Slightly slower than usual, here’s the NSIDC release for two days ago. In a couple of weeks, they’ll be producing their summary of September, and probably drawing the page to a close for the year not long after that. Before the monthly means are posted, though, these are the numbers we have so far for this year.
At it’s lowest, sea ice extent reached around 4.13 Mkm2, during the week of the 16th September; not especially late (the previous week has been the average over recent years), and earlier than some years. What is unusual about this month is the ‘flat-line’ appearance of the ice extent; normally, the refreeze starts as soon as the thaw has ended, and we get the familiar curved graph of extent; this month, the ice has persistently refused to start increasing in extent, in spite of the steadily decreasing temperatures. As a result, the September monthly mean is likely to come in around 4.2 Mkm2. How does this compare to previous years?
The Long-term mean Summer sea ice extent is 7.7 Mkm2. So, this month will have been around 3.5 Mkm2 lower than that mean; around 45% less. The previous lowest ever was two Septembers ago: 5.32 Mkm2. We’re around 1.1 Mkm2 lower than that; about 21% down.
In the meantime, ice extent in the Antarctic has pushed close to record high anomalies; more than 1 Mkm2+ at its’ height. I haven’t yet seen any analysis of the data from here, or any novel explanations, but it still doesn’t look like the Antarctic is trending positively in a consistent way much beyond it’s natural variability; I’ll look for more on this later.
Ignoring the plight of Polar bears and local inhabitants for the moment, does the change in sea-ice matter? It does seem to be prima facie evidence of the ‘Polar Amplification’ hypothesis – not just this year’s low, but the trend over nearly thirty years, and the apparent (this year and 2005 could still turn out to be exceptional, rather than habitual) acceleration of the rate of decline in the NH.
But this year, at first glance, it at least looks as if the conditions in the Arctic are, somehow, different to previous years, not least because of the loss of a chunk of perennial sea ice in the Central Arctic Ocean, and the exceptionally low quantity of multi-year ice in the ocean as a whole, as well as the apparent inertia in the system this month.
What I am confused about, though, is how the huge amount of heat-loss from the ocean compared to previous years will effect the area next year. One the one hand, we may well see a slow and deficient refreeze throughout the Autumn and Winter, with a very large extent of vulnerable first-year ice. On the other, a vast amount of the heat transported in to Arctic Ocean via the NwAC and the Bering Strait will probably be removed from the climate system into the stratospher over the coming months. This is likely to have an impact on this Winter’s weather in Siberia and Northern Canada/Alaska, and more so in the Chukchi Sea area than anywhere else. There is also the possibility that the internal downwelling and upwelling circulation, as well as the boundary layer heights, will be affected.
It is far too soon to be claiming a systemic change in the Arctic yet, but the next four seasons may be critical in helping us understand what is and isn’t happening, and how the global climate system might respond.
Old man in a cave 
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September 29, 2007 at 12:30 am
Gareth
Hi Fergus,
You might like to read Jeff Masters’ views here.
The Arctic ocean (as a whole) has a lot more heat in it at the start of the freeze than in any recent year. It has to lose that heat, and that may be why the onset of an increase in extent is (apparently) delayed. Once that hurdle is overcome, the freeze-up will be as rapid as usual. However, the winter season is defined by orbital parameters, not the atmosphere/ocean, and so in heat budget terms a good chunk of the seasonal heat loss will have gone into initiating the freeze. That would suggest that either this winter’s extent will also be low, or that the volume of ice formed will be less. In either case, when the spring melt starts the system will be primed for rapid loss – and perhaps a new record low. To stop/delay this process – which looks like a classic positive feedback to me – we will need an extremely cold winter. A Pinatubo or two?
Meanwhile, as Masters points out, some of the heat lost by the Arctic ocean goes into the atmosphere, and impacts on the NH autumn and winter. If the next few months are comparable with last year, then we will have witnessed a sudden and dramatic climate change in the NH. If that’s the case, I suspect we will have an Arctic that’s substantially ice-free (barring a slushy ledge north of Greenland and the Canadian islands) within a decade.
An ice-free and warming Arctic ocean will supply a lot of moisture to the atmosphere, which will have to go somewhere. Sounds like a recipe for wet summers to me.
September 29, 2007 at 1:08 am
fergusbrown
Hi Gareth. I was wondering whether we might see a large increase in winter precipitation, especially over Siberia, late this year. I’m also looking forward to seeing when Barrow gets it’s freeze-over.
No question that extar heat in the system would affect synoptics all round the NH, but I’m not sure what effect Stratospheric ozone levels might have on large scale heat loss (basically, allowing it all to ‘leak out’ into space).
This is why I emphasise the next four seasons, one year, to see if there is corroborative evidence for the mechanisms under discussion here.
Regards,
September 30, 2007 at 8:35 am
Steve Bloom
NASA may be laying the groundwork for something big on Monday. Have alook at the extent plot at the first link and see if you see what I saw. It looks rather different from NSIDC’s.
Also on EurekAlert is a press release (no accompanying paper) with some pretty graphic descriptions of this summer’s High Arctic response to the heat. It’s interesting to see how very unpersistent ice is in the face of 20C surface temperatures. See also this regarding changes in the northernmost North American lake. There are some interesting implications for the Holocene Thermal Maximum, at least as experienced in that part of the High Arctic. I also wonder how that lake did this summer.
September 30, 2007 at 10:22 am
fergusbrown
Aha! They look like they are using ‘area’ rather than ‘extent’; hence the difference. But they aren’t using the CT areal calculation, that’s for sure. I suspect this is why there is a difference in the graphics.
Thanks for all the linking; it all contributes to our understanding of the picture as it stands.
Do you think NASA is going to suggest an ‘irreversible feedback’ or something? Can’t see it myself – it’s too soon to say. We’ll find out tomorrow…
October 1, 2007 at 3:46 am
Steve Bloom
In a sense it willl be too soon right up until the moment the ice is gone. Regarding NASA (actually Josefino Comiso), it’s possible that they’ll have some sort of new projection based on recent behavior (recalling that the model results are all off the mark). Jay Zwally is in the same section, so the effect on the GIS is another possibility. As you say, we’ll see.
October 1, 2007 at 10:16 pm
Gareth
Here’s the NASA press release.
And:
October 1, 2007 at 11:29 pm
fergusbrown
Steve; there was a piece a few days ago on recent model calculations of Greenland Ice Sheet mass balance; this suggested that previous estimates may have been about 100% out (or even more) about the rate and volume of loss. Even with the revised numbers, though, the sea-level contribution of the glaciers only amounted to about 8cm in a century.
I wonder if the new material is derived similarly from GRACE data, combined with other stuff.
Thanks for that link, Gareth. There’s a nugget buried in that throwaway at the end, isn’t there: ‘…will require new physical insights and understanding…’, meaning…?
October 2, 2007 at 1:56 am
Gareth
I liked the need for “urgent reassessment of sea ice forecast model predictions and of potential impacts to local* weather and climate”.
(* Where some definitions of “local” include the entire NH above 50N)
October 2, 2007 at 2:08 am
Steve Bloom
Meaning, I think, that the ones we’ve been using aren’t working any longer!
Any chance you could locate that GIS piece or give me a clue as to how to find it?
October 2, 2007 at 9:47 am
fergusbrown
Steve; looking for the reference, I may have combined two results inadvertently. One of the papers is: http://www.the-cryosphere-discuss.net/1/351/2007/tcd-1-351-2007.html
[open access], which analyses sea-level in relation to GIS changes. There are several recent papers in GRL (September) which feature discussion of the GIS and glacial melt rates. I can’t find the other paper that stimulated this response at the moment.