image of Jakobshavn Glacier. The lines show the position of the
calving front of the Jakobshavn Glacier since 1851. The date of this image is 2006 and the calving front of the glacier can be seen at the 2006 line. The area stretching from the calving front to the sea (towards the bottom left corner) is the
Ilulissat icefjord. Courtesy of NASA Earth Observatory Jakobshavn is one of the fastest moving glaciers, flowing at its terminus at speeds that used to be around per day but are over 45 metres (150 ft) per day when averaged annually, with summer speeds even higher (as measured 2012–2013). The speed of Jakobshavn Glacier varied between per year between 1992 and 2003. Jakobshavn Isbrae retreated from 1850 to 1964, followed by a stationary front for 35 years. Jakobshavn has the highest mass flux of any glacier draining the
Greenland Ice Sheet. The glacier terminus region also had a consistent velocity of per day (maximum of per day in the glacier center), from season to season and year to year, the glacier seemed to be in balance from 1955 to 1985. and from the longer-duration forces exerted on the solid Earth during the capsize of very large (e.g., > 1 km3) calved ice volumes. Especially large calving events at Jakobshavn have produced glacial earthquakes that are detectable on seismographs worldwide with moment magnitudes in excess of 5.0. A large calving of approximately 7 km2 took place on 15 February 2015. On 16 August 2015 a calving was identified via satellite images as the largest ever recorded at Jakobshavn, with an area of 12.5 km2.
Mechanisms The first mechanism for explaining the change in velocity is the "Zwally effect" and is not the main mechanism, this relies on meltwater reaching the glacier base and reducing the friction through a higher basal water pressure. A
moulin is the conduit for the additional meltwater to reach the glacier base. This idea, proposed by Jay Zwally, was observed to be the cause of a brief seasonal acceleration of up to 20% on the Jakobshavns Glacier in 1998 and 1999 at Swiss Camp. The acceleration lasted 2–3 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". The acceleration of the three glaciers had not occurred at the time of this study and they were not concluding or implying that the meltwater increase was the cause of the aforementioned acceleration. Examination of 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 outlet glaciers. where a small imbalance of forces caused by some perturbation can cause 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, even becoming afloat at the calving front, and is responsive to tidal changes. The reduced friction due to greater buoyancy allows for an increase in velocity. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension in what R. Thomas calls a backforce reduction. The cause of the thinning could be a combination of increased surface
ablation and basal ablation as one report presents data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of Greenland, and suggests that the changes in Jakobshavn Glacier are due to the arrival of relatively warm water originating from the
Irminger Sea near Iceland. Evidence also exists for a deep subglacial trench beneath the glacial outlet, identified through
seismic reflection methods. There are theories that Greenland consists of three large islands under the ice sheet, separated at the coast by three narrow straits, one of them Jakobshavn Glacier. ==
Chasing Ice ==