Melting Ice - Part 2 - Antarctica

And now for part 2 of my melting ice mini-series – Antarctica. This is a tricky one because although the Antarctic ice-sheets have been losing mass, on average Antarctic sea ice is increasing by ~1.5% a decade (IPCC). So what’s going on? And how is that rather large accumulation of ice on the other side of the world affecting sea level?

 

Antarctica: Making Australia look small since 35 Ma
(planetobserver)

So first, a little information about Antarctica, I am continually amazed by how big Antarctica actually is, I just don’t think pictures and maps get it across. Antarctica is bigger than the USA, and much of the continent is technically a desert (albeit rather colder than what initially comes to my mind when I think of deserts), with ice sheets up to 5km deep compressing the bedrock below. These ice sheets are made up of ~90% of the World's freshwater, and if they melted completely this would cause a ~70 metre rise in sea level (BAS). So clearly any changes in Antarctic ice mass are really, really important in terms of sea level.

 

 

Antarctica is divided by the Transantarctic Mountains which separate the East Antarctic Ice Sheet from the West Antarctic Ice Sheet. The ice sheets are reacting differently to climate change and will therefore affect sea level in different ways.

 

 

The Eastern Antarctic Ice Sheet (EAIS) is the more stable of the two because it rests on bedrock which is above sea level. While some parts of the ice sheet are showing decrease in ice mass, there are signs that Dronning Maud Laud and Wilkes Land are actually gaining mass as a result of increased snowfall. This doesn't necessarily mean it's getting colder though, more snow could actually be a result of atmospheric warming. The EAIS is currently growing at a rate of 25 Gt y^-1 (Shepherd and Wingham, 2007).

 

Meanwhile on the other side, the Western Antarctic Ice Sheet (WAIS) is resting on bedrock below sea level (NASA have compared West Antarctica to Hawaii because it is made up of lots of little islands!). Much of the base of this ice sheet is in contact with sea water - in some places the ice sheet base is up to 1700 metres below sea level. This is important because while the water is very cold there, it is still water. In this area the ocean is ~0.5°C  above freezing (Shepherd et al, 2004), this is significantly warmer than the ice sheets themselves. So in effect the very very cold sea water is warming the base of the glacier causing thinning of the ice sheet and ice to discharged into the oceans (Payne et al, 2004). The WAIS is currently shrinking at a rate of 50 Gt yr^-1 which easily cancels out any growing done by the EAIS (Shepherd and Wingman, 2007).
 

 

Rignot et al (2011) measured acceleration of ice sheet depletion between 1992 and 2009. They found acceleration in Antarctic ice depletion to be 21.9 Gt/yr² over this period, almost double that of mountain glaciers and ice caps. They suggest that if this trend continues the combined effect of melting ice in Greenland and Antarctica will become the main contributor towards rising sea level this century.

Ice Mass Change 2002-2009 (NASA)

 

So the ice sheets of Antarctica area changing a lot, and the huge volume of ice locked up at the South Pole has the capacity to have a huge effect on sea level around the world. Even though some parts of the Antarctic are accumulating ice, overall changes in ice at the South Pole are contributing towards sea level rise, and will become more important in the future. Both poles are being affected by climate change, and will undergo big changes in the future. I'm going to look into how changes in ice volume at the North Pole will affect sea level in the next episode of my melting ice mini- series (like all good series I'm finishing with a taster of what's to come), but if you are interested in how the other ways the Poles are being affected by climate change do check out Fiona's blog.