polar ice packs
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.
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 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, consequently much Arctic sea ice is multi-year ice, and thicker: up to 3–4 meters thick over large areas, with ridges up to 20 meters thick.
The amount of sea ice around the poles in winter varies from the Antarctic with 18,000,000 km² to the Arctic with 15,000,000 km². The amount melted each summer is affected by the different environments: the cold Antarctic pole is over land so sea ice is around edge, and the Antarctic sea ice is in the freely-circulating Southern Ocean.
Climatic importance
Sea ice has an important effect on the heat balance of the polar oceans, since it acts to insulate the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. Especially when covered with snow, sea ice has a high albedo — about 0.8 — 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) and the remaining surface, 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.
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 more dense and sink, sometimes to great depths (2500 m 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 trends of polar ice packs
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.[1].
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.
In a modelling study of the 52-year period from 1948 to 1999 Rothrock and Zhang (2005) find 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.
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[2]. Climate models simulated this trend in 2002[3], and attributed it to anthropogenic forcing.
In September 2002, sea ice in the Arctic reached a record minimum[4], 4% lower than any previous September since 1978, and 14% lower than the 1978–2000 mean minimum extent of about 7 million km². In the past, a low ice year would be followed by a rebound to near-normal conditions, but 2002 was followed by two more low-ice years, both of which almost matched the 2002 record. The September ice extent trend for 1979–2004 is declining by 7.7% per decade[5].
A new record low-ice year in 2005 in which a minimum extent of 5.32 million km² was reached. September 2006 saw a slight recovery.
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 4.14 million km², 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.[6]
While the Northern Hemisphere sea ice reached new record lows, on September 12 2007 the Southern Hemisphere sea ice area reached 15.91 million km², close to the maximum recorded of 16.02 million km².[7]
The Antarctic increase is 0.8% per decade[8] although this depends on the period being considered. Vinnikov et al[9] 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"[1].
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 2.92 million km² and minimum extent of 4.14 million km² were reached. These numbers shattered the previous (September 20 2005) record absolute minima; the 2007 minimum extent was 22% or 1.19 million km² smaller (approximately the size of Texas and California, or five United Kingdoms, combined) and 41% below the 1978-2000 average summer minimum [1]. The area was even 27% below the previous record and 46% below the average, reflecting the poorer quality of the remaining ice packs [7]. The northernmost ice edge ever was recorded in September at 85.5 degrees North (near 160 degrees East), i.e. just 4.5 degrees from the north pole.
Based on the extreme drop in 2007, scientists at the US National Snow and Ice Data Center (NSIDC) reduced their estimates on when the first ice free arctic ocean would appear, predicting this to happen as early as 2030 [10].
The Arctic Sea has not been ice free for a period of at least one million years, and probably much longer.[11][12]
The NSIDC also reported that, for the first time in recorded history, the Northwest Passage opened to ships without the need of icebreakers[1][13]. The main channel of this passage (Lancaster Sound to M'Clure Strait) has been open since about August 11. As of September 10, the Northeast Passage remained blocked by a narrow band of sea ice around Severnaya Zemlya[1].
Summer melting
In the Arctic, the overlying snow layer typically begins to melt from late May to early June. Melting of the snowcover leads to the development of melt ponds (meltwater pools) on the surface of the ice. On first year ice, which has a smooth upper surface at the end of winter (except where ridged), the pools are initially very shallow, forming in minor depressions in the ice surface, or simply being retained within surviving snow pack as a layer of slush. As summer proceeds, however, this initial random structure becomes more fixed as the pools melt their way down into the ice through preferential absorption of solar radiation by the water, which reflects only 15–40% of the radiation falling on it compared to 40–70% for bare ice.[14].
As the melt pools grow deeper and wider they may eventually drain off into the sea, over the side of floes, through existing cracks, or by melting a thaw hole right through the ice at its thinnest point or at the melt pool's deepest point. The downrush of water when a thaw hole opens may be quite violent, and on very level ice, such as fast ice, a single thaw hole may drain a large area of ice surface. From the air such thaw holes give the appearance of "giant spiders", with the "body" being the thaw hole and the "legs" channels of melt water draining radially towards the hole.
The underside of the ice cover also responds to the surface melt. Directly underneath melt pools the ice is thinner and is
absorbing more incoming radiation. This causes an enhanced rate of bottom melt so that the ice bottom develops a topography of
depressions to mirror the melt pool distribution on the top side. In this way an initially smooth first-year ice sheet acquires
by the end of summer an undulating topography both on its top and bottom sides. Some of the drained melt water may in fact gather
in the underside depressions to form under-ice melt pools, which refreeze in autumn and
partially smooth off the underside, leaving it with bulges but not depressions.[14].
| Extent of the Arctic ice-pack 2002–2004 (NSIDC) | ||
 |
 |
 |
A final and most important role of the melt water is that some of it works its way down through the ice fabric through minor pores, veins and channels, and in doing so drives out much of the remaining brine. This process, called flushing, is the most efficient and rapid form of brine drainage mechanism, and it operates to remove nearly all of the remaining brine from the first-year ice. The hydrostatic head of the surface meltwater provides the driving force, but an interconnecting network of pores is necessary for the flushing process to operate. Given that the strength properties of sea ice depend on the brine volume, this implies that the flushing mechanism creates a surviving ice sheet which during its second winter of existence has much greater strength than in its first winter.[14].
References
- ^ a b c d e National Snow and Ice Data Center
- ^ Cavalieri et al. 2003.
- ^ Harvard.
- ^ Serreze et al. 2003.
- ^ Stroeve et al. 2005.
- ^ and NSIDC "Arctic Ice Retreating More Quickly Than Computer Models Project"
- ^ a b
- ^ Sea ice, NSIDC.
- ^ GRL, 2006.
- ^ David Adam Ice-free Arctic could be here in 23 years The Guardian September 5 2007
- ^ Pew Center on Global Climate Change " Arctic Ice Cap at Record Minimum Size"
- ^ "Arctic System on Trajectory to New, Seasonally Ice-Free State"
- ^ North-West Passage is now plain sailing
- ^ a b c National Oceanic and Atmospheric Administration
See also
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 of 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.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
