Friday, February 1, 2013

Antarctica in a changing climate


As you probably know, this year saw the Arctic summer sea ice reach its minimum extent since records began. It was recorded at 4.1 million square kilometres; around 50% lower than the 1979-2000 average. (see post here: http://on.fb.me/URg3KE)

Interestingly, over the same period, a study by the British Antarctic Survey (BAS) and NASA found that sea ice in the Antarctic has been increasing. The key words here are “sea ice”.

Unlike the Arctic, Antarctica is composed of both sea and land ice. This consideration is the most important thing we must remember when talking about the effects of climate change on the Antarctic. Sceptical arguments which suggest that Antarctica is “gaining ice” are relying on an error of omission; essentially ignoring the fundamental differences between land and sea ice.

The Antarctic land ice sheet is the largest single mass of ice on our planet, covering an area of almost 14 million square kilometres. It contains an enormous 30 million km3 of ice, containing around 90% of fresh water on Earth- the equivalent of 70 metres of water in the world’s oceans. (British Antarctic survey, 2012). It is this land ice mass that is the best representation of how climate change is affecting the continent. A study published in the journal Nature based on GRACE (Gravity Recovery and Climate Experiment) satellite data, has found that some areas of the Antarctic are not only losing ice mass rapidly, but also at an accelerating rate. Also, NASA scientists are currently keeping a close eye on a huge crack in the Pine Island Glacier in the Antarctic. Should this calf, it will produce an iceberg the size of New York City and will be the largest contribution to sea level rise from Antarctica to date.

On the other hand, it is true that while the land ice mass is decreasing, the sea ice extent is increasing. This is not unprecedented.

The Oceans around Antarctica consists of layers of cold water at the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures increase, the amount of rain and snowfall also increases. This “freshens” the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007)

Another reason for an increase in sea ice is attributable to another environmental problem; the hole in the ozone layer. The drop in ozone levels over the Antarctic has caused cooling in the stratosphere (Gillet 2003). As stratospheric temperatures affect wind velocity, the cyclonic winds that circle the Antarctic have strengthened (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas lead to increased sea ice production (Turner 2009).

The message to take away today is that the Antarctic is not the Arctic. It is a continent, surrounded by water, while the Arctic is a sea, surrounded by land.

You don’t have to be a scientist to understand that despite being polar; they are extremely different regions and each will be affected by a changing climate in different ways.

Information and references:

British Antarctic Survey: http://www.antarctica.ac.uk/press/press_releases/press_release.php?id=1967

NASA Climate Change: http://climate.nasa.gov/news/?FuseAction=ShowNews&NewsID=813

Previous TES post: http://on.fb.me/SNT7f3

Turner, J., J. C. Comiso, et al. (2009). "Annular atmospheric circulation change induced by stratospheric ozone depletion and its role in the recent increase of Antarctic sea ice extent." Geophys. Res. Lett. 36(8): L08502.

Zhang, Jinlun, 2007: Increasing antarctic sea ice under warming atmospheric and oceanic conditions. J. Climate, 20, 2515–2529.

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