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, currently 28% of Arctic basin sea ice is multi-year ice, thicker than seasonal: 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, which is bordered by sea ice in the freely-circulating Southern Ocean.
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.
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.
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.
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. Climate models simulated this trend in 2002, and attributed it to anthropogenic forcing.
In September 2002, sea ice in the Arctic reached a record minimum, 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.
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.
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".
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 . 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 degrees North (near 160 degrees East), i.e. just 4.5 degrees 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.
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 . Professor Wieslaw Maslowski has gone further, predicting a possible ice-free Arctic Ocean by as early as 2013..
The Arctic Ocean has not been ice free for a period of at least one million years, and perhaps much longer.
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) 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.
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 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.
Critical Arctic sea ice this winter made a tenuous partial recovery from last summer's record melt, U. S. scientists said Tuesday. But that's an illusion, like a Hollywood movie set, The ice is very thin and vulnerable to heavy melting again this summer. - Walter Meier of the National Snow and Ice Data Center
"We're in for a world of hurt this summer. Depending on the weather, there could be as much melting this year as last, maybe more." - NSIDC senior scientist Mark Serreze