A SAR-derived long-term record of glacier evolution in North-West Greenland

Abstract

Observations show that the Greenland ice sheet is losing mass with accelerating pace. Ice discharge trough outlet glaciers contributes approximately for half of the mass loss. However, the role of marine-terminating outlet glaciers on the response of the Greenland ice sheet to climate change is relatively unknown. In recent years, observations have shown dramatic changes in the velocity and front position in a number of marine-terminating outlet glaciers, but a questions remains why some outlet glaciers are stable and others are not. Consequently, predictions on the evolution of the Greenland ice sheet are uncertain until a realistic representation of marine-terminating outlet glaciers is possible. More specifically, significant uncertainty exists on the link between climate forcing and marine-terminating outlet glacier behaviour. This link has been associated to glaciers-specific factors, such as bedrock topography and fjord width, but more glaciers need to be studied. Here we present a study that focussed on the response of different glaciers to a similar climate forcing, thereby taking into account their topographic situation. SAR observations of seven marine-terminating glaciers in the Uummannaq-bay (West-Greenland) are used to estimate terminus positions and glacier flow velocity. The observations are acquired between 1991 - 2014 using ERS, Envisat and TerraSAR-X, thereby extending the length of the state-of-the-art records. Terminus positions are manually digitized and an equivalent position is determined on the glacier flowline, a significant improvement with respect to the box-method. Glacier flow velocities are obtained using ICC offset tracking. The estimated offsets of ERS and Envisat image pairs were noisy, but have been filtered using the along-track velocity profile that was accurately estimated with TerraSAR-X image pairs. The results showed that the outlet glaciers in this region have been stable during the 1990's, and that the warm winter of 2003 initiated retreat. Three glaciers (Lille Gletscher, Umiammakku Isbræ and Inngia Isbræ) have found to show terminus retreat, of which Lille Gletscher and Inngia Isbræ show long-term speed-up. All other glaciers have a stable terminus position, and show no long-term acceleration. Surface air temperature and sea surface temperature show an increasing trend since 1980, whereas the duration of high ice concentrations (sea-ice melange) becomes shorter each year. The retreat of Umiammakku Isbræ and Inngia Isbræ starts after the winter of 2003, in which surface air temperatures were exceptionally high and the period of sea ice was short. The years after, this retreat continued even under normal climatic conditions. Inngia Isbræ showed an ongoing retreat of approximately 6 km between 2003 and 2013, and its flow velocity is estimated to tripled from 500 m/yr to 1500 m/yr. On the contrary, Umiammakku Isbræ ceased its 4 km retreat in 2010, and did not show an increase in flow velocity. Next to long-term patterns, seasonal patterns are identified to differ from glacier to glacier. The terminus position of Rink Isbræ shows a seasonal variation of 1 km, whereas the terminus position of Støre Gletscher only fluctuates around 200 m. The three retreating glaciers are located in a shallow fjord (< 300 m depth) with a reversed bedrock slope. Their retreat into deeper waters initiated a positive feedback-loop leading to multi-year retreat. Glaciers located in a deep fjord (Rink Isbræ and Støre Gletscher) are exposed to warm Atlantic waters, and consequently subject to submarine-melting, but our results show that their front position remained stable and their flow velocity did not increase. The fact that the retreating glaciers are located in a shallow fjord, and exposed to cold Polar water, could indicate that submarine-melting is not a major factor controlling glacier retreat. This also indicates that the bedrock topography is important for a glaciers sensitivity to climate forcing. It is recommended that more effort is put into observations of the submarine-melting process and high resolution bedrock topography and bathymetry, as the relative importance of submarine-melting with respect to the surface mass balance and calving is still obscure. Nevertheless, our results suggest that there is an important link between the fjord depth at the terminus location and the sensitivity to climate forcing.