Google Earth Community: Greenland ice sheet supraglacial lakes

News that the net loss in volume and hence sea level contribution of the Greenland Ice Sheet has doubled in recent years from 90 to 220 cubic kilometers/year has been on our radar (Rignot and Kanagaratnam, 2007). The main cause of this increase is the acceleration of several large outlet glaciers of the Greenland Ice Sheet (GIS) (Rignot and Kanagaratnam, 2007). There has also been an astonishing increase in the number of photographs of meltwater draining into a moulin somewhere on the GIS, often near Swiss Camp on the Jakobshavn Glacier. 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. Is this the story?.

First a brief history of terminus and velocity change. Jakobshavn Glacier, West Greenland, retreated 30 km from 1850-1964, followed by a stationary front for 35 years. Note terminus paths in attached file. 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 1989 paper. 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 the next six years. Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring, but slower. 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. 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 35 km inland from the calving front (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 supraglacial lake drainage documented short terms velocity changes due to such events, but little significance to the annual flow of the large glaciers outlet glaciers (Joughin and Das et.al, 2008)

The second mechanism is a Jakobshavns effect, coined by Terry Hughes, (1986), where a force 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 upglacier. Thinning causes the glacier to be more buoyant, even becoming afloat at the calving front, and making it more responsive to tidal changes. The reduced friction due to greater buoyancy allows for an increase in velocity at the calving front. This is akin to letting off the emergency brake a bit. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension in what R. Thomas calls a backforce reduction (Thomas, 2003 and 2004).

For ice streaming sections of large outlet glaciers there is always water at the base of the glacier that helps lubricate the flow. If the Zwally effect is the key than since meltwater is a seasonal input, velocity would have a seasonal signal and the acceleration would be initiated where the melting input had increased. If the Jakobshavn effect is the key the velocity increase will first occur at the terminus and propogate upglacier, tidal fluctuations of the calving front will become more notable and the velocity change will not be seasonal.

On Jakobshavn the acceleration began at the calving front and spread upglacier 20 km in 1997 and up to 55 km inland by 2003 (Joughin et al., 2004). On Helheim the thinning and velocity propagated upglacier from the calving front. Each of the glaciers fronts did respond to tidal variations indicating they had become afloat, detached from their bed (Hamilton et al, 2006). This had been the case at Jakobshavn for the last 50 years, but not for Helheim or Kangerdlussuaq. Note all of the icebergs in front of the Jakobshavn indicating the great volume loss accomplished via calving.

In each case the major outlet glaciers accelerated by at least 50%, much larger than the impact noted due to summer meltwater increase. There were no seasonal velocity changes near the calving front. On Jakobshavn the acceleration was not restricted to the summer, persisting through the winter when surface meltwater is absent. Each of the three glaciers has a reduced velocity in 2006 and 2007 despite some exceptional melt conditions in 2007. All three of these observations suggest that enhanced meltwater is not primary driving force behind the acceleration of the main Greenland ice Sheet outlet glaciers. As a result of the above Luckman et al.,( 2006) concluded “The most plausible sequence of events is that the thinning eventually reached a threshold, ungrounded the glacier tongues and subsequently allowed acceleration, retreat and further thinning. It is reasonable to believe that the 1998 Jakobshavn speed-up, also following a long period of stability, was triggered by the same processes of thinning but occurred earlier and after a shorter period of thinning because the tongue was already afloat.”

Temporarily there appears to not be a force in balance at the glacier fronts. This will reduce the annual contribution to rising sea level from glacier dynamic changes. The bad news is that the degree of acceleration that can occur via the Jakobshavn effect is greater than that from the Zwally effect. The Zwally effect is real and does represent increased velocities that will be more important in sections of the Greenland Ice Sheet where calving processes are not key.

The Jakobshavn is of particular importance as it has a bed below sea level for at least 80 km inland from the terminus. In this reach there are no significant pinning points, or abrupt changes in slope or width (Clarke and Echelmeyer, 1996), that would help stabilize the glacier during retreat. It is the only outlet glacier of GIS to lack these, and can then via backforce reductions tap into the heart of GIS. Surface melting is a slow process for raising sea level. As Greenland’s major outlet glacier have recently shown, a rapid acceleration can quickly deliver large volume of ice to the ocean.

Edited by mspelto (05/16/08 01:23 PM)