18 October 2012
A record year in the recent history of the arctic sea ice
2012 has set a new record in terms of the extent of the arctic sea ice. On 16 September 2012, the extent of the ice cover reached a low point unmatched since the introduction of satellite monitoring in 1979. The frozen ocean surface area in the northern hemisphere reached the value of 3.41 million km², beating by some 750,000 km² –about 1.4 times the size of metropolitan France – the previous record of 4.17 million km² recorded on 18 September 2007.
Arctic sea Ice extent on September 15th 2012 (right) compared with minimum Sea Ice extent on September 16th 2007 - © Cryosphere Today
On the basis of the monthly averages of the extent of the ice, 2012 also holds the record for the month of September, with 3.61 million km² compared to 4.30 million km² in 2007 – almost 50% less than the September average over the 1979-2000 period. But the record also holds true for August. The seasonal retreat of the ice this year accelerated in early August so that by the 26 August, we knew that the extent of the ice would beat the record of 18 September 2007.
While every September the extent of the arctic ice reaches its low point at the end of the summer thaw, this extent varies strongly from one year to another, and these variations can be seen on the September averages. Thus after 2007’s record, September 2008 and September 2009 saw the extent of the ice cover increase. 2012’s record follows persistent diminution of the September ice cover since 2009.
Beyond these major inter-year fluctuations which are difficult to predict, an average trend of cover reduction is observed over the longer term. The scale of this trend depends greatly on the length of the period considered. Studies based on historic reconstructions suggest that it began at the end of the 19th century. If we refer to the beginning of the period of satellite observations, this trend towards diminution has accelerated every year since 1996, when the extent of the September ice cover reached an unequalled record over the satellite period, with a value of 7.88 million km² exceeding that of 1979. Such a large cover in September has not been recorded since and the slow average diminution of 36,000 km²/year over the 1979-2007 period has increased over the following decade to reach 72,000 km²/year over the 1979-2007 period: double that. Following this year’s record, the diminishing trend of the extent of September ice cover has reached more than 90,000 km²/year on average over the full period, with the 2007-2012 period including the six Septembers with the lowest ice cover over the whole period. Every year, the deficit of ice compared to the beginning of the period is such that it contributes to accentuate a little further the trend of diminution, despite considerable inter-year fluctuations.
What makes 2012 a remarkable year is the abnormally high loss of sea ice between March’s winter high point and September’s summer low point. Almost 12 million km² thus disappeared between 20 March and 16 September, representing 50% more than what was consensually considered as the "usual" seasonal loss in the years 1979-2000.
Arctic sea Ice extent on September 16th 2012 compared with median Sea Ice extent for 1979 - 2000 period - © NSICD
Beyond their overall extent, characterization of the evolution of the arctic ice must also be based on their geographical distribution. Looking at September alone, we can already see great contrasts in distribution from one year to another. Let's compare the two key years 2007 and 2012. The second differs from the first in the very marked abnormal loss of sea ice cover in the Atlantic sector, particularly in the Barents Sea and the Kara Sea. The thawing of this sector was already visible in 2007 but it hadn’t reached a sufficient amplitude to fully free the ice of the Northern Sea Route (NSR). An ice shelf persisted all through the summer to the east of the Severnaya Zemlya archipelago. In 2012, on the other hand, the NSR was fully free of ice for the first time since satellites gave us an overall view.
This year is also characterized by a total loss of ice in the Beaufort Sea sector. The Northwest Passage (NWP) remains sensitive however to the drift of multi-year ice: thawing to the north of the passages may also free pieces of drifting ice which may block ship channels. This happened this summer in the Parry Channel which, unlike in 2007, remained almost continually blocked with ice.
The causes of the new record annual ice cover low point are still widely debated. The geographical distribution of the ice cover, and more specifically its anomalies (compared notably to a climate norm) is important information which, once linked to the geographical distribution of atmospheric (wind and surface temperature) or oceanic (currents, surface temperature and vertical structure of the water column) anomalies can guide us in seeking the responsible mechanisms.
However, the year 2012 is a singular year in this respect that the history of meteorological situations of summer 2012 is insufficient to highlight particularly notable or persistent anomalies of oceanic and atmospheric conditions. The exceptional storm which developed in early August in the far East of Siberia (the Great Arctic Cyclone) and which then moved towards the centre of the Arctic certainly promoted the break-up of the pack-ice in these regions and therefore the absorption of solar rays in the fractures created. The ice thus becomes more vulnerable to the warming of the ocean and the action of the winds.
However, this storm was also the cause of local cooling which we were able to detect. Regardless, these effects are difficult to link to the loss of ice cover in the Atlantic sector. The 2012 situation contrasts with that of 2007 where a dipolar atmospheric pressure regime established between Greenland and the north of Eurasia promoted both the arrival of masses of hot air on the sector of the Chukchi and East Siberian Seas and the export of ice via the Fram Strait. Instead of such a dipole, in summer 2012 we rather observed a difference in pressure in the Beaufort Sea. The structure was favourable to warm winds from the south and warming in this region while a cold anomaly developed in the sector of the Barents Sea concerned by a particularly notable loss of ice cover.
There is also reason to take an interest in the role of the ocean in the abnormal melting of sea ice during summer 2012. We know it played a major role in the changes in the ice cover during summer 2007, particularly in the Pacific sector. In 2012, ocean surface temperatures were almost 5°C higher than normal in the Beaufort, Chukchi and Laptev Seas. But the issue is whether it was a cause or a consequence of the loss of ice cover.
Evolution of Arctic sea ice extent as of October 15, 2012, along with daily ice extent data for the previous five years. 2012 is shown in blue, 2011 in orange, 2010 in pink, 2009 in navy, 2008 in purple, and 2007 in green - © NSICD
The studies underway should make it possible to identify the mechanisms responsible for the event of 2012, in particular to assess it in the light of the variability we have seen over the last 30 years. Ice is a complex environment. Its response to atmospheric and oceanic heat flows, responsible for melting or growth, and to winds and currents, which are responsible for its drift, is largely modulated by its internal structure: thickness, temperature, salinity and resistance; all characteristics which we can only very partially observe to date. However, this structure partially determines the evolution of the ice, as it makes up the memory of past events and allows the ice to develop a unique variability. Thus a thin ice sheet and a thick ice sheet will react differently to a wind anomaly. This ice-memory applies at both the seasonal and multi-year levels. It could have contributed to preconditioning the ice cover before the particularly marked episode of 2012.
Pour en savoir plus :
Voir le site du Polar view center de l'Université de Bremen (Germany)
Voir le site du NSIDC (National Snow and Ice Data Center - USA)
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