Polar Ice Packs

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Polar ice packsFrom Wikipedia, the free encyclopedia

NOAA Projected Arctic changes

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 theAntarctic ice pack of the Southern Ocean, fringing the Antarctic ice sheet. Polar packssignificantly change their size during seasonal changes of the year. However, underlying thisseasonal 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. TheAntarctic ice cover is highly seasonal, with very little ice in the austral summer, expanding to anarea roughly equal to that of Antarctica in winter. Consequently, most Antarctic sea ice is firstyear ice, up to 1 meter (3.28 ft) thick. The situation in the Arctic is very different (a polar seasurrounded by land, as opposed to a polar continent surrounded by sea) and the seasonalvariation much less, currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal:up to 34 meters (9.813.1 ft) thick over large areas, with ridges up to 20 meters (65.6 ft) thick.


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

The area of sea ice around the poles in winter is about 15,600,000 km 2 (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 eachsummer 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.


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C limatic importance

Methane restraint

M ain article: Arctic methane release

Decrease of the Arctic ice-pack 19822007(NSIDC)

Sea ice helps to constrain methane in permafrost and in clathrates. Arctic methane releasetriggered by a breakdown in sea ice could cause an abrupt climate change event, potentiallysimilar in some ways to the Paleocene-Eocene thermal maximum event, or to the great dying, amass extinction event.

Albedo effects

M ain 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 thesurface. The sea ice cycle is also an important source of dense (saline) "bottom water". Whilefreezing, water rejects its salt content (leaving pure ice). The remaining surface water, madedense by the extra salinity, sinks, leading to the productions of dense water masses such asAntarctic Bottom Water. This production of dense water is a factor in maintaining the

thermohaline circulation, and the accurate representation of these processes is an additionaldifficulty to climate modelling.


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Hy drological effects

In the Arctic, a key area where pancake ice forms the dominant ice type over an entire region isthe 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 7274N

during the winter because of the presence of very cold polar surface water in the JanM

ayenCurrent, which diverts some water eastward from the East Greenland Current at that latitude.M ost of the old ice continues south, driven by the wind, so a cold open water surface is exposedon which new ice forms as frazil and pancake in the rough seas. The salt rejected back into theocean from this ice formation causes the surface water to become denser and sink, sometimes togreat depths (2,500 m/8,200 ft or more), making this one of the few regions of the ocean wherewinter convection occurs, which helps drive the entire worldwide system of surface and deepcurrents known as the thermohaline circulation.


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Ex tent and volume of sea ice and their trend

M ain article: Arctic shrinkage

M onthly mean ice area, northern and southern hemispheres, in square meters, 19792003,

showing the annual cycle in the two hemispheres. Blue is NH, black is SH.Records of Arctic Sea ice from the United Kingdoms Hadley Centre for Climate Prediction andResearch go back to the turn of the 20th century, although the quality of the data before 1950 isdebatable. 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, theScanning M ultichannel M icrowave Radiometer (S MM R) on Seasat (1978) and Nimbus 7 (1978 87) satellites provided information that was independent of solar illumination or meteorologicalconditions. The frequency and accuracy of passive microwave measurements improved with thelaunch of the D M SP F8 Special Sensor M icrowave/Imager SS M I 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 atleast 15% sea ice.


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

A modeling study of the 52-year period from 1948 to 1999 found a statistically significant trendin Arctic ice volume of 3% per decade; splitting this into wind-forced and temperature forcedcomponents 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 areaconsiderations.

The trends from 1979 to 2002 have been a statistically significant Arctic decrease and anAntarctic increase that is probably not significant, depending exactly on which time period isused. The Arctic trends of 2.5% 0.9% per decade; or about 3% per decade. Climate modelssimulated 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.


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

In 2007 the ice melt accelerated. The minimum extent fell by more than a million squarekilometers, the biggest decline ever. The minimum extent fell to 4,140,000 km 2 (1,600,000sq 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 ClimateChange in preparing its 2007 assessments.

While the Northern Hemisphere sea ice reached new record lows, on September 12, 2007 theSouthern Hemisphere sea ice area reached 15,910,000 km 2 (6,143,000 sq mi), close to themaximum recorded of 16,020,000 km 2 (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.


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Scientific parameter to quantify the extent of sea ice

In the overall mass balance, the volume of sea ice depends on the thickness of the ice as well asthe 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 thissummers sea ice cover and the slow onset of freeze-up portends lower than normal ice extentthroughout autumn and winter, and the ice that grows back is likely to be fairly thin".


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S ea ice in recent years

2007 record low Arctic sea ice

M ain 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, newhistoric Arctic sea ice minima were observed. Around September 16, 2007, a minimum area of 2,920,000 km 2 (1,130,000 sq mi) and minimum extent of 4,140,000 km 2 (1,600,000 sq mi) werereached. These numbers shattered the previous (September 20, 2005) record absolute minima;the 2007 minimum extent was 22% or 1,190,000 km 2 (459,000 sq mi) smaller (approximatelythe 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 iceedge ever was recorded in September at 85.5N (near 160E), i.e. just 4.5 from the North Pole.A 2007 NASA study concluded that the shrinkage was the result of "unusual atmosphericconditions [which] set up wind patterns that compressed the sea ice, loaded it into the Transpolar


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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 Passageopened to ships without the need of icebreakers. The main channel of this passage (Lancaster

Sound toM

'Clure Strait ) opened as early as August 11. However, the Northeast Passageremained blocked by a narrow band of sea ice around Severnaya Zemlya.


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W inter 2007/2008 Arctic ice growth

Arctic Sea ice age in February 2008 compared to the average for 19852000 (NASA)

Extremely cold temperatures for the Northern Hemisphere in the Winter of 2007/2008 helped theArctic ice pack to grow to more near normal levels i n terms of surface area covered . The ice wasalso 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, whichreached its lowest coverage ever recorded in the summer of 2007," according to Gilles Langis, asenior 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 citedreference) reveals. The February 2008 ice pack (right) contained much more young ice than thelong-term average (left). In the past, more ice survived the summer melt season and had thechance to thicken over the following winter. In the mid- to late 1980s, over 20 percent of Arcticsea ice was at least six years old; in February 2008, just 6 percent of the ice was six years old or older.

S ummer 2008 Arctic ice shrinking

The 2008 minimum was slightly larger than 2007. On August 27, both the Northwest Passageand 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 beencircumnavigated., although the icebreaker Polarstern was the only ship to actually make thecircumnavigation. The Beluga Group of Bremen, Germany, announced plans to send the firstship 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.


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The significant reduction in the extent of the summer sea ice cover and the decrease in theamount of relatively older, thicker ice continued in 2009. The extent of the 2009 summer sea icecover was the fourth lowest value of the satellite monitoring record and more than 25% below

the 19792000 average.

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


On September 19, 2010, 10 days later than the usual minimum, the arctic ice cover reached4,600,000 km 2 (1,800,000 sq mi), its third lowest value since satellite monitoring began. The2010 minimum was just 37,000 km 2 (14,000 sq mi) above that of 2008, and 2,110,000 km 2 (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 DanielGavrilov arriving first, and the Norwegian "Northern Passage" with captain Brge Ousland.

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