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Every time I read a news article about Antarctic ice extent, I don't seem to have a clear answer as to what the deal is. If I look at the February sea ice extent from the National Snow and Ice Data Center (NSIDC), I get no trend that I can see: February extent.

If I look at the September data from NSIDC, then I get a clear increase.September extent. NSIDC even tells you what the trend is.

Monthly Antarctic September ice extent for 1979 to 2014 shows an increase of 1.3% per decade relative to the 1981 to 2010 average.

Finally, if I look at the monthly values it is even trickier: monthly extent

What I can tell is that the minimum extent seems to be unchanged, while there is an increase in the maximum extent. If I where to analyze the trend using all the data, I wonder if any significant trend would be extracted.

I don't want to create any controversies, as I realize that the gains in the Antarctic (small increase of ~100,000 km2 per decade) are much smaller than the loss in the Arctic (decrease of ~500,000 km2 per decade) (NSIDC).

My questions are: Considering the level of variability in coverage, can the Antarctic sea ice extent be considered at "record levels"? If so, what is the consensus on the causes of this increase?

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    $\begingroup$ Very important: The increase in the Antarctic is in sea ice. When you look at the Antarctic as a whole, the trend is very sharply downward. The slight increase in sea ice in Antarctic winter is just that, a slight increase. The loss of ice over the continent of Antarctica more than offsets this slight increase. $\endgroup$ – David Hammen Jan 17 '15 at 5:18
  • $\begingroup$ Do you want to add this aspect to the question? I could try as well $\endgroup$ – arkaia Jan 17 '15 at 5:23
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    $\begingroup$ This is a very good point, and has come up in the literature (see the 4th point at the bottom of my answer), the sea ice extraction of freshwater melt is a very small percentage of what is contributed from the melting of the continental ice. I have added a statement making that explicitly clear. $\endgroup$ – user889 Jan 17 '15 at 5:27
  • $\begingroup$ I do not know if this is true or not but if the ice is thinner it would be more spread out for the same volume $\endgroup$ – Catprog Jan 17 '15 at 9:28
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There have been a number of studies based on observations and modelling of the Antarctic Sea Ice trends. One major observation is that since continuous satellite coverage began in late 1978 (3), there has been an increase in annual Antarctic sea ice extent (1)(2)(3)(4), that reach ~28% of the Arctic sea ice loss (1).

Critically, according to Simmonds (2015) (1) (and Fan et al. 2014 (3)), that despite interannual, sub-decadal and multidecadal variations, there is a statistically significant increasing trend in sea ice extent for all seasons since late 1978 (As shown in the image below).

enter image description here

Caption (from the NSIDC): Arctic and Antarctic Sea Ice Extent Anomalies, 1979-2012: Arctic sea ice extent underwent a strong decline from 1979 to 2012, but Antarctic sea ice underwent a slight increase, although some regions of the Antarctic experienced strong declining trends in sea ice extent. Thick lines indicate 12-month running means, and thin lines indicate monthly anomalies.

Essentially, the sea ice extent annual trend is at record-levels since records began with continuous satellite coverage (from late 1978).

As to why this trend is occurring, there is a considerable amount of uncertainty (1)(2)(3)(4), particularly in reconciling modeled sea ice with the extent observed (1). Simmonds (2015) (1) suggest that this could just be a consequence of the different dynamics between the Arctic and Antarctic - i.e. one model does not fit both ice caps.

Observations documented by Fan et al (2014)(3) indicate that since 1979, there has been a decrease in sea surface temperatures (SST) and surface air temperatures around Antarctica, with the exception of the Antarctic Peninsula and surrounding seas. The trends indicate that in the southern summer (December-February) there is a trend of SST dropping by 0.6°C corresponding to a 9% increasing trend in sea ice extent during the same time - this is also noted by Simmonds (2015) (1) as being the greatest trend in sea ice extent. This is accompanied by intensified westerly winds and a strengthened meridional sea level pressure gradient, both in the southern summer and in the annual mean.

Fan et al. (2014) (3) suggest that there is evidence for a multidecadal climate variability regime occurring in the region - this stands to reason considering that the time of observations of sea ice extent has only been ~35 years.

There have been several theories suggested in the literature, below are a few of them:

  • Fan et al. (2014) (3) describe an earlier idea that the increase in greenhouse gases and ozone depletion may cause an increase in the increased westerly winds, there is considerable uncertainty in the interaction between these factors (4).

  • Models from Holland et al. (2014) (4) suggest a link between Antarctic-wide ice thickness and area trends contributing to overall sea ice trends.

  • Related to the models above, several papers suggest that the mechanism is related to increases in meltwater fluxes from the Antarctic continent itself (2)(3). The meltwater flux is modelled to have increased during the southern summer, depositing greater amounts of cool freshwater into the surface layers of the surrounding Southern Ocean, resulting in cooler SSTs in the southern summer, potentially causing the increase in summer sea ice extent trend.

  • However, most critically, Holland et al. (2014) (4) note that the freshwater 'extraction' by sea ice is a small percentage of the actual fresh meltwater flux. Instead they model that instead of freshwater flux, the driver is based on trends of Antarctic ice thickness and area.

This means that the increase in sea ice is not a 'counter balance' for the ice melted from the continental ice sheets, but rather, according to the models, the increasing sea ice extent appears to be a symptom of the continental ice sheet melting.

References

(1) Simmonds 2015 Comparing and contrasting the behaviour of Arctic and Antarctic sea ice over the 35 year period 1979–2013 Annals of Glaciology

(2) Bitanja et al. 2015 The effect of increased fresh water from Antarctic ice shelves on future trends in Antarctic sea ice Annals of Glaciology

(3) Fan et al. 2014 Recent Antarctic sea ice trends in the context of Southern Ocean surface climate variations since 1950 Geophysical Research Letters

(4) Holland et al. 2014 Modeled Trends in Antarctic Sea Ice Thickness, Journal of Climate (Full text here)

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    $\begingroup$ Always a pleasure to get an answer from you $\endgroup$ – arkaia Jan 17 '15 at 4:55
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In my survey of sea ice extent for the same period (1979-2015) I found that the apparent positive trend in dispersed sea ice in the Antarctic in September is anomalous and possibly spurious for two reasons: (1) there was no trend in concentrated sea ice extent or in the degree of sea ice dispersion, and (2) the residuals of the OLS trend line showed patterns that might violate OLS assumptions. Please see http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2598152

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