Glacier Dynamics May Not be Fully Understood

Jakobshavn Isbræ, an outlet glacier off the west coast of Greenland, is losing mass faster than previously thought, due to increased melt water passing through it, as reported in a new paper. The glacier’s rapid trajectory of thinning may well represent the Greenland Ice Sheet (GrIS) ice loss rate in the near future, which could mean faster sea level rise than currently projected.

 

A team of researchers affiliated with the IMAU developed models of  the glacier’s dynamics, and ran 50 tests of these models with different sets of parameterizations such as ocean temperature, ice mass and basal melt until they were able to best match the models with the observed acceleration of Jakobshavn Isbræ. These parameters were then put in place to run the rest of the research. The 2012 acceleration was not captured by the models because it was an exceptional warm melt season that exceeded the mean length of the melt seasons of the previous 20 years; this season caused an exceptionally large amount of meltwater to pass through Jakobshavn Isbræ.

“An intense and long melt year leads to strong thinning of the ice, steepening surface slopes, and has the potential to further sustain the initial acceleration of [Jakobshavn Isbræ],” the authors wrote in their paper. Extreme warming events “may have created the conditions under which the winter slowdowns can no longer compensate for the summer accelerations leading to an increase in the mean annual flow,” they added.

 

Jakobshavn Isbræ calving front
Jakobshavn Isbræ calving front

The acceleration of the ice mass loss of Jakobshavn Isbræ was modelled by 3D representations of the glaciers. There were two known accelerations in the distant past, in 1998 and 2004. The 2003-04 observed event was so intense that the models were unable to represent the amount of mass loss that it sustained. The floating tongue of Jakobshavn Isbræ was ultimately thinned to the point of collapse in the 2003 acceleration event; this collapse led to even more thinning and increased mass loss.

The floating tongue of a glacier provides a terminus point and helps stabilize the glacier. Once a glacier begins to erode to the floating tongue, the acceleration tends to increase and there is even more mass loss than before the break up. (Further explanations of glacier terms can be found here.)

“Findings suggest that the speed observed today at [Jakobshavn Isbræ] is a result of thinning induced changes and a reduction in resistive stress (buttressing) near the terminus correlated with inland steepening slopes,” the authors wrote.

Jakobshavn Isbræ is an important indicator of future sea level rise since it is the largest drainage outlet glacier from the GrIS. It has seen a doubling in acceleration of mass loss and melt water velocity which shows the GrIS is experiencing higher than normal melt seasons.

Jakobshavn Isbræ ice loss and retreat
Jakobshavn Isbræ ice loss and retreat

Due to constraints of typical global models to represent the increased acceleration of ice flow over outlet glaciers during warming events, there is an underestimation of this ice flow’s contribution to overall sea level rise. Increased acceleration of the Jakobshavn Isbræ may be an important piece of the puzzle to help scientists more accurately portray sea level rise in their global models.

This research points to the importance of replicating such analysis in other regions. Other GrIS drainage areas have seen a recent slowdown of ice melt, so this finding may be confined only to one area. However, Jakobshavn Isbræ’s status as the largest of the drainage areas suggests that it may well provide a kind of climatological foreshadowing of future events. According to the authors, climate modelers need to better incorporate the dynamics of glacier mass loss acceleration into their models to better represent potential sea level rise.

 

GRIS ice discharge 1 from AMAP on Vimeo.

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