”

Present State and Prospects of Ice Sheet and Glacier Modelling

“…For Hansbreen, Spitsbergen, the time scales of the modelled fast retreats over bed depressions correspond well with observed retreat rates (Vieli et al 2002). Nick et al (2007) applied a simplified model to Columbia Glacier, Alaska, and confirmed the importance of bed geometry for calving rates, especially if moraines modify the bed topography near the calving front.…”

“…A few grounded calving fronts have been observed over time periods of decades, such as Columbia Glacier, Alaska (Krimmel 2001) and Hansbreen, Spitsbergen (Vieli et al 2002). These observations clearly reveal a connection between the calving rate and resulting advance or retreat rates and the bed geometry (Van der Veen 1996).…”

“…These results were found by using two-dimensional flowline models and do not consider other factors to be influencing the stability. There are indications that this partly applies to three-dimensional valley glaciers, e.g., Hansbreen, Spitsbergen (Vieli et al 2002) and Columbia Glacier (Nick et al 2007). However, Nick et al (2010) show that stable equilibria may exist even if a grounded calving tongue or a grounding line lies on backsloping beds, if calving is determined by crevassing at the surface and at the bed, and not directly by buoyancy.…”