



Melting on the Greenland ice cap. Sarah Das, Woods Hole

We tend to think of glacial as a synonym for slow, but summer on an ice sheet can be pretty dynamic. Lakes can form on ice sheets as melt water pools during warm summer days. These lakes decrease the albedo of the surface, leading to more melting. Once enough water collects, the pressure it creates can cause a fracture in the underlying (less dense) ice, allowing the lake to drain catastrophically.

These events take on added significance due to what happens to that water. If the lake drains to the bottom of the ice sheet, it can lubricate the bed of the ice sheet, and thus increase rates of movement. That movement can lead to an increased flux of water into the ocean, thus raising sea levels.

A report in today's Science describes how researchers recorded the drainage of one such lake in Greenland. The lake was roughly 5.6 km2, but drained completely in less than an hour and a half. The lake's contents rapidly made their way down to the bottom of the ice sheet, 980 m below the surface. During this period, the average drainage rate was 8700 m3/s. For reference, the average flow rate for Niagara Falls is only 5700 m3/s.

In the 16 hours prior to the catastrophic drainage, they noted a slow decrease in water level. They suggest that this corresponds to the slow initial fracturing of the ice. During this period, the lake was only filling the fracture it was creating. Once this fracture reached the base, the researchers believe that the water spread out in an existing subglacial network, which facilitated the rapid lake drainage.

Along with this drainage, the team recorded movement of the ice sheet (both vertically and horizontally), along with a corresponding increase in glacial seismic activity. As the lake drained, the ice sheet was temporarily lifted by a meter. As the water spread out at the base of the ice sheet, the surface elevation slowly decreased back to its initial level over the course of 24 hours. Along with this lifting came a very rapid shift north of almost one meter, followed by a slow drift back south and movement west of less than one meter.

The fracture created by this lake remained on the surface as a conduit for future melt water. The team also noted that other post-drainage lake beds they came across exhibited the same type of fractures, suggesting that the process they recorded is quite common. Indeed, thousands of lakes are formed each summer season, and they must go somewhere.

A related study also published in Science today looked at the change in ice movement related to basal lubrication. The study found that, while the water lubricating the base of the ice sheet caused seasonal speedups of its motion on the order of 50 to 100 percent, the speedup of outlet glaciers, where the ice sheet drains into the ocean, was only about 15 percent. While other factors are causing outlet glaciers to speed up, lubrication of the base doesn't seem to be the main cause.

Science, 2008. DOI: 10.1126/science.1153360

Science, 2008. DOI: 10.1126/science.1153288

