Making sense of Antarctic sea ice changes

December 9, 2014 1:13 pm10 comments

Antarctic sea ice is one of those things in the climate system that seems to confuse people. Antarctic sea ice, on average, is increasing. How can there be global warming if sea ice is increasing in the Antarctic? Some have gone so far as to average the Arctic sea ice loss with the Antarctica sea ice gains, and imply that globally sea ice isn’t changing. That’s just silly. Even so, it’s fair to say that most of the popular explanations for Antarctic sea ice expansion haven’t been very convincing.

In this essay, I’ll try to explain where the confusion about Antarctic sea ice changes comes from, and to highlight a few recent papers in the scientific literature that add important new clarity to the picture. The bottom line is that scientific understanding Antarctic sea ice trends is actually pretty solid.

S_03_trnd

Antarctic sea ice trend for March (late summer), 1979 through 2014. Areas where sea ice is expanding are shown in red. Source: National Snow and Ice Data Center (NSIDC.org).

First, let’s talk about what determines how much Antarctic sea ice there is. As with Arctic sea ice, it’s a combination of thermodynamics and dynamics: the rate of cooling to the atmosphere vs. the delivery of heat from the ocean below, and the movement of sea ice by the winds and by surface ocean currents. In general, Antarctic sea ice forms near the coastline, where upwelling waters cool to the atmosphere. It melts when the winds and currents push it into areas of warmer water to the north. In the summer, it melts pretty much all the way back to the coast; the 24-hour sunlight provides plenty of energy to make that happen.

An efficient way to form lots of Antarctic sea ice during the autumn growth season is to have strong winds that push the ice away from the coastline. Pushing sea ice away leaves open water that can lose heat to the atmosphere, creating more sea ice. Near the coastline, the major source of this “push” is the katabatic winds, resulting from the flow of very cold dense air from the high elevations of Antarctic continent; katabatics are especially strong in fall and winter and conspire with the extreme cold of the polar night to cause the annual growth of sea ice. Further north, the persistent circumpolar westerlies of the “screaming sixties” help the sea ice break up, and push it into warmer waters. (Owing to the Coriolis effect, westerly winds cause northward-flowing surface ocean currents in the Southern Hemisphere). Sea ice rarely extends northward of about 55°S (the tip of South America). Note that the role of wind in the Antarctic is different from the situation in the Arctic, where sea ice movement towards warmer climes is restricted by the continents.1

Author: Dr. Eric Steig

Author: Dr. Eric Steig

The importance of the winds in controlling Antarctic sea ice leads to the obvious idea that changing winds can explain the increase that has been observed over the last several decades. In particular, it follows from the above discussion that if either the katabatic flow from the Antarctic continent, or the westerlies were to increase, we would expect Antarctic sea ice to expand. There has indeed been a substantial increase in the circumpolar westerlies; this is very well established from observations and is associated with the oft-discussed increase in the “Southern Annular Mode” (SAM) index2. Averaged over the year, the SAM index has increased nearly monotonically since the 1970s (e.g., Marshall et al., 2003).

So at first glance, it really is very simple: the westerly winds have increased, so sea ice has increased too. Furthermore, there is good evidence that the increasing westerlies are a response to anthropogenic climate forcing from CO2 and other greenhouse gas increases in the troposphere, along with ozone declines in the stratosphere (Thompson and Solomon, 2002; Thompson et al., 2011). This would suggest that the observed increase in Antarctic sea ice extent is anthropogenic in origin, just like the Arctic sea ice decline, but for very different reasons.

It sounds pretty good right? Reduced ozone in the stratosphere, and increased CO2 in the troposphere — both climate forcings that are unequivocally anthropogenic — cause increased westerly winds, which cause Antarctic sea ice to expand.

Of course, it’s not that simple. For one thing, the average increase of Antarctic sea ice is actually a small number that is the difference of two big numbers — modest increases over a large area, mostly in the Eastern Hemisphere, and very large decreases over a smaller area in the Western Hemisphere. The map below, showing change in the length of the sea ice season over the last 30 years, illustrates this point well. In spite of the average increase, there are very rapid declines in the Bellingshausen and Amundsen Seas, comparable to sea ice declines in the Arctic.

Trend in the length of the sea ice season, 1979-2010. Blue and purple areas show areas where sea ice is declining, orange and red where it is increasing. Source: Maksym et al., 2012.

Another problem with explaining Antarctic sea ice expansion in terms of the circumpolar winds is that the only season is which there is a significant trend in the westerlies is austral summer. There is a weak positive trend in fall, but both spring and winter show no trend; the SAM trends in these seasons may even be slightly negative, depending on which data are used (Ding et al., 2012). Yet the pattern of sea ice change is quite similar in all seasons: decreasing along the Pacific coast of West Antarctica, and increasing around most of East Antarctica, and in the Ross and Weddell Seas.

Another problem is that modeling studies that have examined the relationship between the westerly winds and Antarctic sea ice have come up with results that appear to be in direct opposition to the observations. When fully coupled climate models are run with increased CO2 and decreased stratospheric ozone, the westerly winds increase as has been observed, but sea ice decreases around most of Antarctica. In fact, in at least one study (Bitz and Polvani, 2012), the pattern of trends is the mirror image of the observations, with increases, rather than decreases in the Amundsen and Bellingshausen Seas3!

bitzpolvani_f1d

Annual mean response of sea ice concentration to ozone depletion in a fully coupled climate model (CCSM3, and 1° resolution). Thick black contour shows the marks the winter edge (15% concentration); thin black lines show areas where the change is statistically significant. Note that in this figure, red means a decrease in sea ice. Source: Bitz and Polvani, 2012, Figure 1d.

So what’s really going on? Before I get to that, I want to note a couple of other ideas that have been suggested but which probably don’t provide the key answer. One idea is that model resolution might be a problem. Big climate models are generally run at relatively low spatial resolution, typically at about 2° latitude and longitude. That’s very high compared with a decade ago but is insufficient to resolve key processes such as ocean heat transport by small scale eddies. Bitz and Polvani looked at this using a very high resolution model (0.1°). Their results show only a somewhat weaker response to the ozone forcing experiment but with the same (incorrect) sign. Another idea is that changes in ocean stratification might be important. One way to increase sea ice growth is to increase the amount of fresh water getting into the Southern Ocean, which could happen from increased rainfall or from glacier melt. The latter is happening, of course, in spades (see the latest data from Sutterly et al., 2014 for example). Fresh water forms a sort of buoyant lid on the ocean, limiting the ability of heat from the warmer water below to get to the sea ice and melt it. A study by Bintanja et al. (2013) showed that it was a least plausible that this explains the Antarctic sea ice change. A basic problem, though, is that the greatest discharge of meltwater is occurring in the Amundsen Sea, exactly where sea ice is declining.

Here’s what I think is really going on.

First, comparing observations with the results of model experiments like those of Bitz and Polvani (2012) is misleading. Most such experiments are equilibrium experiments: What’s done is to run a model under “preindustrial” conditions, and then to run it again with reduced ozone and increased CO2, and to look at the difference. This provide a measure of what will eventually happen (at least in the model) after many decades or centuries. This is a sensible thing to do, because it is much more computationally expensive to do transient simulations. Also, is often reasonable to assume that the short-term (transient) response will look like the long-term (equilibrium) response; it will just be smaller in magnitude. But for this particular problem, things don’t work this way — the sign actually changes through time. When you look at the transient response to changes in the circumpolar winds, as Marshall et al (2014) have done, it turns out that two important things happen. The winds tend to push the sea ice boundary northward, as we would have expected. But also, the winds push the surface ocean northward too, and cause a slow rise in the isopycnal surfaces (surfaces of constant density). This brings relatively warm deep water closer to the surface, eventually melting sea ice after a period of a few decades, countering the initial increase in sea ice. These results explain why equilibrium model calculations find sea ice decreasing in response to ozone forced changes in the circumpolar winds, and also why observations show the opposite. Not enough time has passed for the equilibrium response to be manifested. Theses results suggest that some time in the next few decades, there will reverse, and average sea ice will begin to decline.4

Second, there’s a whole lot more going on with the winds than just “increased westerlies”. In fact, in the areas where the big sea ice losses have occurred, the concept of “circumpolar westerlies” isn’t very relevant. A far more important measure of wind variability in the Amundsen and Bellingshausen Seas is the Amundsen Sea Low (ASL).5 The ASL describes the average location of storms systems the bring heat and moisture into West Antarctica. Changes in the ASL may occur for myriad reasons, but one big hammer that can make it ring is the propagation of atmospheric planetary wave arising out of the tropics, more-or-or less associated with ENSO (El Niño-Southern Oscillation) variability. It’s been clear for many years that ENSO variability play a significant role in sea ice variability in those regions, and recent work shows that this can explain the trends pretty well too (e.g. Yuan and Li, 2008; Stammerjohn et al., 2008). Not incidentally, the adjacent land areas of the Antarctic Peninsula and the West Antarctic Ice Sheet have warmed significantly over the last few decades (Steig et al, 2009; Orsi et al., 2013Bromwich et al, 2013), and those changes can also be attributed largely to tropical climate variability (Schneider and Steig, 2008; Ding et al., 2011; Schneider et al., 2012; Steig et al., 2013). The cause of temperature and sea ice change is the same: more warm air is being steered into West Antarctica, and the atmospheric flow tends to push sea ice against the continent, keeping it from expanding.6

So, do we get the right answer if we take into account all of the wind changes that have occurred over the last few decades? The answer is yes. This is nicely illustrated in a study by Holland and Kwok (2012), who showed that wind, ice motion, and ice concentration changes match each other remarkably well. Where the wind has been increasingly northward, concentrations are increasing; where wind and ice motion changes are toward the continent, ice concentrations are decreasing.

ngeo1627-f3

Trends in sea ice concentration (colors) and ice drift (arrows) at top, compared with trends in sea level pressure (bottom) and wind speed near the surface (arrows) at bottom. From Holland and Kwok (2012).

Moreover, this year, Holland et al. (2014), showed that when they drive an ocean and sea ice model with observed winds — not just increased westerlies, but the full range of wind changes, as calculated by the ECMWF (European Center for Medium Range Weather Forecasting) –- they correctly simulate the overall expansion of sea ice, and they also get the pattern of changes pretty much spot-on. To be sure, authors note that not all the details are explained and highlight the possibly greater importance of thermodynamic consideration (i.e. ocean temperature/stratification) in some areas than in others.  Further, the period they address (1992-2010 only) is short.  Still, the results are nevertheless pretty compelling. Just like the observations, their calculations show large decreases in the Amundsen and Bellinghausen seas, but increases nearly everywhere else.7

jcli-d-13-00301.1-f7

Modeled vs. observed changes in sea ice, 1992-2010. Source: Holland et al, 2014.

 

 

 

 

 

 

 

 

 

 

Taken as a whole, these results show that there is no significant contradiction between our understanding of Antarctic sea ice and the observation that it is, in average, expanding. We can explain sea ice trends in the Antarctic rather well if we take into account the full range of changes in winds that have occurred. The average expansion of Antarctic sea ice was not anticipated, but it hardly represents any sort of existential threat to our fundamental understanding of the climate system as a whole. It’s merely an interesting scientific challenge.

Of course, predicting how sea ice will change in the future is another matter. Turner et al. (2013) show that “hindcasts” of the late 20th and early 21st century, using the latest-generation fully coupled ocean-atmosphere models, generally get the trends wrong. Our track record, in other words, isn’t very good. But the uncertainty here lies not so much in our understanding of sea ice per se, but rather in our ability to predict the details of the wind and ocean changes that drive sea ice change. Various model imperfections, and in particular the representation of the tropical atmosphere, can result in wind biases and therefore sea-ice biases around the Antarctic continent (e.g. Song et al., 2011). We aren’t going to get projections (or hindcasts) in the Amundsen Sea area right if we don’t get details of the tropical Pacific right — and that’s a notoriously difficult problem and an active area in climate dynamics research.


Featured Image: “NASA’s DC-8 Flying Over the Weddell Sea” (11/17/2011) by NASA GISS via Flickr.

Notes.

1This is not to say that winds don’t matter in the Arctic – they do. It does not, however, appear to be as dominant a factor. See for example Lindsay et al.’s (2009) paper on the 2007 Arctic sea ice anomaly.

2The SAM index can be defined a number of different ways, but essentially a measure of the pressure or geopotential height difference between the high and mid latitudes. For example, the commonly used Marshall SAM index is based on the difference of sea level pressure (SLP) between 40°S and 65°S. Since winds flow clockwise around regions of low pressure in the Southern Hemisphere, an increase in the SAM index — which means decreasing pressure at high latitudes relative to low latitudes — has to be associated with increasing westerlies.

3Whether the mirror image character of Bitz and Polvani’s (2012) results is just chance, or might tell us something fundamental, is an interesting question that, as far as I’m aware, has yet to be addressed.

4Call me a skeptic, but I think that the results of Marshall et al. (2014), which suggest that eventually sea ice will start to shrink in response to wind-driven changes in the ocean, needs further evaluation. It’s one thing to bring more warm water upwards towards the surface. It’s another thing for that heat to diffuse through the stratified upper water column, and melt sea ice.

5Some researchers talk about the variability in the Amundsen Sea as being part of the “asymmetrical response of the SAM” but I find this terminology misleading. Changes in the ASL may sometimes be associated with changes in the overall circumpolar westerlies, but they may also occur quite independently. Further, changes in the ASL can change the SAM index, even when there is no change in the circumpolar westerlies per se. Ding et al., 2012 estimated that 25% of the variability in the SAM index is due to tropically-forced variability in the ASL, entirely independent of CO2 and ozone forcing (see also L’heureux and Thompson, 2012 and Seager, 2003).

6These wind changes may also figure prominently in the thinning of West Antarctic glaciers, but that’s a subject for another essay, which I’ll write up for RealClimate.org. For those that are interested, see Steig et al. (2012) and Dutrieux et al., (2014).

7 The full magnitude of average sea ice increase modeled by Holland et al. (2014) is not quite as large (about 70% as big) as the observations suggest, but this is hardly a reason for concern. Some recent results suggest, in any case, that errors in the observations have led to an overestimate of the trend (Eisenman et al, 2014).

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THE FORUM'S COMMENT THREAD

  • Eric –
    Wouldn’t it be accurate to say that most experts have been confused by the growth of Antarctic Sea Ice? It wasn’t predicted. Even now, the Marshall et al. paper, the only one you mention that provides an explanation for the inability of equilibrium models to produce a larger sea ice extent, is one you regard as needing further substantiation.
    Meanwhile, since you attribute the winds that drive the Antarctic Sea Ice growth largely to AGW, would it be reasonable to consider this growth as a negative feedback, at least during the early non-equilibrium stages of warming? Has anybody attempted to calculate the size of the sea ice negative feedback under that assumption?
    – John

  • John,

    Thanks for the comments and questions.

    First, let me emphasize that I am not making an argument that the sea ice trend is anthropogenic; nor am I arguing the reverse. What I was trying to do was to explain what I think has become a sort of “standard answer” (westerly winds) and then point out the various problems with that idea (doesn’t explain the seasonal changes; doesn’t explain the decline in sea ice in the Pacific sector; appears not to work in models). The most recent work from Marshall et al. (2014) addresses only the last of these.

    With respect to further evaluation of the Marshall work, their results are unequivocal as to why equilibrium experiments do not show the increase in sea ice that would be expected from increased westerlies. The part that I think needs further evaluation is the projection implied by equilibrium models that eventually there will be widespread decline, and that this will be due to rising isopycnal surfaces. That’s quite different from the question of whether the increased westerlies explain part of the sea ice trend that has been observed. Of course, certainly more evaluation there is needed on all these aspects. I don’t mean to imply (to borrow a phrase) that the “science is settled”. For example, the Holland et al. paper does not equivocally support increased westerlies as being the primary driver either. They imply that for the East Antarctica, thermodynamic considerations not fully accounted for may still play a major role

    That said, there is no question that changing winds as a whole – not just the westerlies, but everything — go a very long way to explaining what we have observed. The major point I am trying to get across here is that there is absolutely no reason why we’d expect, a priori, that the winds would change in a way that would lead to sea ice decline. It is true that the increase was not anticipated, but nor should anyone with a basic understanding of the processes controlling Antarctic sea ice have expected decline, either. Simple one-cause answers for observed changes in the system are all too easy to make. Even among experts it’s perhaps a little too easy to think only about thermodynamics (warming earth = warming ocean = sea ice loss) and forget about dynamics. It’s equally easy to go with the “it’s all winds” idea, and ignore the other processes going on. But most important to my point: it’s also too easy to assume that just because some model experiments don’t match observations that we “can’t model” or “don’t understand” Antarctic sea ice. The Holland et al. (2014) makes it clear (at least to me) that we can, and do.

    As for they question about feedbacks, I think you pose an interesting question, but one I don’t feel qualified to answer at the moment. Perhaps this is a good theoretical study someone could do. Of course, there are certainly papers that look at the magnitude of climate feedbacks expected from the Antarctic sea ice response to climate forcing, but I’m not very up to speed on that literature. In any case, the answer may change quite a bit, once the recent discovery that Antarctic sea ice may actually be quite a lot thicker than once thought, is taken into account. See Maksym et al., 2014). There’s also a reasonable write up on this, here: http://www.livescience.com/48880-antarctica-sea-ice-thickness-mapped.html (but please ignore the statement about “the mystery” of Antarctic sea ice; this is exactly the kind of misleading reporting that I’m responding to with my essay!).

  • Eric –
    Thanks! On to somewhat broader issues…

    There’s a general tendency in science to assume that you understand the reason for something happening if the outcome agrees with your prediction. I see this in particular with some physicists (no offense intended) who think they understand atmospheric dynamics. Occasionally a completely off-the-wall cause-and-effect statement will appear in print, completely wrong but stated confidently by a physicist who in most other circumstances is probably much smarter than I!

    In that spirit, my impression as an interested non-expert has been that the experts assumed that Antarctic sea ice would probably decline (the thermodynamic mechanism) and was surprised that wind changes would be large enough to dominate the thermodynamic mechanism.

    Meanwhile, in the Arctic, when the dramatic 2007 summer ice loss took place (also unexpected in GCM projections at the time), the message that seemed to emerge was that the thermodynamic explanation is correct, it’s just a lot stronger than we thought. Still at the time it was apparent that winds accounted for most of the interannual variability in summertime Arctic sea ice extent, and that winds were particularly important in 2007. This quickly polarized (no pun intended), with skeptics arguing that winds were the cause and mainstream scientists criticizing skeptics for pretending or imagining that short-term variability equalled long-term variability.

    So, at the science end, have there been any studies that address long-term sea ice loss in the Arctic (wind vs temp) the same way that the studies you discuss above address long-term sea ice loss in the Antarctic (wind vs temp)? Ideally using the same model? From first principles, I would expect Arctic winds to play less of a role overall, since during winter the Arctic fills with sea ice and the winds can’t blow ice away to expose additional open ocean to freezing.

    At the public end, we seem to be in the following position. I’m oversimplifying, but the public won’t get the nuances.

    Arctic Ice: Scientists say temperature, skeptics say wind
    Antarctic Ice: Scientists say wind, skeptics say temperature (and therefore there’s no global warming)

    It’s so easy for a critic to say that scientists are picking whatever answer serves their purposes. On the other hand, if a theory is correct, every correct answer WILL be consistent with that theory. The trap that scientists need to avoid is the converse: assuming without thinking too hard about it that an answer that’s consistent with the theory is the correct answer.

  • Thanks for the delightfully detailed analysis. If we go further back to the analysis of the 1976-1998 period by Zwally et al (2002), http://onlinelibrary.wiley.com/doi/10.1029/2000JC000733/abstract

    They noted that ” It is also qualitatively consistent with the counterintuitive prediction of a global atmospheric-ocean model of increasing sea ice around Antarctica with climate warming due to the stabilizing effects of increased snowfall on the Southern Ocean.” How does the more recent research evaluate this sentence?

  • Jimmy Hansen (real name?) in the open comments section seems to have missed the point of the post, which is precisely that although the Antarctic sea ice growth has been presented as a “contradiction”, it isn’t. It only appears to be a contradiction if one sticks with a very simplistic conceptual model of how the planet work.

    John notes that “if a theory is correct, every correct answer will be consistent with that theory” This is of course true, but not very useful. A more subtle but equally accurate statements is that if a theory is incomplete (which it will always be, for as complex a system as our planet), then observations will often appear to be in conflict with the theory, even when they are not (and similarly, observations will often appear to in agreement with the theory, when they are not). This is applied physics, not elementary particle physics.

  • Eric – I agree. It’s in scientists’ interest in papers and press releases to maximize the extent to which their results are new and unexpected. Then the press, which needs to tell a story if it wants to engage most of the public, plays up the surprise angle even more.

    The net result of all of that is the the public receives a false picture of science dealing with a whole lot of surprise and conflict when usually (not always) everything just gradually makes more and more sense (the normal science of Kuhn).

  • I should add that skeptic blogs treat everything that doesn’t fit with their imagined view of consensus science as being contradictory. Few such blogs seem to care about how things actually work.

  • John, regarding your last comments on “skeptic blogs”, your comment about “normal science” in the Kuhnian sense is interesting. What explains the “skeptic” view, then, is the belief that there is a scientific revolution underway — that the scientific understanding of climate is about to undergo a paradigm shift. In that context, it makes perfect sense that each and every small thing that seems, at first glance, to be a contradiction to the paradigm must actually be a contradiction. Most scientists, on the other hand, don’t think such a revolution is likely. (Of course, one could also argue that we have recently undergone such a revolution — until the mid 20th century, most scientists didn’t really think we could have such a huge impact on the atmosphere as we are clearly having. Some still hold to that view, because they are stuck in the pre-revolutionary paradigm. In other words, the “skeptics” (and I use that word advisedly) think they are revolutionaries, but really they may be reactionaries. Only time will tell for sure of course.

  • Eric,

    Thank you for this very helpful and clearly-written post. Sorry to be late to the comment thread, but I see this as helping to clarify the processes as well as dispel common misconceptions. I certainly plan to refer to this in my teaching.

    My reading is that the very common misconception you’re dressing is that simple “thermodynamics” controls the extent of sea ice. That is, we simplistically assume that sea ice extent is all about “freezing and melting.” But actually a lot of the year-to-year and even decade-to-decade variations are controlled by “dynamics,” which means much greater variability.

    Unfortunately, the distinction between “dynamics” and “thermodynamics” is part of climate scientists’ jargon, and these phrases are usually gobbledygook for a lay reader. But almost anybody can understand the idea that when wind pushes ice away form the frozen coast, more ice can form behind it; and alternately when ice is pushed into warmer waters at lower latitudes it is likely to melt.

    A key and very helpful distinction you make is the interesting asymmetry between the Arctic (a polar ocean surrounded by land) and Antarctic (a polar continent surrounded by ocean), which is really an accident of geography. Arctic sea ice is confined to the highest latitudes by the surrounding coastline, whereas Antarctic sea ice is frequently buffeted around by winds.

    In addition to the “corralling” presence of Arctic Canada, Greenland, and Eurasia, Arctic sea ice is subject to the very strong positive albedo feedback when bright polar ice is replaced by dark ice-free ocean. In Antarctica, by contrast, the pole will remain white for many centuries or millennia even under catastrophic warming because it’s covered by thousands of meters of land ice. There’s also a very important but less-recognized long wave feedback due to increased water vapor (and maybe clouds) in the Arctic winter as the ice-free area expands (Melissa Burt presented this last week at AGU). Neither of these strong radiative feedbacks can operate poleward of about 70 S, but both are very powerful north of 70 N.

    I read your post as explaining that (1) the trend in mean sea ice extent in the Antarctic is best explained as the small difference between much stronger but opposing trends in waters off of East and West Antarctica; and (2) that these regional trends are driven by notoriously noisy patterns in winds linked to tropical climate variability. Neither of these is a simple GHG-driven trend. Rather we should see these rapid increases (in the East) and decreases (in the West) in the same light that we view changes in the behavior of the winter jet or the tropical Trade Winds. These changes are really part of “climate variability” rather than secular “climate change.”

    Again, thanks for the time and effort you invested here in explaining and summarizing an interesting and technical literature so that nonspecialists can understand it!

    Scott Denning

  • Scott,

    Thanks for the comments. I agree entirely with your comments.

    Many readers — particularly on my RealClimate post on this — seem to have thought I was arguing that the sea ice trends are clearly anthropogenic phenomena. I am not.
    There *may* be a forced component to the sea ice increase, but it’s too early to tell.

    Eric Steig

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PUBLIC COMMENT THREAD

  • http://atmo.tamu.edu/profile/JNielsen-Gammon John Nielsen-Gammon

    Eric –
    Wouldn’t it be accurate to say that most experts have been confused by the growth of Antarctic Sea Ice? It wasn’t predicted. Even now, the Marshall et al. paper, the only one you mention that provides an explanation for the inability of equilibrium models to produce a larger sea ice extent, is one you regard as needing further substantiation.
    Meanwhile, since you attribute the winds that drive the Antarctic Sea Ice growth largely to AGW, would it be reasonable to consider this growth as a negative feedback, at least during the early non-equilibrium stages of warming? Has anybody attempted to calculate the size of the sea ice negative feedback under that assumption?
    – John

  • Eric Steig

    John,

    Thanks for the comments and questions.

    First, let me emphasize that I am not making an argument that the sea ice trend is anthropogenic; nor am I arguing the reverse. What I was trying to do was to explain what I think has become a sort of “standard answer” (westerly winds) and then point out the various problems with that idea (doesn’t explain the seasonal changes; doesn’t explain the decline in sea ice in the Pacific sector; appears not to work in models). The most recent work from Marshall et al. (2014) addresses only the last of these.

    With respect to further evaluation of the Marshall work, their results are unequivocal as to why equilibrium experiments do not show the increase in sea ice that would be expected from increased westerlies. The part that I think needs further evaluation is the projection implied by equilibrium models that eventually there will be widespread decline, and that this will be due to rising isopycnal surfaces. That’s quite different from the question of whether the increased westerlies explain part of the sea ice trend that has been observed. Of course, certainly more evaluation there is needed on all these aspects. I don’t mean to imply (to borrow a phrase) that the “science is settled”. For example, the Holland et al. paper does not equivocally support increased westerlies as being the primary driver either. They imply that for the East Antarctica, thermodynamic considerations not fully accounted for may still play a major role

    That said, there is no question that changing winds as a whole – not just the westerlies, but everything — go a very long way to explaining what we have observed. The major point I am trying to get across here is that there is absolutely no reason why we’d expect, a priori, that the winds would change in a way that would lead to sea ice decline. It is true that the increase was not anticipated, but nor should anyone with a basic understanding of the processes controlling Antarctic sea ice have expected decline, either. Simple one-cause answers for observed changes in the system are all too easy to make. Even among experts it’s perhaps a little too easy to think only about thermodynamics (warming earth = warming ocean = sea ice loss) and forget about dynamics. It’s equally easy to go with the “it’s all winds” idea, and ignore the other processes going on. But most important to my point: it’s also too easy to assume that just because some model experiments don’t match observations that we “can’t model” or “don’t understand” Antarctic sea ice. The Holland et al. (2014) makes it clear (at least to me) that we can, and do.

    As for they question about feedbacks, I think you pose an interesting question, but one I don’t feel qualified to answer at the moment. Perhaps this is a good theoretical study someone could do. Of course, there are certainly papers that look at the magnitude of climate feedbacks expected from the Antarctic sea ice response to climate forcing, but I’m not very up to speed on that literature. In any case, the answer may change quite a bit, once the recent discovery that Antarctic sea ice may actually be quite a lot thicker than once thought, is taken into account. See Maksym et al., 2014). There’s also a reasonable write up on this, here: http://www.livescience.com/48880-antarctica-sea-ice-thickness-mapped.html (but please ignore the statement about “the mystery” of Antarctic sea ice; this is exactly the kind of misleading reporting that I’m responding to with my essay!).

  • http://atmo.tamu.edu/profile/JNielsen-Gammon John Nielsen-Gammon

    Eric –
    Thanks! On to somewhat broader issues…

    There’s a general tendency in science to assume that you understand the reason for something happening if the outcome agrees with your prediction. I see this in particular with some physicists (no offense intended) who think they understand atmospheric dynamics. Occasionally a completely off-the-wall cause-and-effect statement will appear in print, completely wrong but stated confidently by a physicist who in most other circumstances is probably much smarter than I!

    In that spirit, my impression as an interested non-expert has been that the experts assumed that Antarctic sea ice would probably decline (the thermodynamic mechanism) and was surprised that wind changes would be large enough to dominate the thermodynamic mechanism.

    Meanwhile, in the Arctic, when the dramatic 2007 summer ice loss took place (also unexpected in GCM projections at the time), the message that seemed to emerge was that the thermodynamic explanation is correct, it’s just a lot stronger than we thought. Still at the time it was apparent that winds accounted for most of the interannual variability in summertime Arctic sea ice extent, and that winds were particularly important in 2007. This quickly polarized (no pun intended), with skeptics arguing that winds were the cause and mainstream scientists criticizing skeptics for pretending or imagining that short-term variability equalled long-term variability.

    So, at the science end, have there been any studies that address long-term sea ice loss in the Arctic (wind vs temp) the same way that the studies you discuss above address long-term sea ice loss in the Antarctic (wind vs temp)? Ideally using the same model? From first principles, I would expect Arctic winds to play less of a role overall, since during winter the Arctic fills with sea ice and the winds can’t blow ice away to expose additional open ocean to freezing.

    At the public end, we seem to be in the following position. I’m oversimplifying, but the public won’t get the nuances.

    Arctic Ice: Scientists say temperature, skeptics say wind
    Antarctic Ice: Scientists say wind, skeptics say temperature (and therefore there’s no global warming)

    It’s so easy for a critic to say that scientists are picking whatever answer serves their purposes. On the other hand, if a theory is correct, every correct answer WILL be consistent with that theory. The trap that scientists need to avoid is the converse: assuming without thinking too hard about it that an answer that’s consistent with the theory is the correct answer.

  • http://glacierchange.wordpress.com/ Mauri Pelto

    Thanks for the delightfully detailed analysis. If we go further back to the analysis of the 1976-1998 period by Zwally et al (2002), http://onlinelibrary.wiley.com/doi/10.1029/2000JC000733/abstract

    They noted that ” It is also qualitatively consistent with the counterintuitive prediction of a global atmospheric-ocean model of increasing sea ice around Antarctica with climate warming due to the stabilizing effects of increased snowfall on the Southern Ocean.” How does the more recent research evaluate this sentence?

  • Eric Steig

    Jimmy Hansen (real name?) in the open comments section seems to have missed the point of the post, which is precisely that although the Antarctic sea ice growth has been presented as a “contradiction”, it isn’t. It only appears to be a contradiction if one sticks with a very simplistic conceptual model of how the planet work.

    John notes that “if a theory is correct, every correct answer will be consistent with that theory” This is of course true, but not very useful. A more subtle but equally accurate statements is that if a theory is incomplete (which it will always be, for as complex a system as our planet), then observations will often appear to be in conflict with the theory, even when they are not (and similarly, observations will often appear to in agreement with the theory, when they are not). This is applied physics, not elementary particle physics.

  • http://atmo.tamu.edu/profile/JNielsen-Gammon John Nielsen-Gammon

    Eric – I agree. It’s in scientists’ interest in papers and press releases to maximize the extent to which their results are new and unexpected. Then the press, which needs to tell a story if it wants to engage most of the public, plays up the surprise angle even more.

    The net result of all of that is the the public receives a false picture of science dealing with a whole lot of surprise and conflict when usually (not always) everything just gradually makes more and more sense (the normal science of Kuhn).

  • http://atmo.tamu.edu/profile/JNielsen-Gammon John Nielsen-Gammon

    I should add that skeptic blogs treat everything that doesn’t fit with their imagined view of consensus science as being contradictory. Few such blogs seem to care about how things actually work.

  • Eric Steig

    John, regarding your last comments on “skeptic blogs”, your comment about “normal science” in the Kuhnian sense is interesting. What explains the “skeptic” view, then, is the belief that there is a scientific revolution underway — that the scientific understanding of climate is about to undergo a paradigm shift. In that context, it makes perfect sense that each and every small thing that seems, at first glance, to be a contradiction to the paradigm must actually be a contradiction. Most scientists, on the other hand, don’t think such a revolution is likely. (Of course, one could also argue that we have recently undergone such a revolution — until the mid 20th century, most scientists didn’t really think we could have such a huge impact on the atmosphere as we are clearly having. Some still hold to that view, because they are stuck in the pre-revolutionary paradigm. In other words, the “skeptics” (and I use that word advisedly) think they are revolutionaries, but really they may be reactionaries. Only time will tell for sure of course.

  • http://biocycle.atmos.colostate.edu/ Scott Denning

    Eric,

    Thank you for this very helpful and clearly-written post. Sorry to be late to the comment thread, but I see this as helping to clarify the processes as well as dispel common misconceptions. I certainly plan to refer to this in my teaching.

    My reading is that the very common misconception you’re dressing is that simple “thermodynamics” controls the extent of sea ice. That is, we simplistically assume that sea ice extent is all about “freezing and melting.” But actually a lot of the year-to-year and even decade-to-decade variations are controlled by “dynamics,” which means much greater variability.

    Unfortunately, the distinction between “dynamics” and “thermodynamics” is part of climate scientists’ jargon, and these phrases are usually gobbledygook for a lay reader. But almost anybody can understand the idea that when wind pushes ice away form the frozen coast, more ice can form behind it; and alternately when ice is pushed into warmer waters at lower latitudes it is likely to melt.

    A key and very helpful distinction you make is the interesting asymmetry between the Arctic (a polar ocean surrounded by land) and Antarctic (a polar continent surrounded by ocean), which is really an accident of geography. Arctic sea ice is confined to the highest latitudes by the surrounding coastline, whereas Antarctic sea ice is frequently buffeted around by winds.

    In addition to the “corralling” presence of Arctic Canada, Greenland, and Eurasia, Arctic sea ice is subject to the very strong positive albedo feedback when bright polar ice is replaced by dark ice-free ocean. In Antarctica, by contrast, the pole will remain white for many centuries or millennia even under catastrophic warming because it’s covered by thousands of meters of land ice. There’s also a very important but less-recognized long wave feedback due to increased water vapor (and maybe clouds) in the Arctic winter as the ice-free area expands (Melissa Burt presented this last week at AGU). Neither of these strong radiative feedbacks can operate poleward of about 70 S, but both are very powerful north of 70 N.

    I read your post as explaining that (1) the trend in mean sea ice extent in the Antarctic is best explained as the small difference between much stronger but opposing trends in waters off of East and West Antarctica; and (2) that these regional trends are driven by notoriously noisy patterns in winds linked to tropical climate variability. Neither of these is a simple GHG-driven trend. Rather we should see these rapid increases (in the East) and decreases (in the West) in the same light that we view changes in the behavior of the winter jet or the tropical Trade Winds. These changes are really part of “climate variability” rather than secular “climate change.”

    Again, thanks for the time and effort you invested here in explaining and summarizing an interesting and technical literature so that nonspecialists can understand it!

    Scott Denning

  • Eric Steig

    Scott,

    Thanks for the comments. I agree entirely with your comments.

    Many readers — particularly on my RealClimate post on this — seem to have thought I was arguing that the sea ice trends are clearly anthropogenic phenomena. I am not.
    There *may* be a forced component to the sea ice increase, but it’s too early to tell.

    Eric Steig

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