In the late 1990s, repeat-pass airborne laser altimetry measurements revealed that the Greenland Ice Sheet was nearly in balance in the interior, but was thinning along its periphery, with deterioration concentrated along the channels occupied by outlet glaciers (Krabill et al, 1999). Overall, the ice sheet was thinning because the coastal changes dominated the balance. This result was consistent with a recent warming in air temperature and an increase in summer melt intensity and extent in Greenland compared to the 1980s. The larger rates of thinning on glaciers, however, suggested that thinning from changes in glacier dynamics could be significant (Abdalati et al, 2001). In the interior, shallow ice cores revealed large interdecadal variations in snowfall, but no clear long-term trend in snowfall.
Subsequent surveys conducted in the late 1990s and early 2000s confirmed the mass loss of the ice sheet, but also revealed that the mass loss was increasing with time (Krabill et al, 2004). Air temperatures were also on the rise. In addition, several glaciers were starting to change behavior. Jakobshavn Isbrae, the largest discharger of ice on the Greenland's west coast, saw its floating section collapse in 2000-2003, accompanied by a doubling in glacier speed (Joughin et al, 2004). The mass loss consecutive to the glacier acceleration was much larger than that caused by the melting of ice and snow.
In the southeast sector of Greenland, where snowfall is the highest and ten times that of the dry north, glaciers were significantly out of balance in the 1990s (Rignot et al, 2004). Few of these glaciers have been studied in any sort of detail; most of them are unnamed. They flow through narrow passages in a complex alpine landscape. Ice thinning was detected all the way to the ice divide in the early 1990s (Krabill et al, 1999), at more than 2000 m elevation. Glacier velocities were more than 50 per cent too large to maintain the ice sheet in a state of mass balance. In 1996, this sector was already contributing the largest mass loss from the Greenland Ice Sheet into the ocean.
More changes took place between 2000 and 2005, which progressively altered the velocity structure of the entire Greenland Ice Sheet (Rignot and Kanagaratnam, 2006). The glaciers in southeast Greenland accelerated 30 per cent on average between 1996 and 2000, and another 57 per cent in 2000-2005, to bring their mass balance even more into the negative. In 2004-2005, Jakobshavn Isbrae was still receding and profusely calving, with a speed stabilized at about twice its former value, and an ice front well inland of anything the glacier had experienced since 1850s, including a warm period in the 1940s. On the east coast, two of the largest glaciers accelerated almost at the same time: Kangerdluqssuaq Glacier in the north and Helheim Glacier in the south. In the case of the former, we had no evidence for any major flow change since at least the 1960s (Thomas et al, 2000). But the glacier suddenly accelerated 250 per cent in 2004, its surface dropped by more than 100 m, and its calving front retreated 5 km (Rignot and Kanagaratnam, 2006). Figure 5.2 (Plate 4) shows how the catastrophic retreat left remnants of ice stranded on the side mountains. Helheim Glacier, 200 km south, sped up 60 per cent and its ice front retreated several kilometers. The mass loss caused by the speed up of these two large glaciers was sufficient to double the mass deficit from east Greenland. In 2006, the two glaciers slowed down near the front, but the thinning wave was propagating inland and the mass loss remained significant (Howat et al, 2007). Similarly, the glaciers in the southeast coast slowed down 10-20 per cent in 2006 but remain largely out of balance with interior accumulation.
Note: The photograph shows massive calving activity at the ice front as well as elevated ice margins (red arrow) compared to the glacier center as a result of the massive drawdown of the ice surface caused by the doubling in speed of the glacier. Source: courtesy of J. Sonntag.
Few large glaciers exist along the southern tip and southwest coast of Greenland because of its large ablation area and shallower slopes, which, together, have the effect of causing the loss of most of the ice before the glaciers reach the ocean even though this fast loss allows modest flow speeds. This sector experienced higher snowfall in the 1980-1990s, which triggered partial advance of ice fronts (Weidick, 1995), except for tidewater glaciers that have been retreating steadily since the Little Ice Age. Over the last few years, glacier acceleration was detected in that sector as well. For instance, Kangiata-nunata Glacier accelerated 38 per cent in 1996-2006.
North from Jakobshavn Isbrae, few glaciers were observed to speed up, but many glaciers were already out of balance and discharging more ice to the sea than required to maintain mass balance in the 1990s. If and when these glaciers will undergo a major speed up, they will add considerably to the mass deficit of Greenland because they are responsible for some of the largest discharge of ice in Greenland.
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