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Sea ice arises as
seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
freezes. Because
ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...
is less
dense Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematically ...
than water, it floats on the ocean's surface (as does
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
ice, which has an even lower density). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world's sea ice is enclosed within the polar ice packs in the Earth's
polar region The polar regions, also called the frigid geographical zone, zones or polar zones, of Earth are the regions of the planet that surround its geographical poles (the North Pole, North and South Poles), lying within the polar circles. These high l ...
s: the
Arctic ice pack The Arctic ice pack is the sea ice cover of the Arctic Ocean and its vicinity. The Arctic ice pack undergoes a regular seasonal cycle in which ice melts in spring and summer, reaches a minimum around mid-September, then increases during fall a ...
of the
Arctic Ocean The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
and the
Antarctic ice pack Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer, getting closer and closer to the coastline every year due to sea ice melting. Sea ice is frozen ...
of the
Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the World Ocean, generally taken to be south of 60° S latitude and encircling Antarctica. With a size of , it is regarded as the second-small ...
. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the
Arctic ecology Arctic ecology is the scientific study of the relationships between biotic and abiotic factors in the arctic, the region north of the Arctic Circle (66 33’). This region is characterized by stressful conditions as a result of extreme cold, low ...
, including the ocean's ecosystems. Due to the action of winds, currents and temperature fluctuations, sea ice is very dynamic, leading to a wide variety of ice types and features. Sea ice may be contrasted with
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s, which are chunks of
ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...
or
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
s that calve into the ocean. Depending on location, sea ice expanses may also incorporate icebergs.


General features and dynamics

Sea ice does not simply grow and melt. During its lifespan, it is very dynamic. Due to the combined action of winds, currents, water temperature and air temperature fluctuations, sea ice expanses typically undergo a significant amount of deformation. Sea ice is classified according to whether or not it is able to drift and according to its age.


Fast ice versus drift (or pack) ice

Sea ice can be classified according to whether or not it is attached (or frozen) to the shoreline (or between
shoal In oceanography, geomorphology, and geoscience, a shoal is a natural submerged ridge, bank, or bar that consists of, or is covered by, sand or other unconsolidated material and rises from the bed of a body of water to near the surface. It ...
s or to grounded
iceberg An iceberg is a piece of freshwater ice more than 15 m long that has broken off a glacier or an ice shelf and is floating freely in open (salt) water. Smaller chunks of floating glacially-derived ice are called "growlers" or "bergy bits". The ...
s). If attached, it is called landfast ice, or more often,
fast ice Fast ice (also called ''land-fast ice'', ''landfast ice'', and ''shore-fast ice'') is sea ice that is "fastened" to the coastline, to the sea floor along shoals or to grounded icebergs.Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer ...
(from ''fastened''). Alternatively and unlike fast ice,
drift ice Drift ice, also called brash ice, is sea ice that is not attached to the shoreline or any other fixed object (shoals, grounded icebergs, etc.).Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer-Verlag. Unlike fast ice, which is "fasten ...
occurs further offshore in very wide areas and encompasses ice that is free to move with currents and winds. The physical boundary between fast ice and drift ice is the ''fast ice boundary''. The drift ice zone may be further divided into a ''shear zone'', a ''marginal ice zone'' and a ''central pack''. Drift ice consists of ''floes'', individual pieces of sea ice or more across. There are names for various floe sizes: ''small'' – ; ''medium'' – ; ''big'' – ; ''vast'' – ; and ''giant'' – more than . The term ''pack ice'' is used either as a synonym to ''drift ice'', or to designate drift ice zone in which the floes are densely packed. The overall sea ice cover is termed the ''ice canopy'' from the perspective of submarine navigation.


Classification based on age

Another classification used by scientists to describe sea ice is based on age, that is, on its development stages. These stages are: ''new ice'', ''nilas'', ''young ice'', ''first-year'' and ''old''.


New ice, nilas and young ice

''New ice'' is a general term used for recently frozen sea water that does not yet make up solid ice. It may consist of
frazil ice Frazil ice is a collection of loose, randomly oriented ice crystals millimeter and sub-millimeter in size, with various shapes, e.g. elliptical disks, dendrites, needles and of an irregular nature. Frazil ice forms during the winter in open-wate ...
(plates or spicules of ice suspended in water),
slush Slush, also called slush ice, is a slurry mixture of small ice crystals (e.g., snow) and liquid water. In the natural environment, slush forms when ice or snow melts or during mixed precipitation. This often mixes with dirt and other pollutant ...
(water saturated snow), or ''shuga'' (spongy white ice lumps a few centimeters across). Other terms, such as
grease ice Grease ice is a very thin, soupy layer of frazil crystals clumped together, which makes the ocean surface resemble an oil slick. Grease ice is the second stage in the formation of solid sea ice after ice floes and then frazil ice. New sea ice f ...
and
pancake ice Pancake ice is a form of sea ice that consists of round pieces of ice with diameters ranging from 30 centimetres (12 in) to 3 metres (9.8 ft) and thicknesses up to 10 centimetres (3.9 inches), depending on the local conditions. It forms as a resu ...
, are used for ice crystal accumulations under the action of wind and waves. When sea ice begins to form on a beach with a light swell,
ice eggs Ice eggs, or ice balls, are a rare phenomenon caused by a process in which small pieces of sea ice in open water are rolled over by wind and currents in freezing conditions and grow into spheroid pieces of ice. They may collect into heaps of balls ...
up to the size of a football can be created. ''Nilas'' designates a sea ice crust up to in thickness. It bends without breaking around waves and swells. Nilas can be further subdivided into ''dark nilas'' – up to in thickness and very dark and ''light nilas'' – over in thickness and lighter in color. ''Young ice'' is a transition stage between nilas and first-year ice and ranges in thickness from to , Young ice can be further subdivided into ''grey ice'' – to in thickness and ''grey-white ice'' – to in thickness. Young ice is not as flexible as nilas, but tends to break under wave action. Under compression, it will either raft (at the grey ice stage) or ridge (at the grey-white ice stage).


First-year sea ice

''First-year sea ice'' is ice that is thicker than ''young ice'' but has no more than one year growth. In other words, it is ice that grows in the fall and winter (after it has gone through the ''new ice – nilas – young ice'' stages and grows further) but does not survive the spring and summer months (it melts away). The thickness of this ice typically ranges from to . First-year ice may be further divided into ''thin'' ( to ), ''medium'' ( to ) and ''thick'' (>).


Old sea ice

''Old sea ice'' is sea ice that has survived at least one melting season (''i.e.'' one summer). For this reason, this ice is generally thicker than first-year sea ice. Old ice is commonly divided into two types: ''second-year ice'', which has survived one melting season and ''multiyear ice'', which has survived more than one. (In some sources, ''old ice'' is more than two years old.) Multi-year ice is much more common in the
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
than it is in the
Antarctic The Antarctic ( or , American English also or ; commonly ) is a polar region around Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica, the Kerguelen Plateau and other ...
. The reason for this is that sea ice in the south drifts into warmer waters where it melts. In the Arctic, much of the sea ice is land-locked.


Driving forces

While
fast ice Fast ice (also called ''land-fast ice'', ''landfast ice'', and ''shore-fast ice'') is sea ice that is "fastened" to the coastline, to the sea floor along shoals or to grounded icebergs.Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer ...
is relatively stable (because it is attached to the shoreline or the seabed), drift (or pack) ice undergoes relatively complex deformation processes that ultimately give rise to sea ice's typically wide variety of landscapes. Wind is the main driving force, along with ocean currents. The
Coriolis force In physics, the Coriolis force is an inertial or fictitious force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the ...
and sea ice surface tilt have also been invoked. These driving forces induce a state of stress within the drift ice zone. An
ice floe An ice floe () is a large pack of floating ice often defined as a flat piece at least 20 m across at its widest point, and up to more than 10 km across. Drift ice is a floating field of sea ice composed of several ice floes. They may caus ...
converging toward another and pushing against it will generate a state of ''compression'' at the boundary between both. The ice cover may also undergo a state of ''tension'', resulting in divergence and fissure opening. If two floes drift sideways past each other while remaining in contact, this will create a state of ''
shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...
''.


Deformation

Sea ice deformation results from the interaction between ice floes, as they are driven against each other. The result may be of three types of features: 1) '' Rafted ice'', when one piece is overriding another; 2) '' Pressure ridges'', a line of broken ice forced downward (to make up the ''keel'') and upward (to make the ''sail''); and 3) ''
Hummock In geology, a hummock is a small knoll or mound above ground.Bates, Robert L. and Julia A. Jackson, ed. (1984). “hummock.” Dictionary of Geological Terms, 3rd Ed. New York: Anchor Books. p. 241. They are typically less than in height and ...
'', a hillock of broken ice that forms an uneven surface. A ''shear ridge'' is a pressure ridge that formed under shear – it tends to be more linear than a ridge induced only by compression. A ''new ridge'' is a recent feature – it is sharp-crested, with its side sloping at an angle exceeding 40 degrees. In contrast, a ''weathered ridge'' is one with a rounded crest and with sides sloping at less than 40 degrees. Stamukhi are yet another type of pile-up but these are grounded and are therefore relatively stationary. They result from the interaction between
fast ice Fast ice (also called ''land-fast ice'', ''landfast ice'', and ''shore-fast ice'') is sea ice that is "fastened" to the coastline, to the sea floor along shoals or to grounded icebergs.Leppäranta, M. 2011. The Drift of Sea Ice. Berlin: Springer ...
and the drifting pack ice. ''Level ice'' is sea ice that has not been affected by deformation and is therefore relatively flat.


Leads and polynyas

Leads Lead is a chemical element with symbol Pb and atomic number 82. Lead or The Lead may also refer to: Animal handling * Leash, or lead * Lead (leg), the leg that advances most in a quadruped's cantering or galloping stride * Lead (tack), a lin ...
and
polynyas A polynya () is an area of open water surrounded by sea ice. It is now used as a geographical term for an area of unfrozen seawater within otherwise contiguous pack ice or fast ice. It is a loanword from the Russian полынья (), which ref ...
are areas of open water that occur within sea ice expanses even though air temperatures are below freezing and provide a direct interaction between the ocean and the atmosphere, which is important for the wildlife. Leads are narrow and linear – they vary in width from meter to km scale. During the winter, the water in leads quickly freezes up. They are also used for navigation purposes – even when refrozen, the ice in leads is thinner, allowing icebreakers access to an easier sail path and submarines to surface more easily. Polynyas are more uniform in size than leads and are also larger – two types are recognized: 1) ''Sensible-heat polynyas'', caused by the upwelling of warmer water and 2) ''Latent-heat polynyas'', resulting from persistent winds from the coastline. File:Sea ice near coast of Labrador -a.jpg, Aerial view showing an expanse of drift ice offshore Labrador (Eastern Canada) displaying floes of various sizes loosely packed, with open water in several networks of
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
s. (Scale not available.) File:Ikaasaartik-strait-icefloes-aerial.jpg, Aerial view showing an expanse of drift ice in southeastern Greenland, comprising loosely packed floes of various sizes, with a
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...
developing in the centre.(Scale not available.) File:Greenland East Coast 7.jpg, Aerial view showing an expanse of drift ice consisting mostly of water. (Scale not available.) File:Ryuhyoh 03.jpg, Close-up view inside a drift ice zone: several small rounded floes are separated from each other by slush or grease ice. (Bird at lower right for scale.) File:Hummock.JPG, Example of hummocky ice: an accumulation of ice blocks, here about in thickness (with a thin snow cover). File:Pressure_ridge_--_where_two_ice_floes_meet.jpg, Field example of a pressure ridge. Only the sail (the part of the ridge above the ice surface) is shown in this photograph – the keel is more difficult to document. File:Chukchi Sea.JPG, Aerial view of the Chukchi Sea between Chukotka and Alaska, displaying a pattern of
leads Lead is a chemical element with symbol Pb and atomic number 82. Lead or The Lead may also refer to: Animal handling * Leash, or lead * Lead (leg), the leg that advances most in a quadruped's cantering or galloping stride * Lead (tack), a lin ...
. Much of the open water inside those leads is already covered by ''new ice'' (indicated by a slightly lighter blue color)(scale not available).


Formation

Only the top layer of water needs to cool to the freezing point. Convection of the surface layer involves the top , down to the
pycnocline A pycnocline is the Cline (hydrology), cline or layer where the density gradient () is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, temperature and salinity differences, wind, Coriolis effec ...
of increased density. In calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than . Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent – that is the ice called ''nilas''. Once nilas has formed, a quite different growth process occurs, in which water freezes on to the bottom of the existing ice sheet, a process called ''congelation'' growth. This growth process yields first-year ice. In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere. The uppermost layer of the ocean is
supercooled Supercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its melting point without it becoming a solid. It achieves this in the absence of a seed crystal or nucleus around which a crystal ...
to slightly below the freezing point, at which time tiny ice platelets (frazil ice) form. With time, this process leads to a mushy surface layer, known as
grease ice Grease ice is a very thin, soupy layer of frazil crystals clumped together, which makes the ocean surface resemble an oil slick. Grease ice is the second stage in the formation of solid sea ice after ice floes and then frazil ice. New sea ice f ...
. Frazil ice formation may also be started by
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout ...
fall, rather than supercooling. Waves and wind then act to compress these ice particles into larger plates, of several meters in diameter, called
pancake ice Pancake ice is a form of sea ice that consists of round pieces of ice with diameters ranging from 30 centimetres (12 in) to 3 metres (9.8 ft) and thicknesses up to 10 centimetres (3.9 inches), depending on the local conditions. It forms as a resu ...
. These float on the
ocean surface The ocean (also the sea or the world ocean) is the body of Saline water, salt water that covers approximately 70.8% of the surface of Earth and contains 97% of Water distribution on Earth, Earth's water. An ocean can also refer to any of the ...
and collide with one another, forming upturned edges. In time, the pancake ice plates may themselves be rafted over one another or frozen together into a more solid ice cover, known as consolidated pancake ice. Such ice has a very rough appearance on top and bottom. If sufficient snow falls on sea ice to depress the freeboard below sea level, sea water will flow in and a layer of ice will form of mixed snow/sea water. This is particularly common around
Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...
. Russian scientist Vladimir Vize (1886–1954) devoted his life to study the Arctic ice pack and developed the ''Scientific Prediction of Ice Conditions Theory'', for which he was widely acclaimed in academic circles. He applied this theory in the field in the
Kara Sea The Kara Sea (russian: Ка́рское мо́ре, ''Karskoye more'') is a marginal sea, separated from the Barents Sea to the west by the Kara Strait and Novaya Zemlya, and from the Laptev Sea to the east by the Severnaya Zemlya archipelago. ...
, which led to the discovery of Vize Island.


Yearly freeze and melt cycle

The annual freeze and melt cycle is set by the annual cycle of solar insolation and of ocean and atmospheric temperature and of variability in this annual cycle. In the Arctic, the area of ocean covered by sea ice increases over winter from a minimum in September to a maximum in March or sometimes February, before melting over the summer. In the Antarctic, where the seasons are reversed, the annual minimum is typically in February and the annual maximum in September or October and the presence of sea ice abutting the calving fronts of
ice shelves An ice shelf is a large floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland, Northern Canada, and the Russian Arctic. The b ...
has been shown to influence glacier flow and potentially the stability of the
Antarctic ice sheet The Antarctic ice sheet is one of the two polar ice caps of Earth. It covers about 98% of the Antarctic continent and is the largest single mass of ice on Earth, with an average thickness of over 2 kilometers. It covers an area of almost and ...
. The growth and melt rate are also affected by the state of the ice itself. During growth, the ice thickening due to freezing (as opposed to dynamics) is itself dependent on the thickness, so that the ice growth slows as the ice thickens. Likewise, during melt, thinner sea ice melts faster. This leads to different behaviour between multiyear and first year ice. In addition,
melt pond Melt ponds are pools of open water that form on sea ice in the warmer months of spring and summer. The ponds are also found on glacial ice and ice shelves. Ponds of melted water can also develop under the ice. Melt ponds are usually darker tha ...
s on the ice surface during the melt season lower the
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
such that more solar radiation is absorbed, leading to a feedback where melt is accelerated. The presence of melt ponds is affected by the permeability of the sea ice (i.e. whether meltwater can drain) and the
topography Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
of the sea ice surface (i.e. the presence of natural basins for the melt ponds to form in). First year ice is flatter than multiyear ice due to the lack of dynamic ridging, so ponds tend to have greater area. They also have lower albedo since they are on thinner ice, which blocks less of the solar radiation from reaching the dark ocean below.


Monitoring and observations

Changes in sea ice conditions are best demonstrated by the rate of melting over time. A composite record of Arctic ice demonstrates that the floes' retreat began around 1900, experiencing more rapid melting beginning within the past 50 years. Satellite study of sea ice began in 1979 and became a much more reliable measure of long-term changes in sea ice. In comparison to the extended record, the sea-ice extent in the polar region by September 2007 was only half the recorded mass that had been estimated to exist within the 1950–1970 period.
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
sea ice extent ice hit an all-time low in September 2012, when the ice was determined to cover only 24% of the Arctic Ocean, offsetting the previous low of 29% in 2007. Predictions of when the first "ice free" Arctic summer might occur vary.
Antarctic sea ice Antarctic sea ice is the sea ice of the Southern Ocean. It extends from the far north in the winter and retreats to almost the coastline every summer, getting closer and closer to the coastline every year due to sea ice melting. Sea ice is froze ...
extent gradually increased in the period of satellite observations, which began in 1979, until a rapid decline in southern hemisphere spring of 2016.


Effects of climate change

Sea ice provides an ecosystem for various polar species, particularly the
polar bear The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear specie ...
, whose environment is being threatened as global warming causes the ice to melt more as the Earth's temperature gets warmer. Furthermore, the sea ice itself functions to help keep polar climates cool, since the ice exists in expansive enough amounts to maintain a cold environment. At this, sea ice's relationship with global warming is cyclical; the ice helps to maintain cool climates, but as the global temperature increases, the ice melts and is less effective in keeping those climates cold. The bright, shiny surface (
albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...
) of the ice also serves a role in maintaining cooler polar temperatures by reflecting much of the sunlight that hits it back into space. As the sea ice melts, its surface area shrinks, diminishing the size of the reflective surface and therefore causing the earth to absorb more of the sun's heat. As the ice melts it lowers the albedo thus causing more heat to be absorbed by the Earth and further increase the amount of melting ice. Though the size of the
ice floes An ice floe () is a large pack ice, pack of Drift ice, floating ice often defined as a flat piece at least 20 m across at its widest point, and up to more than 10 km across. Drift ice is a floating field of sea ice composed of several ice fl ...
is affected by the seasons, even a small change in global temperature can greatly affect the amount of sea ice and due to the shrinking reflective surface that keeps the ocean cool, this sparks a cycle of ice shrinking and temperatures warming. As a result, the polar regions are the most susceptible places to climate change on the planet. Furthermore, sea ice affects the movement of ocean waters. In the
freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid o ...
process, much of the salt in ocean water is squeezed out of the frozen crystal formations, though some remains frozen in the ice. This salt becomes trapped beneath the sea ice, creating a higher concentration of salt in the water beneath ice floes. This concentration of salt contributes to the salinated water's
density Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
and this cold, denser water sinks to the bottom of the ocean. This cold water moves along the ocean floor towards the equator, while warmer water on the ocean surface moves in the direction of the poles. This is referred to as "
conveyor belt A conveyor belt is the carrying medium of a belt conveyor system (often shortened to belt conveyor). A belt conveyor system is one of many types of conveyor systems. A belt conveyor system consists of two or more pulleys (sometimes referred to ...
motion" and is a regularly occurring process. File:Sea-ice-2013-en.svg, Change in extent of the
Arctic Sea The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, a ...
ice between April and August, in 2013 File:Sea Ice off Baffin Island.jpg, Sea ice off
Baffin Island Baffin Island (formerly Baffin Land), in the Canadian territory of Nunavut, is the largest island in Canada and the fifth-largest island in the world. Its area is , slightly larger than Spain; its population was 13,039 as of the 2021 Canadia ...
File:Sea Ice Imitates the Shoreline along the Kamchatka Peninsula.jpg, Sea ice imitates the shoreline along the
Kamchatka Peninsula The Kamchatka Peninsula (russian: полуостров Камчатка, Poluostrov Kamchatka, ) is a peninsula in the Russian Far East, with an area of about . The Pacific Ocean and the Sea of Okhotsk make up the peninsula's eastern and we ...
. File:Antarctic Peninsula, the Larsen Ice Shelf, and the sea ice covered waters around the region.jpg, Clear view of the
Antarctic Peninsula The Antarctic Peninsula, known as O'Higgins Land in Chile and Tierra de San Martín in Argentina, and originally as Graham Land in the United Kingdom and the Palmer Peninsula in the United States, is the northernmost part of mainland Antarctic ...
, the
Larsen Ice Shelf The Larsen Ice Shelf is a long ice shelf in the northwest part of the Weddell Sea, extending along the east coast of the Antarctic Peninsula from Cape Longing to Smith Peninsula. It is named after Captain Carl Anton Larsen, the master of the No ...
and the sea ice-covered waters around the region File:Annual Sea Ice Minimum 1979-2013 with Area Graph.ogv, The Earth showing the annual minimum sea ice with a graph overlay showing the annual minimum sea ice area in millions of square kilometers


Modelling

In order to gain a better understanding about the variability, numerical sea ice models are used to perform sensitivity studies. The two main ingredients are the
ice dynamics Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: ...
and the thermodynamical properties (see
Sea ice emissivity modelling With increased interest in sea ice and its effects on the global climate, efficient methods are required to monitor both its extent and exchange processes. Satellite-mounted, microwave radiometers, such SSMI, AMSR and Advanced Microwave Sounding Un ...
,
Sea ice growth processes Sea ice is a complex composite composed primarily of pure ice in various states of crystallization along with air bubbles and included pockets of brine. Understanding its growth processes is important both for climate scientists for use in simula ...
and
Sea ice thickness Sea ice thickness spatial extent, and open water within sea ice packs can vary rapidly in response to weather and climate. Sea ice concentration are measured by satellites, with the Special Sensor Microwave Imager / Sounder (SSMIS), and the ''Europ ...
). There are a number of sea ice model computer codes available for doing this, including the CICE numerical suite. Many
global climate model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms f ...
s (GCMs) have sea ice implemented in their numerical simulation scheme in order to capture the
ice–albedo feedback Ice–albedo feedback is a positive feedback climate process where a change in the area of ice caps, glaciers, and sea ice alters the albedo and surface temperature of a planet. Ice is very reflective, therefore it reflects far more solar energy ba ...
correctly. Examples include: * The Louvain-la-Neuve Sea Ice Model is a numerical model of sea ice designed for climate studies and operational oceanography developed at
Université catholique de Louvain The Université catholique de Louvain (also known as the Catholic University of Louvain, the English translation of its French name, and the University of Louvain, its official English name) is Belgium's largest French-speaking university. It ...
. It is coupled to the ocean general circulation model OPA (Ocean Parallélisé) and is freely available as a part of the
Nucleus for European Modeling of the Ocean Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...
. * The
MIT General Circulation Model The MIT General Circulation Model (MITgcm) is a numerical computer code that solves the equations of motion governing the ocean or Earth's atmosphere using the finite volume method. It was developed at the Massachusetts Institute of Technology and ...
is a
global circulation model A general circulation model (GCM) is a type of climate model. It employs a mathematical model of the general circulation of a planetary atmosphere or ocean. It uses the Navier–Stokes equations on a rotating sphere with thermodynamic terms ...
developed at
Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a private land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern technology and science, and is one of the ...
includes a package for sea-ice. The code is freely available there. * The
University Corporation for Atmospheric Research The University Corporation for Atmospheric Research (UCAR) is a US nonprofit consortium of more than 100 colleges and universities providing research and training in the atmospheric and related sciences. UCAR manages the National Center for Atmosph ...
develops the Community Sea Ice Model. * CICE is run by the
Los Alamos National Laboratory Los Alamos National Laboratory (often shortened as Los Alamos and LANL) is one of the sixteen research and development laboratories of the United States Department of Energy (DOE), located a short distance northwest of Santa Fe, New Mexico, ...
. The project is
open source Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...
and designed as a component of GCM, although it provides a standalone mode. * The Finite-Element Sea-Ice Ocean Model developed at
Alfred Wegener Institute The Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (German: ''Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung'') is located in Bremerhaven, Germany, and a member of the Helmholtz Association o ...
uses an
unstructured grid An unstructured grid or irregular grid is a tessellation of a part of the Euclidean plane or Euclidean space by simple shapes, such as triangles or tetrahedra, in an irregular pattern. Grids of this type may be used in finite element analysis w ...
. * The neXt Generation Sea-Ice model (neXtSIM) is a Lagrangian model using an adaptive and unstructured triangular mesh and includes a new and unique class of rheological model called Maxwell-Elasto-Brittle to treat the ice dynamics. This model is developed at th
Nansen Center
in Bergen, Norway. The
Coupled Model Intercomparison Project In climatology, the Coupled Model Intercomparison Project (CMIP) is a collaborative framework designed to improve knowledge of climate change. It was organized in 1995 by the Working Group on Coupled Modelling (WGCM) of the World Climate Research ...
offers a standard protocol for studying the output of coupled atmosphere-ocean general circulation models. The coupling takes place at the atmosphere-ocean interface where the sea ice may occur. In addition to global modeling, various regional models deal with sea ice. Regional models are employed for seasonal forecasting experiments and for process studies.


Ecology

Sea ice is part of the Earth's
biosphere The biosphere (from Greek βίος ''bíos'' "life" and σφαῖρα ''sphaira'' "sphere"), also known as the ecosphere (from Greek οἶκος ''oîkos'' "environment" and σφαῖρα), is the worldwide sum of all ecosystems. It can also be ...
. When sea water freezes, the ice is riddled with brine-filled channels which sustain sympagic organisms such as bacteria, algae, copepods and annelids, which in turn provide food for animals such as krill and specialised fish like the bald notothen, fed upon in turn by larger animals such as
emperor penguins The emperor penguin (''Aptenodytes forsteri'') is the tallest and heaviest of all living penguin species and is endemic to Antarctica. The male and female are similar in plumage and size, reaching in length and weighing from . Feathers of th ...
and
minke whales The minke whale (), or lesser rorqual, is a species complex of baleen whale. The two species of minke whale are the common (or northern) minke whale and the Antarctic (or southern) minke whale. The minke whale was first described by the Danish na ...
. A decline of seasonal sea ice puts the survival of Arctic species such as
ringed seal The ringed seal (''Pusa hispida'') is an earless seal inhabiting the Arctic and sub-Arctic regions. The ringed seal is a relatively small seal, rarely greater than 1.5 m in length, with a distinctive patterning of dark spots surrounded by light g ...
s and
polar bear The polar bear (''Ursus maritimus'') is a hypercarnivorous bear whose native range lies largely within the Arctic Circle, encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the largest extant bear specie ...
s at risk.


Extraterrestrial presence

Other element and compounds have been speculated to exist as oceans and seas on extraterrestrial planets. Scientists notably suspect the existence of "icebergs" of solid
diamond Diamond is a Allotropes of carbon, solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the Chemical stability, chemically stable form of car ...
and corresponding seas of liquid
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
on the
ice giants An ice giant is a giant planet composed mainly of elements heavier than hydrogen and helium, such as oxygen, carbon, nitrogen, and sulfur. There are two ice giants in the Solar System: Uranus and Neptune. In astrophysics and planetary science th ...
,
Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...
and
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus (mythology), Uranus (Caelus), who, according to Greek mythology, was the great-grandfather of Ares (Mars (mythology), Mars), grandfather ...
. This is due to extreme pressure and heat at the core, that would turn carbon into a
supercritical fluid A supercritical fluid (SCF) is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist, but below the pressure required to compress it into a solid. It can effuse through porous so ...
.


See also


Ice types or features

* * * * * * * * * * * * * * * * * * *


Physics and chemistry

* * * * * *


Applied sciences and engineering endeavours

* * * * * * * * * * *


References

* * *


Sea ice glossaries

* * *


External links


Daily maps of sea ice concentration from the University of BremenSea ice maps from the National Snow and Ice Data Center
{{Authority control Aquatic ecology Earth phenomena Bodies of ice Articles containing video clips Oceanographical terminology Cryosphere