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Polar ice packs are large areas of pack ice formed from seawater in the Earth's polar regions, known as polar ice caps: the Arctic ice pack (or Arctic ice cap) of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean, fringing the Antarctic ice sheet. Polar packs significantly change their size during seasonal changes of the year. However, underlying this seasonal variation, there is an underlying trend of melting as part of a more general process of Arctic shrinkage.

In spring and summer, when melting occurs, the margins of the sea ice retreat. The vast bulk of the world's sea ice forms in the Arctic ocean and the Southern Ocean, around Antarctica. The Antarctic ice cover is highly seasonal, with very little ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. Consequently, most Antarctic sea ice is first year ice, up to 1 meter (3.28 ft) thick. The Arctic Ocean is very different, being a polar sea surrounded by land rather than to a polar continent surrounded by sea, and its ice shows less seasonal variation. Currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal: up to 3–4 meters (9.8–13.1 ft) thick over large areas, with ridges up to 20 meters (65.6 ft) thick.

The area of sea ice around the poles in winter is about 15,600,000 km (6,000,000 sq mi) either for the Antarctic or Arctic. However, whereas the northern cap is shrinking at a rate of about 3% per decade, the southern cap is expanding at a rate of 0.8% per decade. The amount melted each summer is affected by the different environments: the cold Antarctic pole is over land, which is bordered by sea ice in the freely circulating Southern Ocean. The summer ice cover is about 12% of the winter coverage in the Antarctic and 50% in the Arctic.

Climatic importance

Methane restraint

Sea ice helps to constrain methane in permafrost and in clathrates. Arctic methane release triggered by a breakdown in sea ice could cause an abrupt climate change event, potentially similar in some ways to the Paleocene–Eocene thermal maximum event of 55 million years ago, or to the more catastrophic Permian–Triassic extinction event of 252 million years ago.

Albedo effects

Sea ice has an important effect on the heat balance of the polar oceans, since it insulates the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. sea ice has a high albedo — about 0.6 when bare, and about 0.8 when covered with snow — compared to the sea – about 0.15 – and thus the ice also affects the absorption of sunlight at the surface. The sea ice cycle is also an important source of dense (saline) "bottom water". While freezing, water rejects its salt content (leaving pure ice). The remaining surface water, made dense by the extra salinity, sinks, leading to the productions of dense water masses such as Antarctic Bottom Water. This production of dense water is a factor in maintaining the thermohaline circulation, and the accurate representation of these processes is an additional difficulty to climate modelling.

Hydrological effects

In the Arctic, a key area where pancake ice forms the dominant ice type over an entire region is the so-called Odden ice tongue in the Greenland Sea. The Odden (the word is Norwegian for headland) grows eastward from the main East Greenland ice edge in the vicinity of 72–74°N during the winter because of the presence of very cold polar surface water in the Jan Mayen Current, which diverts some water eastward from the East Greenland Current at that latitude. Most of the old ice continues south, driven by the wind, so a cold open water surface is exposed on which new ice forms as frazil and pancake in the rough seas. The salt rejected back into the ocean from this ice formation causes the surface water to become denser and sink, sometimes to great depths (2,500 m,8,200 ft or more), making this one of the few regions of the ocean where winter convection occurs, which helps drive the entire worldwide system of surface and deep currents known as the thermohaline circulation.

Extent and volume of sea ice and their trends

Records of Arctic Sea ice from the United Kingdom’s Hadley Centre for Climate Prediction and Research go back to the turn of the 20th century, although the quality of the data before 1950 is debatable. Still, these records show a persistent decline in Arctic Sea ice over the last 50 years.

Reliable measurements of sea ice edge begin within the satellite era. From the late 1970s, the Scanning Multichannel Microwave Radiometer (SMMR) on Seasat (1978) and Nimbus 7 (1978–87) satellites provided information that was independent of solar illumination or meteorological conditions. The frequency and accuracy of passive microwave measurements improved with the launch of the DMSP F8 Special Sensor Microwave/Imager SSMI in 1987. Both the sea ice area and extent are estimated, with the latter being larger, as it is defined as the area of ocean with at least 15% sea ice.

A modeling study of the 52-year period from 1948 to 1999 found a statistically significant trend in Arctic ice volume of −3% per decade; splitting this into wind-forced and temperature forced components shows it to be essentially all caused by the temperature forcing. A computer-based, time-resolved calculation of sea ice volume, fitted to various measurements, revealed that monitoring the ice volume is much more significant for evaluating sea ice loss than pure area considerations.

The trends from 1979 to 2002 have been a statistically significant Arctic decrease and an Antarctic increase that is probably not significant, depending exactly on which time period is used. The Arctic trends of −2.5% ± 0.9% per decade; or about 3% per decade. Climate models simulated this trend in 2002, and attributed it to anthropogenic forcing.

The September minimum ice extent trend for 1979–2011 declined by 12.0% per decade.

In 2007 the ice melt accelerated. The minimum extent fell by more than a million square kilometers, the biggest decline ever, to 4,140,000 km (1,600,000 sq mi), then by far the lowest ever. New research shows the Arctic Sea ice to be melting faster than predicted by any of the 18 computer models used by the Intergovernmental Panel on Climate Change in preparing its 2007 assessments. In 2012, a new record low of about 3,500,000 km (1,400,000 sq mi) was reached.

While the Northern Hemisphere sea ice reached new record lows, on September 12, 2007 the Southern Hemisphere sea ice area reached 15,910,000 km (6,143,000 sq mi), close to the maximum recorded of 16,020,000 km (6,185,000 sq mi).

The Antarctic increase is 0.8% per decade although this depends on the period being considered. Vinnikov et al. find the NH reduction to be statistically significant but the SH trend is not.

In the overall mass balance, the volume of sea ice depends on the thickness of the ice as well as the areal extent. While the satellite era has enabled better measurement of trends in areal extent, accurate ice thickness measurements remain a challenge. "Nonetheless, the extreme loss of this summer’s sea ice cover and the slow onset of freeze-up portends lower than normal ice extent throughout autumn and winter, and the ice that grows back is likely to be fairly thin".

As more and more of the sea ice is thinner first-year ice the greater effect storms have on its stability with turbulence resulting from major extratropical cyclones resulting in extensive fractures of sea ice.

Sea ice in recent years

2007 record low Arctic sea ice

Already in early August 2007, about a month before the absolute minimum was expected, new historic Arctic sea ice minima were observed. Around September 16, 2007, a minimum area of 2,920,000 km (1,130,000 sq mi) and minimum extent of 4,140,000 km (1,600,000 sq mi) were reached. These numbers shattered the previous (September 20, 2005) record absolute minima; the 2007 minimum extent was 22% or 1,190,000 km (459,000 sq mi) smaller (approximately the size of Texas and California, or five United Kingdoms, combined) and 41% below the 1978–2000 average summer minimum. The area was even 27% below the previous record and 46% below the average, reflecting the poorer quality of the remaining ice packs. The northernmost ice edge ever was recorded in September at 85.5°N (near 160°E), i.e. just 4.5° from the North Pole. A 2007 NASA study concluded that the shrinkage was the result of "unusual atmospheric conditions set up wind patterns that compressed the sea ice, loaded it into the Transpolar Drift Stream and then sped its flow out of the Arctic." Also the total summertime cloud cover was lower than previous years enhancing the melting.

The NSIDC also reported that, for the first time in recorded history, the Northwest Passage opened to ships without the need of icebreakers. The main channel of this passage (Lancaster Sound to M'Clure Strait) opened as early as August 11. However, the Northeast Passage remained blocked by a narrow band of sea ice around Severnaya Zemlya.

Winter 2007/2008 Arctic ice growth

Extremely cold temperatures for the Northern Hemisphere in the Winter of 2007/2008 helped the Arctic ice pack to grow to more near normal levels in terms of surface area covered. The ice was also found to be 10 to 20 centimeters (3.9 to 7.9 in) thicker than the previous year in some areas. "But it's too soon to say what impact this winter will have on the Arctic summer sea ice, which reached its lowest coverage ever recorded in the summer of 2007," according to Gilles Langis, a senior ice forecaster with the Canadian Ice Service in Ottawa.

While the cold winter did allow sea ice to re-cover much of the Arctic during the Winter of 2007/2008, conditions were far from normal as "this" pair of NASA images (in the cited reference) reveals. The February 2008 ice pack (right) contained much more young ice than the long-term average (left). In the past, more ice survived the summer melt season and had the chance to thicken over the following winter. In the mid- to late 1980s, over 20 percent of Arctic sea ice was at least six years old; in February 2008, just 6 percent of the ice was six years old or older.

Summer 2008 Arctic ice shrinking

The 2008 minimum was slightly larger than 2007 at 4.67 million square kilometers (1.80 million square miles). On August 27, both the Northwest Passage and the Northeast Passage were ice-free. This was the first time in recorded history that both passages were open at the same time. The North Pole could at that point have been circumnavigated., although the icebreaker Polarstern was the only ship to actually make the circumnavigation. The Beluga Group of Bremen, Germany, announced plans to send the first ship through the Northern Sea Route in 2009, thereby cutting 4,000 nautical miles (7,400 km; 4,600 mi) off the voyage from Germany to Japan.

Summer 2009 Arctic ice shrinking

The significant reduction in the extent of the summer sea ice cover and the decrease in the amount of relatively older, thicker ice continued in 2009. The extent of the 2009 summer sea ice cover was the fourth lowest value of the satellite monitoring record and more than 25% below the 1979–2000 average, while a record low sea ice volume was reached.

Summer 2010 Arctic ice shrinking

On September 19, 2010, 10 days later than the usual minimum, the Arctic ice cover reached 4,600,000 km (1,800,000 sq mi), its third-lowest value since satellite monitoring began. The 2010 minimum was just 37,000 km (14,000 sq mi) above that of 2008, and 2,110,000 km (815,000 sq mi) below the 1979 to 2009 average minimum. For the first time, two yachts were able to make the circumnavigation in one season: The Russian Peter 1 with captain Daniel Gavrilov arriving first, and the Norwegian Northern Passage with captain Børge Ousland.

Summer 2011 Arctic ice shrinking

Ice extent reached its minimum of 4.33 million square kilometers (1.67 million square miles) on September 9, 2011 - the second lowest summer extent since satellite monitoring began.

Summer 2012 Arctic sea ice new record low

Arctic sea ice was at a new record low on September 16, 2012 at 3.41 million square kilometers (1.32 million square miles). The new record low was thus 760,000 square kilometers (293,000 square miles) below the previous record minimum extent, which occurred on September 18, 2007. The 2012 value is only half (51%) of the 1979–2000 average extent of arctic sea ice at the September minimum. The six lowest seasonal minimum ice extents in the satellite record have all occurred between 2007 and 2012.

Summer 2013 Arctic ice growth

In August 2013 the ice extent increased to an average of 6.09 million square kilometers. A hole in the ice cover was observed near the North Pole. Antarctic ice cover reached record highs.

Climate

ID - insufficient data

See also

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• Arctic Ocean
• Arctic shrinkage
• Extreme Ice Survey
• Global warming
• Iceberg

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• Ice drift
• Jurassic
• Marine Modeling and Analysis Branch
• Polar ice cap
• Polynya

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• Sea ice
• Shelf ice
• Tabular iceberg
• Triassic

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References

1. Arctic Sea Ice News & Analysis – April 2008
2. Polar Sea Ice Cap and Snow – Cryosphere Today, University of Illinois
3. ^ National Snow and Ice Data Center
4. ^ Zhang, Jinlun and D.A. Rothrock: Modeling global sea ice with a thickness and enthalpy distribution model in generalized curvilinear coordinates, Mon. Wea. Rev. 131(5), 681–697, 2003.
5. Cavalieri et al. 2003.
6. Gregory, J. M. (2002). "Recent and future changes in Arctic sea ice simulated by the HadCM3 AOGCM". Geophysical Research Letters. 29 (24): 2175. Bibcode:2002GeoRL..29x..28G. doi:10.1029/2001GL014575.
7. National Snow and Ice Data Center
8. and NSIDC "Arctic Ice Retreating More Quickly Than Computer Models Project"
9. http://www.ijis.iarc.uaf.edu/en/home/seaice_extent.htm, as of Sept. 17, 2012.
10. http://www.smh.com.au/environment/climate-change/staggering-arctic-ice-loss-smashes-melt-records-20120917-260zu.html
11. ^ The Cryosphere Today
12. Sea ice, NSIDC.
13. Vinnikov, Konstantin Y.; Cavalieri, Donald J.; Parkinson, Claire L. (2006). "A model assessment of satellite observed trends in polar sea ice extents" (PDF). Geophysical Research Letters. 33 (5): L05704. Bibcode:2006GeoRL..3305704V. doi:10.1029/2005GL025282.
14. Andrew Freedman (March 13, 2013). "Large Fractures Spotted in Vulnerable Arctic Sea Ice". Climate Central. Retrieved March 14, 2013.
15. NASA Examines Arctic Sea Ice Changes Leading to Record Low in 2007
16. Kay, J. E., T. L'Ecuyer, A. Gettelman, G. Stephens, and C. O'Dell (2008), The contribution of cloud and radiation anomalies to the 2007 Arctic sea ice extent minimum, Geophys. Res. Lett., 35, L08503, doi:10.1029/2008GL033451
17. North-West Passage is now plain sailing
18. ^ Recent cold snap helping Arctic sea ice, scientists find
19. NASA Earth Observatory "Arctic Sea Ice Younger than Normal"
20. NSIDC Press Room: Sea Ice Minimum 2008. Nsidc.org. Retrieved on 2010-09-05.
21. Tauende Arktis: Nordost- und Nordwestpassage erstmals gleichzeitig eisfrei – SPIEGEL ONLINE – Nachrichten – Wissenschaft. Spiegel.de. Retrieved on 2010-09-05.
22. independent.co.uk, For the first time in human history, the North Pole can be circumnavigated
23. Map of the Polarstern excursion August – October 2008
24. dailymail.co.uk, The North Pole becomes an 'island' for the first time in history as ice melts
25. NOAA Arctic Report Card. Update for 2009
26. Sea Ice Volume Anomaly Measurements 1980 – 2010
27. National Snow and Ice Data Center Updated minimum Arctic sea ice extent, September 27, 2010
28. "Arctic sea ice level third-lowest on record". The Toronto Star. Toronto, Canada. September 15, 2010. Retrieved 2010-09-15. from The Canadian Press
29. Yacht "Peter 1" blog
30. Yacht "Northern Passage" blog
31. BarentsObserver.com: "Around the North Pole in less than three months"
32. National Snow and Ice Data Center Updated minium Arctic sea ice extent, October 4, 2011
33. National Snow and Ice Data Center Arctic sea ice extent settles at record seasonal minimum, September 19, 2012
34. National Snow and Ice Data Center A real hole near the pole, September 4, 2012
35. Environment Canada—Canadian Climate Normals 1971–2000. Retrieved 17 June 2011.
36. Environment Canada—Canadian Climate Normals 1961–1990. Retrieved 25 september 2012.

External links

• Ice in the Sea, chapter from Nathaniel Bowditch's American Practical Navigator.
• Cryosphere Today: current Arctic sea ice conditions.
• The Shrinking Arctic Ice Cap research summary, graphics and animations from NOAA Geophysical Fluid Dynamics Laboratory
• Data source for sea ice picture.
• Everything you ever wanted to know about sea ice but were afraid to ask.
• Animation of the movement of sea ice, September 2003 through May 2004.
• NSIDC Sea Ice Index.
• Arctic summer time: The short summer of 2004. North Pole webcam view.
• Global Sea Ice Extent and Concentration: what sensors on satellites are telling us about sea ice.
• "Arctic ice 'disappearing quickly'", BBC News, 28 September 2005.
• "Ice-free Arctic could be here in 23 years" David Adam, environment correspondent, The Guardian, September 5, 2007, Retrieved September 5, 2007
• "Arctic Melt Unnerves the Experts" Andrew C. Revkin, The New York Times, October 2, 2007, Retrieved October 2, 2007.
• Video on polar ice changes around St. Lawrence Island in the Bering Sea
• The Arctic ice sheet True color map, daily updates during summer.

• Earth phenomena
• Sea ice
• Glaciology
• Climatology
• Geography of the Arctic
• Arctic Ocean