This article explores the potential contribution Antarctica could make to global sea level rise by investigating the current state of Antarctic ice masses and discussing how past Antarctic ice volumes can inform us about what may happen in the future.
It is worth noting that Antarctica is not the only contributor to sea level rise: other melting ice masses (such as Greenland) are contributing water to the oceans, and thermal expansion as the waters warm is increasing the volume of the ocean (NASA, no date).
Figure 1. Sea ice breaking up as 'pack ice' in the Weddell Sea, West Antarctica.
(British Antarctic Survey, 2015)
Learning from the Past
If all of Antarctica’s ice were to melt, global average sea levels would rise by approximately 58 metres (Fretwell et. al., 2013). There’s no need to worry about this happening anytime soon; a realistic estimate of Antarctica's contribution to sea level rise is 1 metre by 2100 (DeConto and Pollard, 2016). However, these estimates remain uncertain. Evidence from the Last Interglacial (130,000-115,000 years ago) and the Pliocene (3 million years ago) suggest that similar climate scenarios to today’s could produce significantly higher sea levels (DeConto and Pollard, 2016). During the Last Interglacial, global mean atmospheric temperatures were between 0-2°C warmer and ocean temperatures were around 4°C higher than today. As a result, sea levels were 6-9.3 metres higher, which is thought to be mainly due to the warmer ocean temperatures. Antarctic melting is thought to have contributed 3.6-7.4 metres during this period, with most of the remaining amount coming from the Greenland Ice Sheet. To put this into context, the Paris Agreement aims to keep atmospheric warming below 2°C above pre-industrial levels, but even this target is thought to be unattainable based on the pledges different nations have made. Also, in the last 100 years, ocean surface temperatures have increased by approximately 0.13°C - this doesn’t sound much, but predictions in the 2013 IPCC Report suggest that mean global ocean temperature is likely to rise by 1-4°C by 2100 (United Nations Climate Change, no date; Rogelj et. al., 2016; IUCN, no date). In short, a similar climate scenario to the Last Interglacial could be recreated by human influence: if ocean temperatures rise by 4°C and atmospheric temperatures by 2°C, will predictions of 1 metre of Antarctic sea level rise by 2100 be accurate?
Figure 2. Ice core data from EPICA Dome C showing the link between temperature and atmospheric carbon dioxide concentrations over the last 800,000 years
(British Antarctic Survey, 2015).
The Last Interglacial gives a good comparison of temperature values to today and the future, but the Pliocene can provide a comparison of atmospheric carbon dioxide (CO₂) concentrations – another important factor to consider with regard to melting ice. Temperature is the direct control over how much ice will be lost or gained, but greenhouse gases – including CO₂ – are closely linked to temperature. Discovering what temperature various greenhouse gas concentrations have occurred alongside in the past could help to predict what temperatures our current greenhouse gas levels will give in the future. Figure 2 shows how temperature and carbon dioxide levels have followed a very similar pattern of fluctuation over the last 800,000 years. During the Pliocene, 3 million years ago, atmospheric CO₂ levels were relatively high at ~400ppm but still slightly lower than today’s levels of ~410ppm (DeConto and Pollard, 2016; NASA, 2019). Sea levels were between 10-30 metres higher than today, with the majority (5-20 metres) of that coming from Antarctica. Alongside high CO₂ levels, the Pliocene had much warmer summer air temperatures which were capable of melting vast quantities of surface ice on a scale large enough to explain the high sea levels. The climate system is significantly more complex than being able to link CO₂ directly to temperature, but the relationship between the two can act as an indicator of what may happen in the future.
Figure 3. Map of Antarctica showing the West and East Antarctic Ice Sheets
(Aster and Barletta, 2018).
What About Now?
The potential contribution of Antarctica to sea level rise, both from the West and East Antarctic Ice Sheets (WAIS and EAIS, Figure 3), make it essential to continually monitor their current condition. The WAIS is deteriorating much more than the EAIS: it may have crossed a tipping point from which there is no going back (Lenton et. al., 2019). A tipping point is reached when abrupt and irreversible change occurs. For the Amundsen Sea coastline of the WAIS, there is evidence to suggest it may have passed a tipping point where the ice has begun to retreat irreversibly. Although this is only one part of the WAIS, models have suggested it could cause the whole WAIS to destabilise and collapse, resulting in 3 metres of sea level rise over a period of hundreds to thousands of years. The rate over which sea level rise will happen depends on the magnitude of atmospheric warming; a 1.5°C warming scenario could take 10x longer than a >2°C warming scenario to raise sea levels by the same amount (Lenton et. al., 2019). Much of the WAIS – particularly the Antarctic Peninsula and Pine Island Glacier – has also experienced dramatic mass loss, with ecological effects already being shown (Boening et. al., 2012; Welch, no date). The much larger EAIS is relatively stable compared to the WAIS: temperature is not the only factor affecting mass loss, the rate at which snow and ice is being replaced by precipitation must also be considered. In fact, between 2009-2011 part of the EAIS experienced a period of high snow accumulation equivalent to 0.32mm per year global sea level decrease due to increased precipitation (Boening et. al., 2012). However, between 1992 and 2017, the WAIS melted enough to contribute ~5.7mm to global sea level rise, whilst the EAIS accumulated snow equivalent to ~1.1mm of global sea level decrease (Shepherd et. al., 2019). So despite the EAIS accumulation, the Antarctic Ice Sheet is still losing mass overall through intense thinning of the ice sheet and retreat of ice shelves in the WAIS.
References
Aster, R. and Barletta, V. R. (2018) The West Antarctic Ice Sheet is in trouble – but the ground beneath it may buy some time. Available at: https://theconversation.com/the-west-antarctic-ice-sheet-is-in-trouble-but-the-ground-beneath-it-may-buy-some-time-98368 (Accessed 13 February 2021).
Boening, C., Lebsock, M., Landerer, F. and Stephens, G. (2012) ‘Snowfall-driven mass change on the East Antarctic ice sheet’. Geophysical Research Letters, 39(21), L21501.
British Antarctic Survey (2015) Ice. Available at: https://www.bas.ac.uk/about/antarctica/geography/ice/ (Accessed 13 February 2021).
DeConto, R. M. and Pollard, D. (2016) ‘Contribution of Antarctica to past and future sea-level rise’. Nature, 531, pp. 591-597.
Fretwell, P., Pritchard, H., Vaughan, D., Bamber, J., Barrand, N., Bell, R., Bianchi, C., Bingham, R., Blankenship, D., Casassa, G., Catania, G., Callens, D., Conway, H., Cook, A., Corr, H., Damaske, D., Damm, V., Ferraccioli, F., Forsberg, R., Fujita, S., Gim, Y., Gogineni, P., Griggs, J., Hindmarsh, R., Holmlund, P., Holt, J., Jacobel, R., Jenkins, A., Jokat, W., Jordan, T., King, E., Kohler, J., Krabill, W., Riger-Kusk, M., Langley, K., Leitchenkov, G., Leuschen, C., Luyendyk, B., Matsuoka, K., Mouginot, J., Nitsche, F., Nogi, Y., Nost, O., Popov, S., Rignot, E., Rippin, D., Rivera, A., Roberts, J., Ross, N., Siegert, M., Smith, A., Steinhage, D., Studinger, M., Sun, B., Tinto, B., Welch, B., Wilson, D., Young, D., Xiangbin, C. and Zirizzotti, A. (2013) Bedmap2: improved ice bed, surface and thickness datasets for Antarctica. The Cryosphere, 7(1), pp. 375-393.
IUCN. Ocean Warming. Available at: https://www.iucn.org/resources/issues-briefs/ocean-warming (no date) (Accessed 13 February 2021).
Lenton, T. M., Rockstrom, J., Gaffney, O., Rahmstorf, S., Richardson, K., Steffen, W. and Schellnhuber, H. J. (2019) ‘Climate tipping points – too risky to bet against’. Nature, 575, pp. 592-595.
NASA (2019) The Atmosphere: Getting a Handle on Carbon Dioxide. Available at: https://climate.nasa.gov/news/2915/the-atmosphere-getting-a-handle-on-carbon-dioxide/ (Accessed 13 February 2021).
NASA. Understanding Sea Level: Thermal Expansion. Available at: https://sealevel.nasa.gov/understanding-sea-level/global-sea-level/thermal-expansion (no date) (Accessed 13 February 2021).
Rogelj, J., den Elzen, M., Höhne, N., Fransen, T., Fekete, H., Winkler, H., Schaeffer, R., Sha, F., Riahi, K. and Meinshausen, M., 2016. Paris Agreement climate proposals need a boost to keep warming well below 2 °C. Nature, 534(7609), pp.631-639.
Shepherd, A., Gilbert, L., Muir, A., Konrad, H., McMillan, M., Slater, T., Briggs, K., Sundal, A., Hogg, A. and Engdahl, M. (2019) ‘Trends in Antarctic Ice Sheet Elevation and Mass’. Geophysical Research Letters, 46(14), pp. 8174-8183.
United Nations Climate Change. The Paris Agreement. Available at: https://unfccc.int/process-and-meetings/the-paris-agreement/the-paris-agreement (no date) (Accessed 13 February 2021).
Welch, C. The Big Meltdown. Available at: https://www.nationalgeographic.com/magazine/graphics/antarctica-climate-change-western-peninsula-ice-melt-krill-penguin-leopard-seal (no date) (Accessed 13 February 2021).
ABOUT THE AUTHOR
Hannah is a BSc Geography graduate of Royal Holloway, University of London and is currently studying MSc Climate Change at UCL. Hannah is interested in a career in sustainability, particularly within the built environment or energy sectors where there is still great potential to reduce greenhouse gas emissions.
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