This project was a collaborative effort between investigators at the Woods Hole Oceanographic Institution and the University of Washington, who are submitting one unified report.

Each summer in Greenland, melt water pools in thousands of large lakes on the surface of the ice sheet. The melt water often is able to breach the thick underlying ice (several thousand feet thick), causing the lakes to drain rapidly (in a matter of hours) through narrow but deep cracks to base of the ice sheet. During some of these drainage events the outflow from the lakes through the ice can exceed the flow over Niagara Falls! Once underneath the ice sheet, the melt water acts as a lubricant, causing the ice to slide more quickly toward the ocean. This behavior raises the issue of whether more ice melt in a warming climate could enhance ice flow, potentially destabilizing the Greenland ice sheet. This project was aimed at better understanding the processes that cause lakes to drain and to understand how lake drainage might extend to higher elevations in a warmer climate.

Water is denser than ice, so as water from summer melting fills the many cracks and crevices on the surface of the ice sheet, the water pressure in the crack tip (greater than the pressure from the surrounding ice) causes the crack to deepen as long as water more water is available to fill in from above. This water-driving “wedging” process is called hydro-fracturing, and it is nature’s analog to the “fracking” processes now widely used in the petroleum industry. Because the ice sheet is so thick (over half a mile in many places), a large volume of water is needed to drive a crack open all the way through to the bottom. Lakes on the surface of the ice sheet provide just such a reservoir, and thus they play a crucial role in hydro-fracturing to the bed and ultimately in increasing summer flow speeds.

The field component of this project deployed an array of GPS receivers around a large melt lake in Greenland, which was known to drain each summer. For three years, the GPS units recorded the motion of the ice-sheet surface as the water wedged the crack open during each summer’s hydro-fracture event. These data were used in conjunction with computer models to gain an improved understanding about how the crack opens and how the water floods the space at the ice sheet’s base. The data and model indicate that drainages are more likely when there is already extra water at the bed (such as from nearby lake drainages), providing initial lubrication that causes slipping and stretching of the ice beneath a lake site. This implies that lakes will likely drain more frequently where they are surrounded by other nearby melt water lakes and/or other drainage conduits that can transport melt from the surface to the bed.

The remote sensing component of this investigation focused on how lake drainages might evolve in a warming world as the summer melt zone continues to expand further inland across the Greenland ice sheet. Analysis of satellite data indicates that the thicker, more slowly flowing, inland ice tends to have many fewer crevasses (surface cracks). Without an initial crack for water to fill, hydro-fracture cannot occur. The lack of cracks on inland ice means that lakes can fill without reaching a crack, causing them to overflow into surface streams.  These streams then  transport the water over distances of 10s of miles before intersecting existing hydro-fracture sites at lower elevations where the melt water eventually drains to the bed.  As a result, on short time-scales warmer temperatures are not likely to lubricate much greater areas beneath the ice sheet as had been feared.

The overall finding of this and a prior related study is that lake-driven hydro-fracture and the resulting lubrication of the bed are not likely to destabilize th...

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