The Greenland Ice Sheet is melting from all sides. Not surprisingly, as air temperatures above it continue to warm, scientists have observed a steadily increasing amount of surface melt each year. What is less known is that where the ice sheet meets the ocean—in valleys and fjords referred to as “marine-terminating glaciers” – the ice is being melted through contact with warm (i.e. greater than 0 degrees Celcius) ocean water. Recent results suggest that the total loss of mass (or ice) from the Greenland Ice Sheet has quadrupled when comparing the periods 1992-2001 with 2002-2011 (see Straneo and Heimbach, Nature 2013). The total mass loss from Greenland includes not only surface melt, but increased melting and glacier calving around the edges.
That warm ocean water would melt a vertical wall of ice isn’t too surprising, but recent work has suggested that it may have contributed to the acceleration of Greenland ice loss in a way that was previously un-thought of. As atmospheric temperature have continued to rise, some of the excess heat being absorbed by the planet has been taken up by the ocean, particularly in the North Atlantic which surrounds Greenland and its massive stores of land-ice. Where Greenland’s fjords are connected to the deep Atlantic Ocean, warmer waters have been able to find their way close to the ice.
When the edges of the ice sheet begin to melt “from below”, the melting can change the shape of the face of the glaciers, causing them to “undercut” (see Rignot, et al, Geophysical Research Letters 2015). This undercutting in turn leads to more calving (when large pieces of ice fall off the edge of the glacier), and the edge of the glacier can start to retreat towards the ice sheet rapidly. Such retreats were observed in many of Greenland’s largest glaciers in the late 1990’s, coinciding with the arrival of warmer ocean waters.
However, because the entire ice sheet has to drain through its glaciers (which don’t span the entire coastline but are concentrated in a few steep-sided valleys), the friction of trying to force huge amounts of flowing ice through a narrow channel has a “buttressing” effect, which stabilizes the ice sheet further inland. As the marine-terminating glaciers began to retreat due to increased calving, the buttressing effect was reduced, and a large-scale acceleration of ice flow was observed across the entire ice sheet.
A further complicating aspect of trying to puzzle out how the ice sheet and ocean interact is that when the surface of the ice sheet melts, the melted freshwater also drains into the ocean through the glacial valleys. This can have (at least) two negative effects:
1. Increased melt-water under the glacier may act to “lubricate” the flowing ice above, causing further acceleration, and
2. The freshwater discharged into the ocean enters at the bottom of the fjord, underneath the glacier, where it subsequently rises towards the ocean surface as a buoyant “plume”
It has become clear recently, that such “melt-water plumes” are a major driver of circulation inside the fjord, and turbulent mixing between the cold rising freshwater and deep warm ocean water acts to deliver heat to the face of the glacier at an even faster rate. Thus, one of the most challenging aspects of determining how much the ice sheet will melt in response to global warming (and therefore how much sea level rise we might expect), is that the interaction of the ice-sheet and the ocean constitutes a “feedback” effect, whereby the more the ice sheet melts from the surface, the more heat can be delivered to the edge of the glaciers, which destabilizes and further accelerates glacier flow.
1. Fiammetta Straneo and Patrick Heimbach, : North Atlantic warming and the retreat of Greenland’s outlet glaciers,” Nature 504 (December 2013):36
2. Eric Rignot, Ian Fenty, Yun Xu, Cilan Cai, and Chris Kemp, “Undercutting of marine-terminating glaciers in West Greenland”, Geophysical Research Letters (2015)
Clark Richards, PhD
RBR Ltd, Ottawa ON, Canada