|Jakobshavn Glacier, Jakobshavn Isbræ or Sermeq Kujalleq|
The calving front of the glacier
|Location||Near Ilulissat, Greenland|
|Area||110 000 km2 (whole catchment)|
|Length||greater than 65 km.|
|Thickness||around 2000 m|
|Terminus||Ocean (was floating now grounded)|
Jakobshavn Glacier, also known as the Jakobshavn Isbræ (in Danish) and Sermeq Kujalleq (in Greenlandic), is a large outlet glacier in West Greenland. It is located near the Greenlandic town of Ilulissat (Danish: Jakobshavn) and ends at the sea in the Ilulissat Icefjord.
Jakobshavn Glacier drains 6.5% of the Greenland ice sheet and produces around 10% of all Greenland icebergs. Some 35 billion tonnes of icebergs calve off and pass out of the fjord every year. Icebergs breaking from the glacier are often so large (up to a kilometer in height) that they are too tall to float down the fjord and lie stuck on the bottom of its shallower areas, sometimes for years, until they are broken up by the force of the glacier and icebergs further up the fjord. Studied for over 250 years, Jakobshavn Glacier has helped develop our understanding of climate change and icecap glaciology.
Jakobshavn has been a name used for this glacier in scientific literature since 1853 when Danish geologist Hinrich Johannes Rink referred to it as Jakobshavn Isstrøm (Danish for Jakobshavn Ice Stream). It is sometimes referred to in the international scientific literature (by glaciologists) as Jakobshavn Isbræ glacier. Isbræ is Danish for glacier. It is also commonly known by the anglicised version, Jakobshavn Glacier.
The local name for this glacier is Sermeq Kujalleq, where "sermeq "is Greenlandic for 'glacier' and "kujalleq" means 'southern'. It lies south of the town Ilulissat (formerly Jakobshavn). UNESCO's World Heritage Site website uses this name, in connection with mention of the Ilulissat Icefjord world heritage site, which includes the downstream end of the glacier.
There is evidence that people have inhabited the area around the glacier for up to 4000 years. The recently abandoned settlement of Sermermiut (Which means 'place of the glacier people') lies just to the north of the glacier, much nearer than Ilulissat.
The glacier is sometimes referred to as Ilulissat Glacier. This form simply replaces Jakobshavn with Ilulissat because of the change in the name of the town.
 Acceleration and retreat
Jakobshavn is one of the fastest moving glaciers, flowing at its terminus at speeds of around 20 metres per day. The speed of Jakobshavn Glacier varied between 5700 and 12600 metres per year between 1992 and 2003. The ice stream's speed-up and near-doubling of ice flow from land into the ocean has increased the rate of sea level rise by about 0.06 millimeters (about 0.002 inches) per year, or roughly 4 percent of the 20th century rate of sea level rise. Jakobshavn Isbrae, retreated 30 km from 1850–1964, 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 20 meters/day (maximum of 26 m/day in glacier center), from season to season and year to year, the glacier seemed to be in balance from 1955-1985. After 1997 the glacier began to accelerate and thin rapidly, reaching an average velocity of 34 m/day in the terminus region. It also thinned at a rate of up to 15 m/year and retreated 5 km in six years. Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring. On Jakobshavn the acceleration began at the calving front and spread up-glacier 20 km in 1997 and up to 55 km inland by 2003. The position of this calving front, or terminus, fluctuated by 2.5 km around its annual mean position between 1950 and 1996. 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 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 outlet glaciers.
The second mechanism is a "Jakobshavn effect", coined by Terry Hughes, 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.
This mechanism is supported by the data indicating no significant seasonal velocity changes at the calving front and the acceleration propagating upglacier from the calving front. 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.
 See also
- Extremes on Earth
- Extreme points of Earth
- Retreat of glaciers since 1850
- Glacial motion
- Glacier mass balance
- Helheim Glacier
- Kangerdlugssuaq Glacier
- Large igneous province
- List of glaciers
- Sullorsuaq Strait
- West Greenland Current
- Bentley Subglacial Trench
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- "Plummer Jakobshavn". University of Kansas. Retrieved 2009-02-17.
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- "Ilulissat Icefjord world heritage site". UNESCO. Retrieved 2009-02-26.
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|Wikimedia Commons has media related to: Jakobshavn Isbræ|
- Jakobshavn Glacier Flow Animation of the glacier by NASA.
- Researchers Witness Overnight Breakup, Retreat of Greenland Glacier - July 2010