Antarctic Sea Ice Increase

There’s no doubt that the amount of sea ice in the Arctic has decreased, by a lot. I would even describe the decrease as “staggering.” Meanwhile, the amount of sea ice in the Antarctic has trended in the opposite direction, i.e. it has grown. Has its increase also been “staggering?” Let’s look at the data.

Here’s Antarctic sea ice extent anomaly, monthly data from NSIDC (the National Snow and Ice Data Center), with the baseline period for “anomaly” calculation selected as the entire time span, together with a lowess smooth (in red):


It certainly looks as though it has increased. The smooth, in particular, suggests an interesting pattern of increase. There seems to be what almost looks like a “step change” around 1993, and there seems to be what looks like a more steady increase starting around 2005.

Of course we know that “looks” isn’t sufficient reason to draw reliable conclusions about genuine changes, especially in noisy data. Linear regression confirms that indeed the change is “statistically significant” (although not necessarily actually “linear”) even when we apply a strict correction for autocorrelation. Yes, Antarctic sea ice has increased, and no, the increase isn’t an artifact of the noise.

We can see the same pattern in annual average Antarctic sea ice extent anomaly, on which I’ve superimposed the smooth based on monthly data:


The annual averages alone aren’t as suggestive of patterns as the monthly data. But they do emphasize that last year (2013) was a record-breaker, by a substantial margin.

The apparent “step change” in the early 1990s may have an explanation in terms of a change in the way data were processed. A paper currently in review at The Cryosphere Discussion (Eisenman et al. 2014, A spurious jump in the satellite record: is Antarctic sea ice really expanding?) states “Here, we show that the increase in the reported trend occurred primarily due to the effect of a previously undocumented change in the way the satellite sea ice observations are processed for the widely-used Bootstrap algorithm dataset, rather than a physical increase in the rate of ice advance.

The most unfortunate aspect is that the change was undocumented. But Eisenman et al. were able to locate data sets representing “before” and “after” the undocumented change, and show a highly suggestive graph of their difference:


The clear impact was to introduce a “step change” at precisely the moment when a sensor change occurred, December 1991. Their paper is specifically relevant to data produced with what’s called the “bootstrap” algorithm (which doesn’t actually relate to the statistical procedure known as “bootstrapping”).

I don’t know whether or not the data I’ve downloaded from NSIDC utilizes the bootstrap algorithm, or whether or not data sets using other algorithms exhibit a similar step change between different versions. But I do know that the moment of the sensor change coincides well with the timing of the apparent “step change” in the NSIDC data.

Therefore I took the NSIDC data from its start up to November 1991 and computed its average, then I took the NSIDC data from December 1991 through the end of 2004 (since the recent increase seems to start right aroung 2005), and computed the difference between their averages. The Dec.91-to-Dec.04 data is, on average, 0.19 million km^2 larger than the Nov.78-to-Nov.91 average. Could that difference be due to some processing issue at the time of a sensor transition?

Next I took the anomaly data and added 0.19 million km^2 to everything before December 1991. That makes the data look like this:


This makes it seem entirely plausible that the apparent step change in the NSIDC monthly data is also due to some processing issue at the time of the sensor transition. I consider this especially true because I don’t know of a plausible physical mechanism which would produce a step change — but I don’t really know much about the physics behind Antarctic sea ice change, so take that opinion with a large dose of salt.

On the basis of the clear step change, Eisenman et al. also suspect that a correction is called for at the time of the sensor change. They go further, asking in the title of their paper the suggestive question “is Antarctic sea ice really expanding?” and stating in the abstract “The results of this analysis raise the possibility that this expansion may be a spurious artifact of an error in the satellite observations, and that the actual Antarctic sea ice cover may not be expanding at all.”

I don’t agree. Even using the “adjusted” data I’ve created after adding 0.19 million km^2 to everything before Dec. 1991, the trend from linear regression is still statistically significant. Even when applying a strict correction for autocorrelation. I think they’ve made a compelling case for a step change having occurred, and a good case that earlier data should be higher (or later data lower), but that the idea “actual Antarctic sea ice cover may not be expanding at all” is contradicted by the data. The increase in Antarctic sea ice cover is “robust.”

So (since this is a “discussion” journal) I submitted a comment to the discussion saying just that. Even before I did, Kevin Cowtan made a similar suggestion, asking the authors to test the trend’s sensitivity to a possible correction. Not long after, Paul Holland pointed out that the other (non-bootstrap) data sets also showed increase, and suggested (and I agree) that the growth of Antarctic sea ice is robust.

There are certainly complications (and potential dangers) with open-discussion journals like The Cryosphere Discussion, but I think in this case the system worked like it’s supposed to. We’ll see how Eisenman et al. respond to these comments.

The more interesting issue, in my opinion, is what has happend since 2005. The linear trend rate over that time span is 77 thousand km^2/yr:


However, the noise is substantial compared to the trend, so there’s considerable uncertainty about the size of the trend; I estimate the 95% confidence interval to be 77 +/- 66 thousand km^2/yr.

That’s quite substantial. But I wouldn’t call it “staggering,” for two reasons. First, it really has only been expanding rapidly since about 2005, which is just not very long (in climatological terms), and as a result the rate of increase is highly uncertain (although, uncertainty cuts both ways and it could be nearly twice the estimated rate). The next few years will dramatically increase our understanding of how Antarctic sea ice is really trending.

The second reason is that, all other things being equal, we expect changes in Antarctic sea ice to be larger than those in the Arctic for purely geographical reasons. As Eisenman has already pointed out, the Arctic is an ocean surrounded by land while the Antarctic is land surrounded by ocean. Even a small change in the location of the Antarctic “ice edge” therefore translates into a much larger change in area or extent than would an equivalent change in location of the ice edge for the Arctic.

I took the Arctic sea ice area (rather than extent) and transformed it to the latitude of the “equivalent ice edge” using the approximation formula published by Eisenman. I also took the Antarctic sea ice area and transformed it to the latitude of the equivalent ice edge, by using a very simple model of the Antarctic sea ice as an annulus extending from latitude 70S to the edge of the ice. It’s a crude model indeed, but it does capture the essential geometry of the situation and I think is at least realistic enough to put the changes in perspective.

I then transformed those “equivalent latitudes” into anomaly values in order to remove the seasonal cycle. This enables us to compare directly the equivalent latitude changes between the two hemispheres. In both cases, increased latitude means migration toward the poles and therefore less ice (like in the northern hemisphere) while decreased latitude means migration away from the poles and therefore more ice (like in the southern hemisphere). Here’s the comparison:


In terms of smoothed equivalent latitude, the Arctic ice pack has receded poleward by 2.6 degrees of latitude while the Antarctic ice pack has advanced by only 0.4. That’s 48 km advance in the south compared to 290 km retreat in the north, larger by a factor of 6.

Latitudinal range isn’t the whole story, and the sheer size of the increase in Antarctic sea ice is important to bear in mind. But the latitudinal range is also an important indicator of how severe are the climatological changes taking place. So here, to put things into one perspective, is a comparison of just the smoothed values:


I think that shows, rather effectively, a very important difference between the changes we’ve seen in the Antarctic ice pack, and those seen in the Arctic.

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23 responses to “Antarctic Sea Ice Increase

  1. What are we measuring? Area or rate of change to amount of ice?

    Because the area of the ice could grow, yet the average thickness of sea ice could be radically smaller. And why do we measure sea ice on the fringes of a continent that has average thickness of 7,000 feet? – gravity pushes that ice toward the sea. The Pine Island Glacier complex of WAIS flows to the sea at rate of 4000 meters per year and “… the Pine Island Glacier has been losing 20 billion tons of ice annually for the past two decades and could lose 100 billion tons annually over the next 20 years. ”

    None of that would show in ice area measurements. Shouldn’t we ask for anomalies to ice thickness above sea level for the entire continent?

    • Chris O'Neill

      The volume of ice flowing into the sea from Antarctica would be far more than the volume of ice flowing into the Arctic ocean from Greenland and the Canadian islands. I wonder what difference this would make to sea ice?

  2. The importance of all this is the radiative effects, no? And that relates to area, not equivalent latitude anomaly.

    [Response: I estimated the relative contributions from albedo change here.

    While area is a big factor in the *effect* of sea ice changes, It seems to me that latitude is a more important indicator of the climatological *cause*.]

  3. > The importance of all this is the radiative effects, no?

    “Pine Island Glacier’s grounding line is probably engaged in an unstable 40 km retreat. The associated mass loss increases substantially …”

  4. B Buckner, if you are concerned about the effect on albedo, you cannot exclude the reduction in NH snow extent from the equation. That makes the radiative effects of NH cryosphere changes much larger than is indicated by Tamino’s analysis of sea ice only shows, and larger again relative to SH changes.

  5. Surely, while the increase in Antarctic winter sea ice extent is something of a puzzle, it’s really of no major interest to anyone except those denying climate science?

    I say this because in the Arctic the most frequently referenced measure of warming is summer sea ice extent; and, as has been discussed, the annual decrease is staggering. So shouldn’t it be the same measure—summer sea ice extent—which is the only meaningful indicator to compare warming in the Antarctic? This is where polar comparisons become meaningless; because the Antarctic summer sea ice extent is as compromised by land as is the Arctic winter sea ice extent. How do we know that in the Arctic winter, without the constraint of the Canadian and Siberian coasts, sea ice wouldn’t be increasing there, which would perhaps make this Antarctic phenomenon less of a peculiarity?

    Mark Brandon’s latest post prompted my response:

  6. “I don’t know whether or not the data I’ve downloaded from NSIDC utilizes the bootstrap algorithm,”

    We don’t know either, NSIDC distributes sea ice concentration data from at least three different algorithms (the third is Nasa Team 2). That is not counting the NOAA/NSIDC Climate Data Record (CDR) which is combination of Bootstrap and Nasa Team, trying to use the strength of each method and avoiding each weak area’s.

    For reasons of both long term data coverage and careful processing for consistency of the data there is a strong reason why the NSIDC Sea Ice Index should be used:

    “Sea Ice Index images depict ice cover and trends in ice cover in the Arctic and Antarctic oceans. Sea Ice Index data files tabulate ice extent in numbers. The images and data are produced in a consistent way that makes the Index time-series appropriate for use when looking at long-term trends in sea ice cover.”

    The index is used in the NSIDC Sea Ice News and Analysis. It is based on the Nasa Team algorithm and is available from 1978-2012. The 2013 data is marked “near-real-time”. This data can and will change when it appears in the set data marked “final”.

  7. I was wondering if the alleged problem affected the Arctic record, too. The full paper is online here. [OK, to link to an 'open access' paper?]

    The main findings discussed in Sect. S2 include [...] (4) that there was also a spurious jump in the Arctic sea ice extent trend between the IPCC AR4 and IPCC AR5 but it was relatively small compared with the real change in trend associated with adding several more years to the record…

    I’m curious to know what “relatively small” means, and in which direction the trend/acceleration may be affected, assuming the paper raises valid points.

    Also, there are a number of research groups that calculate sea ice data, with different algorithms (at least 7 according to this paper), or hybrid versions incorporating Bootstrap and NASA Team algorithms with others. Would comparison provide a useful check on the hypothesis?

    [Response: That would seem, to me, to be a very interesting and possibly fruitful investigation.]

  8. Actually, the increase in sea ice is very interesting. It could be caused by several different factors, and understanding them helps us to understand sea-ice processes. Hypothesis 1: surface layer freshening plus increased snowfall builds ice faster in Antarctic winter. Hypothesis 2: Increased winds move ice away from the coast faster in winter, causing increased ice production in coastal polynyas. Hypothesis 3: Increased South-North heat transport causes Antarctica to stay relatively cool at the surface, allowing more ice to build.

    Interesting science appears when data appear anomalous to understanding. It is clear that Antarctic and Arctic response to global warming are fascinatingly different.

    • To expand on the “surface layer freshening”: increase rate of ice sheet melt over land due to increased air temps -> more freshwater available to freeze. So global warming may counterintuitively lead to more sea ice in the Antarctic.

  9. I keep hearing that Antarctic sea ice is a mystery but when I look at temperature trend maps the ocean area around Antarctica appears to have cooled over the past few decades. Why would we expect anything other than flat to increasing sea ice given this regional sea surface cooling?

    • Interesting point, but there’s a bit of a ’cause vs. effect’ question there, surely, since the presence or absence of sea ice affects both actual SST and the measurement thereof. And even if cooling SSTs were unambiguously causing faster rates of sea ice formation, we’d still have the ‘mystery’ as to why SSTs were cooling when most of the planet is warming.

      • On your first point, yes, although trends for SH Summer, when sea ice cover only really exists around the coasts, also indicate cooling to about the same degree.

        [Response: I don't know what your point is.]

        It’s true we’d still have the mystery of Antarctic SSTs, but talking about that makes more sense to me than the common suggestion that sea ice isn’t responding as expected to climate conditions. As far as we can tell surely Antarctic sea ice actually is responding pretty much as expected, but it’s the conditions themselves which are divergent from most model simulations.

        [Response: I'm skeptical]

        It would interesting to see a model run with a fixed annual sea ice cycle to give some comparative idea for how much sea ice changes contribute to regional surface temperature trends.

  10. [Response: I don't know what your point is.]

    Kevin was suggesting that regional surface cooling might have been caused by the sea ice increase rather than being a causal factor itself. It’s possible of course. However, there has been no increase in open ocean Antarctic sea ice in the SH Summer months because it’s always essentially zero. This could be considered a basic test case for the question of whether the cooling in other seasons has most likely been influenced by sea ice increase. Since we do see cooling for these months it seems reasonable to suggest that sea ice may not be the key driver of the temperature change.

    In spirit of moving to a question of what is causing the cooling I can offer Korhonen et al. 2010, which suggests increasing circumpolar wind speeds may have enhanced sea spray aerosol numbers, and therefore low cloud albedo, around the Southern Ocean.

  11. It seems that the “skeptics” think that if the decline in arctic sea-ice is “proof of global warming”, then surely an increase in sea-ice is “proof of no global warming”. Reality is not as simple as that.
    Let’s forget about any possible cause of warming and just think of the consequences. What would we expect to happen with sea-ice, if there were a global warming?
    1) We might expect increased melting, so less ice
    2) As warmer air can take up more moisture, and what goes up must come down, there should be increased precipitation, which in cold places occurs as snow, so more ice.
    The result is a tug-of-war between those two effects.
    Globally the increased melting should be dominant, but with strong regional and seasonal variations.
    So far we have not used any fancy computer modelling, just high-school physics. Computer models allow to predict when and where which of the effects may be dominant. For instance, they predict that for arctic sea-ice, the “melting” team would be a safe bet, whereas the antarctic sea ice with its annual cycle, any trend is small in relation to the amplitude of seasonal variations, so it is too close to call.
    So it may be a bit more complex than some people think, but not so complex that you could say “models fail” and “scientists have no clue”.

  12. > Kevin McKinney | February 5, 2014 at 8:09 pm
    Very relevant; leads to a lot of material. 10 months since that was published; I didn’t find much new when I started to wonder how long such a lens of fresh melt water can persist holding up a shallower layer of heavier cold salt water — whether the stack breaks up with the springtime melt of sea ice and reforms in the fall, for example.

    There are ARGO instruments in the area, and instruments carried by deep-diving elephant seals.

    I found one mention that “In the Antarctic Zone, the summer [Sea Surface Salinity] signature shows distinct biannual variations that are correlated with upstream sea-ice coverage in the preceding spring.”
    Journal of Marine Systems
    Nineteen-year changes in surface salinity in the Southern Ocean south of Australia
    Volume 129, January 2014, Pages 472–483

  13. Thanks, Hank. Another good link.

    • Confused, what is confusing? You had a big fluctuation downward on a generally downward trend–and what happens after a big downward fluctuation? An upward fluctuation. The year-on-year trend is still down strongly. It’s a difference of whether the Arctic is ice-free in 2025 or 2030.