Arguments
Why is southern sea ice increasing?
The skeptic argument...
Southern sea ice is increasing
'Antarctic sea ice set a new record in October 2007, as photographs distributed by the National Oceanic and Atmospheric Administration showed penguins and other cold-weather creatures able to stand farther north on Southern Hemisphere sea ice than has ever been recorded. The news of expanding Antarctic sea ice stole headlines from global warming alarmists who asserted Arctic sea ice had reached its lowest extent since 1979.' (James Taylor)
What the science says...
| Select a level... |
Basic
|
Intermediate
| |||
| Antarctic sea ice has been growing over the last few decades but it certainly is not due to cooling - the Southern Ocean has shown warming over same period. Increasing southern sea ice is due to a combination of complex phenomena including cyclonic winds around Antarctica and changes in ocean circulation. | |||||
The most common misconception regarding Antarctic sea ice is that sea ice is increasing because it's cooling around Antarctica. The reality is the Southern Ocean surrounding Antarctica has shown strong warming over the same period that sea ice has been increasing. Globally from 1955 to 1995, oceans have been warming at 0.1°C per decade. In contrast, the Southern Ocean (specifically the region where Antarctic sea ice forms) has been warming at 0.17°C per decade. Not only is the Southern Ocean warming, it's warming faster than the global trend. This warming trend is apparent in satellite measurements of temperature trends over Antarctica:
Figure 2: Antarctic surface temperatures as observed by satellites between 1981 and 2007.
Similar trends are found when combining temperature data measured from ships and buoys. The following figure from Increasing Antarctic Sea Ice under Warming Atmospheric and Oceanic Conditions (Zhang 2007) displays trends over the ice-covered Southern Ocean - this is the region where Antarctic sea ice forms.
Figure 3: Linear trend (1979–2004) of surface air temperature over the ice-covered areas of the Southern Ocean (Zhang 2007).
We see strong warming over most of the ice-covered Southern Ocean although there is also some cooling. What is the average trend over the whole region? The overall surface temperature trend over the ice-covered regions of the Southern Ocean shows a warming trend:
Figure 4: Annual mean surface air temperature averaged over the ice-covered areas of the Southern Ocean. Straight line is trend line (Zhang 2007).
Oceanographic data also find that the waters in the Southern Ocean are warming. The waters of the Southern Ocean's Antarctic Circumpolar Current have warmed more rapidly than the global ocean as a whole. From 1960 to 2000, water temperature increased by 0.068°C per decade at depths between 300 and 1000 metres. This warming trend has increased to 0.098°C per decade since the 1980s (Boning 2008).
If the Southern Ocean is warming, why is sea ice increasing? There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). A side-effect is a strengthening of the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas leads to increased sea ice production (Turner 2009).
Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).
Antarctic sea ice is complex and counter-intuitive. Despite warming waters, complicated factors unique to the Antarctic region have combined to increase sea ice production. The simplistic interpretation that it's caused by cooling is false.
Last updated on 5 January 2011 by John Cook.
Sorry to nitpick, but your quote at the top was from James Taylor, not Steve Goddard.
OBSERVATIONS: Much of this depends on the “reanalysis” project, that tries to reconstruct climate data for the period 1957-96.
It seems clear from Boning el al that there is a measured warming trend of the water of the Southern Ocean (between 30 and 60 degrees south, down to 2000 meters). The reanalysis of temperature records seem (Zhang) to show that in the interval 1979-2004 the air surface temperature in the much smaller “ice covered area” around Antarctica has been rising. There is also a measured increase in Antarctic sea ice extent (Turner et al and references therein)
In addition to this there is also a further measured change, related to the Antarctic Oscillation (also called Southern Annular Mode). There are two modes (Thompson & Solomon): High index means cold polar temperatures, strong western winds, and low index means the opposite. This index has been rising, increasing winds and decreasing polar temperature. There is evidence that this development is driven by the “ozone hole”.
All of the above seem to comparatively safe. In particular the simplistic argument “more ice means the South Polar Sea is colder” is nonsense, we do know that the Polar Sea is getting warmer, so that the extent of ice on the Ocean is definitely a bad proxy for temperature,
But when it comes to explanations of what we see, things look a lot more murky to me. Two main players seem to be the layering of the Southern Ocean water (less dense water above denser water) and the constant western winds around 60 degree.
THE EKMAN SPIRAL: The wind possibly contributes to the mixing of layers in an interesting way: the wind drives an ocean current which runs around the Antarctic moving from West to East. This current extends down into the ocean, with the surface water moving fastest, and deeper water moving in the same direction, but slower, Because of the Coriolis effect, the current will try to veer to the left, which means towards the North. Now, since the surface water moves faster than the deeper water, the net effect is stronger at the surface, So surface water will move to the North, which forces deep water to the south. This creates a down-welling North of the current, and an upwelling South of the current. This whole business is called an Ekman spiral.
BONING: The paper by Boning et al. quotes measurements that seem to show that even if the circumpolar winds have been increasing, that “Ekman spiral” has not become stronger. They believe that the reason for this is that the increased wind also produces more eddies, which confuse the whole picture. They note the the models that has been used cannot resolve those eddies (they are too small). If they are right, the lack of enhanced mixing of layers in the ocean is not yet theoretically understood.
ZHANG: Zhang’s paper is a pure model study The main point of the article is that he can construct a model of the South Sea that agrees with two important seemingly contradictory measured factS: it has a warming ocean, but an increase in sea ice.
I think that the mechanism proposed by Zhang is slightly (but not essentially) different from what John describes. The motor driving various changes is the increase in surface temperature. This initially leads to a decrease in production of new sea ice. The top water gets warmer and less salty. Both these changes work in the same direction, they both make the top layer less dense.
The next thing that happens is that since the top layer is gets dense, there is less upwelling of warm water. This means that less ice is melted. So now, both less ice is created and less ice is melted. The model says that the change in melting is bigger than the change in the creation of new ice, so the net effect is that we get less ice.
But wait? If less ice melts in the top layer, the salinity will increase, counteracting the previous effect? Zhang says that this is so, but we still have to take the warming of the top layer into account! This warming makes the top layer lighter, decreasing mixing of layers.
So now there are lots of things going on: Since the top water gets warmer, less ice is produced. On the other hand, less ice is melted by upwelling deeper warm water. Then there is precipitation, but Zhang does not believe that the increase in precipitation is the decisive effect.
And the sum of the three effects "creating less new ice", “destroying less old ice" and "warming the water" is actually that the top layer gets less dense - driving the cycle.
This all seems a bit subtle for my taste, taking the big uncertainties into account. For instance, what happened to Boning’s eddies, which were supposed to be important? But at least this is a testable model.
TURNER: This paper seems to ignore questions of up and down convection in the oceans, the questions that dominated Boning et al. and Zhang.et al. Instead it focuses on the strengthening of the western wind – the high index of the Antarctic oscillation.
Another important point of this paper is that they break down the increase in sea ice into geographical areas and seasons. In particular, the ice in the Ross sea (close to the pole) has been increasing, while the ice in the Bellinghausen-Amundsen sea (father from the pole) has been decreasing. This is clearly important, and the regional differences should be explained.
There is some modeling going on, but the upshot seems to be unclear (they conclude that it could all be natural variability). The paper is often cited for explaining increased sea ice by the polynyas in the Ross sea. It seems to me that they only suggest this mechanism, but they don’t give strong arguments for it. Possibly I’m missing something.
If you know what paper I'm talking about, it deserves a shout-out from this page. Thanks!
GFW, this isn't exactly what you're asking for. However many early models of the 80's/90's showed greatly delayed Antarctic warming compared to rapid Arctic warming. This is due (a) to the very large Southern hemisphere oceans and (b) different S and N polar ocean circulation which gives more efficient mixing of surface and deeper waters in the deep S hemisphere, transferring heat from the surface.
So, quoting from a recent review of ocean circulation modelling in which the mechanisms for hemispheric warming asymmetry are described illustrates that highly delayed Antarctic Circumpolar ocean warming has been predicted since the early 1980’s.
Here’s a bit of a summary from (direct excerpts are in blockquotes):
S. Manabe and R. J. Stouffer (2007) Role of Ocean in Global Warming J. Meterolog. Soc. Jpn. 85B 385-403.
General point about ocean modulation of surface warming:
Discussing the early models of Schneider and Thompson (1981) to evaluate the delay in the response of the sea surface temperature to gradual increase in CO2, Manabe and Stouffer say:
In a later model Bryan et al (1988) made the same sort of analysis, investigating the role of the oceans in modulating the response of surface warming to enhanced greenhouse gases.
It’s not just the oceans per so of course. It’s also ocean and air currents, and particularly the mechanisms governing the efficiency of surface heat transfer into the deeper oceans. If this is efficient, the deep oceans will absorb heat and there might be little measured surface warming, at least for a while. So (speaking of Bryan et al (1988)) again:
Later models predict the same hemispherical asymmetry that is seen in the real world. e.g. discussing the simulations of Manabe et al (1992):
Why is this, one might ask?! Here’s what Manabe and Stouffer say:
n.b. remember this is a prediction from a model involving the response to [CO2] doubling; we’re nowhere near CO2 doubling yet. However these early models predicted what we're seeing in the real world today.
Chris, thanks - Manabe et al (1992) is exactly the paper I was remembering, and I finally found the place I saw it described. Bob Grumbine!
http://moregrumbinescience.blogspot.com/2010/03/wuwt-trumpets-result-supporting-climate.html
Now we have Liu & Curry (2010) that seems to be saying very much what Manabe et al said in 1992. Which I'd say is quite a coup for the latter.
Transient responses of a coupled ocean-atmosphere model to gradual changes of atmospheric CO2. Part I: Annual mean response
Transient responses of a coupled ocean-atmosphere model to gradual changes of atmospheric CO2 Part II: Seasonal response
Want more?
How do the explanations in this article correspond to the 2008 paper by Corr and Vaughn in Nature Geoscience about volcanism affecting the Pine Island Glacier?
newtja, Corr & Vaughan (2008) does not attempt to assess or quantify the effect of volcanic venting on the Pine Island glacier. The eruption discussed occurred roughly 2k years ago. It's possible that venting is helping the break up of PI glacier, but as you can see from the article above, and from Shepherd et al. (2013), there are greater factors involved. You might also check out Vaughan & Corr (2012). Here are the last lines of their abstract:
"We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse."