Why sea ice extent in Antarctica is growing slightly

While the Arctic sea-ice has experienced a dramatic reduction, Antarctic sea ice continues to increase in extent hitting a new record in October this year. So what's going on?

The trend of a slight gradual increase in Sea ice around Antarctica has puzzled scientists. Firstly, the gradual increase in Antarctic sea ice is far less than the amount of sea ice vanishing in the Arctic Sea. Global sea ice trend still shows a marked retreat.

But in Antarctica working out why the sea ice trend has seen a one percent increase per decade since the 1970s has bamboozled climate modelling for Antarctica. The scientists put it down to changes in atmospheric circulation caused by the ozone hole and the growing strength of the westerly circumpolar winds. A new study based upon 18 years of detailed satellite ice motion measurements has put forward that it is primarily local winds pushing ice mainly north creating polyanas in the ice flows where more ice can easily form.

This animated video on Antarctic Sea Ice Extent, 1979-2012 is by the National Snow and Ice Data Center (NSIDC) It presents an Animated map of 2012 sea ice extent shown side-by-side with 1979--2009 climatology.

Antarctica has it's own regional climate peculiarities. For many years scientists thought changed atmospheric circulation caused by the Ozone hole was the primary culprit. The Ozone hole above the continent makes the stratosphere colder which has strengthened the cyclonic winds around the continent. This was originally put forward in studies (See Gillet 2003, Thompson 2002, Turner 2009) as the main reason for increasing Antarctic sea ice. But a study in 2010 by Sigmond and Fyfe (Full paper PDF) developed a climate model using a forced observed stratospheric ozone depletion from 1979 to 2005 that simulated a a year‐round decrease in Antarctic sea ice due to stratospheric ozone depletion. "Our model results strongly suggest that processes not linked to stratospheric ozone depletion must be invoked to explain the observed increase in Antarctic sea ice extent." the authors summarise.

And hence the study by Paul R. Holland & Ron Kwok from NASA's JPL and the British Antarctic survey using Satellite based research for the period 1992 to 2010. The paper has just been published in Nature Geoscience on 11 November 2012. These scientists concluded sea ice drift is largely linked to local winds and is primarily responsible for the increase in sea ice around Antarctica.

Northerly winds blowing off the continent tend to drive the sea-ice away. Sea ice on the northern edge can also drift with the circumpolar westerlies. Sea-ice around the continent varies with some areas increasing more markedly, while others the extent of sea ice suffers a decline. But the overall long term trend since the late 1970's has been a very gradual slight increase in sea ice extent.

From the article - Wind-driven trends in Antarctic sea-ice drift (abstract).

 

Here we present a data set of satellite-tracked sea-ice motion for the period of 1992–2010 that reveals large and statistically significant trends in Antarctic ice drift, which, in most sectors, can be linked to local winds. We quantify dynamic and thermodynamic processes in the internal ice pack and show that wind-driven changes in ice advection are the dominant driver of ice-concentration trends around much of West Antarctica, whereas wind-driven thermodynamic changes dominate elsewhere. The ice-drift trends also imply large changes in the surface stress that drives the Antarctic ocean gyres, and in the fluxes of heat and salt responsible for the production of Antarctic bottom and intermediate waters.

 

Data on sea ice drift around Antarctica was collected from maps created by NASA's Jet Propulsion Laboratory (JPL) using over 5 million individual daily ice motion measurements captured over a period of 19 years by four US Defense Meteorological satellites.

Dr Paul Holland from the British Antarctic Survey and lead author of the study said:

"Until now these changes in ice drift were only speculated upon, using computer models of Antarctic winds. This study of direct satellite observations shows the complexity of climate change. The total Antarctic sea-ice cover is increasing slowly, but individual regions are actually experiencing much larger gains and losses that are almost offsetting each other overall. We now know that these regional changes are caused by changes in the winds, which in turn affect the ice cover through changes in both ice drift and air temperature. The changes in ice drift also suggest large changes in the ocean surrounding Antarctica, which is very sensitive to the cold and salty water produced by sea-ice growth.

"Sea ice is constantly on the move; around Antarctica the ice is blown away from the continent by strong northward winds. Since 1992 this ice drift has changed. In some areas the export of ice away from Antarctica has doubled, while in others it has decreased significantly.”

New Zealand climate scientist from NIWA, Dr James Renwick, explains what changes are occurring in the Antarctic in response to climate change and what's likely to happen in the future. He raises that although the extent of sea ice around Antarctic is increasing, there's a big question around how long this will continue.

 

Climate sceptics leap onto thin (Antarctic) Ice

Various sceptical climate commentators including Australian science commentator Jo Nova - Antarctic sea ice trends at record highs. Fears for shrinking southern ocean, right? - and Anthony Watts - According to NOAA data, all time Antarctic sea ice extent record was set on Sept 22nd, 2012 - leapt onto the initial reports of record Antarctic Sea Ice to attempt to dull the news of the drastic reductions in Arctic sea ice. But the rate of change of sea ice from both poles is markedly different, with the global sea ice trend still showing a pronounced downwards slant.

Anthony Watts followed up with an article criticising and denigrating the research as 'From the “no matter what happens it is climate change” department.' : Bipolar disorder – as in the Arctic, the Antarctic sea ice extent is affected by wind, unless of course it’s ‘climate change’

This video from the Yale Climate Forum highlights the gross inconsistencies in the sceptical arguments with regard to Arctic versus Antarctic Sea Ice. : "Suggestions that modest increases in sea ice around Antarctica offset significant losses in Arctic sea ice are based on a bogus "apples and oranges" comparison. Through interviews with a range of respected experts, Peter Sinclair's newest Yale Forum video explains why such suggestions do not stand up to scientific scrutiny.":

The drastic sea ice reduction in the Arctic is a major concern for it's albedo feedback effect adding to polar warming affecting the Greenland Ice Sheet. While Antarctic sea ice is expanding, mass loss from the Antarctica, particularly the West Antarctic Ice sheet, and from glaciers in the Antarctic peninsula is showing signs of accelerating and contributing to sea level rise. (see Global Warming in Antarctica: Thwaites and Pine Island Glaciers accelerating, West Antarctic Ice Sheet losing mass and Waking the giant: Global Warming in the Weddell Sea, West Antarctic Ice Sheet and sea level rise)

The last word goes to JPL scientist Dr Ron Kwok who commented in the media release: “The Antarctic sea ice cover interacts with the global climate system very differently than that of the Arctic, and these results highlight the sensitivity of the Antarctic ice coverage to changes in the strength of the winds around the continent.”

Sources