A shaft in the glacier—known as a moulin—which transports meltwater from the surface to the base of the glacier. Credit: C F Dow

Despite a recent slowdown, Greenland’s ice sheet may see a faster flow in the near future.

Researchers from the U.K., Canada, Sweden and Norway believe that outlet glaciers slide over wet sediment, not hard rock, making the sediment weaker and more slippery as global temperatures rise and meltwater supply becomes more variable.

Professor Bernd Kulessa, a glaciologist from Swansea University, said the findings, based on seismic surveys, challenge the view that the recent slowdowns in ice flow would continue long-term.

“This discovery leaves us concerned because we have so far accepted the exact opposite—that Greenland's icy conveyor would slow down,” Kulessa said in a statement.

Previous research found that self-regulation—where glaciers slow down at the end of the summer because large networks of channels drain water away at the base, increasing frictional resistance to ice flow, is a possible mechanism for controlling the speed of the ice flow.

Another mechanism shows that changing patterns of water flow alternately weaken or strengthen the soft sediment that underlies glaciers, making it more or less slippery and enhancing or decreasing the speed of ice flow.

The researchers tested the sediment theory by conducting seismic surveys at a surface lake on the Greenland Ice Sheet, where the ice is about 1.2 kilometers thick.

They used a series of controlled explosions to send acoustic waves through the glacier, which then bounce back to a series of small microphones called geophones set up on the glacier.

The researchers then analyzed the data to delineate and characterize the physical properties of the sediment underneath the ice.

This confirmed that sediment plays a key role in controlling ice sheet flow, variability in meltwater supply controls sediment’s slipperiness and the weakening of subglacial sediment leads to accelerated ice flow.

The researchers now conclude that the future response of the ice sheet to climate warming will be extremely complex, and understanding how widespread sediments are and how their properties change with meltwater supply is essential.

“Greenland's margin has many outlet glaciers that act as fast conveyor belts of ice,” Kulessa said. “Thousands of surface lakes act as taps that deliver meltwater to the ice base, turning it into a slippery bathtub.

“Because it is difficult to see beneath them we surmised until recently that the glaciers slip over hard rock, but we realize now that they often slip over sloppy wet sediments instead,” he added. “In a warming Arctic climate more ice will melt and make the sediments even sloppier and more slippery, so that fast ice flow can occur long into the future.”