Air temperatures have warmed by 2-3°C along the coast of southeast and southwest Greenland since the 1980s (see Figure 5.3), which is a large forcing. As temperature records show, there is a strong coherence in the warming trends affecting east and west stations. This symmetry breaks down further north. As a result, further glacier acceleration will likely progress in an erratic manner in the north, for the reasons below.
The increase in air temperature has four consequences:
1. Warming increases the melting of snow and ice, hence the mass deficit of the ice sheet. As the ice sheet thins and becomes lower, melting increases, even if air temperatures remain the same, a phenomena known as topography feedback. The removal of the upper level of fresh snow also reduces the ice sheet albedo, which further increases melting and the spatial extent of melting.
1920 1940 1960 1980 2000 Year
Figure 5.3 Air temperature records for several stations around Greenland showing the strong warming since the 1980s (Angmassalik and Godthab), and a shorter period of warming in the 1930s
2. Warming allows more melt water to reach the glacier beds. When more water lubricates the bed, glaciers flow faster, especially as subglacial water pressure increases and lifts the glaciers off their beds. Recent GPS surveys (Zwally et al, 2002) revealed that even areas not associated with glaciers accelerate by 30-50 per cent in the summer, on short time scales, meaning that melt water goes rapidly from the surface to the glacier base and accelerates ice flow. Natural pathways for that transition are provided by moulins, which guide rivers of melt water down to the ice sheet bottom, or pre-existing networks of crevasses and micro-fractures connecting to the bed. Increased subglacial water pressure or lubrication enhances glacier flow. The rates of speed up remain modest, however, when averaged over the whole year. For instance, we measured an 8-10 per cent speed up over the summer months compared to the winter months over a wide latitudinal range of glaciers in Greenland, almost regardless of the amount of melt water produced. Enhanced sliding from more melt water is therefore not the primary response of the ice sheet to warming.
3. Rather than percolating through the bottom, melt water may pond and form large supraglacial lakes that either freeze in or drain to the bottom of the ice sheet. When they drain to the sea, these lakes send large pulses of melt water, which may travel quickly (1km/hr) over large distances and lift the glacier above its bed, thereby allowing fast flow. Little is known about the effect of these lakes because most of them cannot be seen from the surface, but they certainly play an important role in controlling ice flow.
4. Ice thinning near the glacier's front edge progressively disconnects the glaciers from the ground below, equivalent to removing a part of the glacier that would normally buttress its upstream flow. The thinning of the frontal plug helps glaciers flow faster. Model studies suggest that the effect on speed is enormous, i.e. an order of magnitude larger than that associated with an increase in melt water. This was indeed first illustrated in the Antarctic (Thomas et al, 2004), where melt water is not a factor. Ice front thinning is the main reason for the abrupt and enormous speed up of the southeast Greenland glaciers (Howat et al, 2005). As glaciers re-advance, the speed up may diminish, as observed on Helheim and Kangerdlugssuaq in 2006 (Howat et al, 2007). But the process is unlikely to reverse entirely because the glacier speed up has significantly and almost irreversibly altered the force balance of the glaciers. We come back to that point later in the discussion.
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