A note on ‘Collapse’

There is a lot in the media at the moment about the ‘collapse’ of the West Antarctic Ice Sheet. See my previous blog post for more information. But when we talk about ‘ice sheet collapse’, what do we actually mean? When we talk of people ‘collapsing’, they fall down right in front of us in the street. Buildings collapse. Bridges collapse. It’s a very bad thing. Right? Continue reading

Is the West Antarctic Ice Sheet collapsing?

Marine ice sheet instability

The West Antarctic Ice Sheet (WAIS) is the world’s most vulnerable ice sheet. This is because it is grounded below sea level, and marine ice sheets such as these are susceptible to rapid melting at their base. Fast-flowing ice streams draining the WAIS (Pine Island Glacier and Thwaites Glacier in particular) into the Amundsen Sea have a grounding line on a reverse bed slope, becoming deeper inland. Recession of the grounding line means that the ice stream is grounded in deeper water, with a greater ice thickness. Stable grounding lines cannot be established on these reverse bed slopes1, because ice thickness is a key factor in controlling ice flux across the grounding line. Thicker ice in deeper water drives increased calving, increased ice discharge, and further grounding-line recession in a positive feedback loop2, 3.  This process is called Marine Ice Sheet Instability4.

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Dealing with uncertainty: predicting future sea level rise

How much sea level rise? | Climate change and rising sea levels | The West Antarctic Ice Sheet | How much sea level rise from Antarctica? | Comments |

How much sea level rise?

A 5 m sea level rise would inundate many coastal cities in Europe. Source: CReSIS

A 5 m sea level rise would inundate many coastal cities in Europe. Source: CReSIS

How much will global sea levels rise in our lifetime, or in the lifetime of our children? We need to know the answer to this question if we are to mitigate effectively against sea level rise, particularly when it’s associated with storm surges, hurricanes and extreme weather events, which test our already strained flood defence schemes. However, uncertainty in the response of polar ice sheets to climate change limits our ability to project sea level rise into the future.

Climate change and rising sea levels

Figure 5. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013

Figure 5. Climate change over the last 11,500 years from multiple proxies. From Marcott et al., 2013. Used with permission from the author.

During the Twentieth Century, the Earth warmed by 0.6 ± 0.2°C. Since 1900 AD, a long-term cooling trend that began around 5000 years ago and culminated in the Little Ice Age in the 1750s (with its ice fairs on the frozen River Thames) has been reversed. Global sea level is now rising at a rate of 3.1 mm per year, which will lead to a total rise of 18-59 cm by 2100 AD. Most of this rise is caused by thermal expansion of the ocean and the melting of small ice caps and glaciers. However, the large polar ice sheets have the potential to contribute to sea level rise above and beyond this modest rate. The West Antarctic Ice Sheet alone could raise global sea levels by 3.3 m if it all melted. But how likely is this to happen, and how quickly?

The West Antarctic Ice Sheet

An unstable marine ice sheet

BEDMAP 2, showing that the bedrock on which the West Antarctic Ice Sheet rests is well below sea level.

BEDMAP 2, showing that the bedrock on which the West Antarctic Ice Sheet rests is well below sea level.

The West Antarctic Ice Sheet is currently warming particularly rapidly, and this warming is associated with increased ocean temperatures and changes to atmospheric circulation, which drives increased upwelling of deep, relatively warm oceanic water onto the continental shelf.

The West Antarctic Ice Sheet is drained by fast-flowing, marine-terminating ice streams and it is surrounded by floating ice shelves. Much of the rock on which the ice sheet rests is below current sea level, and the bedrock slopes downwards towards the centre of the ice sheet. Because of this, the ice sheet is unstable, because as water gets deeper, more icebergs are calved, increasing ice discharge.

Ice streams in West Antarctica are also melted rapidly at their base by those warming ocean waters, leading to melting, recession into deeper water and more melting again. The West Antarctic Ice Sheet may therefore be inherently susceptible to ever faster glacier recession, and could pass tipping points that mean rapid sea level rise irrevocably occurs. Pine Island Glacier, one of the fastest ice streams in the world, is already thinning and receding, making it susceptible to rapid recession in ever deeper water.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

Ice streams of Antarctica with Pine Island Glacier and Thwaites glacier highlighted.

Thinning and retreating ice shelves

Warm ocean waters are melting a cavity beneath Pine Island Glacier

Warm ocean waters are melting a cavity beneath Pine Island Glacier

Ice shelves around the West Antarctic Ice Sheet are thinning as they are melted from below by upwelling warm ocean currents. Ice shelves have been known to disintegrate rapidly over the course of just one summer.

Ice shelves ‘buttress’ or hold back glaciers on the interior of the continent. Rapid removal of bounding ice shelves, such as those around Pine Island Glacier, could therefore result in increased thinning and recession of grounded glaciers, initiating a positive-feedback loop that could be catastrophic.

Increased snowfall

It doesn’t end there. Although there may be more snow over the Antarctic Ice Sheet under a warmer climate, this too could lead to changes in glacier dynamics. Increased snow will steepen surface gradients near the edge of the Antarctic Ice Sheet. Glaciers will flow faster, discharging more icebergs into the ocean, negating any impact the increased snowfall would have in mitigating sea level rise.

Increased meltwater from melting ice shelves also produces a layer of cold, fresh water on the ocean’s surface, which easily freezes, increasing winter sea ice extent. Sea surface temperatures are directly related to snowfall, so cooler sea surface temperatures and more sea ice may actually decrease snowfall over Antarctica.

How much sea level rise from Antarctica?

Sea level rise to 2100. Modified from the IPCC sea level rise estimates (from Wikimedia Commons) and using estimates from Bamber and Aspinall 2013, assuming a uniform rate of sea level rise.

Sea level rise to 2100. Modified from the IPCC sea level rise estimates (from Wikimedia Commons) and using estimates from Bamber and Aspinall 2013, assuming a uniform rate of sea level rise.

Because of these factors, the West Antarctic Ice Sheet could rapidly and catastrophically melt, resulting in as much as 3.3 m of sea level rise within 500 years.

Rates such as these are common in the geological record, but these dynamic behaviours are too difficult for even our most complex computer models to solve.

A new paper in the journal Nature Climate Change by Bamber and Aspinall has attempted to untangle this thorny problem. They pooled different assessments by numerous experts in order to reach a consensus on likely sea level rise by AD 2100.

Bamber and Aspinall used a mid-range carbon emissions scenario, with an increase of 3.5°C above pre-industrial temperatures. They found that the average rate of sea level rise from just the Greenland and Antarctic ice sheets agreed upon by these experts was 5.4 mm per year by 2100 AD.

Combined with melting glaciers and ice caps and thermal expansion of the ocean, Bamber and Aspinall gave a range of 33-132 cm, with 62 cm the average estimate, for sea level rise by 2100. It’s still uncertain, but it’s the best estimate we have for now.

Marine ice sheet instability

Introduction | Past evidence of ice sheet collapse | Hypothesis of marine ice sheet instability | References | Comments |

Introduction

Images of the Amundsen Sea Embayment, showing: Landsat image (LIMA); BEDMAP bed elevation (from Lythe et al., 2001); and ice velocity (from Rignot et al. 2011)

In 1978, Mercer was one of the first to identify that rising temperatures could have catastrophic consequences in West Antarctica, triggering a collapse of the West Antarctic Ice Sheet[1]. This is because much of the West Antarctic Ice Sheet lies below sea level[2], making it a Marine Ice Sheet. West Antarctica is currently the world’s largest marine ice sheet, although they may have been common during the Last Glaciation, circa 18,000 years ago. Portions of the Greenland Ice Sheet and East Antarctic Ice Sheet are also marine, but have shallower bathymetries than West Antarctica. The ice sheet is currently stable due to its buttressing ice shelves and local regions where the bathymetry opposes the general trend[3].

The figure panel opposite shows the Pine Island Glacier and Twaites ice streams, which are grounded well below sea level and drain a large proportion of West Antarctica. Their accumulation areas flow from the Transantarctic Mountains and out into the Amundsen Sea. The map below, from the BEDMAP2 database, shows ice sheet thicknesses and a cross section across the entire Antarctic continent. Here, you can clearly see the difference between the West and East Antarctic ice sheets. They are separated by the 2000 m high Transantarctic Mountains. The East Antarctic Ice Sheet is grounded largely above sea level, whereas the West Antarctic Ice Sheet is mostly grounded well below sea level.

The BEDMAP 2 dataset shows how ice thickness across the Antarctic continent is variable, with thin ice over the mountains and thick ice over East Antarctica. The cross section shows how the West Antarctic Ice Sheet is grounded below sea level.

The BEDMAP 2 dataset (Fretwell et al. 2013) shows how ice thickness across the Antarctic continent is variable, with thin ice over the mountains and thick ice over East Antarctica. The cross section shows how the West Antarctic Ice Sheet is grounded below sea level.

The figures below show how, firstly, the West Antarctic Ice Sheet is grounded below sea level, and that both the West and East Antarctic Ice sheet have water (lakes and channels) at their base; secondly, bedrock topography of Antarctica; thirdly, ice streams of Antarctica, and fourthly, what the Antarctic continent would look like if all the ice were to be removed. Note how West Antarctica becomes a series of islands.

Past evidence of ice sheet collapse

Profile through the Antarctic ice sheet (A) Bellingshausen Sea – West Antarctic ice sheet – Ross ice shelf – Ross Sea (B). The profile shows that most of the West Antarctic ice sheet is grounded below sea level which makes it sensitive to sea level rise. If the contact of the ice to the bottom rocks is lost seaward of the grounding line, the ice sheet becomes significantly thinner (some 100 m), forming a shelf ice.
By Hannes Grobe 21:51, 12 August 2006 (UTC), Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany (Own work) [CC-BY-SA-2.5 (www.creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons.

There is some evidence to suggest that, in previous interglacials, the West Antarctic Ice Sheet completely disappeared, leading to sea levels about 5m higher than at present[1].  For example, marine micro-organisms have been found in in glacial sediments at the base of ice cores beneath Ice-Stream B[4]. This occurred during a period of anomalous warmth during MIS 5e in East Antarctica. Evidence from bryozoan and other marine micro-organisms indicates open seaways across West Antarctica at various periods during the last few million years, and even during the past one or more interglacials[3].

 

Hypothesis of marine ice sheet instability

Much of West Antarctica drains through the Pine Island Glacier and Thwaites ice streams into Pine Island Bay. These ice shelves are warmed from below by Circumpolar Deep Water[5], which has resulted in system imbalances, more intense melting, glacier acceleration and drainage basin drawdown[6-8]. This is the “Weak Underbelly” of the West Antarctic Ice Sheet[9], which may be prone to collapse. Pine Island Glacier is currently thinning[10], and, combined with rapid basal melting of the Amundsen Sea ice shelves[11], means that there is concern for the future viability of its fringing ice shelves.

Marine Ice Sheet instability hypothesis flow chart

The Marine Ice Sheet Instability hypothesis is that atmospheric and oceanic warming could result in increased melting and recession at the grounding line on a reverse slope gradient[12]. This would result in the glacier becoming grounded in deeper water and a greater ice thickness. This is because the grounding line in this region has a reverse-bed gradient, becoming deeper inland.  Stable grounding lines cannot be located on upward-sloping portions of seafloor[13]. Ice thickness at the grounding line is a key factor in controlling flux across the grounding line[3], so thicker ice grounded in deeper water would result in floatation, basal melting, increased iceberg production, and further retreat within a positive feedback loop. This would result in a rapid melting of the West Antarctic Ice Sheet, triggering rapid sea level rise.

 

Simplified cartoon of a tributary glacier feeding into an ice shelf, showing the grounding line (where the glacier begins to float).

This could be exacerbated by the removal of fringing ice shelves around the Amundsen Sea sector of the West Antarctic Ice Sheet. Removal of buttressing ice shelves around ice streams tends to result in glacier acceleration, thinning, and grounding line migration[14, 15].

This is a low-probability, high-magnitude event, with a 5% probability of the West Antarctic Ice Sheet contributing 10 mm sea level rise per year within 200 years[16]. The most recent numerical models predict a sea level rise of 3.3 m if this event was to occur[12].

This hypothesis has recently featured prominently in the science news, for example, on the Discovery News.

Further reading

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References


1.            Mercer, J.H., 1978. West Antarctic Ice Sheet and CO2 Greenhouse effect – threat of disaster. Nature, 1978. 271(5643): p. 321-325.

2.            Lythe, M.B., Vaughan, D.G., and the BEDMAP Consortium. 2001. BEDMAP: a new ice thickness and subglacial topographical model of Antarctica. Journal of Geophysical Research, 2001. 106(B6): p. 11335-11351.

3.            Joughin, I. and Alley, R.B., 2011. Stability of the West Antarctic ice sheet in a warming world. Nature Geosci, 2011. 4(8): p. 506-513.

4.            Scherer, R.P., Aldahan, A., Tulaczyk, S., Possnert, G., Engelhardt, H., and Kamb, B., 1998. Pleistocene Collapse of the West Antarctic Ice Sheet. Science, 1998. 281(5373): p. 82-85.

5.            Jacobs, S.S., Jenkins, A., Giulivi, C.F., and Dutrieux, P., 2011. Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geoscience, 2011. 4(8): p. 519-523.

6.            Shepherd, A., Wingham, D., and Rignot, E., 2004. Warm ocean is eroding West Antarctic Ice Sheet. Geophysical Research Letters, 2004. 31(23): p. L23402.

7.            Shepherd, A., Wingham, D.J., Mansley, J.A.D., and Corr, H.F.J., 2001. Inland thinning of Pine Island Glacier, West Antarctica. Science, 2001. 291: p. 862-864.

8.            Wingham, D.J., Wallis, D.W., and Shepherd, A., 2009. Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006. Geophysical Research Letters, 2009. 35: p. L17501.

9.            Hughes, T.J., 1981. The weak underbelly of the West Antarctic Ice Sheet. Journal of Glaciology, 1981. 27: p. 518-525.

10.          Pritchard, H.D., Arthern, R.J., Vaughan, D.G., and Edwards, L.A., 2009. Extensive dynamic thinning on the margins of the Greenland and Antarctic ice sheets. Nature, 2009. 461(7266): p. 971-975.

11.          Pritchard, H.D., Ligtenberg, S.R.M., Fricker, H.A., Vaughan, D.G., van den Broeke, M.R., and Padman, L., 2012. Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature, 2012. 484(7395): p. 502-505.

12.          Bamber, J.L., Riva, R.E.M., Vermeersen, B.L.A., and Le Brocq, A.M., 2009. Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet. Science, 2009. 324(5929): p. 901-903.

13.          Schoof, C., 2007. Ice sheet grounding line dynamics: Steady states, stability, and hysteresis. Journal of Geophysical Research-Earth Surface, 2007. 112(F3).

14.          Scambos, T.A., Bohlander, J.A., Shuman, C.A., and Skvarca, P., 2004. Glacier acceleration and thinning after ice shelf collapse in the Larsen B embayment, Antarctica. Geophysical Research Letters, 2004. 31: p. L18402.

15.          De Angelis, H. and Skvarca, P., 2003. Glacier surge after ice shelf collapse. Science, 2003. 299: p. 1560-1562.

16.          Vaughan, D.G. and Spouge, J.R., 2002. Risk estimation of collapse of the West Antarctic Ice Sheet. Climatic Change, 2002. 52: p. 65-91.

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