Disko Island (70° N, 54° W) is located in central West Greenland (Figure 1) and is the largest island in Greenland, covering 8575 km2 of which about 19% is glacierized (Humlum, 1987).
The climate in the south-western part of the island is polar maritime, and it gradually becomes more continental towards the northeast. The mean annual precipitation in
Qeqertarsuaq (Godhavn) is about 400 mm, whereas it was about 200 mm (1961-1972) in Qullissat, an abandoned mining village on the northeast coast (Humlum et al, 1995). The termination of the Little Ice Age (1150-1920) was marked by a dramatic increase in the regional mean annual air temperature (MAAT) of about 2-4°C between 1921 and 1930. In Qeqertarsuaq (Godhavn), this was followed by a decline in temperature from -3.2°C in 19311960 MAAT to -4.0°C in 1961-1990 MAAT with interannual fluctuations up to 7°C (Humlum, 1999). Since 1991, the temperature has slightly increased. The temperature at sea-level is about 3°C colder in the central part of the island compared to the temperature in Qeqertarsuaq (Godhavn) (Humlum et al., 1995), and the vertical lapse rate is 6-7°C km-1 (Humlum, 1998).
Based on the assumption that the -6°C MAAT isotherm coincides with the position of the southern limit of continuous permafrost, it is suggested that the southern and western coastal areas of Disko Island have discontinuous permafrost, whereas the remaining part of the island has continuous permafrost. The thickness of the permafrost is not known, and there has not been conducted continuous monitoring of active layer thickness. Hence, it is difficult to assess any imbalance between the current climate and the permafrost distribution.
The non-glacierized landscape on Disko Island is characterized by an Early Tertiary basalt plateau at 600-1200 m a.s.l. intersected by several 10-50 km long, U-shaped valleys with glacier-carved cirques and talus deposits along the valley walls. The valley floors contain large areas of relict glacier ice (dead-ice), moraines, outwash plains, alluvial fans formed by tributary glacial meltwater streams, river bank terraces, and periglacial landforms. Most glacial meltwater drains through these valleys before emanating into fjords facing towards the Davis Strait to the west, or directly into the Vaigat Strait to the north or the Disko Bay to the east. At least a thousand homothermal springs are found near sea-level on Disko Island, the warmest being Puilassoq at the head of Mellemfjord with a constant temperature of about 18°C (Steenstrup, 1901; Heide-J0rgensen and Kristensen, 1999). Their presence indicates that aquifers exist within the basaltic rocks, and that the geothermal gradient is enhanced.
The lower layers of the basalt contain shale fragments or beds. In the eastern part of Disko Island, the basalt rocks overlay formations of Cretaceous sandstone and Paleocene mud- and sandstones with coal seams. The basement of Disko Island consists of gneisses, which only are exposed at sea-level along a north-south ridge through Qeqertarsuaq (Godhavn).
Disko Island contains 1070 glaciers, icefields and snowfields of which 350 are larger than 1 km2 (Weidick et al., 1992). Of these, 247 are glaciers, while the remaining comprises ice and snowfields (Yde and Knudsen, 2007). The two major ice caps, Sermersuaq (Storbraen) and Brapasset, cover the central part of the island, sending several outlet glaciers into the surrounding valleys, while valley glaciers, cirque glaciers and icefields are widespread all over the island. All glaciers on Disko Island terminate on land. A recent study has classified 75 glaciers on Disko Island as surge-type glaciers (Yde and Knudsen, Chapter 10). Surge-type glaciers experience a non-climatic-related cyclic behavior consisting of a short active period (a few years) of rapid frontal advance, where the glacier becomes heavily crevassed and water discharge is increased, and a long quiescent period (more than 100 years). Glacier surging may have a devastating effect on proglacial permafrost and landforms as the glacier front may advance more than 10 km down-valley.
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