CR
Cryospheric Sciences

Submarine Melt

Image of the Week — Cavity leads to complexity

Aerial view of Thwaites Glacier [Credit: NASA/OIB/Jeremy Harbeck].

 

A 10km-long, 4-km-wide and 350m-high cavity has recently been discovered under one of the fastest-flowing glaciers in Antarctica using different airborne and satellite techniques (see this press release and this study). This enormous cavity previously contained 14 billion tons of ice and formed between 2011 and 2016. This indicates that the bottom of the big glaciers on Earth can melt faster than expected, with the potential to raise sea level more quickly than we thought. Let’s see in further details how the researchers made this discovery.


Thwaites Glacier

Thwaites Glacier is a wide and fast-flowing glacier flowing in West Antarctica. Over the last years, it has undergone major changes. Its grounding line (separation between grounded ice sheet and floating ice shelf) has retreated inland by 0.3 to 1.2 km per year in average since 2011. The glacier has also thinned by 3 to 7 m per year. Several studies suggest that this glacier is already engaged in an unstoppable retreat (e.g. this study), called ‘marine ice sheet instability’, with the potential to raise sea level by about 65 cm.

Identifying cavities

With the help of airborne and satellite measurement techniques, the researchers that carried out this study have discovered a 10km-long, 4km-wide and 350m-high cavity that formed between 2011 and 2016 more than 1 km below the ice surface. In Figure 2B, you can identify this cavity around km 20 along the T3-T4 profile between the green line (corresponding to the ice bottom in 2011) and the red line (ice bottom in 2016). According to the researchers, the geometry of the bed topography in this region allowed a significant amount of warm water from the ocean to come underneath the glacier and progressively melt its base. This lead to the creation of a huge cavity.

Fig. 2: A) Ice surface and bottom elevations in 2014 (blue) and 2016 (red) retrieved from airborne and satellite remote sensing along the T1-T2 profile identified in Fig. 2C. B) Ice surface and bottom elevations in 2011 (green) and 2016 (red) along the T3-T4 profile. C) Changes in ice surface elevation between 2011 and 2017. The ticks on the T1-T2 and T3-T4 profiles are marked every km [Credit: adapted with permission from Figure 3 of Milillo et al. (2019)].

What does it mean?

In order to make accurate projections of future sea-level rise coming from specific glaciers, such as Thwaites Glacier, ice-sheet models need to compute rates of basal melting in agreement with observations. This implies a correct representation of the bed topography and ice bottom underneath the glacier.

However, the current ice-sheet models usually suffer from a too low spatial resolution and use a fixed shape to represent cavities. Thus, these models probably underestimate the loss of ice coming from fast-flowing glaciers, such as Thwaites Glacier. By including the results coming from the observations of this study and further ongoing initiatives (such as the International Thwaites Glacier Collaboration), ice-sheet models would definitely improve and better capture the complexity of glaciers.

Further reading

Edited by Sophie Berger


David Docquier is a post-doctoral researcher at the Earth and Life Institute of Université catholique de Louvain (UCL) in Belgium. He works on the development of processed-based sea-ice metrics in order to improve the evaluation of global climate models (GCMs). His study is embedded within the EU Horizon 2020 PRIMAVERA project, which aims at developing a new generation of high-resolution GCMs to better represent the climate.

 

Image of The Week – Plumes of water melting Greenland’s tidewater glaciers

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Figure 1: Simulation of a plume at a tidewater glacier in a general circulation model (MITgcm). Left – water temperature and right – time-averaged submarine melt rate in metres per day. Shown are face-on views of a tidewater glacier, as if you were under the water in front of the glacier, looking towards the calving front. 250 m3/s of fresh water emerges into the ocean from a channel at the bottom of the glacier, forming a plume. As the plume rises towards the fjord surface it mixes turbulently with warm ocean water, causing the plume to warm with height. Further details of this simulation can be found here: Slater et al. 2015.

Loss of ice from The Greenland Ice Sheet currently contributes approximately 1 mm/year to global sea level (Enderlin et al., 2014). The most rapidly changing and fastest flowing parts of the ice sheet are tidewater glaciers, which transport ice from the interior of the ice sheet directly into the ocean. In order to better predict how Greenland will contribute to future sea level we need to know more about what happens in these regions.


Tidewater glaciers meet the ocean at the calving front (Fig. 2), where ice undergoes melting by the ocean (“submarine melting”) and icebergs calve off into the sea. In recent decades, tidewater glaciers around Greenland have retreated (due to increased loss of ice at the calving front) and started flowing faster. This in turn causes more ice to be released into the ocean, contributing to sea level. Understanding the cause of these changes at tidewater glaciers is an ongoing topic of research.

Figure 2: Kangiata Nunata Sermia, a large tidewater glacier in south-west Greenland. The expression of a plume originating at the base of the calving front is visible on the fjord surface as turbid sediment-rich water. [Credit: Peter Nienow]

Figure 2: Kangiata Nunata Sermia, a large tidewater glacier in south-west Greenland. The expression of a plume originating at the base of the calving front is visible on the fjord surface as turbid sediment-rich water. [Credit: Peter Nienow]

One possible cause of change is an observed warming of the ocean around Greenland (Straneo and Heimbach, 2013). A warming of the ocean is likely to lead to increased submarine melt rates, which may in turn influence iceberg calving if, for example, melting results in instability of the ice at the calving front. Submarine melt rates are thought to be increased further by upwelling of warm water at the calving front (Fig. 1 and Fig. 2).

This upwelling water, called a plume, may be initiated by submarine melting of the ice, or by fresh glacial meltwater from the ice sheet surface. This fresh glacial meltwater penetrates to the base of the glacier and flows into the ocean from beneath the glacier, which may be hundreds of metres underwater. Once in the ocean, the meltwater rises buoyantly because of a density difference between the meltwater and ocean water, forming a plume. In order to better understand the effect of plumes on submarine melting, we can model plumes using a numerical model (e.g. MITgcm). Our image of the week (Fig. 1) shows such a model, which we can use to estimate submarine melt rates. In combination with simpler analytical approaches (Jenkins et al., 2011; Slater et al., 2016), we can estimate how submarine melt rates may change over time and from glacier to glacier (Carroll et al., 2016), and begin to assess the effect of submarine melting on tidewater glaciers and ultimately on future sea level rise.

Edited by Teresa Kyrke-Smith and Emma Smith


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Donald Slater is a PhD student in the Glaciology and Cryosphere Research Group at the University of Edinburgh. His research focusses on understanding the effect of the ocean on the Greenland Ice Sheet. For more information look up his website or follow him on twitter @donald_glacier.