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.