What is the ice volume of Thwaites Glacier?

Thwaites Glacier in West Antarctica is currently the focus of a major scientific campaign. Why is Thwaites Glacier of so much interest, however? How much ice is there, and how much would sea levels rise if it all melted?

Thwaites Glacier is roughly the size of UK (176 x103 km2). The glacier terminus is nearly 120 km wide, and the bed of the glacier reaches to >1000 m below sea level. Pine Island Glacier and Thwaites Glacier together account for 3% of grounded ice-sheet area, but they receive 7% of Antarctica’s snowfall1.

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What is the global volume of land ice and how is it changing?

How much land ice is there in the World?

Most (99.5%) of the permanent ice volume in the world is locked up in ice sheets and glaciers. The Antarctic Ice Sheet is the largest store of frozen freshwater; it would raise sea levels by 57.9 m (its “sea level equivalent”, or SLE) on full melting (BedMachine). The Antarctic Ice Sheet covers 8.3% of the Earth’s land surface.

The Greenland Ice Sheet has a sea level equivalent ice volume of 7.42 m, and covers 1.2% of the global land surface (BedMachine).

Finally, glaciers and ice caps have a sea level equivalent ice volume of 0.32 m, covering just 0.5% of the global land surface (Figure 1). There is a nice illustration of this here.

Global glaciers (in yellow) and ice sheets (white). From IPCC AR5

Figure 1. Global land ice. Glaciers are highlighted in yellow, ice shelves in green, ice sheets in white.

Other sources of global ice

There are also small amounts of ice stored in the ground in permafrost regions, frozen lakes and rivers, seasonal snow cover, and so on.

Sea ice (frozen sea water) and ice shelves (frozen floating extensions of land ice; green on Figure 1 above) do not have a “sea level equivalent” of ice volume as they are already floating, so would not raise sea levels on full melting.

Measuring changes in global ice volume

Changes in global ice volume are often expressed in gigatonnes per year (yr-1). A gigatonne is 1,000,000,000 tonnes. 1 kmwater = 1 Gt water; 361.8 Gt of ice will raise global sea levels by 1 mm.

Greenland Ice Sheet

Mass balance of the Greenland Ice Sheet

The Greenland Ice Sheet has been losing mass for over 20 years. The most recent estimates suggest that the Greenland Ice Sheet from 2012 to 2016 had a negative mass balance, losing 247 ± 15 Gigatonnes (Gt) per year of ice volume, contributing 0.69 ± 0.04 mm per year to sea level rise[2]. The mass balance of Greenland has been increasingly negative since 1995, and it is now equivalent to the global contribution to sea level rise from glaciers and ice caps (Figure 2).

Figure 2. Cumulative ice mass loss from Greenland ice sheet 1992–2012[1] (from IPCC AR5).

Driven by changes in surface mass balance

These changes have largely been driven by changes in surface mass balance. While in Greenland 60% of mass loss is through ice discharge across the grounding line to the ocean (as icebergs or melting in the ocean), 40% of mass loss is from surface melt. Increases in surface melt (ablation) are largely responsible for the increasing melting of Greenland [3].

On June 15, 2016, the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 satellite acquired a natural-color image of an area just inland from the coast of southwestern Greenland (120 kilometers southeast of Ilulisat and 500 kilometers north-northeast of Nuuk). From Wikimedia Commons

Figure 3. Surface meltwater on the Greenland Ice Sheet.

The estimates of Greenland Ice Sheet mass balance above include the peripheral glaciers surrounding the larger ice sheet. These peripheral glaciers account for around 15-20% of the total mass imbalance of the ice sheet[2, 4].

These increases in surface melt and mass losses from Greenland are due to recent increases in winter and summer air temperatures, with increases in the size of the ice sheet ablation area (the area with net melting over one year). This is associated with changes in the surface albedo, as ice has a lower albedo than white snow, exacerbating melt. Overall, this is leading to a lowering of the Greenland Ice Sheet surface elevation (Figure 4), and a decrease in ice volume.

Acceleration in outlet glaciers

Ice discharge from the major outlet glaciers of the Greenland Ice Sheet has also increased, with glaciers accelerating in western Greenland (e.g. Jakobshavn Isbrae, JI) (Figure 4). This faster ice flow leads to these outlet glaciers discharging more ice volume to the ocean as icebergs than is replaced by snow, so the outlet glaciers are also thinning, as can be seen by the red on the figure below.

Figure 4. Average rates of surface elevation change (dh/dt) through time (2010-2017) for the Greenland and Antarctic Ice Sheets[2].

Antarctic Ice Sheet

Antarctic Ice Sheet ice volume

The best estimates of Antarctic volume come from BEDMAP2 [5]. BEDMAP2 provides us with a detailed map of the base of the ice sheet, derived mostly from radar data. There are three ice sheets in Antarctica, each with their own unique characteristics. They are the larger East Antarctic Ice Sheet (EAIS), with an SLE of 53.3 m, the West Antarctic Ice Sheet (WAIS), with an SLE of 4.3 m, and the Antarctic Peninsula Ice Sheet (APIS) with an SLE of 0.2 m.

Surface elevation of the Greenland and Antarctic ice sheets (IPCC AR5)

Figure 5. BEDMAP2 (Fretwell et al., 2013; IPCC AR5).

Antarctica surface mass balance

It is very cold in Antarctica, with very limited surface melt [6]. There is abundant accumulation in the coastal parts of Antarctica, especially western West Antarctica and on the APIS.  The figure below shows where surface mass balance is highest; reds and yellows indicate far more snowfall than is lost through surface melting. It is cold and dry in the centre of the East Antarctic Ice Sheet, with very little snowfall or surface melt.

The average ice-sheet integrated surface mass balance of Antarctica is +2418 ± 181 Gt yr-1 [6].

Figure 6. Mean (1979–2010) surface mass balance [mm w.e. y−1]. [6]

Changes in Antarctic mass balance

Most mass loss in Antarctica is driven through ocean melting and iceberg calving[7, 8]. This ice discharge to the ocean through the grounding line is increasing as outlet ice streams are accelerating and grounding lines are retreating (see here). Thus increased ice flow in Antarctica accounts for almost all recent increases in mass losses.

The sea level rise contribution from Antarctica was 0.49 – 0.73 mm yr-1 from 2012-2017, mostly from the APIS and WAIS and due to acceleration of outlet glaciers in Amundsen Sea Embayment (e.g. Pine Island Glacier/Thwaites Glacier) (Figure 4; 7)[2].

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

Figure 7. Location of Pine Island and Thwaites Glacier in Antarctica, with ice velocity from Rignot et al. 2011

Including ice gained and lost through all mechanisms, the current mass balance of Antarctica from 1992 to 2017 was:

  • EAIS: +5 ± 46 Gt yr-1
  • WAIS: –94 ± 27 Gt yr-1
  • APIS: –20 ± 15 Gt yr-1
  • Total Antarctic Ice Sheet: -109 ± 56 Gt yr-1

Antarctic Ice Sheet mass balance changed from 2012 to 2017 to -219 ± 43 Gt yr-1 [8] . Mass losses from West Antarctica are driving most of the total mass losses from Antarctica, with the mass balance of East Antarctica showing negligible changes [8].

Shepherd et al. 2018

Figure 8. Mass changes in Antarctica (Shepherd et al. 2018).

Glaciers and Ice caps

Glacier extent

The amount of ice contained in global glaciers and ice caps is mapped by the Randolph Glacier Inventory[9, 10]. This inventory uses satellite imagery and a formalised methodology to organise researchers working on mapping glaciers and glacier change. The Randolph Glacier inventory estimates that there are 198,000 glaciers worldwide (Figure 9); however, this is an arbitrary number as it depends on:

  • Subdivision of glaciers and mapping of ice divides
  • Accuracy of the digital elevation model used
  • Minimum area threshold; it is hard to map glaciers smaller than 0.2 km2 and so this is usually set as a minimum area threshold. There could be up to 400,000 glaciers if small glacierettes are included (but they only account for 1.4% of glacierised area).

Bamber et al. 2018

Figure 9. Global glaciers (yellow) and their area (pie charts) [2, 10].

The RGI estimates a total glacierised area of: 726,000 km2

  • Subantarctic and Antarctic: 132,900 km2
  • Arctic Canada North: 104,900 km2
  • Asia: 62,606 km2
  • Low latitudes: 2346km2
  • 44 % is in Arctic regions, 18% in Antarctic & Subantarctic.

Global glacier ice volume

An estimate of global ice volume in glaciers and ice caps remains a “grand challenge” in glaciology; there are few glaciers with direct measurement by radar [11]. Bed topography and thus ice thickness is usually then estimated, either by volume-area scaling [12, 13], inversions of ice surface slope and velocity [14, 15], or from numerical modelling of ice flow [16].

Our best current estimate of global glacier ice volume is[16]:

  • 170 x 103 ± 21 x 103 km3 (moutain glaciers & ice caps outside Greenland & Antarctica)
  • = 0.43 ± 0.06 m SLE.

Glacier recession

Glaciers worldwide are receding. The key methods for mapping glacier change include:

  • Satellite images (1970s-present)[17]
  • Topographic maps (~1900 to present)
  • Geomorphological evidence of glacier extent (LIA/sig. advances)
  • Automated and manual mapping from satellite imagery
  • Limit realistically of mapping glaciers min. 0.2 km2

Mass loss can also be quantified from analysis of glacier surface elevation change (dh/dt)[18, 19] using digital elevation model differencing, satellite gravimetry or altimetry, and in-situ surface mass balance measurements [20].

The figure below shows the current best estimates of ice volumes lost from Antarctica and Greenland from 2012-2016 (taken from Bamber et al. 2018) and from glaciers around the world. Bamber et al. 2018 do not provide an individual assessment of ice volume lost from each area, so here I have plotted ice volumes lost from 2003-2009 from Gardner et al. 2013. Each region corresponds to those mapped out in Figure 9 and glacier outlines are from GLIMS and the Randalph Glacier Inventory.

Note that peripheral glaciers around Greenland and Antarctica are included in the assessment for the ice sheets (cf. Bamber et al. 2018). These glaciers are however changing rapidly, and indeed account for a large portion of the overall change.

World glaciers and ice sheets mass balance

Figure 10. Global glacier mass budgets from 2012-2016 by Bamber et al. 2018 (ice sheets) and 2003-2009 (glaciers; Garder et al. 2013).

These data, recently compiled by Bamber et al. 2018, give a global estimate of mass loss from glaciers of -227 ± 31 Gt yr-1 (2012-2016). This does not include losses from peripheral glaciers around Greenland and Antarctica, which are included in the ice sheet mass balance assessments.

IPCC AR5

Figure 11. Global glacier melt (IPCC AR5)[1]

This has led the World Glacier Monitoring Service (WGMS) to state: “rates of early 21st-century mass loss are without precedent on a global scale, at least for the time period observed and probably also for recorded history” [21].

This global melt is a challenge for society. While the sea level rise from glaciers is ultimately constrained by their small ice volume globally, they remain important as sources of freshwater [22]; their melting poses new hazards to mountain communities[23-25], and they remain important for local economies [26].

Summary

Global changes in land ice volume were recently summarised by Bamber et al. (2018):

Ice mass Total ice volume % Global land surface Volume change 2012-2016 Sea level contribution 2012-2016
EAIS 53.3 m SLE 8.3% -19 ± 20 Gt yr-1 0.05 ± 0.06 mm yr-1
WAIS & APIS 4.5 m SLE -172 ± 27 Gt yr-1 0.48 ± 0.08 mm yr-1
Greenland 7.36 m SLE 1.2% -247 ± 15 Gt yr-1 0.69 ± 0.04 mm yr-1
Global glaciers and ice caps* 0.43 m SLE

(113,915 to 191,879 Gt)

0.5% -227 ± 31 Gt yr-1 0.63 ± 0.08 mm yr-1
Total 12.5% -665 ± 48 Gt yr-1 1.85 ± 0.13 mm yr-1

*excl. glaciers peripheral to ice sheets

Accelerating mass loss from land ice

Mass loss is accelerating (Figure 12), with changes in ocean melt driving recession in Antarctica, increased ice discharge and surface melt driving changes in Greenland, and negative surface mass balances largely driving glacier recession worldwide. Losses from Greenland are now the most significant contributor to global sea level rise (this includes the peripheral glaciers around the ice sheet), recently overtaking glaciers as the largest contributor.

Bamber et al. 2018

Figure 12. Mass losses from glaciers and ice sheets, annually (Bamber et al. 2018)

Below is a nice summary of the key changes and processes from the IPCC AR4:

Figure 13. Summary of global changes in land ice, IPCC AR5 (2013).

Further reading

References

  1. Vaughan, D.G., et al., Observations: Cryosphere, in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, T.F. Stocker, et al., Editors. 2013, Cambridge University Press: Cambridge, UK. p. 317-382.
  2. Bamber, J.L., et al., 2018, Environmental Research Letters.
  3. van den Broeke, M., et al., 2017, Current Climate Change Reports. 3, 345-356.
  4. Bolch T, et al., 2013, Geophys. Res. Lett. . 40, 875-881.
  5. Fretwell, L.O., et al., 2013, The Cryosphere. 7, 375-393.
  6. Lenaerts, J.T.M., et al., 2012, Geophysical Research Letters. 39, L04501.
  7. Pattyn, F., et al., 2018, Nature Climate Change.
  8. Shepherd, A., et al., 2018, Nature. 558, 219-222.
  9. Arendt, A., et al., Randolph Glacier Inventory [v2.0]: A Dataset of Global Glacier Outlines. 2012, Global Land Ice Measurements from Space: Boulder Colorado, USA.
  10. Pfeffer, W.T., et al., 2014, Journal of Glaciology. 60, 537.
  11. Gärtner-Roer, I., et al., 2014, Global and Planetary Change. 122, 330-344.
  12. Bahr, D.B., Estimation of glacier volume and volume change by scaling methods, in Encyclopedia of Snow, Ice and Glaciers. 2014, Springer. p. 278-280.
  13. Bahr, D.B., W.T. Pfeffer, and G. Kaser, 2014, Reviews of Geophysics.
  14. Carrivick, J.L., et al., 2018, Geografiska Annaler: Series A, Physical Geography, 1-23.
  15. Carrivick, J.L., et al., 2016, Global and Planetary Change. 146, 122-132.
  16. Huss, M. and D. Farinotti, 2012, Journal of Geophysical Research: Earth Surface. 117, F04010.
  17. Davies, B.J. and N.F. Glasser, 2012, Journal of Glaciology. 58, 1063-1084.
  18. Willis, M.J., et al., 2011, Remote Sensing of Environment. 117, 184-198.
  19. Willis, M.J., et al., 2012, Geophys. Res. Lett. 39, L17501.
  20. Gardner, A.S., et al., 2013, Science. 340, 852-857.
  21. Zemp, M., et al., 2015, Journal of Glaciology. 61, 745-762.
  22. Immerzeel WW, van Beek L P H, and B.M.F. P, 2010, Science. 328, 1382–85.
  23. Emmer, A., 2017, Quaternary Science Reviews. 177, 220-234.
  24. Emmer, A., Glacier Retreat and Glacial Lake Outburst Floods (GLOFs), in Oxford research Encyclopedias–Natural Hazard Science. 2017, Oxford University Press. p. 1-38.
  25. Harrison, S., et al., 2017.
  26. al, H.M.e., 2017 Earth’s Future 5 418-35.

Choosing the future of Antarctica

In a new article in the journal Nature, Stephen Rintoul and colleagues present two very different visions of Antarctica’s future, from the perspective of an observer looking back from 2070. In one vision, humanity continues to exploit Earth’s natural resources (such as fossils fuels) and does little to protect the environment, and in the other, there is a global movement towards conservation. The article shows how Antarctica will change over the next 50 years, should either of these two situations occur.

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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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Antarctic Ice Sheet mass balance

How does mass balance vary over Antarctica? | Surface mass balance in the past | Surface mass balance in the future | References | Comments |

How does mass balance vary over Antarctica?

Is Antarctica currently losing or gaining mass? Will this massive ice sheet grow or shrink in the future? And what effect will increased snowfall have over coming centuries? In order to answer these questions, we must analyse the surface mass balance of the Antarctic Ice Sheet.

First, let’s introduce some definitions.

  • Mass balance is the sum of all processes of accumulation and ablation, including those at the ice surface and at the bed, but does not include mass changes due to ice flow1. See this page (Introduction to Glacier Mass Balance) for more information.
  • Surface mass balance is the net balance between the processes of accumulation and ablation on a glacier’s surface (it does not include dynamic mass loss and basal melting)1.
  • Climatic mass balance includes surface mass balance and internal accumulation1.
  • Ice dynamical changes may include changes to ice discharge and acceleration or deceleration of flow, which can lead to dynamic thinning or thickening, ice-shelf collapse, marine ice sheet instability, and other factors resulting in changes in the glacier beyond surface mass balance.

Surface mass balance

Surface mass balance varies extensively over Antarctica. The Antarctic Peninsula has the highest accumulation rates (up to 1500 mm per year), followed by coastal West Antarctica, which has around 1000 mm accumulation per year2. Compare this with the interior of the Antarctic Ice Sheet, where it is dry and cold; here accumulation can be less than 25 mm per year.

Surface mass balance of the Antarctic and Greenland ice sheets. From Van den Broeke et al., 2011.

Surface mass balance of the Antarctic and Greenland ice sheets. From Van den Broeke et al., 2011.

Surface mass balance estimates are constantly improving as scientists gain better understandings of glacio-isostatic adjustment, improve glacier modelling techniques and gain access to higher resolution satellite datasets over longer timescales3. Surface mass balance estimates therefore tend to improve over time, but are subject to large uncertainties4. For this reason, there tends to be differences between the results of different techniques used to measure surface mass balance. Surface mass balance of the grounded Antarctic Ice Sheet is currently estimated at ~2000 gigatonnes per year2, 5, 6, and it is subject to large variability across the ice sheet and through time.

Total mass balance

The figure below shows some recent estimates for total mass balance (including basal processes) over Antarctica7. Each box is bounded by the time interval studied and the uncertainties identified.

Summary of estimates of rates of ice mass change for Antarctica and Greenland. Reprinted by permission from Macmillan Publishers Ltd: [Nature] (Hanna et al., 2013) copyright (2013)

Summary of estimates of rates of ice mass change for Antarctica and Greenland. Reprinted by permission from Macmillan Publishers Ltd: [Nature] (Hanna et al., 2013) copyright (2013)

Overall, a recent estimate puts Antarctic net mass balance at -71 ± 53 gigatonnes per year8, so just negative over the 19 year survey. Mass losses are increasing in West Antarctica and the Antarctic Peninsula. The mass balance of West Antarctica is dominated by dynamic losses from the Amundsen Sea sector, and dynamic gains from the Kamb Ice Stream8. From the period 2005-2010, Shepherd et al. (2012) estimate the mass balance of the entire Antarctic Ice Sheet to be -81 ± 37 gigatonnes per year8.

An unweighted average of recent estimates suggests that Antarctica moved from a weakly negative mass balance in the 1990s to a faster rate of mass loss at a rate of between -45 and -120 gigatonnes per year7. Larger dynamic losses in West Antarctica are being partially offset by increases in accumulation over East Antarctica.

The total mass balance of Antarctica was recently updated here.

Accelerating total mass losses from Antarctica

The GRACE (Gravity Recovery and Climate Experiment) satellite gravity mission shows that total mass loss in Antarctica is accelerating over time. They found that total mass loss increased by 26 ± 14 gigatonnes per year from 2002 to 20099. Rignot et al. (2011) found a smaller acceleration of 14.5±2 gigatonnes per year from 1993-20115, but this change is still three times larger than that found for mountain glaciers and ice caps.

Surface mass balance of Antarctica in the past

How has the surface mass balance of Antarctica changed in the past? Firn and ice-core records can hold the key to providing a longer perspective on surface mass balance than is currently available from satellite records. Frezzotti et al. used 67  of these cores to reconstruct surface mass balance over the last 800 years. They found that current surface mass balance is not exceptionally high compared with the last 800 years10. Periods of high accumulation occurred in the past, in the 1370s and 1610s AD, but there has been an increase of 10% in snow accumulation in some coastal regions since 1850 – a fact that agrees with independent work on the Antarctic Peninsula11.

Surface mass balance of Antarctica in the future

Climate models predict that, for a generally warmer climate, snowfall will increase over Antarctica7. Surface melt will increase around the more northerly Antarctic Peninsula, and dynamic changes such as increased ice discharge12, ice-shelf collapse and grounding line recession13, and marine ice-sheet instability are likely to offset any increases in precipitation7. However, if no dynamical ice response is assumed, then increases in snowfall over the entire continent of 6-16% to 2100 AD and 8-25% to 2200 AD are likely to result in a drop in sea level of 20-43 mm in 2100 and 73-163 in 2200, compared with today14. However, it is more likely that the Greenland and Antarctic ice sheets will lose mass over the next century, with rapid coastal changes, increases in ice flow and ice-shelf collapse all likely4. As a result of these complex expected changes, there are a number of uncertainties in past, present and future ice sheet mass balance.

Further reading

References


1.            Cogley, J.G., Hock, R., Rasmussen, B., Arendt, A., Bauder, A., Braithwaite, R.J., Jansson, P., Kaser, G., Moller, M., Nicholson, L., & Zemp, M. Glossary of Glacier Mass Balance and related terms. Paris: IHP-VII Technical Documents in Hydrology No. 86, IACS Contribution No. 2, UNESCO-IHP. 124 (2011).

2.            Lenaerts, J.T.M., van den Broeke, M.R., van de Berg, W.J., van Meijgaard, E., & Kuipers Munneke, P. A new, high-resolution surface mass balance map of Antarctica (1979–2010) based on regional atmospheric climate modeling. Geophysical Research Letters. 39, L04501 (2012).

3.            Van den Broeke, M., Bamber, J., Lenaerts, J., & Rignot, E. Ice Sheets and Sea Level: Thinking Outside the Box. Surveys in Geophysics. 32, 495-505 (2011).

4.            Alley, R.B., Spencer, M.K., & Anandakrishnan, S. Ice-sheet mass balance: assessment, attribution and prognosis. Annals of Glaciology. 46, 1-7 (2007).

5.            Rignot, E., Velicogna, I., Van den Broeke, M., Monaghan, A., & Lenaerts, J. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters. 38, (2011).

6.    Agosta, C., Favier, V., Krinner, G., Gallée, H., Fettweis, X., & Genthon, C. High-resolution modelling of the Antarctic surface mass balance, application for the twentieth, twenty first and twenty second centuries. Climate Dynamics. 41, 3247-3260 (2013).

7.            Hanna, E., Navarro, F.J., Pattyn, F., Domingues, C.M., Fettweis, X., Ivins, E.R., Nicholls, R.J., Ritz, C., Smith, B., Tulaczyk, S., Whitehouse, P.L., & Zwally, H.J. Ice-sheet mass balance and climate change. Nature. 498, 51-59 (2013).

8.            Shepherd, A., Ivins, E.R., A, G., Barletta, V.R., Bentley, M.J., Bettadpur, S., Briggs, K.H., Bromwich, D.H., Forsberg, R., Galin, N., Horwath, M., Jacobs, S., Joughin, I., King, M.A., Lenaerts, J.T.M., Li, J., Ligtenberg, S.R.M., Luckman, A., Luthcke, S.B., McMillan, M., Meister, R., Milne, G., Mouginot, J., Muir, A., Nicolas, J.P., Paden, J., Payne, A.J., Pritchard, H., Rignot, E., Rott, H., Sørensen, L.S., Scambos, T.A., Scheuchl, B., Schrama, E.J.O., Smith, B., Sundal, A.V., van Angelen, J.H., van de Berg, W.J., van den Broeke, M.R., Vaughan, D.G., Velicogna, I., Wahr, J., Whitehouse, P.L., Wingham, D.J., Yi, D., Young, D., & Zwally, H.J. A Reconciled Estimate of Ice-Sheet Mass Balance. Science. 338, 1183-1189 (2012).

9.            Velicogna, I. Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE. Geophysical Research Letters. 36, (2009).

10.            Frezzotti, M., Scarchilli, C., Becagli, S., Proposito, M., & Urbini, S. A synthesis of the Antarctic surface mass balance during the last 800 yr. The Cryosphere. 7, 303-319 (2013).

11.            Thomas, E.R., Marshall, G.J., & McConnell, J.R. A doubling in snow accumulation in the western Antarctic Peninsula since 1850. Geophysical Research Letters. 35, L01706 (2008).

12.          Winkelmann, R., Levermann, A., Martin, M.A., & Frieler, K. Increased future ice discharge from Antarctica owing to higher snowfall. Nature. 492, 239-243 (2012).

13.          Barrand, N.E., Hindmarsh, R.C.A., Arthern, R., Williams, C.R., Mouginot, J., Scheuchl, B., Rignot, E., Ligtenberg, S.R.M., van den Broeke, M.R., Edwards, T.L., Cook, A.J., & Simonsen, S.B. Computing the volume response of the Antarctic Peninsula Ice Sheet to warming scenarios to 2200. Journal of Glaciology. 59, 397-409 (2013).

14.          Ligtenberg, S.R.M., Berg, W.J., Broeke, M.R., Rae, J.G.L., & Meijgaard, E. Future surface mass balance of the Antarctic ice sheet and its influence on sea level change, simulated by a regional atmospheric climate model. Climate Dynamics. 41, 867-884 (2013).

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.

Antarctic datasets

The following is a list of publically available Antarctic datasets. These datasets are often freely accessible, usually providing that you cite the source and often a relevant paper. Do not use data without correct attribution. Many of these datasets have been used in the creation of figures on this website, and I thank all the authors that have contributed their data. There are many more datasets around, but these ones concentrate on Antarctic Glaciers. I hope this list is useful.

Much of this data is aimed at academics rather than as outreach, and assumes users are familiar working with ASCII data or various other GIS formats such as GEOTIFFs. However, many of the Antarctic datasets listed may be interesting to teachers and educators as well as researchers.

Browse the datasets:

BEDMAP2

BEDMAP 2 preview

BEDMAP 2 preview. From BAS.

BEDMAP2 is a compilation of bedrock topography data from around the Antarctic continent. The dataset includes bed topography, ice surface elevation, a rock mask, ice shelf mask, ice thickness and more. Data are available as ASCII files, suitable for loading into a GIS.

Download the PDF from The Cryosphere.

Download BEDMAP2 data.

Citation: Fretwell, L.O., H. D. Pritchard, D. G. Vaughan, J. L. Bamber, N. E. Barrand, R. Bell, C. Bianchi, R. G. Bingham, D. D. Blankenship, G. Casassa, G. Catania, D. Callens, H. Conway, A. J. Cook, H. F. J. Corr, D. Damaske, V. Damm, F. Ferraccioli, R. Forsberg, S. Fujita, P. Gogineni, J. A. Griggs, R. C. A. Hindmarsh, P. Holmlund, J. W. Holt, R. W. Jacobel, A. Jenkins, W. Jokat, T. Jordan, E. C. King, J. Kohler, W. Krabill, M. Riger-Kusk, K. A. Langley, G. Leitchenkov, C. Leuschen, B. P. Luyendyk, K. Matsuoka, Y. Nogi, O. A. Nost, S. V. Popov, E. Rignot, D. M. Rippin, A. Riviera, J. Roberts, N. Ross, M. J. Siegert, A. M. Smith, D. Steinhage, M. Studinger, B. Sun, B. K. Tinto, B. C. Welch, D. A. Young, C. Xiangbin & Zirizzotti, A., 2013. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. The Cryosphere 7, 375-393.

BedMachine

BedMachine is a self-consistent dataset of the bed topography of Antarctica (also for Greenland). It is freely available at the NSIDC.

BedMachine provides a NetCDF on a 450m resolution grid, and has a nominal date of 2012. It gives surface elevation, ice thickness and bed topography, and is an update of BedMap2.

There are some great visualisations of the data set here.

This is an image of the bed topography under the Denman Glacier in Antarctica colored by the elevation. Areas below sea level are colored in shades of blue while areas above sea level are colored in green, yellow and brown. From the NASA Science Visualisation Studio.

Citation:

Morlighem, M., Rignot, E., Binder, T. et al. Deep glacial troughs and stabilizing ridges unveiled beneath the margins of the Antarctic ice sheet. Nature Geoscience 13, 132–137 (2020)

Ice velocity

Ice streams of Antarctica. From Rignot et al. (2011).

Ice streams of Antarctica. From Rignot et al. (2011).

The Rignot et al 2011 dataset allows users to download ice velocity across the Antarctic continent. The digital ice motion map is available as a MEaSUREs Earth Science Data Record (ESDR) at the National Snow and Ice Data Centre, Boulder, CO.

Citation: Rignot, E., Mouginot, J. & Scheuchl, B., 2011. Ice Flow of the Antarctic Ice Sheet. Science 333, 1427-1430.

Download the data.

Grounding line data

Rignot et al. 2011 also have a Grounding Line dataset available for download. Download the Grounding Line Data.

Citation: Rignot, E., Mouginot, J. & Scheuchl, B., 2011. Antarctic grounding line mapping from differential satellite radar interferometry. Geophys. Res. Lett. 38, L10504.

Landsat Image Mosaic Antarctica (LIMA)

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Download a beautiful Landsat image mosaic of Antarctica. This dataset was created in support of the International Polar Year (IPY 2007-2008) with a virtually cloudless, seamless, high resolution satellite view of Antarctica. It was created by the USGS, the British Antarctic Survey and NASA.

MOA (MODIS Mosaic of Antarctica) image map

The NSIDC and the University of New Hampshire have assembled a digital image map of Antarctica and surrounding islands. The MODIS Mosaic of Antarctica is 260 swaths of Terra and Aqua MODIS (Moderate Resolution Imaging Spectroradiometer) images acquired between 20th November 2003 and 29th February 2004.

To download the data, users will need to register.

Citation: Haran, T., J. Bohlander, T. Scambos, T. Painter, and M. Fahnestock compilers. 2005, updated 2006. MODIS mosaic of Antarctica (MOA) image map. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

Scambos, T., T. Haran, M. Fahnestock, T. Painter, and J. Bohlander. 2007. MODIS-based Mosaic of Antarctica (MOA) data sets: continent-wide surface morphology and snow grain size. Remote Sensing of Environment 111(2): 242-257. 10.1016/j.rse.2006.12.020.

Antarctic Digital Database (ADD)

Research stations and summer camps in Antarctica, using data from the ADD.

Research stations and summer camps in Antarctica, using data from the ADD.

A whole host of resources are available through the ADD. The SCAR Antarctic Digital Database is managed by the British Antarctic Survey and the Scientific Committee for Antarctic Research. Users can use a map viewer to download multiple features across Antarctica in various formats. There is a combination of point (vector) and raster data.

Data includes: hillshade and bathymetry, LIMA mosaic, sea mask, BEDMAP2, coastlines, ice shelf margins, coastal change, contours, elevations, moraines, place names, lakes, streams, subglacial lakes, stations, and bases.

Go to the Map Viewer to access data. Users will need to register to access data.

IBCSO (International Bathymetric Chart of the Southern Ocean)

IBCSO bathymetric charts

IBCSO bathymetric charts

The IBCSO website provides information on sea floor topography. Users can download a PDF chart or Geotiff data of the entire ocean floor.

Access the data.

When using any data from the IBCSO project please cite:

Arndt, J.E., H. W. Schenke, M. Jakobsson, F. Nitsche, G. Buys, B. Goleby, M. Rebesco, F. Bohoyo, J.K. Hong, J. Black, R. Greku, G. Udintsev, F. Barrios, W. Reynoso-Peralta, T. Morishita, R. Wigley, “The International Bathymetric Chart of the Southern Ocean (IBCSO) Version 1.0 – A new bathymetric compilation covering circum-Antarctic waters”, Geophysical Research Letters, doi: 10.1002/grl.50413

ASTER GDEM of the Antarctic Peninsula

Antarctic Peninsula 100 m DEM. Cook et al., 2012.

Antarctic Peninsula 100 m DEM. Cook et al., 2012.

This dataset, created by Alison Cook, provides a 100 m resolution surface topography DEM of the Antarctic Peninsula. Data are available via FT in GeoTIFF and ASCII formats.

Download the Data. Users must register to download the data.

Citation: Cook, A. J., T. Murray, A. Luckman, D. G. Vaughan, and N. E. Barrand. 2012. Antarctic Peninsula 100 m Digital Elevation Model Derived from ASTER GDEM. [indicate subset used]. Boulder, Colorado USA: National Snow and Ice Data Center. http://dx.doi.org/10.7265/N58K7711.

Global Land-Ice Measurements from Space (GLIMS)

The GLIMS dataset provides platform-independent GIS shapefiles of glaciers worldwide, including some from the Antarctic Peninsula. Users can browse glacier outlines using the GLIMS viewer, and download relevant glacier shapefiles. Users should cite the author of the shapefile as well as the GLIMS database.

Marine Geoscience Data System (MGDS)

The MGDS provides access to data portals for the NSF. These portals provide free public access to a wide variety of marine geoscience data collected during expeditions across the World’s oceans. Users can explore data using the GeoMapApp. Data available through the Antarctic and Southern Ocean Data Portal include seafloor bathymetry, subbottom profiling, trackline gravity and magnetics, meterological and water column data.

DIVA-GIS

The DIVA-GIS website provides free GIS data for any country in the world, including country boundaries, inland water, roads, railways, population and more.

IPCC data

The IPCC Data Distribution Centre (DDC) for the International Panel on Climate Change (IPCC) provides climatic, socio-economic and environmental data, from the past and also for projections into the future.

Further resources

The NSIDC has a list here of available Antarctica datasets, including:

  • Antarctic 5 km DEM
  • GEOSAT Radar Altimeter DEM Atlas of Antarctica
  • ICESat 500m Laser Altimetry DEM of Antarctica
  • Ice Thickness and Surface Elevation, Southeastern Ross Embayment
  • MODIS MOA image map
  • Radarsat Antarctic Mapping Project DEM V2.

The NSIDC has a great list of easy to use resources that are suitable for K-12 teachers and students, the press, the general public and non-cryospheric researchers. Datasets include:

  • MASIE measurements of daily sea ice extent
  • Sea ice index
  • Frozen ground maps
  • Glacier photographs
  • Atlas of the Cryosphere

Google Earth has fabulous satellite images of Antarctica, and it lets you explore the continent from the comfort of your sofa.

If you know of any more datasets, please add a comment in the box below!

Go to top.

Sea level rise over the next 2000 years

A new paper by Levermann et al. in PNAS uses the record of past rates of sea level rise from palaeo archives and numerical computer models to understand how much sea level rise we can expect per degree of warming in the future. These data suggest that we can expect a global sea level rise of 2.3 m per 1°C of warming within the next 2000 years: well within societal timeframes. A 2°C of warming would result in a global sea level rise of 4.8 m within 2000 years. This would inundate many coastal cities in Europe alone, and cause untold economic and societal damage.

Continue reading

Pine Island Glacier

Investigating Pine Island Glacier | Why is Pine Island Glacier important? | Pine Island Glacier ice shelf | Pine Island Glacier: the longer term view | Conclusions | References | Comments |

Investigating Pine Island Glacier

A fast-flowing ice stream

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

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

Pine Island Glacier is one of the largest ice streams in Antarctica. It flows, together with Thwaites Ice Stream, into the Amundsen Sea embayment in West Antarctica, and the two ice streams together drain ~5% of the Antarctic Ice Sheet1. Pine Island Glacier flows at rates of up to 4000 m per year2. It is of interest to scientists because it is changing rapidly; it is thinning, accelerating and receding3, all of which contribute directly to sea level, and its future under a warming climate is uncertain. Pine Island Glacier is buttressed by a large, floating ice shelf, which helps to stabilise the glacier, but this ice shelf is itself thinning and recently calved a huge iceberg.

Just watch how fast the ice flows in the video below, and notice especially how the ice speeds up when it reaches the floating ice shelf.

Caption: Visualisation of ice flow in the Antarctic ice sheet model PISM-PIK. The white dots show how particles move with the ice which are initially randomly distributed over the ice surface. Colours in addition show the flow speed. By Youtube user pikff1.

An inaccessible location

Autosub near the ice (from http://www.krapp.org/rupert/archiv.html/2007_08_01_archive.html)

Autosub near the ice (from http://www.krapp.org/rupert/archiv.html/2007_08_01_archive.html)

Despite this interest, Pine Island Glacier is difficult to access. It is remote from any research bases, so flying there means making multiple short flights, making fuel depots to allow scientists to hop to the location. Low lying cloud often makes flying hazardous. The ice stream is heavily-crevassed and dangerous, so walking on it is difficult. Sea ice keeps ships away, making it difficult to access the ice stream from the ocean. However, scientists have several ingenious ways in which they can observe changes to this fragile, important ice stream. They can measure changes in ice extent and thinning from satellites4,5, and they have fired javelins loaded with sensors onto the ice surface, into places with too many crevasses for people to travel. Finally, scientists on board ships have deployed ‘Autosub’ beneath the very ice shelf, to make observations where no man can go.

Exploring Pine Island Glacier

You can use Google Earth below to explore the ice stream. Can you identify the ice shelf? If you zoom in far enough, you’ll be able to see the huge crack in the ice shelf. You can also see how the surface of both the ice stream and ice shelf is heavily crevassed, making it difficult to walk on the surface of the ice.


View Pine Island Glacier in a larger map

Why is Pine Island Glacier important?

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

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

Pine Island Glacier drains much of the marine-based West Antarctic Ice Sheet, and it has a configuration susceptible to rapid disintegration and recession. The ice sheet in this area is grounded up to 2000 m below sea level, making it intrinsically unstable6 and susceptible to rapid melting at its base, and to rapid migration of the grounding line up the ice stream7 (see Marine Ice Sheet Instability). The images below show how much of the West Antarctic Ice Sheet, especially around Pine Island Glacier, is grounded well below sea level.

Pine Island Glacier is one of the most dynamic features of the Antarctic Ice Sheet. It is buttressed by a large ice shelf that is currently thinning8, and the ice stream itself has a negative mass balance (the melting is not replaced by snowfall)3, it is flowing faster9,  and the grounding line is retreating further and further up into the bay. The grounding line receded by more than 20 km from 1996 to 20092. The ice stream is steepening, which increases the gravitational driving stress, helping it to flow faster, and there is no indication that the glacier is approaching a steady state10.

Possible future collapse?

A1B warming scenarios from the IPCC. A1B is the "Business as Usual" scenario, with emissions continuing to increase in line with present-day rates of increase.

A1B warming scenarios from the IPCC. A1B is the “Business as Usual” scenario, with emissions continuing to increase in line with present-day rates of increase. The grey bars at the right indicate the best estimate and likely range of temperatures.

Pine Island Glacier could collapse – stagnate and retreat far up into the bay, resulting in rapid sea level rise – within the next few centuries, raising global sea levels by 1.5 m11,12, out of a total of 3.3 m from the entire West Antarctic Ice Sheet13. Some studies have suggested that the entire main trunk of Pine Island Glacier could unground and become afloat within 100 years14, but more recent modelling efforts suggest that much longer timescales are needed to unground the entire trunk2. These numerical computer models indicate that annual rates of sea level rise from Pine Island Glacier could reach 2.7 cm per 100 years2. Under the A1B “Business as Usual” emissions scenario from the IPCC (2.6°C warming by 2100), Gladstone et al. (2012) predict recession over the next 200 years with huge uncertainty over the rate of retreat, and full collapse of the trunk of Pine Island Glacier during the 22nd Century remains a possibility15.

It remains difficult to assess how soon a collapse of Pine Island Glacier could occur, but a new paper by Bamber and Aspinall (2013) suggest that there is a growing view that the West Antarctic Ice Sheet could become unstable over the next 100 years16. The largest contibution to global sea level rise from the Greenland and Antarctic ice sheets combined is around 16.9 mm per year, but is more likely to be around 5.4 mm per year by 2100. This gives a total of 33 to 132 cm of global total sea level rise by 2100. Uncertainty over the future behaviour of Pine Island Glacier in West Antarctica is one of the largest constraints on accurately predicting future sea level rise16.

Current behaviour

Reprinted by permission from Macmillan Publishers Ltd: Nature (Rignot et al., 2008), copyright 2008

Reprinted by permission from Macmillan Publishers Ltd: Nature
(Rignot et al., 2008), copyright 2008

Pine Island Glacier is currently flowing very quickly and it is accelerating, causing thinning. The velocity is well above that required to maintain mass balance – so the ice stretches longitudinally, and thins vertically3. In the figure right, from Rignot et al. 2008, you can see that mass losses from Pine Island Glacier and Thwaites Glacier dominate Antarctic Ice Sheet ice losses. Mass loss from this basin doubled from 1996 to 2006, and it is the largest ice loss in Antarctica.

Pine Island Glacier ice shelf

Pine Island Glacier has a large ice shelf, which supports the glacier. Removal of the ice shelf would likely result in rapid acceleration, thinning and recession as the glacier adjusts to new boundary conditions; these reactions have been observed following ice shelf collapse around the Antarctic Peninsula17-21. The ice shelf around Pine Island Glacier is currently thinning, and it is warmed from below by Circumpolar Deep Water that flows onto the continental shelf22,23. This melts the ice shelf from below24, and this melting is probably the cause of the observed ice stream thinning, acceleration and grounding line recession25, which is contributing to a sea level rise of 1.2 mm per decade3.

Calving Icebergs

NASA’s DC-8 flies across the crack forming across the Pine Island Glacier ice shelf on Oct. 26, 2011. The ice shelf is in the midst of a natural process of calving a large iceberg, which it hasn’t done since 2001. Credit: Jefferson Beck/NASA

NASA’s DC-8 flies across the crack forming across the Pine Island Glacier ice shelf on Oct. 26, 2011. The ice shelf is in the midst of a natural process of calving a large iceberg, which it hasn’t done since 2001. Credit: Jefferson Beck/NASA

Pine Island Glacier ice shelf periodically calves huge icebergs. The ice shelf currently loses around 62.3 ± 5 Gigatonnes per year of ice through calving, and loses 101.2 ± 8 Gigatonnes per year through basal melting24. It calved a large iceberg in 2001, and in 2011 a huge rift developed on the ice shelf.  This iceberg was finally calved in July 2013. It’s about eight times the size of New York, or half the size of Greater London, at 720 km2. However, this iceberg calving event is a natural process, part of how the ice shelf regularly calves – this ice shelf spawns huge icebergs every 6-10 years. Releasing a huge iceberg, by itself, is a normal process, unrelated to warming, but increased calving may occur in the future if the ice shelf continues to thin, which would make it susceptible to plate bending and hydrofracture processes21. This threshold has yet to be passed.

Current melting, thinning and acceleration

What is concerning is the current intense melting, thinning and glacier acceleration observed on Pine Island Glacier ice shelf22. Measurements from the British Antarctic Survey’s Autosub, the intrepid sub-ice shelf explorer, help scientists understand sub-ice conditions. Autosub is a remotely operated vehicle, loaded with sensors that measure temperature, salinity, pressure and so on, and it can map the sea bed using downward-pointing swath bathymetry. It can dive to 1600 m and travel 400 km, and it has a clever collision avoidance system.  It’s a dangerous business; several iterations of Autosub have been lost under the ice. However, data from Autosubs that did return indicates that more warm Circumpolar Deep Water has been in Pine Island Bay in recent summers22. Meltwater production underneath the ice shelf increased by 50% from 1994 to 2011; this increased melting results from stronger sub-ice-shelf circulation. As the ice shelf thins, more water is able to circulate beneath it22, exacerbating the problem and encouraging further melting.

Warm ocean waters are melting a cavity beneath Pine Island Glacier

Warm ocean waters are melting a cavity beneath Pine Island Glacier. After Schoof, 2010, Nature Geosci, 3, 450-451.

Pine Island Glacier ice shelf now has one of the fastest rates of ice-shelf thinning in Antarctica24,25.

Antarctic ice shelf thickness changes. Note the rapid thinning of Pine Island Glacier ice shelf in West Antarctica. From Pritchard et al., 2012, Nature. Reprinted by permission from Macmillan Publishers Ltd: Nature (Pritchard et al. 2012), copyright (2012).

Antarctic ice shelf thickness changes. Note the rapid thinning of Pine Island Glacier ice shelf in West Antarctica. From Pritchard et al., 2012, Nature. Reprinted by permission from Macmillan Publishers Ltd: Nature
(Pritchard et al. 2012), copyright (2012).

Pine Island Glacier: the longer term view

It is important that we take a longer-term perspective of the current changes observed on Pine Island Glacier. Are these on-going changes unprecedented, or are they part of the normal behaviour for the glacier? Marine sediment cores and swath bathymetry from ships can image the sea floor and detect and date the former behaviour of this ice stream. These data suggest that the recession of this ice stream was largely controlled by sea level rise, with a 55 m in sea level rise during deglaciation resulting in 225 km of grounding-line recession26. At the Last Glacial Maximum, circa 18,000 years ago, the ice stream was at the continental shelf edge27. It rapidly shrank back from around 16400 years ago, when rising sea levels made this ice stream more buoyant, causing lift-off, decoupling from the ice sheet’s bed, and recession.  The ice stream continued to recede from 16400 to 12300 years ago, controlled by global sea level rise. It reached its current position around 10,000 years ago27.

The recession of the ice stream was also controlled by the presence or absence of ice shelves. From 12300 to 10600 years ago, there was a large ice shelf throughout the Amundsen Sea Embayment. This ice shelf collapsed after 10600 years ago28, when warmer waters flowed onto the continental shelf. The grounding line of the ice stream retreated rapidly following ice-shelf collapse26.

It seems that the glacier is capable of very rapid recession within millennial timescales27, and that the dynamics between ice shelf and ice stream are intrinsically linked.  More work at a higher resolution, combined with modelling studies, is required to fine-tune and better understand the longer-term history of Pine Island Glacier.

Conclusions

Pine Island Glacier is a cause for concern, because it’s thinning rapidly, steepening, accelerating and receding. It is out of balance. Huge amounts of meltwater are generated in a large cavity beneath the ice shelf. It periodically, every 10 or so years, calves large icebergs – but on their own, they are not worrisome. The recently calved iceberg may be 720 km2, but that’s the least of this ice stream’s worries. This ice stream is unlikely to collapse in our lifetime – but the same cannot be said for future generations.

Pine Island Glacier is one of the largest ice streams in Antarctica, and drains much of the West Antarctic Ice Sheet. Because it is grounded in ever deeper sea water, it is vulnerable to melting at its base and rapid grounding line migration. A collapse of Pine Island Glacier could occur within 1000-2000 years, raising sea levels by up to 1.5 m, but it is unlikely to contribute to more than 2.7 cm of sea level rise over the next 100 years.

Wider Reading

Go to top or jump to Marine Ice Sheet Instability.

References


1.            Vaughan, D.G., Smith, A.M., Corr, H.F.J., Jenkins, A., Bentley, C.R., Stenoien, M.D., Jacobs, S.S., Kellogg, T.B., Rignot, E. & Lucchitta, B.K. A Review of Pine Island Glacier, West Antarctica: Hypotheses of Instability Vs. Observations of Change. in The West Antarctic Ice Sheet: Behavior and Environment 237-256 (American Geophysical Union, 2001).

2.            Joughin, I., Smith, B.E. & Holland, D.M. Sensitivity of 21st century sea level to ocean-induced thinning of Pine Island Glacier, Antarctica. Geophysical Research Letters 37, L20502 (2010).

3.            Rignot, E., Bamber, J.L., van den Broeke, M.R., Davis, C., Li, Y., van de Berg, W.J. & van Meijgaard, E. Recent Antarctic ice mass loss from radar interferometry and regional climate modelling. Nature Geosci 1, 106-110 (2008).

4.            Rignot, E., Mouginot, J. & Scheuchl, B. Ice Flow of the Antarctic Ice Sheet. Science (2011).

5.            Shepherd, A., Wingham, D.J., Mansley, J.A.D. & Corr, H.F.J. Inland thinning of Pine Island Glacier, West Antarctica. Science 291, 862-864 (2001).

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

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

8.            Rignot, E. Ice-shelf changes in Pine Island Bay, Antarctica, 1947-2000. Journal of Glaciology 48, 247-256 (2002).

9.            Rignot, E. Changes in ice dynamics and mass balance of the Antarctic ice sheet. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 364, 1637-1655 (2006).

10.          Scott, J.B.T., Gudmundsson, G.H., Smith, A.M., Bingham, R.G., Pritchard, H.D. & Vaughan, D.G. Increased rate of acceleration on Pine Island Glacier strongly coupled to changes in gravitational driving stress. Cryosphere 3, 125-131 (2009).

11.          Hughes, T. A simple holistic hypothesis for the self-destruction of ice sheets. Quaternary Science Reviews 30, 1829-1845 (2011).

12.          Vaughan, D.G. West Antarctic Ice Sheet collapse – the fall and rise of a paradigm. Climatic Change 91, 65-79 (2008).

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

14.          Wingham, D.J., Wallis, D.W. & Shepherd, A. Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006. Geophysical Research Letters 36, L17501 (2009).

15.        Gladstone, R.M., Lee, V., Rougier, J., Payne, A.J., Hellmer, H., Le Brocq, A., Shepherd, A., Edwards, T.L., Gregory, J. & Cornford, S.L. Calibrated prediction of Pine Island Glacier retreat during the 21st and 22nd centuries with a coupled flowline model. Earth and Planetary Science Letters 333–334, 191-199 (2012).

16.          Bamber, J. L., and Aspinall, W. P. (2013). An expert judgement assessment of future sea level rise from the ice sheets. Nature Clim. Change 3, 424-427.

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

18.          De Angelis, H. & Skvarca, P. Glacier surge after ice shelf collapse. Science 299, 1560-1562 (2003).

19.          Rott, H., Rack, W., Skvarca, P. & De Angelis, H. Northern Larsen Ice Shelf, Antarctica: further retreat after collapse. Annals of Glaciology 34, 277-282 (2002).

20.          Glasser, N.F., Scambos, T.A., Bohlander, J.A., Truffer, M., Pettit, E.C. & Davies, B.J. From ice-shelf tributary to tidewater glacier: continued rapid glacier recession, acceleration and thinning of Röhss Glacier following the 1995 collapse of the Prince Gustav Ice Shelf on the Antarctic Peninsula. Journal of Glaciology 57, 397-406 (2011).

21.          Scambos, T., Fricker, H.A., Liu, C.-C., Bohlander, J., Fastook, J., Sargent, A., Massom, R. & Wu, A.-M. Ice shelf disintegration by plate bending and hydro-fracture: Satellite observations and model results of the 2008 Wilkins ice shelf break-ups. Earth and Planetary Science Letters 280, 51-60 (2009).

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

23.          Jenkins, A., Dutrieux, P., Jacobs, S.S., McPhail, S.D., Perrett, J.R., Webb, A.T. & White, D. Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat. Nature Geoscience 3, 468-472 (2010).

24.          Rignot, E., Jacobs, S., Mouginot, J. & Scheuchl, B. Ice Shelf Melting Around Antarctica. Science (2013).

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

26.          Kirshner, A.E., Anderson, J.B., Jakobsson, M., O’Regan, M., Majewski, W. & Nitsche, F.O. Post-LGM deglaciation in Pine Island Bay, West Antarctica. Quaternary Science Reviews 38, 11-26 (2012).

27.          Lowe, A.L. & Anderson, J.B. Reconstruction of the West Antarctic ice sheet in Pine Island Bay during the Last Glacial Maximum and its subsequent retreat history. Quaternary Science Reviews 21, 1879-1897 (2002).

28.          Jakobsson, M., Anderson, J.B., Nitsche, F.O., Dowdeswell, J.A., Gyllencreutz, R., Kirchner, N., Mohammed, R., O’Regan, M., Alley, R.B., Andandakrishnan, S., Eriksson, B., Kirshner, A., Fernandez, R., Stolldorf, T., Minzoni, R. & Majewski, W. Geological record of ice shelf break-up and grounding line retreat, Pine Island Bay, West Antarctica. Geology 39, 691-694 (2011).

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Antarctica

The Antarctic continent | The Antarctic Ice Sheets | Ice streams, subglacial lakes and ice shelves in Antarctica | Wildlife of Antarctica | Exploration of Antarctica | References | Comments |

The Antarctic continent

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Landsat Image Mosaic of Antarctica (LIMA) showing location of key ice shelves.

Antarctica: the enigmatic, romantic, remote white continent. Antarctica lies at the bottom of the world and all waters south of 60°S latitude are designated Antarctic, where no country owns the land and where only scientific and peaceful operations may take place. Military activity is banned in Antarctica, and it is a haven for wildlife.

Unlike the Arctic, where floating sea ice annual melts and refreezes, Antarctica is a solid ice sheet lying on a solid continent1. The Antarctic summer is during the northern Hemisphere winter. Antarctica may be remote and isolated, but the dynamics of Antarctic glaciers affect us all.

Antarctica is huge. The Earth’s southernmost continent is twice the size of Australia, and 98% of it is covered by ice. Antarctica is cold (the coldest recorded temperature is -89°C, from Vostok), but the peripheral islands and Antarctic Peninsula may have positive air temperatures in summer.

There is no permanent human population in Antarctica, but around 1000 people, mostly scientists and support staff, overwinter each year. Summer populations can be as high as 5000 (excluding the many hundreds of visitors who briefly visit on tourist ships). The British Antarctic Survey maintains eight research stations and operates many summer field camps each year.

Research stations and summer camps in Antarctica.

Research stations and summer camps in Antarctica.

You can use Google Earth to explore Antarctica for yourself. You can see how the great continent is surrounded by cold ocean waters. Note the Antarctic Peninsula, the thin spine of mountains pointing towards South America, the huge flat and floating ice shelves, and the large, high, East Antarctic Ice Sheet.

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Antarctic mountains

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 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 Antarctic continent lies on a large landmass. Underneath that smooth ice sheet there are mountains and valleys.

The surface of the Antarctic Ice Sheet is up to 4000 m high, and in places the ice is 4000 m deep, but the Gamburtsev Mountain range is up to 2,700 m high and lies underneath the East Antarctic Ice Sheet.

The Transantarctic Mountains divide East and West Antarctica. This mountain range is 3500 km long and 100-300 km wide. The summits of these mountains poke through the ice to form some of the only ice-free areas of Antarctica; these ‘nunataks’ are up to 4,500 m high.

The Transantarctic Mountains contain some of the oldest glacial sediments in Antarctica, and the Sirius Group, from Mount Sirius,  indicates that there has been ice here for at least 15 million years. This webpage has beautiful photographs of the Transantarctic Mountains.

You can use the Google Map below to easily explore the Transantarctic Mountains. You can see how they go through the ice sheet. In this map, the Byrd and Shackleton glaciers are in the centre, and they flow into the giant, floating, flat Ross Ice Shelf. How does this compare with the BEDMAP2 figures above and below?

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The first ice in Antarctica grew on the Transantarctic Mountains and Gamburtsev Mountains around 34 million years ago2, when global air temperatures were around 4°C warmer than today. Since then, with on-going cooling, the ice sheets have fluctuated, growing and shrinking at different timescales.

During the Quaternary Period, the ice sheets fluctuated first at 41,000 year timescales, and after around 1 million years ago, they fluctuated at 100,000 year timescales. These huge ice sheets came to dominate and influence the Earth’s climate and global sea levels. The last glacial cycle ended around 11,000 years ago and the Last Glacial Maximum was around 18,000 years ago.

Antarctica's subglacial topography, with main mountain ranges shown.

Antarctica’s subglacial topography, with main mountain ranges shown, using the BEDMAP2 dataset (Fretwell et al., 2012).

The Antarctic ocean

Simplified schematic map of ocean currents of the Southern Ocean.

Simplified schematic map of ocean currents of the Southern Ocean.

There is a strong circumpolar circulation around Antarctica. This results in a cooler continent, as heat exchange from the tropics is limited. The circulation in the Weddell Sea brings ice bergs and cold water north, up the Antarctic Peninsula, and is one of the reasons why the eastern Antarctic Peninsula is much warmer than the western Antarctic Peninsula.

Antarctica is globally important, and not just because melting Antarctic glaciers have the potential to raise global sea levels. Cold, salty water forms around Antarctica, which sinks to the sea floor and drives global ocean currents. The Global Thermohaline Circulation drives large currents around the world, and brings the warm Gulf Stream to Britain, moderating its climate.

Global thermohaline circulation. From: Wikimedia Commons

Global thermohaline circulation. From: Wikimedia Commons

The Antarctic Ice Sheets

East Antarctic Ice Sheet

Satellite image of the Dry Valleys

Satellite image of the Dry Valleys

There are three ice sheets in Antarctica; the East Antarctic Ice Sheet (EAIS), the West Antarctic Ice Sheet and the Antarctic Peninsula Ice Sheet. Each of these ice sheets has its own unique characteristics and behaviour. East Antarctica is grounded mostly above sea level and forms the bulk of the Antarctic Ice Sheet; if it melted, the East Antarctic Ice Sheet would raise global sea levels by 53 m3.

The EAIS holds the bulk of frozen fresh water on planet Earth, and it’s the highest, driest, coldest and windiest ice sheet in Antarctica by far.

In fact, the East Antarctic Ice Sheet is so cold and dry, it is the world’s most southerly desert. The Dry Valleys of East Antarctica receive around 10 mm of precipitation per year, and the mean annual air temperature is -19.8°C, making this one of the harshest places in the world.

West Antarctic Ice Sheet

Antarctic Ice Sheet without ice

Isostatically corrected Antarctic continent with the ice removed. From the Global Warming Art Project

The West Antarctic Ice Sheet is grounded largely below sea level. If it melted, it would raise global sea levels by a mere 3.3 m4, but unlike the East Antarctic Ice Sheet, rapid ice-sheet melt is a threat and a possibility.

The West Antarctic Ice Sheet is grounded well below sea level and the base of the ice sheet deepens landwards; it is therefore known as a “Marine Ice Sheet“.

The West Antarctic Ice Sheet is located in a region of rapid warming, and warm ocean waters threaten to melt the ice sheet at its base5.

During past interglacials, it is likely that the West Antarctic Ice Sheet almost entirely disappeared, and was left as a series of islands – as shown in the figure opposite. A future collapse of the West Antarctic Ice Sheet could rapidly raise global sea levels6. The likely hood of this happening, when it would happen and how long it would take is currently a topic of hot debate7.

Antarctic Peninsula Ice Sheet

Antarctic Peninsula Ice Shelves

Antarctic Peninsula Ice Shelves

The Antarctic Peninsula Ice Sheet is the smallest, holding only 0.24 m of sea level equivalent. However, this small ice sheet, situated on a mountain range, is perhaps the most vulnerable to climate change.

The glaciers of the Antarctic Peninsula are small and located in a region of rapid warming8. This has already resulted in numerous observable changes: collapsing ice shelves9, thinning and accelerating glaciers10-12, and widespread glacier recession13.

Ice streams, subglacial lakes and ice shelves in Antarctica

Ice Streams

Ice streams of Antarctica. From Rignot et al. (2011).

Ice streams of Antarctica. From Rignot et al. (2011).

The Antarctic Ice Sheets are not just domes of ice spreading slowly out to their margins. The Antarctic Ice Sheets are drained by fast-flowing ice streams14. The Twaites Ice Stream and Pine Island Glacier, for example, together drain 30% of the West Antarctic Ice Sheet.

Pine Island Glacier moves at about 4000 metres per year, and the stability and dynamics of this ice stream is essential for the stability of the larger Antarctic Ice Sheet. Ice streams send dendritic fingers deep into the Antarctic continent, and you can see on the figure of ice velocities the slow-moving ice divides at the centre of the different ice sheets.

Recent data published by Rignot et al. 2011 shows the ice flow across the Antarctic continent. This image, made from data downloaded from the NSIDC[5] is shown on alogarithmic scale. This emphasises the ice divides clearly. You can see, by comparing with the BEDMAP figure above, that these tend to follow the mountain ranges. Large ice streams drain into fast-flowing, floating ice shelves.

Subglacial Lakes

379 subglacial lakes have now been identified beneath the Antarctic continent. This map, using data from Wright and Siegert 2012 [1] shows that many are located in ice-stream onset zones as well as underneath slow-moving ice domes.

379 subglacial lakes have now been identified beneath the Antarctic continent. This map, using data from Wright and Siegert 2012 [1] shows that many are located in ice-stream onset zones as well as underneath slow-moving ice domes.

Despite being so cold, there is water at the base of the Antarctic Ice Sheet. The huge weight of the ice above melts ice at the base of the ice sheet, aided by geothermal heating. This water lubricates the base of the ice sheet, and helps the ice streams achieve their great speeds.

The water ponds in lows and hollows beneath the ice sheet, and it may exist at huge hydrostatic pressure, enabling water to flow uphill.

379 subglacial lakes have now been mapped across Antarctica15, and more are being found all the time. These subglacial lakes influence the behaviour of the ice streams of Antarctica, and drainage of lakes may add more water to the base of an ice stream – helping it to flow faster16.

Ice Shelves

Larsen Ice Shelf in 2004

Larsen Ice Shelf in 2004

Antarctica is fringed with ice shelves; in fact, 75% of the Antarctic continent is buttressed with ice shelves. Ice shelves are floating extensions of Antarctic glaciers, supplemented by snow fall directly onto the ice shelves and freezing of marine waters below5.

Ice shelves cover ~1.561 million km2, which is similar in area to the Greenland Ice Sheet. Ice shelves collect 20% of Antarctica’s snowfall and cover 11% of its area.

Ice shelves lose mass by melting from below and by calving ice bergs. In fact, basal melting from ice shelves accounts for most of the ice loss from Antarctica, and most of this ice loss comes from a few small ice shelves in West Antarctica and along the western Antarctic Peninsula5.

Sea Ice

Breaking sea ice with HMS Protector, northern Antarctic Peninsula, March 2012.

Breaking sea ice with HMS Protector, northern Antarctic Peninsula, March 2012.

Sea ice is seasonal and consists of frozen sea water, together with icebergs calved from Antarctic glaciers and ice shelves. Winter sea ice around Antarctica is increasing, in contrast with winter sea ice in the Arctic, which is decreasing.

This seasonal increase in sea ice may be due to colder, fresher water, released from the melting ice shelves, which accumulates in a cool, fresh surface layer and shields surface waters from the warmer, deeper waters that are melting the ice shelves17.

Wildlife of Antarctica

Adelie penguin

Adelie penguin

Antarctica is a wild continent. It is also largely deserted; all the wildlife lives in the ocean. It may come ashore briefly, but all the food is in the ocean. Small mites and springtails are the only animals that actually live on the small land oases around Antarctica. Birdlife, however, is prevalent in Antarctica.

Flying birds include Albatross, terns, cormorants, gulls, skuas, petrels and fulmar.

Penguins are Antarctica’s poster child, and Antarctica has seven species: Adélie penguins, chinstrap penguins, emperor penguins, Gentoo penguins, king penguins, rockhopper penguins and royal penguins.

The warm upwelling ocean currents around Antarctica make it a haven for sea animals, and this accounts for the high numbers of Antarctic whales and seals. There are true (earless) seals and fur seals, which have ear flaps.

Whales are common in Antarctica and for decades were hunted, in some cases nearly to extinction. The Antarctic Treaty has allowed some species to recover, although some are still vulnerable.

Exploration of Antarctica

Antarctica was first explored in the 19th Century. Captain James Cook, in the ships HMS Resolution and HMS Adventure, crossed the Antarctic Circle for the first time on 17th January 1773, and was repeatedly beaten back by sea ice.  Land was first sighted, probably around 1820. James Clark Ross sailed through the Ross Sea in 1841, and sailed near the Ross Ice Shelf.

Ernest Shackleton lead the Nimrod expedition in 1907 and reached the magnetic South Pole. An expedition led by Roald Amundsen reached the geographic South Pole on 14th December 1911.

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References

1.        Siegert, M.J. Antarctic subglacial topography and ice-sheet evolution. Earth Surface Processes and Landforms 33, 646-660 (2008).

2.        Siegert, M.J., Barrett, P., Decont, R., Dunbar, R., Cofaigh, C.O., Passchier, S. & Naish, T. Recent advances in understanding Antarctic climate evolution. Antarctic Science 20, 313-325 (2008).

3.        Fretwell, L.O., H. D. Pritchard, D. G. Vaughan, J. L. Bamber, N. E. Barrand, R. Bell, C. Bianchi, R. G. Bingham, D. D. Blankenship, G. Casassa, G. Catania, D. Callens, H. Conway, A. J. Cook, H. F. J. Corr, D. Damaske, V. Damm, F. Ferraccioli, R. Forsberg, S. Fujita, P. Gogineni, J. A. Griggs, R. C. A. Hindmarsh, P. Holmlund, J. W. Holt, R. W. Jacobel, A. Jenkins, W. Jokat, T. Jordan, E. C. King, J. Kohler, W. Krabill, M. Riger-Kusk, K. A. Langley, G. Leitchenkov, C. Leuschen, B. P. Luyendyk, K. Matsuoka, Y. Nogi, O. A. Nost, S. V. Popov, E. Rignot, D. M. Rippin, A. Riviera, J. Roberts, N. Ross, M. J. Siegert, A. M. Smith, D. Steinhage, M. Studinger, B. Sun, B. K. Tinto, B. C. Welch, D. A. Young, C. Xiangbin & Zirizzotti, A. Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. The Cryosphere 7, 375-393 (2013).

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

5.        Rignot, E., Jacobs, S., Mouginot, J. & Scheuchl, B. Ice Shelf Melting Around Antarctica. Science (2013).

6.        Vaughan, D.G. West Antarctic Ice Sheet collapse – the fall and rise of a paradigm. Climatic Change 91, 65-79 (2008).

7.        Bamber, J.L. & Aspinall, W.P. An expert judgement assessment of future sea level rise from the ice sheets. Nature Clim. Change 3, 424-427 (2013).

8.        Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carelton, A.M., Jones, P.D., Lagun, V., Reid, P.A. & Iagovkina, S. Antarctic climate change during the last 50 years. International Journal of Climatology 25, 279-294 (2005).

9.        Cook, A.J. & Vaughan, D.G. Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. The Cryosphere 4, 77-98 (2010).

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

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

12.      Pritchard, H.D. & Vaughan, D.G. Widespread acceleration of tidewater glaciers on the Antarctic Peninsula. Journal of Geophysical Research-Earth Surface 112, F03S29, 1-10 (2007).

13.      Cook, A.J., Fox, A.J., Vaughan, D.G. & Ferrigno, J.G. Retreating glacier fronts on the Antarctic Peninsula over the past half-century. Science 308, 541-544 (2005).

14.      Rignot, E., Mouginot, J. & Scheuchl, B. Ice Flow of the Antarctic Ice Sheet. Science (2011).

15.      Wright, A. & Siegert, M. A fourth inventory of Antarctic subglacial lakes. Antarctic Science 24, 659-664 (2012).

16.      Smith, B.E., Fricker, H.A., Joughin, I.R. & Tulaczyk, S. An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008). Journal of Glaciology 55, 573-595 (2009).

17.      Bintanja, R., van Oldenborgh, G.J., Drijfhout, S.S., Wouters, B. & Katsman, C.A. Important role for ocean warming and increased ice-shelf melt in Antarctic sea-ice expansion. Nature Geosci advance online publication(2013).

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