Greenland Ice Sheet - Ice Sheet Acceleration

Ice Sheet Acceleration

Two mechanisms have been utilized to explain the change in velocity of the Greenland Ice Sheets outlet glaciers. The first is the enhanced meltwater effect, which relies on additional surface melting, funneled through moulins reaching the glacier base and reducing the friction through a higher basal water pressure. (It should be noted that not all meltwater is retained in the ice sheet and some moulins drain into the ocean, with varying rapidity.) This idea, was observed to be the cause of a brief seasonal acceleration of up to 20% on Sermeq Kujalleq in 1998 and 1999 at Swiss Camp. (The acceleration lasted two-three months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the "coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming". Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large glaciers outlet glaciers. The second mechanism is a force imbalance at the calving front due to thinning causing a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, reducing frictional back forces, as the glacier becomes more afloat at the calving front. The reduced friction due to greater buoyancy allows for an increase in velocity. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension because of the backforce reduction. For ice streaming sections of large outlet glaciers (in Antarctica as well) there is always water at the base of the glacier that helps lubricate the flow. This water is, however, generally from basal processes, not surface melting.

If the enhanced meltwater effect is the key then since meltwater is a seasonal input, velocity would have a seasonal signal and all glaciers would experience this effect. If the force imbalance effect is the key the velocity will propagate up-glacier, there will be no seasonal cycle, and the acceleration will be focused on calving glaciers. Helheim Glacier, East Greenland had a stable terminus from the 1970s-2000. In 2001–2005 the glacier retreated 7 km (4.3 mi) and accelerated from 20 to 33 m or 65.6 to 108.3 ft/day, while thinning up to 130 meters (430 ft) in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960 to 2002. The glacier velocity was 13 m or 42.7 ft/day in the 1990s. In 2004–2005 it accelerated to 36 m or 118 ft/day and thinned by up to 100 m (328 ft) in the lower reach of the glacier. On Sermeq Kujalleq the acceleration began at the calving front and spread up-glacier 20 km (12 mi) in 1997 and up to 55 km (34 mi) inland by 2003. On Helheim the thinning and velocity propagated up-glacier from the calving front. In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. On each glacier the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent.

An examination of 32 outlet glaciers in southeast Greenland indicates that the acceleration is significant only for marine terminating outlet glaciers. That is glaciers that calve into the ocean. Further, noted that the thinning of the ice sheet is most pronounced for marine terminating outlet glaciers. As a result of the above, all concluded that the only plausible sequence of events is that increased thinning of the terminus regions, of marine terminating outlet glaciers, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. Enhanced meltwater induced acceleration does exist but is of a notably smaller magnitude and duration.