22
$\begingroup$

As can be seen from the below data, for the past year or so (2014), sea ice surrounding Antarctica has been well above normal. Currently, Antarctic sea ice is at an all-time record high for this time of year, 1,344,000 square kilometers above normal (or about the size of Spain, France and Germany put together).

Is there any explanation for such an extreme excess of sea ice around Antarctica recently?

Antarctic Sea Ice

Antarctic See Ice anomaly

Source of data is the US National Snow and Ice Data Center sea ice news for April 2014 (for the upper graph) and University of Illinois's "The Cryosphere Today" (for the lower graph).

In the above link, the National Snow and Ice Data Center states "Antarctic sea ice extent for April 2014 reached 9.00 million square kilometers (3.47 million square miles), the largest ice extent on record by a significant margin."

Update 6/30/2014 :

Now the anomalously high southern sea ice has reached a record level for any time of year, at more than 2 million square kilometers above normal.

enter image description here

$\endgroup$
  • $\begingroup$ RELATED: Is the Arctic sea ice volume increasing? $\endgroup$ – blunders May 13 '14 at 15:53
  • 3
    $\begingroup$ @blunders Maybe there is a superficial relationship in that both questions are about sea ice. However, the situations in the arctic versus antarctic are much different. In the arctic, there is a strong long-term downward trend, with ice nowhere near record levels, whereas in the antarctic there is somewhat of a long-term upward trend, and ice has been at record levels recently. $\endgroup$ – DavePhD May 13 '14 at 16:21
  • 3
    $\begingroup$ @blunders yes, from the US National Snow and Ice Data Center monthly sea ice news at: nsidc.org/arcticseaicenews "Antarctic sea ice extent for April 2014 reached 9.00 million square kilometers (3.47 million square miles), the largest ice extent on record by a significant margin." $\endgroup$ – DavePhD May 13 '14 at 16:58
  • 3
    $\begingroup$ @blunders and also from the University of Illinois "The Cryosphere Today" website (for the second plot in the question), at arctic.atmos.uiuc.edu/cryosphere $\endgroup$ – DavePhD May 13 '14 at 17:02
  • 1
    $\begingroup$ The questions are related, even though the Arctic and Antarctic are very different environments. $\endgroup$ – gerrit May 13 '14 at 19:28
21
$\begingroup$

Antarctic sea ice extent for April 2014 reached 9.00 million square kilometers (3.47 million square miles), the largest ice extent on record by a significant margin.

Is there any explanation for such an extreme excess of sea ice around Antarctica recently?

I'll give four separate answers:

  1. It's not as significant an increase as you think.
  2. The Interdecadal Pacific Oscillation is strongly in its negative phase.
  3. The Southern Annular Mode is at a 1000 year high.
  4. Natural and man-made variations have collectively caused increased melting of the Antarctica ice sheet.

It's not as significant an increase as you think.

It's important to remember what that "largest ice extent on record" means. That "on record" means in the satellite era. Data from before the satellite era (pre-1981) are rather spotty. Except they aren't.

There's a nice proxy for sea ice extent, at least during Antarctic summer, in the locations at which whalers made their catches and established their factory ships (Williams 1997). In the first half of the 20th century, whalers sought species that concentrated near the edge of the Antarctic sea ice. By the early 1930s whalers had developed a practice of anchoring factory ships within 20 km of the ice edge and followed the ice southward as summer progressed. This practice continued throughout the 1930s and resumed after WWII through to the mid 1950s. The practice was later restarted in the mid to late 1970s. Every whale caught by Norwegian whalers was tagged with date, latitude, and longitude. The practice of setting up shop near the ice edge gave Williams a nice way of estimating the pre-satellite era ice extent.

What Williams found was that summer Antarctic sea ice suffered a massive decline that continued throughout the 1950s and 1960s. The ice levels we are seeing now, at least during Antarctic summer, are on par with the levels seen eighty years ago.

The Interdecadal Pacific Oscillation is strongly in its negative phase.

The Interdecadal Pacific Oscillation (IPO) is highly correlated with a hiatus in perceived global warming and also with increased precipitation in the Antarctic (Meehl 2013). An increase in precipitation means that surface waters are less salty (increasing the freezing point) and cooler. The combination of reduced sea surface temperatures and reduced salinity makes for increased ice formation during the Antarctic winter during periods when the IPO is in its negative phase.

The Southern Annular Mode is at a 1000 year high.

It has long been suspect that stronger winds are one of the causes of the increase in the Antarctic sea ice levels (Zhang 2007, Zhang 2014). Winds help create polynyas, open areas of water surrounded by ice. That open water is subject to freezing during the Antarctic winter. Not only are the winds strong now, they are stronger than they have been for at least 1000 years (Abram 2014).

Natural and man-made variations have collectively caused increased melting of the Antarctica ice sheet.

Just as increased precipitation leads to increased ice formation, so does increased melting of the continental Antarctic ice sheet. And the ice sheet is melting (Shepherd 2012, Rignot 2013). Gravity measurements from the GRACE satellites, combined with ice height measurements from ICESat collectively show that Western Antarctica and the Antarctica Peninsula are decreasing in mass, and at a significantly greater rate than the slight increase in the mass of the Eastern Antarctica ice sheet. This influx of cold, fresh water during the Antarctic summer makes for increased sea ice production during the Antarctic winter.


References

Abram, N. et al., "Evolution of the Southern Annular Mode during the past millennium," Nature Clim. Change, (2014)

Meehl, G. et al., "Externally forced and internally generated decadal climate variability associated with the interdecadal pacific oscillation,"J. Climate 26 (2013), 7298–7310.

Shepherd, A. et al., "A Reconciled Estimate of Ice-Sheet Mass Balance," Science 338.6111 (2012): 1183-1189.

Rignot, E. et al., "Ice-Shelf Melting Around Antarctica," Science 341.6143 (2013): 266-270.

William, K. "Abrupt mid-twentieth-century decline in Antarctic sea-ice extent from whaling records," Nature 389.6646 (1997): 57-60.

Zhang, J., "Increasing Antarctic sea ice under warming atmospheric and oceanic conditions," J. Climate, 20 (2007), 2515-2529.

Zhang, J., "Modeling the Impact of Wind Intensification on Antarctic Sea Ice Volume," J. Climate, 27 (2014), 202–214.

$\endgroup$
  • 1
    $\begingroup$ Thanks for the answer, I really appreciate the whaling reference with data from 1931-87, it's worth an upvote for that alone. However, your statement "The ice levels we are seeing now, at least during Antarctic summer, are drastically reduced (by 25% or more) from the levels seen eighty years ago" is incorrect. William's data shows a summer ice decline of 25% to 1987. As can be see in this data: arctic.atmos.uiuc.edu/cryosphere/… all the years 1979-87 had annual minima < 2 million sq.km, but 2013 and 2014 minima where ~2.5 million sq. km. $\endgroup$ – DavePhD May 14 '14 at 0:30
  • $\begingroup$ Great answer (much better than mine). By a hiatus in perceived global warming, I suppose you are talking exclusively about mean atmospheric temperatures (not considering the ocean heat content)? It could be good to point that out explicitly (I make this comment wherever the “hiatus” is mentioned...). $\endgroup$ – gerrit May 14 '14 at 15:11
8
$\begingroup$

(Note: this is based on what I found in literature. Sea ice is not my expertise.)

Short answer: We don't know.

It may be related to changes in atmospheric temperature, wind stress, precipitation, ocean temperature, changes in coastal polynas, or other factors. The usual way to explain observed behaviour is through models. However, models are currently unable to represent the observed behavior.

The IPCC AR5 WG1 report, chapter 4 (Vaughan et al., 2013), addresses Antarctic sea ice. Firstly, as your question addresses overall winter sea ice, it is important to note that trends vary as a function of season and location:

The seasonal trends are significant mainly near the ice edge, with the values alternating between positive and negative around Antarctica. Such an alternating pattern is similar to that described previously as the Antarctic Circumpolar Wave (ACW) (White and Peterson, 1996) but the ACW may not be associated with the trends because the trends have been strongly positive in the Ross Sea and negative in the Bellingshausen/Amundsen seas but with almost no trend in the other regions (Comiso et al., 2011). In the winter, negative trends are evident at the tip of the Antarctic Peninsula and the western part of the Weddell Sea, while positive trends are prevalent in the Ross Sea. The patterns in spring are very similar to those of winter, whereas in summer and autumn negative trends are mainly confined to the Bellingshausen/Amundsen seas, while positive trends are dominant in the Ross Sea and the Weddell Sea.

IPCC AR5 WG1 Figure 4.7
(a) Plots of decadal averages of daily sea ice extent in the Antarctic (1979–1988 in red, 1989–1998 in blue, 1999– 2008 in gold) and a 4-year average daily ice extent from 2009 to 2012 in black. Maps indicate ice concentration trends (1979–2012) in (b) winter, (c) spring, (d) summer and (e) autumn (updated from Comiso, 2010). Source: Vaughan et al. (2013)

A recent article on the topic is by Holland and Kwok (2012). In the abstract, they summarise:

The sea-ice cover around Antarctica has experienced a slight expansion in area over the past decades1, 2. This small overall increase is the sum of much larger opposing trends in different sectors that have been proposed to result from changes in atmospheric temperature or wind stress3, 4, 5, precipitation6, 7, ocean temperature8, and atmosphere or ocean feedbacks9, 10.

The article itself is behind a paywall. Back to the IPCC AR5 WG1 chapter 4 (Vaughan et al., 2013):

An increase in the extent of coastal polynyas in the Ross Sea caused increased ice production (latent heat effect) that is primarily respon- sible for the positive trend in ice extent in the Antarctic (Comiso et al., 2011). Drucker et al. (2011) show that in the Ross Sea, the net ice export equals the annual ice production in the Ross Sea polynya (approximately 400 km3 in 1992), and that ice production increased by 20 km3 yr–1 from 1992 to 2008.

So, the best answer we can give so far is: we don't really know.


Vaughan, D.G., J.C. Comiso, I. Allison, J. Carrasco, G. Kaser, R. Kwok, P. Mote, T. Murray, F. Paul, J. Ren, E. Rignot, O. Solomina, K. Steffen and T. Zhang, 2013: Observations: Cryosphere. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

Holland, Paul R., and Ron Kwok. "Wind-driven trends in Antarctic sea-ice drift." Nature Geoscience 5.12 (2012): 872-875.

$\endgroup$
3
$\begingroup$

I looked at trends in Antarctic sea ice extent in the satellite era from 1979 to 2015 for both the summer minimum (February) and the winter maximum (September) and did not find a statistically significant trend. Please see:

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2598152

$\endgroup$
  • $\begingroup$ But your paper explicitly states in section 7.1: "The data provide sufficient evidence of a positive trend in dispersed sea ice in the Antarctic in September for the sample period 1979-2014. Dispersed winter sea ice extent in the Antarctic grew larger over the sample period at a rate that cannot be explained by random and unbiased measurement error or by random natural variations." Why are you contradicting your own paper here? $\endgroup$ – DavePhD May 8 '15 at 15:01
  • $\begingroup$ good point. thanks. if you look at dispersed extent in isolation it looks like a rising trend but when you consider dispersed extent together with concentraded extent and degree of dispersion the finding for dispersed extent looks spurious. the more accurate way to say that is that my test for trends in the antarctic in sept is inconclusive. thank you again for your astute comment. $\endgroup$ – Jamal Munshi May 10 '15 at 14:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.