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Cryospheric Sciences

ice-ocean interactions

Did you know… about the fluctuating past of north-east Greenland?

Did you know… about the fluctuating past of north-east Greenland?

Recent geological data shows that during a very cold phase of our Earth’s climate (between 40,000 and 26,000 years ago), there was a huge expansion of polar ice sheets, yet the north-eastern part of the Greenland ice sheet was less extensive than today. How could this have occurred? In this post we shed light on the potential causes of this ice sheet behaviour.


What do we know about present-day north-east Greenland?

The North East Greenland Ice Stream is one of the most interesting icy features of the present-day Greenland Ice Sheet. Extending for more than 600 km from the ice-sheet interior to the ocean, this ice stream drains almost 12% of the whole ice sheet and terminates through a system of fast-flowing outlet glaciers, known as Nioghalvfjerdsfjorden glacier (also called 79N), Zachariae Isstrøm and Storstrømmen (Fig. 1).

These three outlet glaciers belong to the disappearing category of marine-terminating glaciers, i.e. glaciers flowing into the ocean that possess an ice tongue (or small ice shelf) floating on the seawater. These tongues are connected to the main trunk of the glacier that is grounded over the continent, and the zone where the glacier starts to float is called the grounding line. The grounding line can move forward (or backward) if the glacier gains (loses) sufficient mass and can then expand (or retreat).

But how can a glacier gain mass? This is possible if the glacier mass balance is positive, i.e. if the glacier accumulates more ice (through snow precipitation) than is melted away (through surface and basal melting). Therefore, if surface air or oceanic temperatures increase, the glacier will likely lose mass, reducing its floating tongue and potentially making the grounding line retreat.

Recently, the 79N and Zachariae Isstrøm glaciers have lost a huge amount of ice mass via this mechanism due to rising temperatures. This suggests that this part of the ice sheet is very sensitive to climate change. Under a warmer climate, these glaciers could even lose their whole floating tongue, potentially causing irreversible consequences to the stability of the region.

How did north-east Greenland behave in the ancient past?

On millennial timescales, the north-east Greenland has had a very animated past too. Understanding the history of polar ice sheets requires a lot of effort both collecting and analysing paleoclimatic data (i.e. data archives providing information on the past evolution and climate of the ice sheet, e.g. from ice cores, marine sediments, …), which are usually very sparse, and running numerical simulations. Thanks to both disciplines, we know today that during the last glacial period (when Homo Sapiens were still in the Stone Age), the north-eastern part of the ice sheet expanded by hundreds of kilometers away from the coast into the ocean. During its maximum phase (in the Last Glacial Maximum, 21,000 years ago), this north-eastern part very likely reached the continental shelf break, the edge of shallow waters (see Fig. 1). However, very little information is available on the evolution of north-east Greenland before the Last Glacial Maximum.

One of the most interesting contribution to this gap in knowledge came less than two years ago from a group of researchers headed by Dr. Nicolaj Larsen, who ran an expedition at 79°N for new evidence. They focused their research on marine material found on moraines (eroded material deposited by the glacier). There, they collected and dated fragments of marine shells and were able to pinpoint when the moraines were formed, indicating the timing of glacier retreat. Their new data suggested that these moraines were a few thousand years older than the Last Glacial Maximum, meaning that the edge of Zachariae Isstrøm was located at least 20 km upstream of its current position at that date (Fig. 2, right).

Fig. 2: Left: Sample site at Zachariae Isstrøm [Credit: Supplementary Material of Larsen et al., 2018]. Right: Reconstruction of the North East Greenland Ice Stream grounding line distance from its present-day position for the last 45,000 years in the Marine Isotope Stage 3 (MIS-3) and Last Glacial Maximum (LGM) [Credit: I. Tabone, data from Larsen et al., 2018].

How could north-east Greenland be less extensive than today during a cold climate?

How could it be possible that these glaciers were less extensive than today, despite the cold, dry climatic conditions during the last glacial period? Well, it is a complex story! We are actually talking about a glacial time interval called Marine Isotope Stage 3 (~ 60,000-25,000 years ago), a period studded with several rapid abrupt climate events in which polar latitudes received larger incoming summer radiation than during the rest of the glacial period.

During Marine Isotope Stage 3, summer air temperatures were about 6 to 8ºC higher than during the Last Glacial Maximum (Fig 3, middle panel). This is likely due to long-term variations in incoming solar radiation associated with changes in the Earth’s orbit. Marine Isotope Stage 3 was also very dry, with low accumulation (snowfall) rates in northern Greenland (Fig 3, low panel). These warm temperatures and low accumulation could be a possible explanation for the expansion of the north-east Greenland ice sheet during Marine Isotope Stage 3 (Larsen et al., 2018). However, summer air temperatures at high latitudes were still well below 0°C during this time, so it is unlikely that they could have strongly affected ice mass loss in the region. The conclusion is: the atmosphere alone probably could not be the only climatic driver of this strong ice fluctuation at 79°N.

Fig. 3: Grounding line (GL) distance from the present-day (PD) position (a); summer temperature at 79N (b); accumulation rate at North Greenland Eemian Ice Drilling (NEEM) ice-core site [Credit: I. Tabone, data from Rassmussen et al., 2013, figure inspired from Larsen et al., 2018].

So it’s not the atmosphere… What about the ocean?

As the ocean has been recently argued to have an important role in the present-day retreat of the North East Greenland Ice Stream (see this study, this one and this one), it is reasonable to think about the ocean as a possible player in this large past fluctuation. Thanks to a 3D ice-sheet model, we performed simulations of north-east Greenland evolution in the past to assess how sensitive the ice-sheet margins were to variable oceanic conditions during the last glacial cycle. We represented these different oceanic states in the model by imposing melt at the grounding line and at the ice-shelf bases (also called submarine melt). Past submarine melt rates are inferred from past oceanic temperatures changes, because any increase in oceanic temperature results in increased submarine melting. Over long-term timescales, this means submarine melt rates were higher during Marine Isotope Stage 3 compared to during the Last Glacial Maximum.

Results of these model runs are pretty neat: with sufficiently high submarine melting along its margins, the North East Greenland Ice Stream retreats several tens of kilometres upstream from its maximum glacial position. Importantly, this suggests that increasing oceanic temperatures during Marine Isotope Stage 3 could have driven this large instability in the north-eastern ice-sheet margin during the last glacial. This is likely to have caused large reorganisations of the entire region and major ice discharge into the ocean (here is an animation showing the modelled north-east Greenland evolution during the last 45,000 years).

Fig. 4: Evolution of the north-east Greenland grounding-line (GL) distance from its present-day (PD) position simulated by our ice-sheet model. The model was run with no submarine melting (red line) and with progressively higher melting (other coloured lines). The dashed black line shows the reconstruction by Larsen et al. (2018). The three horizontal black dotted lines show the today’s NEGIS grounding-line position (0 km) and the maximum (300 km ± 50 km) reconstructed advance of the north-east Greenland during the Last Glacial Maximum according to Funder et al. (2011) [Credit: I. Tabone].

More investigation is needed!

Today we are aware that the north-east Greenland ice sheet is one of the most vulnerable regions of the ice sheet to current climate change. Figuring out its past evolution will help to understand its behaviour in a warming world, and its importance in the future stability of the entire Greenland ice sheet. However, as our study is the first attempt to look at the causes of this anomalous retreat from a modelling point of view, further work is needed and many questions are still unanswered. Was the ocean the major player in this past fluctuation? To what extent were surface air conditions also a factor? How much abrupt atmospheric warming events have influenced this margin fluctuation at smaller timescales? Further modelling work and observations at the North East Greenland Ice Stream are needed to unravel this icy riddle…

Further reading

Edited by Jenny Arthur and Clara Burgard


Ilaria Tabone is a Postdoc Researcher at the University Complutense of Madrid (Spain) in the Paleoclimatic Analysis and Modelling (PalMA) research group. She investigates the evolution of the Greenland Ice Sheet in the past glacial-interglacial cycles by working with ice-sheet models of continental scale. Her research focuses on ice-ocean and ice-atmosphere interactions. Contact Email: itabone@ucm.es

Image of the Week – How geometry limits thinning in the interior of the Greenland Ice Sheet

Image of the Week –  How geometry limits thinning in the interior of the Greenland Ice Sheet

The Greenland ice sheet flows from the interior out to the margins, forming fast flowing, channelized rivers of ice that end in fjords along the coast. Glaciologists call these “outlet glaciers” and a large portion of the mass loss from the Greenland ice sheet is occurring because of changes to these glaciers. The end of the glacier that sits in the fjord is exposed to warm ocean water that can melt away at its face (a.k.a. its “terminus”) and force the glacier to retreat. As the glaciers retreat, they thin and this thinning can spread into the interior of the ice sheet along the glacier’s flow, causing glaciers to lose ice mass to the ocean as is shown in our Image of the Week. But how far inland can this thinning go?

Not all glaciers behave alike

NASA’s GRACE mission measures mass changes of the Earth and has been used to measure ice mass loss from the Greenland ice sheet (see Fig. 1a). The GRACE mission has been extremely valuable in showing us where the largest changes are occurring: around the edge of the ice sheet. To get a closer look, my colleagues and I use a technique called photogrammetry.

Using high-resolution satellite photos, we created digital elevation models of the present-day outlet glacier surfaces. The imagery was collected by the WorldView satellites and has a resolution of 50 cm per pixel! When we compared our present-day glacier surfaces with surfaces from 1985, with the help of an aerial photo survey of the ice sheet margin (Korsgaard et al., 2016), we found that glacier thinning was not very uniform in the West Greenland region (see our Image of the Week, Fig. 1b). Some glaciers thinned by over 150 meters at their termini but others remained stable and may have even thickened slightly! Another observation is that, of the glaciers that have thinned, some have thinned only 10 kilometers into the interior while others have thinned hundreds of kilometers inland (Felikson et al., 2017).

But atmospheric and ocean temperatures are changing on much larger scales – they can’t be the cause of these huge differences in thinning that we observe between glaciers. So what could be the cause of the differences in glacier behaviour? My colleagues and I used kinematic wave theory to help explain how each glacier’s unique shape (thickness and steepness) can control how far inland thinning can spread…

A kinematic wave of thinning

As a glacier’s terminus retreats, it thins and this thinning can spread upglacier, into the interior of the ice sheet, along the glacier’s flow. This spreading of thinning can be modeled as a diffusive kinematic wave (Nye, 1960). This means that the wave of thinning will diffuse in the upglacier direction while the flow of ice advects the thinning in the downglacier direction. An analogy for this process is the spreading of dye in a flowing stream. The dye will spread away from the source (diffusion) and it will also be transported downstream (advection) with the flow of water.

The relative rates of diffusion and advection are given by a non-dimensional value called the Peclet number. For glacier flow, the Peclet number is a function of the thickness of the ice and the surface slope of the ice. Where the ice is thick and flat, the Peclet number is low, and thinning will diffuse upglacier faster than it advects downglacier. Where the ice is thin and steep, the Peclet number is high, and thinning will advect downglacier faster than in diffuses upglacier.

Let’s take a look at an example, the Kangilerngata Sermia in West Greenland

Figure 2: Thinning along the centreline of Kangilerngata Sermia in West Greenland. (a) Glacier surface profile in 1985 (blue), present-day (red), and bed (black). (b) Dynamic thinning from 1985 to present along the profile with percent unit volume loss along this profile shown as colored line. (c) Peclet number along this profile calculated from the geometry in 1985 with Peclet number running maxima highlighted (red). [Credit: Denis Felikson]

There, dynamic thinning has spread from the terminus along the lowest 33 kilometers (see Fig. 2). At that location, the glacier flows over a bump in the bed, causing the ice to be thin and steep. The Peclet number is “high” in this location, meaning that any thinning here will advect downglacier faster than it can spread upglacier. Two important values are needed to further understand the relationship between volume loss and Peclet number. On the one hand, we compute the “percent unit volume loss”, which is the cumulative thinning from the terminus to each location normalized by the total cumulative thinning, to identify where most of the volume loss is taking place. On the other hand, we identify the “Peclet number running maxima” at the locations where the Peclet number is larger than all downglacier values. These locations are critical because if thinning has spread upglacier beyond a local maximum in the Peclet number, and accessed lower Peclet values, then thinning will continue to spread until it reaches a Peclet number that is “large enough” to prevent further spreading. But just how large does the Peclet number need to be to prevent thinning from spreading further upglacier?

Figure 3: (a) Percent unit volume loss against Peclet number running maximum for 12 thinning glaciers in West Greenland. (b) Distances from the termini along glacier flow where the Peclet number first crosses 3. Abbreviations represent glacier names [Credit: Denis Felikson]

If we now look at the percent unit volume loss versus Peclet number running maxima for not only one but twelve thinning glaciers in the region, we see a clear pattern: as the Peclet number increases, more of the volume loss is occurring downglacier (see Fig. 3). By calculating the medians of the glacier values, we find that 94% of unit volume loss has occurred downglacier of where the Peclet number first crosses three. All glaciers follow this pattern but, because of differences in glacier geometry, this threshold may be crossed very close to the glacier terminus or very far inland. This helps explaining the differences in glacier thinning that we’ve observed along the coast of West Greenland. Also, it shows that the Peclet number can be a useful tool in predicting changes for glaciers that have not yet retreated and thinned.

Further reading