Disintegration of Northern Larsen Ice Shelf, Antarctic Peninsula, and Ice Retreat in Patagonia

The dynamic behaviour and mass balance of ice shelves on the northern Antarctic Peninsula and of glaciers in Patagonia and their dependence on climatic factors were investigated. Atmospheric warming and drastic ice retreat were observed during recent years in this region which extends between 50 and 70 southern latitude. The project activities focussed on various section of the northern Larsen Ice Shelf and on two outlet glaciers of the Patagonian Icefield in Argentina. The work was based on a synthesis of glaciological field measurements, satellite data analysis, and model calculations. The field activities were carried out in close cooperation with scientists of Instituto Antrtico Argentino, Buenos Aires. The northern Larsen Ice Shelf is close to the climatic limit for the existence of ice shelves. The collapse of its two northernmost sections within a few days in January 1995, which was investigated in the previous FWF project P10709-GEO, drew the attention of the scientific community to this region. The investigations in this follow-on project confirmed that the limit for ice shelves is shifting further south. Since early 1995 the area of Larsen B, the presently northernmost section of Larsen Ice Shelf, decreased by 40%, from 11500 km 2 to 6800 km 2 . Field observations revealed increasing ice velocity, negative surface mass balance, and opening of rifts. In addition, maps of ice motion, derived from interferometric Synthetic Aperture Radar (SAR) images of the European ERS satellites, enabled detailed studies of the dynamic behaviour. With this interferometric data set, based on SAR images over one day time intervals, accuracies of a few millimetres per day were achieved for the ice velocity. The analysis confirmed the importance of fracture mechanisms for ice shelf disintegration. In the region where the ice shelf had disappeared in 1995, the behaviour of glaciers and ice streams, which previously nourished the ice shelf and now calve directly into the sea, was investigated by means of SAR interferometry. All major glaciers retreated inland of the previous grounding line and showed a significant acceleration of flow. This is the first direct prove that the removal of ice shelves can lead to a rise of eustatic sea level through increasing the ice export from grounded outlet glaciers above. Also the majority glaciers of the Southern Patagonian Icefield shows pronounced retreat, though there are some exceptions. The mass fluxes and dynamics of two adjoining outlet glaciers of the Icefield, which calve into freshwater lakes, were investigated to learn about the reasons for the different behaviour. Moreno Glacier is nearly stationary since 80 years, whereas Ameghino Glacier retreated by several kilometres during the last 30 years. Mass balance measurements were carried out on both glaciers in the field and complemented by interferometric motion maps derived from SAR data of the Space Shuttle. The studies showed that the differences of glacier surface topography and of the geometry of the calving front are the main reasons for the different reactions of the two glaciers. This agrees with the observations of the outlet glaciers on the Antarctic Peninsula and confirms that calving glaciers and ice streams are particularly sensitive to climate change.