The NSIDC press release of April 4th: http://nsidc.org/news/press/20070403_winterrecovery.html matches the data now available at Cryosphere Today (see my previous comment about this). This Winter’s sea ice level was just short of a record low. Complicating a clear understanding of the measurements, now both CT and the NSIDC use two separate measures of the sea ice; extent and cover (which NSIDC calls ‘concentration’).
There is a good reason, though, why two methods of defining sea ice level are used: the extent measure is part of a long-term dataset going back 100 years or so. As such, it allows us to see trends in a more realistic context, so it is maintained, even though it is a less accurate measure than cover/concentration. This second method has only been possible in the satellite era and only goes back to 1978. Basically, it is extent minus open water within the ice pack (NSIDC says ‘less than 15% coverage’ of the sea). This is generally accepted as being more accurate, but the brevity of the dataset makes long-term climate analysis more speculative.
What do the two sites tell us about the Winter just gone and the season to come? Firstly, they show that the recent downward trend in Arctic sea ice level is continuing. Then they show us that the pattern of decline is consistent with previous years. but there are a few other pointers which need closer inspection.
Assuming the CT data has now been ‘fixed’, a look at the anomaly record (‘tale of the tape’, as it is called) is revealing. Based on a 1979-2000 average, the period 1979-89 shows generally a small positive anomaly in sea ice levels. The period from ’90-’95 fluctuates, but is broadly ‘neutral’. From late ’95 to 2002, a noticeable negative general trend is observable, within the context of several positive anomalies during some months/seasons. But since 2002, there has not been a single day when the sea ice level was above the long-term average. That is four continuous years of negative anomaly. For the past year or more, that negative anomaly has stayed consistently around the 1 million Km2 mark. During the past year, the negative anomaly has hit a record amount briefly; in excess of 1.5 MKm2.
What about the coming seasons? Last summer’s level was just above the record low. There was some recovery during the Autumn, but then the Winter level was also close to a record low. Normally, a less anomalous Winter would be expected to follow a Autumn ‘recovery’, but this year it didn’t happen that way. It is also worth noting that the period of maximum extent was in late February, rather than the more normal March, implying an earlier than usual shift in the seasonal influence. Given the already low level of Winter ice, the early onset of the thaw season, and the recent trends, it is reasonable to forecast that this year, the sea ice levels in the Arctic will hit a new record low. This is my prediction for the coming seasons. I have not mentioned other factors such as the Arctic Oscillation and Sea Surface Temperature Anomalies, though they have been considered in this forecast. The AO is broadly neutral at the moment, but may be shifting phase; the SSTAs are positive. Both of these influences are consistent with a forecast of low ie levels this year.
Another concern is the extent of the ice anomaly in the Barents Sea, the part of the North-West Atlantic which is so crucial for the ocean circulation. Here, the anomaly is over 0.5M Km2 already. SSTA has been consistently high in the region for about a year and a half. It is possible that there are ongoing processes in this area which are yet to be published, which indicate complications with the ocean circulatory system; only time and research will tell.
The situation in the Antarctic is much less clear. The anomalies have fluctuated much more wildly from positive to negative and back again, over the past five years. Although there is a slight general upward trend in the 1979-2006 period as a whole, it is possible to detect the early signs that this is about to change. Last year’s overall anomaly was strongly negative (though nowhere near a record low), and there appears to be a trend of larger anomaly swings between seasons and years than previously, though this may simply be a statistical ‘accident’. Though it is much less certain, my forecast for the coming seasons in the Antarctic is that recovery will be slighly below average, the Southern Winter will end earlier than average, and next year will show a near-record negative anomaly.
While I am on, a quick note of thanks to Michael Tobis, on ‘In it for the Gold’, for the name check, and to the various correspondents on Climate Science with whom I have been sharing a lively debate on the virtues or otherwise of supporting another new website about climate, Icecap, hosted by an eminent meteorologist and climate student, Joe d’Aleo; I wish him good luck in his venture.