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Geosciences Column: How fast are Greenland’s glaciers melting into the sea?

Geosciences Column: How fast are Greenland’s glaciers melting into the sea?

The Greenland ice sheet is undergoing rapid change, and nowhere more so than at its margins, where large outlet glaciers reach sea level. Because these glaciers are fed by very large reservoirs of ice, they don’t just flow to the coast, but can extend many kilometres out into the ocean. Here, the ice – being lighter than water – will float, but remain connected to the ice on the mainland. This phenomenon is called an ice shelf or, if it is confined to a relatively narrow fjord, an ice tongue. Ice shelves currently exist in Antarctica as well as in high Arctic Canada and Greenland.

Ice shelves already float on the ocean so that their melting does not affect sea level, but they are a crucial part of a glacier’s architecture. The mass of an ice shelf, as well as any contact points with fjord walls, mean that it acts as a buttress for the rest of the glacier, slowing down its flow speed and stabilising it. When ice shelves melt, therefore, this can lead to the whole glacier system behind them flowing faster and thus delivering more land-based ice to the ocean.

Ice shelves lose mass as icebergs calve off at their seaward end, and through melting on their surface – but, unlike glaciers on land, they are with the ocean below. This ice-ocean interface is an important source of melting for a number of glaciers in northern Greenland; instead of the large volume of icebergs produced by many glaciers further south, the large ice tongues reaching into the ocean mean that a lot of ice is instead lost through submarine melting.

This ice-ocean interface is an environment that was, until recently, very difficult to accurately observe and study, and accordingly there is relatively little data on the impact of submarine melting on ice shelves. But the changes that take place here, at the ice-ocean interface, can have important implications for the entire glacier system, as well as for the ice sheet as a whole.

Over the last 30 years, a number of Arctic ice shelves and ice tongues have dramatically shrunk or disappeared entirely. In the Canadian Arctic, the Ellesmere ice shelf broke up into a number of smaller shelves over the course of the 20th century, most of which are continuing to shrink. In Greenland, meanwhile, the dramatic retreat of the Jakobshavn Glacier’s ice tongue during the 2000s has been particularly well documented.

The largest remaining ice tongues in Greenland are now all located in the far north of the island. But even here, at nearly 80°N and beyond, ice tongues are changing rapidly. Warming air temperatures probably play a role in this development, but submarine melting is thought to be the key driver of these rapid changes.

Submarine melting of ice tongues thus appears to be an important variable in ice-sheet dynamics. A new study in the EGU’s open access journal The Cryosphere has now used satellite imagery to produce a detailed map of submarine melt under the three largest ice tongues in northern Greenland. They are the ones belonging to Petermann and Ryder Glaciers in far northwestern Greenland and 79N Glacier – named after the latitude of its location – in the northeast of the island. Each of these ice shelves extends dozens of kilometres from where the glacier stops resting on bedrock and begins to float (the so-called grounding line) and is up to several hundred metres thick.

The locations of Petermann (PG), Ryder (RG) and 79N Glaciers in northern Greenland. From Wilson et al. (2017).

Previous attempts to estimate submarine melt rates relied on an assumption of steady state: that the ice shelf is becoming neither thicker nor thinner. Given the recent changes in all these ice shelves and the glaciers above them, this is not a tenable assumption in this case. Petermann and Ryder Glaciers, in particular, have recently experienced large calving events that were probably related to unusual melt patterns under the waterline.

Lead author Nat Wilson, a PhD student at MIT and Woods Hole Oceanographic Institution, and his colleagues used satellite images spanning four years to create a number of digital elevation models of the Petermann, Ryder and 79N ice shelves. A digital elevation model, or DEM, is a three-dimensional representation of a surface created – in this case – from satellite-based elevation data. By comparing DEMs from different points in time to each other, the team could deduct changes in the height – and therefore volume – of the ice shelves. This method also allowed them to track visible features of the glaciers between images from different years, providing estimates of how fast the ice was flowing down into the ocean.

However, using digital elevation models in a marine setting is not always a straightforward matter. Tides can affect the elevation of ice shelves by a significant amount, especially as the distance from the grounding line increases, and their effect needed to be accounted for in the results. Similarly, the team had to account for the changes on the surface of the ice shelf, where snowfall and melting can affect its volume.

What Wilson and his colleagues were left with was a map of melt rates across the ice shelves. In some respects, the findings were unsurprising. Melt rates were greatest near the coast, where the ice shelves were thickest, because at these points they would be in contact with the ocean at depths of several hundred metres. At such depths, fjords around Greenland often contain warm, dense water that flows in from the continental shelf and contributes to rapid ice melt. As the ice shelves thin towards their outer edges, they are in contact with shallower, colder water that doesn’t melt the ice as quickly.

Submarine melt rates at Greenland’s largest ice tongues are shown in colour shading; the arrows show the direction of ice flow. PG – Petermann Glacier; RG -Ryder Glacier. From Wilson et al. (2017).

All three ice shelves lost between 40-60m per year to submarine melting at their thickest points, while this decreased to about 10m per year in thinner sections. This equates to billions of tonnes of ice melting in contact with the ocean. Each of the ice shelves lost at least five times as much ice to melting underwater than to melting on the surface. This highlights what an important contribution submarine melting makes to the mass balance of Greenland’s ice shelves, and that this remote environment is deserving of our interest and study.

The team found that at Ryder Glacier’s ice shelf, mass loss from melting (from both above and below) is not significantly greater than the amount of ice entering the ice shelf from land: the ice shelf appears to be relatively stable for the time being. The situation is similar at Petermann Glacier, although its ice shelf has been in rapid retreat and lost some 250 km in the decade leading up to 2010. With the extra submarine melting from that area, melting would likely have exceeded incoming ice! It remains to be seen whether Petermann Glacier and its ice shelf will stabilise in their new configuration.

Finally, at 79N Glacier, the results indicate the ice shelf is losing mass faster than it is replenished from upstream. The ice tongue loses some 1.3% of its mass to melting each year – and that’s before iceberg calving is included in the equation. This finding is consistent with satellite imagery that suggests that the ice shelf at 79N has been thinning in recent decades.

This new study shows that there is considerable variability in submarine melting of ice shelves, both in space and in time. 79N glacier’s ice shelf – the biggest one remaining in Greenland – exhibited the highest mass deficit in this study, suggesting that we may see major changes in this glacier in future. With this type of melt making up for the bulk of mass loss of northern Greenland’s ice shelves, its accurate prediction plays an important role in understanding how these huge glaciers – and the whole ice sheet itself – will change in coming years.

By Jon Fuhrmann, freelance science writer

References

Wilson, N., Straneo, F., and Heimbach, P.: Satellite-derived submarine melt rates and mass balance (2011–2015) for Greenland’s largest remaining ice tongues, The Cryosphere, 11, 2773-2782, https://doi.org/10.5194/tc-11-2773-2017, 2017.

Hodgson, D. A. First synchronous retreat of ice shelves marks a new phase of polar deglaciation. Proc. Natl. Acad. Sci. U. S. A. 108, 18859-18860, doi:10.1073/pnas.1116515108 (2011).

Münchow, A., L. Padman, P. Washam, and K.W. Nicholls. 2016. The ice shelf of Petermann Gletscher, North Greenland, and its connection to the Arctic and Atlantic OceansOceanography 29(4):84–95, https://doi.org/10.5670/oceanog.2016.101.

Reeh N. (2017) Greenland Ice Shelves and Ice Tongues. In: Copland L., Mueller D. (eds) Arctic Ice Shelves and Ice Islands. Springer Polar Sciences. Springer, Dordrecht. https://doi.org/10.1007/978-94-024-1101-0_4

Truffer, M., and R. J. Motyka, Where glaciers meet water: Subaqueous melt and its relevance to glaciers in various settings, Rev. Geophys., 54, 220– 239. doi:10.1002/2015RG000494,  (2016)

Imaggeo on Mondays: A dramatic avalanche from Annapurna South

Imaggeo on Mondays: A dramatic avalanche from Annapurna South

The Annapurna massif is located in an imposing 55 km long collection of peaks in the Himalayas, which behave as a single structural block. Composed of one peak (Annapurna I Main) in excess of 8000 m, a further thirteen peaks over 7000 m and sixteen more of over 6000 m, the massif forms a striking structure within the Himalayas. Annapurna South (pictured in today’s featured image), the 101st tallest peak in the world, towers 7219 m above sea level.

Glaciers in High Mountain Asia, a region that includes the Himalayas, contain the largest volume of ice outside the polar regions. The water trapped, as ice, in the glaciers of the Himalayas is an important source of drinking water, water for irrigation and water for hydropower generation throughout the region. As the Earth’s climate changes and negatively affects glaciers world-wide, scientists are working hard to understand what increased glacier melting means for the communities which depend on them.

Emily Hill is one such scientist. Her and a team of colleagues spent 2 weeks at Annapurna Base Camp in Nepal conducting measurements on the debris covered South Annapurna Glacier.

“We frequently heard avalanches but often they were over too quick to capture on camera. Fortunately, this was one of the largest and the camera was at the ready. These avalanches are an important source of mass for the glacier below,” reminisces Emily.

Glaciers accumulate ice throughout the winter months, as snow adds to the glacial column during the cold months. In addition, avalanches deliver additional snow throughout the year.

“I’m not too sure of the scale of the avalanche, it could probably have been a couple of 100 m across. The avalanche occurred early afternoon when the solar radiation was highest and increased melt is likely to have caused the failure,” describes Emily.

Avalanches in the region are not only an important source of mass accumulation for many of the glaciers, they also pose a hazard not only to climbers of these mountains but also further down along the tourist trail up to Annapurna Base Camp, where there is an avalanche risk section of the route.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: Tongue of a small giant

Imaggeo on Mondays: Tongue of a small giant

In a world where climate change causes many mountain glaciers to shrink away, bucking the ‘melting’ trend is not easy. In today’s post, Antonello Provenzale, a researcher in Italy, tells us of one glacier in the Alps which is doing just that.

Mountain glaciers are retreating worldwide, with the possible exception of the Karakoram area. For most glaciers, ablation (ice melt) during the warm season is stronger than the accumulation of new ice by snowfall. As a result, while glacier ice flows downhill, the accelerated melting at lower elevation forces the terminus of the glacier to retreat uphill, with a net loss of ice volume.

Such behavior is especially evident on the southern flank of the Alps, where many mountain glaciers have dramatically reduced their dimensions, often fragmenting into smaller, detached pieces.

An important exception is represented by the Miage glacier in Val Veny, Val d’Aosta, northwestern Italy, at the base of the Mount Blanc massif. This glacier is covered with a thick layer of debris, which protects the underlying ice from the direct heating by sunlight. The rocks which make up the debris are poor heat conductors and thus preserve the ice beneath them, making this glacier particularly stable.

This glacier is so stationary that vegetation and trees have grown on its margins and on the debris. Several ponds punctuate the surface of the glacier, as well as some areas on its sides. The Miage lake, for example, is directly in contact with the slowly flowing ice and it is sometimes run by large outburst waves generated by huge blocks of ice and rock falling into the lake water.

This picture was taken in September 2014, during a field excursion of the Italian Glaciological Committee. The image is a composition (stitch) of several images taken with a moderate wide angle lens on a rangefinder digital camera.

By Antonello Provenzale studies Geophysical Fluid Dynamics, Earth System processes and Geosphere-Biosphere interactions at the Institute of Geosciences and Earth Resources of the National Research Council of Italy.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

Imaggeo on Mondays: Why does a Norwegian glacier look blue?

Imaggeo on Mondays: Why does a Norwegian glacier look blue?

This picture shows the outlet glacier Engabreen running down from the plateau of Svartisen in Norway. Svartisen ice cap comprises two glacier systems of which the Vestre (western) Svartisen is Norway’s second largest glacier. Located right at the polar circle, Svartisen covers a total of 369 km² of the Nordland region. These coastal mountains accumulate a snowpack of 5-7 m depth through the winter season, which feeds the glaciers.

Actually, Svartisen means black ice. However, the ice of the glacier tongue of Engabreen, an outlet glacier of Svartisen ice cap, looks pretty blue in the flat light of a late afternoon in August.  The ice, which is mostly free of air bubbles, transmits the blue colour more than the rest of the visible spectrum of light. Thus, by having to travel a distance of approx. 3 m through the ice body, the blue light is particularly visible.

More than the colour, it is the hydrology and the ice flow of Engabreen which are studied with attention by the Norwegian Water Resources and Energy Directorate (NVE). A tunnel system was built partly underneath the glacier in the 1990’s to collect the waters from Svartisen for hydro power production., The Svartisen Subglacial Laboratory  is located at one end of the tunnel, providing a unique opportunity for direct access to the bed of a temperate glacier. The privileged location means that sub-glacial parameters can be obtained from experiments right at the bottom of a 200 m thick ice pack.

The waters of Holandsfjord have to be crossed to visit the beautiful area of the glacier lake Svartisvatnet and Engabreen. This can be either done by boat shuttle or, as we did, by kayak [ find a magazine article about Kay’s adventure here – in German -].

 By Kay Helfricht, researcher at the Institute for Interdisciplinary Mountain Research of the Austrian Academy of Sciences, Austria

Editor’s note: this text was revised on Tuesday 25th October 2016 following comments from Terje Solbakk.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.