Google Earth Community: Greenland ice sheet supraglacial lakes

This file examines the area of 43 supraglacial lakes on a transect from Epiq Sermia Glacier in the north across the Jakobshavns Glacier basin to an area just south of Mint Julep base. The average area of the 43 glacier is 1.23 km2

The 1975 Mountain Glaciers of the Northern Hemisphere Atlas (Field, 1975) indicates the presence of many supraglacial lakes on the surface of the Greenland Ice Sheet a short distance inland from the western margin. These meltwater lakes some seasonal, some perennial are not a new feature. However, their importance and possible increase in size and extent are crucial questions. Working on Jakobshavns Isbrae in 1985 some of the lakes were clearly ephermeral, here today and gone the next day. Such a rapid drainage, was one of the reasons emphasized by Terry Hughes at the University of Maine, that the Jakobshavns was the fastest moving non-surging glacier in the world.

Twenty years ago Terry Hughes proposed the Jakobshavns Effect (JE). The JE as explained by Hughes (1986) results from an imbalance of horizontal hydrostatic forces at the grounding line. With positive feedback mechanisms that sustain rapid ice discharge: ubiquitous surface crevassing, high summer rates of surface melting, which eventually reaches the glacier base, extending creep flow, progressive basal uncoupling, lateral uncoupling, and rapid iceberg calving. The glacier lakes than could be important in delivering water to the base of the glacier. The more melting the greater the number and volume of lakes and delivery of this water to the glacier base. The shear volume of the lakes is also a measure of the amount of melting on the glacier in the region of lake formation. The region occupied by lakes also indicates a region of the glacier dominated by melt processes. A change in this region would indicate a significant change in the glacier mass balance regime for portions of the ice sheet. Lastly the lakes themselves once formed, negatively impact mass balance, as the lakes absorb more radiation than the surrounding ice and snow. This results in more ablation. This has been observed both in Alaska and Greenland.

Are the recent outlet glacier accelerations indicative of the Jakobshavns Effect at work is the reduction in back stress allowing the ice to be pulled out of the ice sheets, or is reduced basal coupling solely enhancing basal sliding?

The Jakobshavn Isbrae has long been viewed as the fastest sustained tidewater glaciers in the world. After nearly 50 years of stability (Pelto and others 1989), a remarkable retreat of the ice front and an increasing flow velocity have been noted (Dietrich, Maas, Baessler, Ruelke, Schwalbe, 2005). In August 2004 they determined the flow velocity of the ice front and last 5 km of the glacier. They obtained velocities range up to 40 m/day, a dramatic acceleration from the 20-23 m/day that had been observed over a sustained period (Pelto and others, 1989). (Mayer and Herzfeld, 2005) observed that in 2002, this ice stream with a 12 km long floating tongue, suddenly entered a phase of rapid retreat. The ice front started to break up, the floating tongue disintegrated, the production of icebergs increased. Thomas (2004) argues for acceleration via back force reduction. The thinning and acceleration of the glacier immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. He assumes that the force perturbation associated with such weakening is swiftly transmitted up-glacier, to a maximum point 10 km upglacier. The conclusion is that the initial observed changes in flow are consistent with the comparatively small perturbation associated with the calving.

The importance of the lakes has attracted the attention of several research teams. One team looks to survey in detail two supraglacial lakes south of Jakobshavn Isbrae and compare geophysical measurements to remotely sensed data to better understand how supraglacial lakes deliver meltwater to the ice sheet bed and regulate ice flow in the short
term.

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