Polar ice packs

 

Ice losses in Antarctica are concentrated on its western coast, while Greenland's thawing has spread from the south to the northwest since 2005.

NOAA Projected Arctic changes.

Annual growth and retreat of the polar ice packs from SeaWiFS images.

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 situation in the Arctic is very different (a polar sea surrounded by land, as opposed to a polar continent surrounded by sea) and the seasonal variation much less^{[citation needed]}, 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 km² (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 1.8% 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 icer cover is about 12% of the winter coverage in the Antarctic and 50% in the Arctic.

This animation shows the Arctic Ocean melt during the summer of 2011.

NASA Cryosphere Program manager Tom Wagner in his video interpretation of the 2011 sea ice record.

Climatic importance

Methane restraint

Main article: Arctic methane release

Decrease of the Arctic ice-pack 1982–2007(NSIDC).

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, or to the great dying, a mass extinction event.

Albedo effects

Main article: Albedo

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

Main article: Arctic shrinkage

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 ice extent trend for 1979–2004 is declining by 7.7% per decade.

In 2007 the ice melt accelerated. The minimum extent fell by more than a million square kilometers, the biggest decline ever. The minimum extent fell to 4,140,000 km² (1,600,000 sq mi), 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.

While the Northern Hemisphere sea ice reached new record lows, on September 12, 2007 the Southern Hemisphere sea ice area reached 15,910,000 km² (6,143,000 sq mi), close to the maximum recorded of 16,020,000 km² (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".

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  Monthly mean ice area, northern and southern hemispheres, in square meters, 1979–2003, showing the annual cycle in the two hemispheres. Blue is NH, black is SH.

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  The development of arctic sea ice volume as determined by measurement corrected numerical simulation shows probability of total sea ice loss in summer for the near future.

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  Record Low Arctic Sea Ice in 2007 – Showing the Northwest passage open.

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  Scientific parameter to quantify the extent of sea ice.

Sea ice in recent years

2007 record low Arctic sea ice

Main article: Arctic shrinkage

Decline in ice extent on September 9, 2007 from historic average.

In 2007 the Arctic Sea ice melted early, and rapidly fell to record low levels.

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 km² (1,130,000 sq mi) and minimum extent of 4,140,000 km² (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 km² (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 [which] 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

Arctic Sea ice age in February 2008 compared to the average for 1985–2000 (NASA).

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. 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.

Despite the fact that the extent did not reach a new minimum in 2009 due to unfavorable wind conditions in August, it did set a new record for minimum ice volume.

Summer 2010 Arctic ice shrinking

On September 19, 2010, 10 days later than the usual minimum, the arctic ice cover reached 4,600,000 km² (1,800,000 sq mi), its third lowest value since satellite monitoring began. The 2010 minimum was just 37,000 km² (14,000 sq mi) above that of 2008, and 2,110,000 km² (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.

See also

Arctic Ocean Arctic shrinkage Extreme Ice Survey Global warming Iceberg

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

Sea ice Shelf ice Tabular iceberg Triassic

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.