'Plumbing System' Slows Greenland Ice Sheet

The subglacial "plumbing system" beneath Greenland is slowing the ice sheet's movement toward the sea as the summer progresses, according to new research.

"Everyone wants to know what's happening under Greenland as it experiences more and more melt," study co-author Ginny Catania, a research scientist at the University of Texas at Austin's Institute for Geophysics, said in a statement. "This subglacial plumbing may or may not be critical for sea level rise in the next 100 years, but we don't really know until we fully understand it."

This underground plumbing system consists of meltwater that drains from the surface into moulins - naturally formed pipes that drain water through more than half a mile of ice to passageways between the bedrock and overlying ice. The higher the water level (or pressure) in the moulin, the faster the ice sheet slides. And in recent decades, the amount of summer surface melting has increased.

Interestingly, scientists found that while the extra meltwater does cause the ice sheet to pick up the pace, at the same time the ice gradually becomes less and less sensitive to melting over the course of the summer season. The research team suggests that this phenomenon can be explained by the subglacial plumbing system, and its ability to adapt to the increased meltwater.

To solve the mystery, lead author Lauren Andrews and her colleagues drilled 13 holes through ice up to 2,300 feet thick in the Paakitsoq region of western Greenland in the 2011 and 2012 summer months.

Measurements of water pressure, meltwater flow and ice velocity showed that water draining through moulins forms a network of subglacial channels that regulates daily changes in ice flow. Likewise, there are also large regions of the ice bed that are isolated from the channel network. So when the two converge over the course of the summer season, it causes a decrease in the ice sheet's sensitivity to melting, according to Andrews.

"Thus, the ice sheet may not move as much as the surface melting might indicate," she added.

The findings were published in the journal Nature.