Moulin
Climate Change Daily Geosphere Repost

Moulins, meltwater, and a disappearing ice sheet

Greenland’s water disappears into ice caves

When standing on the ground, you may not be able to see a gaping hole punched through Greenland’s ice sheet. But from the air, on a clear day, the hole would be visible along with thousands of others just like it, all across the icy continent. The hole is a vertical ice cave called a moulin.

Moulins can be over ten meters wide and extend hundreds of meters below the ice sheet’s surface, often reaching the underlying bedrock. These ice-caves are part of a large hydrologic system moving meltwater from the ice sheet to the ocean. But they can also destabilize individual glaciers that make up the ice sheet. 

Recent work by two geologists has demonstrated that Greenland’s moulins can be larger than initially expected. Matt Covington from the University of Arkansas and Jason Gulley from the University of South Florida descended into several of these vertical ice caves to observe and measure their internal dimensions. Caves with surface openings of under ten meters can widen to many times that diameter below ground, sometimes creating an underground network connecting multiple moulins.

As global temperatures rise, Greenland loses more ice to both meltwater and the calving of coastal glaciers into the ocean. In some cases, these two processes are inter-related. The faster a glacier moves, the more ice it delivers to the sea. One of the factors controlling glacial flow rates is the frictional resistance between the glacier and underlying bedrock. Less friction allows the glacier to slide more rapidly.

The Moulins enter the equation at this point because they act as drains for surface meltwater. During hot spells, the ice melts, and meltwater flows into moulins and then ultimately to the base of the ice sheet. Water is a great lubricator, so more water flowing along the base of a glacier translates to less friction and increased flow rates. 

Eventually, water from multiple moulins can combine to form underground rivers carrying meltwater along the rock-ice interface, all the way to the ocean. Through a combination of meltwater and calving ice, Greenland has lost over four trillion tons of ice since 1992, with meltwater accounting for about half those losses. As Earth warms, the ice loss will increase, and understanding the role of moulins and meltwater in this process is a critical part of the story.

Sources

Scientists descended into Greenland’s perilous ice caverns — and came back with a worrying message (By Chris Mooney; The Washington Post) – https://www.washingtonpost.com/climate-environment/2020/12/23/climate-moulins-greenland/

Feature Image: Moulin (Modified by ArcheanWeb) – Original Credit: By Halorache – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23270860

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.