What is an ice tongue?
Ice tongues are simply floating platforms of ice which are attached to the front of marine-terminating glaciers and extend into the sea. Ice tongues differ from ice shelves as they are confined by valley walls and have a narrow width relative to their length, hence their name (1). They typically range anywhere between 0.5-69 km from glacier to the ocean in Greenland today (2).
Unlike the Antarctic ice sheet where the coastline consists of ~75% floating ice shelves, Greenland’s floating ice margins account for a much smaller proportion of the coastline and are restricted generally to the Northern fjords above ~77°N latitude (3). There are some very small ice tongues (most of which only exist in winter (e.g., 4)) in Southern Greenland, but they are confined to the largest Southern glaciers (5).
Petermann Glacier Ice Tongue
One famous ice tongue is in front of Petermann Glacier, in northern Greenland. Petermann Glacier flows northwards into the Arctic Ocean. This tidewater glacier has an ice tongue that is around 15 km wide. You can explore the ice tongue in the Google Map below. You can see that the flat, floating ice tongue is confined within the fjord.
Where does ice come from? Where does ice go?
Floating ice tongues gain ice from freezing sea water at their bases, snow falling directly on top, and mostly from ice flowing in from the glacier they are attached to as it flows into the sea.
Over 80% of the mass of large ice tongue mass is lost through basal melting from relatively warm sea water penetrating underneath the ice tongue. Most of the rest of the mass is lost from the breaking off (or calving) of icebergs (3; 7).
Finally, ice tongues can also melt on the surface due to rising air temperatures. This can actually be seen with the naked eye in the summer, when large cracks form, which fill with water. Some are so large they can even be seen in satellite images! (8) See the below image from the Petermann ice tongue.
When icebergs do break away from ice tongues, they are particularly large. They are often several kilometres wide (Figure 4) (8). The figure below shows a large iceberg calving from Petermann Glacier’s ice tongue in 2012.
The calving of such large, wide icebergs can be very hazardous for ships and trans-arctic transport. There is no greater example of this than the sinking of the RMS Titanic on 14th April 1912. The iceberg which the Titanic struck (shown in the photograph below) is believed to have come from a Greenland fjord (9), most likely from Jakobshavn isbrae (10) whose floating ice tongue was thinning at this time due to increasing atmospheric and sea surface temperatures (11; 12). We can’t be certain of this, but an iceberg of this size (>100m wide) is very likely to have come from a floating ice tongue and followed a trajectory from South-Western Greenland fjord (9).
Why are ice tongues important?
Glaciers which flow into the seas and oceans act very differently to glaciers on land and are often considered to be unstable (13; see marine ice sheet instability). A floating ice tongue can effectively hold back or ‘buttress’ a glacier in a stable position (14). The aftermath of large icebergs breaking off, or ‘calving’, can cause big increases in the speed of upstream ice. This effect is similar to popping the cork on a bottle of champagne: with the ice tongue acting like the cork, holding back the ice further in land (or in our example the champagne).
Two great examples of this effect were 1) in the image above at the Petermann glacier; 2) the glacier Jakobshavn Isbrae. The speeds of these glaciers doubled after the loss of their floating ice tongues (15; 16; 17). Therefore, although ice tongues do not directly contribute to sea level rise as they are already situated at sea level, the hangover of ice tongue collapse can result in a persistent loss of ice due to the increased ice flow.
Ice tongue collapse: a natural cycle or a symptom of a changing world?
Some debate remains among researchers as to whether ice tongue collapses in Greenland are related to global warming through rising ocean temperature (18; 19; 20) and rising air temperatures (8; 21), or whether it is part of a natural cycle of breakups and reformations of the ice shelf (17). Over the past century, the latitude at which stable ice tongues can persist has shifted further north, and collapses are becoming more frequent (21). This indicates that climate change is likely the biggest driving factor in determining ice tongue stability, with both atmospheric and ocean warming being contributing factors.
Projecting sea level rise? Not so fast! Ice tongues are very complicated!
The calving of large icebergs from floating ice tongues doesn’t directly contribute to sea level rise, because the ice tongues are already floating and displacing water. However, the loss of the ice tongues could act to destabilise the tributary glaciers, leading them to accelerate and contribute to sea level rise at a faster rate.
However, whether this will cause catastrophic collapse of glaciers like in Antarctica remains uncertain (22). This is because unlike Antarctica, Greenland glaciers are in enclosed fjords and water is not as deep which both help stabilise glaciers (see marine ice sheet instability). Collapses of the Petermann ice tongue in 2010 and 2012 produced contradicting and confusing results regarding the response of the glacier after the events. The 2010 event resulted in no difference in up glacier ice speed, while the 2012 event caused a doubling of glacier ice velocity (17).
Outlet glaciers that are near to each other have also responded to the same ice tongue collapse trigger in very different manners in the past. While studying the landforms and sediments on the seafloor in NW Greenland, researchers have observed contrasting speeds of glacier retreat inland. One lost a lot of ice very quickly (Petermann glacier) and one very slowly (Ryder glacier) despite the two glaciers being in very close proximity following a period of ice tongue collapses both originating from the warming of sea water temperature (23; 24).
The complicated story of how marine-terminating glaciers act is the single biggest reason for the large uncertainties in sea level rise projections (25; 26). Without improving our understanding of how the incredibly fragile ice tongues interact with the air, ocean and parent glaciers, it will remain very difficult to accurately predict the contribution of Greenland ice to sea level rise.
About the author
Neil McDonald is a PhD student at the University of Stirling where his research focuses on the complex relationships between marine ice sheet dynamics, ice shelves and climate. Specifically, he examines the signature of ice shelf/ice tongue presence and removal in the palaeo-record by analysing mud retrieved from seafloor sediment cores. Adopting a multiproxy approach (combining sedimentology, micropalaeontology, and geochemistry) his research aims to better understand ice shelf dynamics and their role in the deglaciation of the NW sector of the last British-Irish ice sheet.
Neil graduated with a BSc in Environmental Geography from the University of Stirling in 2018, where his undergraduate thesis examined past evidence of icebergs in the Minch basin (NW Scotland) during deglaciation. His interest in science communication encouraged him to follow a PGDE from the University of Strathclyde in 2018-19. Before returning to the University of Stirling to take up his PhD which expands on much of the work from his undergraduate honours project.
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