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Why is Antarctic sea ice increasing?

Posted on 2 April 2009 by John Cook

While Arctic sea ice loss has shown dramatic decline over the past few decades, Antarctic sea ice has shown long term growth since satellites began measurements in 1979. This is an observation that has been often cited by skeptics as proof against global warming. However, in all the skeptic articles I've read, not one has raised the crucial question: why is Antarctic sea ice increasing?

Figure 1: annual mean sea ice extent, observed by satellite. Straight line is the trend line (Zhang 2007).

The implicit assumption is that if Antarctic sea ice is growing, it must be cooling around Antarctica. This is decidely not the case. In fact, the Southern Ocean has been warming faster than other oceans in the world. The average global ocean temperature trend has been 0.1°C per decade from 1955 to 1995. In contrast, the Southern Ocean has been warming at 0.17°C per decade. Not only is the Southern Ocean warming, it is warming faster than the global trend.

Figure 2: Linear trend (1979–2004) of surface air temperature over the
ice-covered areas of the Southern Ocean.

So this raises the big question: if the Southern Ocean is warming, why is Antarctic sea ice increasing? The paper Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions (Zhang 2007) attempts to answer this question.

The paper uses a coupled ocean/sea ice model to find the predominant reason that sea ice is increasing is due to a decrease in upward ocean heat transport. Eg - less heat is being carried up by ocean convection to melt sea ice. The reason for this is a complex chain of events.

When surface temperature increases, the upper ocean warms and ice growth decreases. This leads to a decrease in salt rejection from new ice. The salinity of the upper ocean falls. Lower salinity and warmer water results in lower water density in the upper ocean. With fresher, less dense upper water, there is now increased stratification of ocean layers which weakens convective overturning. Less ocean heat is transported upwards. This leads to a decrease in ice melting from ocean heat. Hence we observe an increase in net ice production - sea ice increases.

While all that is a bit of a mouthful, it's actually a simplification of the process as there are various feedbacks along the process. Warming air increases upper ocean temperature which affects air temperature through air-sea interactions. Warming temperature leads to increased precipitation which increases sea ice growth. More sea ice means less atmospheric heat can penetrate waters.

The bottom line is the answer to Antarctic sea ice isn't simple - the Southern Ocean is a complex system with a number of factors likely contributing. One factor certainly isn't a contributor - the simplistic explanation that it must be cooling around Antarctica is not the case. Warming is happening - how it affects specific areas is complicated.

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Comments 1 to 31:

  1. John, what affect could the water from the melting ice cap have on the size of the ice pack? The water surrounding the Antarctic should be less salty and thus freeze at a higher temperature. Could this be partially responsible?
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    Response: Funny you should mention that - when I first started researching this topic, that was the first thought I had. Well, not the impact of fresh meltwater but of calving land ice. None of the papers I read find calving land ice or meltwater have an impact on sea ice levels. Nevertheless, I ran the idea by Eric Rignot who has done a lot of research into Antarctic land ice loss. His response:
    Glacier ice and sea ice do not have much in common. My paper is about the loss of land ice; it has nothing to do with sea ice. Vice versa. The stability of the sea ice cover in the antarctic does not mean anything in terms of land ice. Almost feels like comparing water from the ocean and water from lakes.
    Sorry I don't have any harder numbers for you on that (if anyone else cares to chase down hard figures, I'd be appreciative :-)
  2. This appears to be the same Zhang paper I linked in the Arctic Sea Ice thread, thanks for picking up on it John, interesting stuff.
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    Response: It is interesting stuff, I was very surprised to find the Southern Ocean has been warming so much - I thought it must be cooling if sea ice was increasing. But I must confess I missed your link (will have another look). I've been sitting on this paper for over 6 months - I started writing this post last September!
  3. Actually, I did not link it but gave the reference and abstract in post #253 of the thread. It appears to be the very same paper but published in the Journal of Climate instead of UW. I would have linked the pdf from UW if I had known it was there. Author(s): Zhang JL Source: JOURNAL OF CLIMATE Volume: 20 Issue: 11 Pages: 2515-2529 Published: JUN 1 2007 Times Cited: 1 References: 34 Abstract: Estimates of sea ice extent based on satellite observations show an increasing Antarctic sea ice cover from 1979 to 2004 even though in situ observations show a prevailing warming trend in both the atmosphere and the ocean. This riddle is explored here using a global multicategory thickness and enthalpy distribution sea ice model coupled to an ocean model. Forced by the NCEP-NCAR reanalysis data, the model simulates an increase of 0.20 x 10(12) m(3) yr(-1) (1.0% yr(-1)) in total Antarctic sea ice volume and 0.084 x 10(12) m(2) yr(-1) (0.6% yr(-1)) in sea ice extent from 1979 to 2004 when the satellite observations show an increase of 0.027 x 10(12) m(2) yr(-1) (0.2% yr(-1)) in sea ice extent during the same period. The model shows that an increase in surface air temperature and downward longwave radiation results in an increase in the upper-ocean temperature and a decrease in sea ice growth, leading to a decrease in salt rejection from ice, in the upper-ocean salinity, and in the upper-ocean density. The reduced salt rejection and upper-ocean density and the enhanced thermohaline stratification tend to suppress convective overturning, leading to a decrease in the upward ocean heat transport and the ocean heat flux available to melt sea ice. The ice melting from ocean heat flux decreases faster than the ice growth does in the weakly stratified Southern Ocean, leading to an increase in the net ice production and hence an increase in ice mass. This mechanism is the main reason why the Antarctic sea ice has increased in spite of warming conditions both above and below during the period 1979-2004 and the extended period 1948-2004
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  4. Good to have you back John. Obviously a complex topic. Similar to the increase in snow fall on Antarctica as a result of the seas warming. In both cases the outcome is counter-intuitive to what would be "common sense" and therefore gets picked up by denialists as evidence of failure to warm, when in fact it is the opposite. The contrast to the Arctic (where estimates of time to an ice free summer have just been revised downward again) is stark. But of course the Arctic and Antarctic are similar only in being polar and icy. In most other ways they are quite different, and their responses to a warming planet are also consequently different.
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  5. sorry this is not about antarctic ice. john, i recently came across two new "skeptic arguements" you may want to add to your list: 1)"crustal movement" (by brian valentine of the heartland institute) 2)"england temperature record doesn't show much warming" tamino has addressed the england temp record claim at his website. i thought the more creative one was valentine's "crustal movement". he and bob ryan had an blog discussion over at capital weather and here's what brian said: "The earthquake that led to the terrible Tsunami in the Indian Ocean was a collosal event; an event of that magnitude must have been preceded by crustal shifts that resulted in the immense stresses released by the earthquake, and as I stated I have resolved a slight nutation that has a resultant North, meaning the northern hemisphere was slighly biased toward the Sun during a period of some years" the true effects of "crustal movement" on earth's orbit can be found here:
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  6. Also the latest NOAA report finds that in the Antarctica the "Wordie Ice Shelf, which had been disintegrating since the 1960s, is gone and the northern part of the Larsen Ice Shelf no longer exists." I guess this is an obvious counterpoint to the "more sea ice" finding, but I also wonder if loss/fragmentation of ice shelfs is contributing to the sea ice. That is, more sea ice is confirmation of the melting on and near land?
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  7. David Horton, do not confuse ice shelves and sea ice. Sea ice is frozen sea water, whereas ice shelves are pieces of land based glaciers floating on the sea. Ice shelves are governed by land based ice dynamics and their interactions with the marine environment. However, it is true that they have been going pretty fast. The Wilkins is about ready to break loose as well.
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  8. Phillippe, no I wasn't confusing them. But I was wondering if a break up of ice shelves may be contributing to greater sea ice. Or are icebergs irrelevant to sea ice extent?
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  9. i am less educated than most other forum members and i hope i am not lowering the tone with my interjections. i was just thinking that melted land ice would lower the temperature of the adjacent sea water contributing to more sea ice. the trouble with this simple argument though i reckon is that land ice isn't as salty so melted land ice and may raise the temperature of sea water. normally when i talk about ice its to do with keeping my drinks cool, that was the inception of my chain of thought to do with increasing sea ice. it imparts coldness
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  10. David, afaik, icebergs would be relevant mostly to the extent that they can prevent normal movement of sea ice if they happen to be very large and located where they can interefere with wind driven sea ice motion. About the changes in temp/density/salinity, I really don't know, would have to look at the existing litterature. Let us know if you find pointers.
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  11. It doesn't look to me like we need any explanation other than random variation. "The paper uses a coupled ocean/sea ice model to find the predominant reason that sea ice is increasing" Translation: another computer model unverified and perhaps related to reality, perhaps not. Seems to me a stretch. We don't have the data to know much about sea ice trends either way. 30 years of records with plenty of problems isn't really something worth building models to explain, and certainly isn't enough to be useful.
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    Response: The take home I took from the paper is that empirical measurements, not model results, show the Southern Ocean is warming - in fact, warming faster than the rest of the oceans of the world. So the conclusion that increasing Antarctic sea ice shows cooling Antartica which disproves global warming is a false conclusion. In fact, the answer is a lot more complicated than that - and a lot more interesting.
  12. Just a thought--when did all that ice down there form? Perhaps the Antarctic ice grows during the warmer periods. If it's too cold, the air is too dry for precip. So the oceans around it grow a little warmer, more snow falls down there. Ice grows larger. Either way, you guys don't know why it's growing, but it is. Sounds like yet another poorly understood system falling under the 'science is settled' banner.
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  13. Tommybar, this article discusses sea ice, which is frozen sea water. It is not dependent on snow fall.
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  14. Really...."Warming temperature leads to increased precipitation which increases sea ice growth."
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    Response: This is something I discussed with Zhang, the author of the paper - he clarified that precipitation had small impact on the overall sea ice trend compared to the decrease in upward ocean heat transfer.

    When I first read the paper, there was one immediate question I had about his conclusion and it was the first thing I asked Zhang - I'm surprised noone has asked it here yet. I wonder who'll bring it up... :-)
  15. Paragraph d of the paper clearly shows that P is not nearly as much a factor as the increased stratification, although you're right in the sense that it is a factor. I don't know what you have in mind, John. I am nowhere close to your background in physics but questions I have would be the following: since the net increase is due more to less melt than more growth, there should be some seasonal variations corresponding to that, i.e. increased summer extent, and also decreased amplitude between summer and winter. Is it visible in the data? Since there is decreased convection from the deeper ocean, there should be a relative increase of heat content at greater depths, is it the case (not sure there are datasets for that)? Finally, this appears to be bound to change at some point. How much of an equilibrium is it really? What happens when the upper layer is disturbed? Shouldn't the underlying water at some point get warm enough to displace the lower salinity layer? Then what happens? Now are you going to tell us what your question was or keep us hanging :-)?
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  16. From what I understood from the paper the warmer air and ocean temps means the sea ice forms more slowly which reduces the salt rejection which weakens convective overturning. This would suggest to me that autumnal ice formation would be slower than normal and that the spring thaw would come later. Thing is over the last couple of years, at least, the amount of ice in autumn has been well above the long term average. This last summer the ice went right back to around average levels but currently ice levels are again well above average, up near 1 millions sq km's above normal. This means there has been a rapid increase in new ice through the autumn. Am I missing something?
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  17. For one thing--Antarctica is massive. Is the ice build up in one particular area, and not so much in another?
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  18. Oh well, I'll try for the prize. One obvious question is - how much sea ice build up since 2004? Another is - if less sea ice leads to "to a decrease in salt rejection from ice" then when sea ice builds up, this trend should be reversed. That is we should see an oscillation, not a trend.
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  19. There is a prize John?
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    Response: No prize other than the warm satisfaction of guessing it. But now I'm starting to feel the weight of expectation - it wasn't that big a question and I'm regretting talking it up (particularly as Phillipe Chantreau's questions in #15 were much more interesting than mine) :-)
  20. Ah, I was expecting at least a tee shirt with "I guessed the right answer on Skeptical Science and all I got was this tee shirt" written on it.
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  21. I have an idea. Even better than "the warm satisfaction of guessing it" would be the even warmer satisfaction of seeing the article re-written to include the latest information. As it is, too many readers will not see the point of it. There is nothing to "sink your teeth into". Finally, I promise that this warmer satisfaction will not contribute to AGW;)
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    Response: Agreed, will update the article. I'll give a clue - as I read the paper, my immediate thought was "this is the first question my skeptic readers will ask when I post this article" (Quietman, Wondering Aloud, I'm looking at you).
  22. Ah, that explains it. Who can think like a "skeptic"?
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  23. This is an interesting post, John. Your blogposts have been missed in my free time. Something that comes to my mind is: "Is this a strong negative feedback?"
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  24. Given the breaking away of several major ice shelves in the recent past, the Wilkens shelf being the latest, are'nt there more icebergs floating around the continent now?
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  25. Lee, I guess that's what David was asking in post #8 and I must say I have no idea. One would think that satellites are blind to the real origin of the ice. Hence, some glacial ice is bound to be part of the extent and area. I don't know how significant that is.
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  26. Over at the thread on Real Climate that is devoted to the collapse of the ice bridge that helped support the Wilkins Ice Shelf, someone asked, "How does the collapse square with the latest NSIDC data which shows an increasing Antartic ice coverage?" Gavin responded, "Look in the region where this is happening." There is the warm West Wind Drift which is resulting in ice melt in along the West Antarctic Peninsula and in the nearby ocean, particularly the Bellingshausen Sea -- where sea ice is declining at -5.3% per decade. This is leading to additional fresh water at the surface which is no doubt leading to some of the stratification suggested in the model. However, if this fresh water gets caught up in the inner cold East Wind Drift, it will be carried to the West Pacific Ocean, where sea ice is growing at 1.2% per decade. That is essentially along the coast of continental West Antarctica. And some will be carried further east where the Ross Sea forms a harbor between West Antarctica and East Antarctica. This is where sea ice is growing at 4.8% per decade. No other place rivals it. In fact, the closest you get is the West Pacific Ocean at 1.2%. So part of what explains the growth in sea ice may be stratification due to fresh water, but also I would suggest the fresh water itself -- which freezes at a higher temperature than saltier seawater. However, with respect to the future, the ozone hole was increasing the strength of the Antarctic Polar Vortex, and now that the ozone hole is being repaired the Antarctic Polar Vortex is weakening, and the cold dry Antarctic climate is giving way to the moist maritime air of the north. Of course with global warming we would expect a poleward shift of storm tracks in any case, but in this case the effects would appear to be mutually reinforcing. In any case, here is a link to my comment where I followed up Gavin's suggestion -- by I would also recommend checking out the comment below it: 8 April 2009 at 8:19 PM
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  27. Very informative, Thanks! From the little bit I've pieced together, I would have thought that the ozone hole - via SAM - would also tend to make the storm tracks shift poleward ?? - but maybe only in winter ????
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  28. Patrick 027 wrote, "Very informative, Thanks! "From the little bit I've pieced together, I would have thought that the ozone hole - via SAM - would also tend to make the storm tracks shift poleward ?? - but maybe only in winter ????" I am no expert. However, I understand that both the Arctic and Antarctic Polar Vortices are essentially persistent cyclones. The eye of the cyclone will become smaller and more well-defined as the cyclone strengthens and larger and less well-defined as the cyclone weakens. But as I understand it, the cyclone itself should become larger as it strengthens and smaller as it weakens. Thus as the Antarctic Polar Cyclone weakens as the result of the ozone hole repairing itself and reducing the temperature differential between the stratosphere and the troposphere, I would expect it to become smaller, resulting in the Antarctic Polar Vortex and its domain of influence giving way to the moist maritime climate to the north. And as it does so, I would expect the storm tracks in the southern hemisphere to move further southward. Likewise, as the Hadley Cells weaken they tend to expand, and this will shift storm tracks poleward. But as I have said, I am no expert, and if someone knows better I would be interested in learning. In the meantime I will do some digging.
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  29. I saw a sattelite photo of Antarctica and noticed a band of icebergs right off and all along the coast. Now it seems to me, with all those icebergs melting that the water along the coast would be less salty, and colder than would otherwise be the case,and thus would freeze at a higher tempurature. I know it sounds simple, but sometimes the simple explanation is the right one.
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  30. Timothy Chase - I have actually tracked down some articles on the subject, though haven't actually gotten to reading most of them yet. I may post the websites sometime... I have a long-running series of comments at: (Starting at comment ~272) about atmospheric circulation, eventually focussing on wave-mean interactions, in particular Rossby waves. By the time of my last comments on that matter so far (~ 491 - 496), I had only just established the basic concepts that may be necessary for understanding storm track variability and troposphere-stratosphere interactions...(I was figuring some of it out as I explained it).
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  31. Hi, isnt everyone missing the point, I thought that the big scare was according to God (Al Gore) that because the ice caps were melting we would need to buy stilts to get around and turn our gardens into rice paddies. As the ice caps are not melting but increasing in size - British Antarctic Survey Report, being released soon - shows that East Antarctica 4 times the size of West has been growing in size (100,000 sqkms per decade since 1970) and that temperatures there are falling not rising. Also and maybe more important the Oxford professor who designs satellites says that data received by satellite observation should be taken with a pinch of salt because they are too remote to be taken as explicitly accurate. It really doesnt matter whether the sea is a little warmer or a little colder 0.17c so long as the variation is within acceptable limits as to keep most things becalmed and liveable. Remember that Australias recent bushfires were caused because the local pacific sea area was colder than usual meaning less precipitation so less rain in Australia causing drought and extensive bush fires. My feeling is that these insignificant variations in temperatures have been occuring since the beginning of time and will continue whether we are here or not, what supposed scientists and pseudo scientists are trying to do is explain the inexplicable and using flawed computer models to try and predict the future when you would have more change of predicting the 4.30 at Cheltenham and they cant even do that when you have more than enough - more than enough data - to be able to achieve the objective. We burned more animals than was necessary in the last big foot and mouth epidemic in the UK that was necessary because of computer modelling, most of the financial meltdown was caused by people relying on computer models in fact if you really dig we seem to be becoming obsessed with anything to do with computers as though once a computer has made its decision it must be right even though the individual who designed the model was a human being who by default are more stupid than clever. No one ever took Al Gore seriously when he pretended to be a politician precisely why people take him seriously when he pretends to be a scientist is and will remain a complete mystery. My overall impression is that there are far too many people on this planet who are unemployed or unemployable and they fill in their time by trying to resolve the mysteries of the universe, by tryin to apply logic to a system that doesnt even understand the word. A scientist reporting on genetics said this morning that we would like to believe that we understand genetics but what most people either fail to understand or dont want to understand is just how ignorant we are. If that is the case and it most likely is then the same logic should be applied to the supposed science surrounding the idea that less than half of one percent of Co2 in the atmosphere is actually going to cause absolute catastrophe when in fact it wont. It maybe the case that a sea rise in certain places will cause some to have to move but the real and absolute disaster will be caused if the planet cools, it will only take the same temperature drop as the predicted rose for everything to stop dead. The last time in cooled in Europe parents were eating their children to survive, those who are captivated by the idea that a rise in temperature would be an absolute disaster need to chew on that piece of historical fact and go somewhere and get a life, because in anycase most of you will have died from old age before the predicted misery even begins to happen!! David Wells
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