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Arctic Ice Cover

A consensus among climate change prediction scenarios using coupled ocean-climate general circulation models (GCMs) is enhanced warming in the Arctic. Annual average near-surface air temperatures across Alaska and the Arctic have increased over the last 50 years at a rate more than twice as fast as the global average temperature. This suggests that changes in the Arctic sea ice cover may provide early indications of global warming. The shrinking and thinning of the arctic sea ice cover appear to be in keeping with the poleward amplification of the global warming induced by increased greenhouse loading of the atmosphere and predicted by interactive climate models.

For the next few decades out to 2040, continuing environmental changes in the Arctic are very likely, and the appropriate response is to plan for adaptation to these changes. For example, it is very likely that the Arctic Ocean will become seasonally nearly sea ice free before 2050 and possibly within a decade or two, which in turn will further increase Arctic temperatures, economic access, and ecological shifts.

Pack ice includes first-year ice, multiyear un-deformed and deformed ice, and ice islands. First-year ice forms in fractures, leads, and polynyas and varies in thickness from inches to more than a meter. Traditional knowledge indicates that in recent years, ice has been less stable, there is less multiyear ice, pack ice is smaller, and large icebergs are rarely seen.

Circulation in the Beaufort Sea is primarily influenced by the large-scale Arctic circulation known as the Beaufort Gyre, which is driven by large atmospheric pressure fields. In the Beaufort Gyre, water moves to the west in a clockwise motion. The long-term direction of ice movement is from east to west in response to the Beaufort Gyre.

The Chukchi open-water system appears to be the result of the general westward motion seen in the Beaufort Gyre and is strongly influenced by the wind direction. Historically, first-year floes off the Chukchi Sea coast had a thickness of about 1.0 to 1.5 meters, and multiyear floes were 3 to 5 meters thick. Sea ice that is thicker than 5 meters is common in Arctic Ocean pack ice and is generally believed to consist of pressure ridges and rubble fields. Increased ridging generally occurs from east to west and in the vicinity of shoals and large necks of land.

During the first half of the 20th-century, ice extent in all seasons remained essentially constant. Beginning about mid-century, the summer minimum extent began to shrink while the winter maximum remained unchanged. Starting in about 1975, the maximum, too, began to shrink.

Information about the average thickness of ice before the advent of nuclear submarines is limited to occasional samples taken from ships and manned ice camps. They suggest an average thickness of 3 to 3.5 meters and little change in time. Declassified submarine sonar observations of the ice draft taken between 1958 and 1976 corroborate this information. However, the unclassified cruises of the SCICEX program, 1993199t show a significant, locally variable, thinning of the ice by 12 meters (Rothrock et al., 2000). It is likely that most of this thinning occurred in the last two decades of the century.

All indicate a reduction of older ice types in the Arctic over the past several decades, in general agreement with observations of a thinning ice pack. The eastern Arctic Ocean is dominated by younger ice, while older ice resides in the western Arctic, in the Beaufort Gyre near the Canadian Archipelago. Multi-year ice accumulates from first-year ice formed primarily in the eastern Arctic and north of Alaska. Some of this ice is entrained into the Beaufort Gyre where it recirculates, ridging and thickening until it is eventually ejected into the Transpolar Drift Stream, which carries ice across the North Pole and out of the Arctic through the Fram Strait.

The Beaufort Gyre is the largest reservoir of freshwater in the Arctic Ocean. The quantity of fresh water stored in the Beaufort Gyre increased substantially in 2004-2005, 2006-2007 and 2007-2008, and remained at high levels since 2008. These increases in freshwater content were due to inputs of sea-ice melt water. Since the start of the satellite record in 1979, scientists have observed the continued disappearance of older "multiyear" sea ice that survives more than one summer melt season. Until recently, the majority of Arctic sea ice survived at least one summer and often several. But things have changed dramatically.

Arctic sea ice during the 2007 melt season plummeted to the lowest levels since satellite measurements began in 1979. The average sea ice extent for the month of September was 4.28 million square kilometers (1.65 million square miles), the lowest September on record, shattering the previous record for the month, set in 2005, by 23 percent.

Arctic sea ice receded so much that the fabled Northwest Passage completely opened for the first time in human memory during 2007. Explorers and other seafarers had long recognized that this passage, through the straits of the Canadian Arctic Archipelago, represented a potential shortcut from the Pacific to the Atlantic. Roald Amundsen began the first successful navigation of the route starting in 1903. It took his group two-and-a-half years to leapfrog through narrow passages of open water, with their ship locked in the frozen ice through two cold, dark winters. More recently, icebreakers and ice-strengthened ships have on occasion traversed the normally ice-choked route. However, by the end of the 2007 melt season, a standard ocean-going vessel could have sailed smoothly through. On the other hand, the Northern Sea Route, a shortcut along the Eurasian coast that is often at least partially open, was completely blocked by a band of ice in 2007.

In 2007 National Snow and Ice Data Center (NSIDC) Senior Scientist Mark Serreze said, "The sea ice cover is in a downward spiral and may have passed the point of no return. As the years go by, we are losing more and more ice in summer, and growing back less and less ice in winter. We may well see an ice-free Arctic Ocean in summer within our lifetimes." The scientists agree that this could occur by 2030.

Arctic sea ice extent had been shrinking at a relatively modest rate of 3-4% per decade (annually averaged) but after 1996 this speeded up to 10% per decade and in summer 2007 there was a massive collapse of ice extent to a new record minimum of only 4.1 million km(2). Thickness has been falling at a more rapid rate (43% in the 25 years from the early 1970s to late 1990s) with a specially rapid loss of mass from pressure ridges. The summer 2007 event may have arisen from an interaction between the long-term retreat and more rapid thinning rates.

A challenging characteristic of the summer sea ice extent is that its decay has exceeded the predictions of models. The observed extent began to deviate from the ensemble mean of models used by Intergovernmental Panel on Climate Change (IPCC) in the 1970s and by the 1990s it was more than one standard deviation less than the mean. The 2007 extent was less than the most extreme member of the ensemble. These results strongly suggest that existing climate models are inadequate in predicting Arctic sea ice extent and that some important physics was missing.

In 2012 Peter Wadhams at the University of Cambridge reviewed thickness monitoring techniques that showed the greatest promise on different spatial and temporal scales, and for different purposes. This showed results from some recent work from submarines, and speculated that the trends towards retreat and thinning will inevitably lead to an eventual loss of all ice in summer, which can be described as a 'tipping point' in that the former situation, of an Arctic covered with mainly multi-year ice, cannot be retrieved.

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Page last modified: 23-07-2018 13:41:03 ZULU