Antarctic ice shelves – the hidden villain

Sea ice and ice shelves

What is sea ice? Sea ice is frozen sea water; it perennially expands and contracts during each year’s winter and summer. Amongst the sea ice are icebergs calved from tidewater glaciers and ice shelves. Melting sea ice does not contribute directly to sea level rise (ice floats and displaces the same volume of water), but sea ice is important because it enhances climate warming. It changes the reflectivity of the sea water, reflecting lots of sunlight back (it has a high albedo), and is therefore an important component of the climate and cryospheric (icey) system.

Shrinking sea ice in the Arctic

Recent observations of sea ice extent in the Arctic have shown that the extent of sea ice in the Arctic has plummeted, with observed decreases over the last 50 years meaning that the annual sea ice now is at a minimum not reached for the last 1450 years¹. The decreasing extent of sea ice in the Arctic is often used as evidence for the impact of climate change on our world, and graphically illustrates how sensitive our cryospheric system is.

Growing sea ice in the Antarctic

However, sea ice extent in Antarctica is actually growing ². Year on year, there is more sea ice in Antarctica during the winter months. Surely this disproves the notion that climate change is occurring, and that our world is not as sensitive as we thought? Actually, no. There is a whole raft of data showing that the glaciers in Antarctica are responding rapidly to a changing climate, ranging from ice shelf collapse ³ as a result of melting from below and above ^4-7, to glacier recession ⁸, thinning ⁹ and acceleration ¹⁰. It turns out that all this activity has an impact on the ocean around Antarctica.

 

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).

Ice shelves melt because they are warmed from below by upwelling Circumpolar Deep Water¹¹. Circumpolar Deep Water is relatively warm, and accesses the undersides of ice shelves through deep submarine trenches. This warm subsurface water has enhanced the basal melting of ice shelves, leading to increased amounts cold, fresh water being released from the ice shelves. Together with the release of large quantities of floating ice to the Southern Ocean, the basal melting from the undersides of ice shelves also cools the surface waters of the Southern Ocean and results in a cold, fresh surface layer on the ocean’s surface – and decreased sea surface temperatures (SSTs) ¹². A paper just published in Nature Geoscience by Bintanja et al. (2013) indicates that this cool, fresh surface layer insulates sea ice from the warm upwelling waters that are melting ice shelves from below. A 100 m thick layer of sea water is separated from the saltier water below, and is cooled even more by the cold air above. Because these waters are cool and fresh (not salty), they freeze more easily, resulting in peaks in Antarctic sea ice in the autumn and early winter¹².

Sea ice extent and sea surface temperatures (SSTs) around Antarctica. From: Bintanja et al., 2013, Nature Geoscience. Reprinted by permission from Macmillan Publishers, Nature Geoscience, Bintanja et al. 2013, copyright 2013.

 

Changes in snowfall over Antarctica

Winkelman et al. (2013) argued that climate change may result in increased snowfall in Antarctica¹³ (although this is likely to be mostly moderated by increases in ice discharge). Most numerical models also predict strong decreases in Antarctic sea ice over coming decades. However, Bintanja et al. (2013) argue that increases in ocean temperature will continue to melt ice shelves from below, releasing cool waters onto the surface of the ocean and maintaining high sea ice cover. Precipitation (snowfall) over Antarctica is related to sea surface temperatures, so increased ice cover and decreasing sea surface temperatures over coming decades may well offset this projected precipitation increase. This may mean that the contribution to sea level rise from Antarctica over coming decades may be underestimated ¹².

References

1. Kinnard, C., Zdanowicz, C.M., Fisher, D.A., Isaksson, E., de Vernal, A. & Thompson, L.G. Reconstructed changes in Arctic sea ice over the past 1,450 years. Nature 479, 509-512 (2011).
2. Turner, J. & Overland, J. Contrasting climate change in the two polar regions. Polar Research 28, 146-164 (2009).
3. 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).
4. 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).
5. Rott, H., Skvarca, P. & Nagler, T. Rapid collapse of northern Larsen Ice Shelf, Antarctica. Science 271, 788-792 (1996).
6. 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).
7. De Angelis, H. & Skvarca, P. Glacier surge after ice shelf collapse. Science 299, 1560-1562 (2003).
8. 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).
9. 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).
10. 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).
11. 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).
12. 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).
13. Winkelmann, R., Levermann, A., Martin, M.A. & Frieler, K. Increased future ice discharge from Antarctica owing to higher snowfall. Nature 492, 239-243 (2012).