Moulins, Calving Fronts and Greenland Outlet Glacier Acceleration

Guest Commentary by Mauri Pelto

The net loss in volume and hence sea level contribution of the Greenland Ice Sheet (GIS) has doubled in recent years from 90 to 220 cubic kilometers/year has been noted recently (Rignot and Kanagaratnam, 2007). The main cause of this increase is the acceleration of several large outlet glaciers. There has also been an alarming increase in the number of photographs of meltwater draining into a moulin somewhere on the GIS, often near Swiss Camp (35 km inland from the calving front). The story goes—warmer temperatures, more surface melting, more meltwater draining through moulins to glacier base, lubricating glacier bed, reducing friction, increasing velocity, and finally raising sea level. Examining this issue two years RealClimate suggested this was likely the correct story. A number of recent results suggest that we need to take another look at this story.

The Acceleration:

Jakobshavn Glacier, West Greenland, retreated 30 km from 1850-1964, followed by a stationary front for 35 years. Jakobshavn has the highest mass flux of any glacier draining an estimated 6% of the GIS. The glacier terminus region also had a consistent velocity of 19 meters/day (maximum of 26 m in glacier center), from season to season and year to year, the glacier seemed to be in balance, as I noted in a 1989paper. This is the fastest glacier in the world, no steroids needed. After 1997 it began to accelerate and thin rapidly, reaching an average velocity of 34 m/day in the terminus region. The glacier 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, but likewise is.

Helheim Glacier, East Greenland had a stable terminus from the 1970’s-2000. In 2001-2005 the glacier retreated 7 km and accelerated from 20 m/day to 33 m/day, while thinning up to 130 meters in the terminus region. Kangerdlugssuaq Glacier, East Greenland had a stable terminus history from 1960-2002. The glacier velocity was 13 m/day in the 1990’s. In 2004-2005 it accelerated to 36 m/day and thinned by up to 100 m in the lower reach of the glacier. Helheim and Kangerdlugssuaq combined drain 8 % of GIS. Hence, they are more than canaries in the coal mine. In 2006, the velocity of Helheim and Kangerdlugssuaq decreased to near the 2000 level, the terminus of Helheim advanced a bit (Howat et al., 2007).

The first mechanism for explaining the change in velocity is the “Zwally effect”, which 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 (Zwally et al., 2002). 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”. 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. However, many others have made this assertion and are investigating (Stearns and Hamilton, 2007). 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 (Das, 2008).

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