Sea ice forms asseawater freezes. Becauseice is lessdense than liquid water, it floats on the ocean's surface (just likefresh water ice). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans.[1][2][3] Much of the world's sea ice is enclosed within the polar ice packs in the Earth'spolar regions: theArctic ice pack of theArctic Ocean and theAntarctic ice pack of theSouthern Ocean. Polar packs naturally undergo significant yearly cycling, reaching their greatest surface extent in winter and retreating in summer.
Within the ice, salty brine channels provide habitat formicroorganisms that form the base of unique food webs. The presence or absence of sea ice also shapes navigation routes, regional weather, and globalocean circulation. Sea ice plays a key role in Earth's climate. Its white surface reflects the Sun's energy back into space, helping to keep the planet cool in a process known as thealbedo effect. Sea ice also insulates the ocean below, limiting the transfer of heat, water vapor, and gases such ascarbon dioxide between the sea and the atmosphere.
Satellite records have shown a markeddecline in Arctic sea ice extent and thickness in recent decades, a trend linked to globalclimate change. Antarctic sea ice shows more regional variability but is recently also experiencing declines.
Sea ice is dynamic, due to the action of winds, currents and temperature fluctuations, which lead to a wide variety of ice types and features. Sea ice differs fromicebergs, which are chunks ofice shelves orglaciers thatcalve into the ocean. Depending on location, sea ice may contain embedded icebergs.
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.
Sea ice is a composite material made up of pure ice, liquid brine, air, and salt. The volumetric fractions of these components—ice, brine, and air—determine the key physical properties of sea ice, including thermal conductivity, heat capacity, latent heat, density, elastic modulus, and mechanical strength.[4] Brine volume fraction depends on sea-ice salinity and temperature, while sea-ice salinity mainly depends on ice age and thickness. During the ice growth period, its bulk brine volume is typically below 5%.[5] Air volume fraction during ice growth period is typically around 1–2 %, but may substantially increase upon ice warming.[6] Air volume of sea ice in can be as high as 15 % in summer[7] and 4 % in autumn.[8] Both brine and air volumes influence sea-ice density values, which are typically around 840–910 kg/m3 for first-year ice.[9] First-year ice has a strong seasonality of its density, with higher values around 910–920 kg/m3 in winter and lower values around 860–880 kg/m3 in summer.[10] Density of second- and multiyear ice typically has a weaker seasonality and lower density than for first-year ice. Sea-ice density is a significant source of errors in sea-ice thickness retrieval using radar and laser satellite altimetry, resulting in uncertainties of 0.3–0.4 m.[11]
Sea ice can be classified according to whether or not it is attached (or frozen) to the shoreline (or betweenshoals or to groundedicebergs). If attached, it is called landfast ice, or more often,fast ice (as infastened). Alternatively and unlike fast ice,drift ice 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 thefast ice boundary. The drift ice zone may be further divided into ashear zone, amarginal ice zone and acentral pack.[12] Drift ice consists offloes, individual pieces of sea ice 20 metres (66 ft) or more across. There are names for various floe sizes:small – 20 to 100 m (66 to 328 ft);medium – 100 to 500 m (330 to 1,640 ft);big – 500 to 2,000 m (1,600 to 6,600 ft);vast – 2 to 10 kilometres (1.2 to 6.2 mi); andgiant – more than 10 km (6.2 mi).[13][14] The termpack ice is used either as a synonym ofdrift ice,[13] or to designate drift ice zone in which the floes are densely packed.[13][14][15] The overall sea ice cover is termed theice canopy from the perspective of submarine navigation.[14][15]
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 andold.[13][14][15]
New ice is a general term used for recently frozen sea water that does not yet make up solid ice. It may consist offrazil ice (plates or spicules of ice suspended in water),slush (water saturated snow), orshuga (spongy white ice lumps a few centimeters across). Other terms, such asgrease ice andpancake ice, are used for ice crystal accumulations under the action of wind and waves.[citation needed] When sea ice begins to form on a beach with a light swell,ice eggs up to the size of a football can be created.[16]
Nilas designates a sea ice crust up to 10 centimetres (3.9 in) in thickness. It bends without breaking around waves and swells. Nilas can be further subdivided intodark nilas – up to 5 cm (2.0 in) in thickness and very dark andlight nilas – over 5 cm (2.0 in) in thickness and lighter in color.
Young ice is a transition stage between nilas and first-year ice and ranges in thickness from 10 cm (3.9 in) to 30 cm (12 in), Young ice can be further subdivided intogrey ice – 10 cm (3.9 in) to 15 cm (5.9 in) in thickness andgrey-white ice – 15 cm (5.9 in) to 30 cm (12 in) 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 is ice that is thicker thanyoung 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 thenew 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 0.3 m (0.98 ft) to 2 m (6.6 ft).[13][14][15] First-year ice may be further divided intothin (30 cm (0.98 ft) to 70 cm (2.3 ft)),medium (70 cm (2.3 ft) to 120 cm (3.9 ft)) andthick (>120 cm (3.9 ft)).[14][15]
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. The thickness of old sea ice typically ranges from 2 to 4 m.[17] Old ice is commonly divided into two types:second-year ice, which has survived one melting season andmultiyear ice, which has survived more than one. (In some sources,[13]old ice is more than two years old.) Multi-year ice is much more common in theArctic than it is in theAntarctic.[13][18][needs update] 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.
Leads andpolynyas are areas of open water that occur within sea ice expanses even though air temperatures are below freezing. They provide a direct interaction between the ocean and the atmosphere, which is important for the wildlife. Leads are narrow and linear, varying in width from meters to kilometers. 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.[13]
Only the top layer of water needs to cool to the freezing point.[19] Convection of the surface layer involves the top 100–150 m (330–490 ft), down to thepycnocline 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 0.3 cm (0.12 in). 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 calledfrazil 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 callednilas. 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 calledcongelation 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 issupercooled 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 asgrease ice. Frazil ice formation may also be started bysnowfall, rather than supercooling. Waves and wind then act to compress these ice particles into larger plates, of several meters in diameter, calledpancake ice. These float on theocean surface 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 aroundAntarctica.
Whilefast ice 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.[1][13] TheCoriolis force and sea ice surface tilt have also been invoked.[13] These driving forces induce a state of stress within the drift ice zone. Anice floe converging toward another and pushing against it will generate a state ofcompression at the boundary between both. The ice cover may also undergo a state oftension, resulting in divergence and fissure opening. If two floes drift sideways past each other while remaining in contact, this will create a state ofshear.
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:[14][15] 1)Rafted ice, when one piece is overriding another; 2)Pressure ridges, a line of broken ice forced downward (to make up thekeel) and upward (to make thesail); and 3)Hummock, a hillock of broken ice that forms an uneven surface. Ashear ridge is a pressure ridge that formed under shear – it tends to be more linear than a ridge induced only by compression.[14][15] Anew ridge is a recent feature – it is sharp-crested, with its side sloping at an angle exceeding 40 degrees. In contrast, aweathered ridge is one with a rounded crest and with sides sloping at less than 40 degrees.[14][15]Stamukhi are yet another type of pile-up but these are grounded and are therefore relatively stationary. They result from the interaction betweenfast ice and the drifting pack ice.
Level ice is sea ice that has not been affected by deformation and is therefore relatively flat.[14][15]
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. The presence of sea ice abutting the calving fronts ofice shelves has been shown to influence glacier flow and potentially the stability of theAntarctic ice sheet.[24][25]
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.[13] Likewise, during melt, thinner sea ice melts faster. This leads to different behaviour between multiyear and first year ice. In addition,melt ponds on the ice surface during the melt season lower thealbedo such that more solar radiation is absorbed, leading to a feedback where melt is accelerated. The presence of melt ponds is affected by thepermeability of the sea ice (i.e. whether meltwater can drain) and thetopography 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.[26]
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.[27] Satellite study of sea ice began in 1979 and became a much more reliable measure of long-term changes in sea ice.
September Arctic sea ice extent is currently decreasing at about 12% per decade, compared to the 1981-2010 average.[28] In comparison to the extended record, the sea-ice extent in the Arctic region by September 2007 was only half the recorded mass that had been estimated to exist within the 1950–1970 period.[29] In September 2012Arctic sea ice reached its lowest level ever recorded, covering just 24% of the Arctic Ocean, down from the previous record low of 29% in 2007.[30][31] A new second-lowest extent was later set in 2020.[32] Predictions of when the first"ice free" Arctic summer might occur vary but are anticipated by mid-century (2035-2067).[33]
Antarctic sea ice extent increased gradually from the start of satellite observations in 1979 until spring 2016, when it began a rapid decline that is still continuing as of 2024.[34][35][36]
Sea ice helps keep polar regions cool by reflecting incomingsolar radiation due to its highalbedo. This reflective surface prevents much of the Sun's energy from being absorbed by the darker ocean below. As sea ice melts, the exposed ocean absorbs more heat, further accelerating warming in a positive feedback loop known as theice–albedo feedback.[41]
Sea ice also influences globalocean circulation. When seawater freezes, most of the salt is excluded from the ice crystals, creating denser, saltier water beneath the ice. This dense water sinks and helps drivethermohaline circulation, a global "conveyor belt" of ocean currents that redistributes heat across the planet.[42]
The polar regions are among the most sensitive areas toclimate change, with consequences for ecosystems, weather patterns, and global sea level.[43] Rising global temperatures from human-causedgreenhouse gas emissions have led to warming of the atmosphere and oceans, accelerating sea ice melt.[43]
Although the melting of floating sea ice has a small effect on global average sea level (because sea ice is less salty and less dense than the seawater it displaces) it has large indirect effects on global climate systems.[44][45] Loss of sea ice lowers the albedo of polar regions, amplifying warming and accelerating the melt of the Greenland and Antarcticice sheets, which contributes substantially tosea level rise.[46] Reduced sea ice alters ocean circulation and wave activity, which can enhance the erosion of coastalice shelves andglaciers.[47]
Melting sea ice also introduces large amounts of freshwater into the surface ocean. This reduces salinity, which can alter water density and influence global ocean circulation, including theAtlantic Meridional Overturning Circulation.[48] These changes alter the transport of heat and nutrients, with consequences for marine ecosystems as well as regional and global climate patterns.[43]
In order to gain a better understanding about the variability, numerical sea ice models are used to performsensitivity studies. The two main ingredients are theice dynamics and the thermodynamical properties (seeSea ice emissivity modelling,Sea ice growth processes andSea ice thickness). There are many sea ice model computer codes available for doing this, including theCICE numerical suite.
Manyglobal climate models (GCMs) have sea ice implemented in their numerical simulation scheme in order to capture theice–albedo feedback correctly. Examples include:
TheCoupled Model Intercomparison Project 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 forprocess studies.
Sea ice provides a unique habitat within the Earth'sbiosphere. As seawater freezes, it traps pockets ofbrine, creating a network of channels and pores that host diverse communities ofmicroorganisms, includingbacteria,archaea,fungi,algae,protozoa, andviruses. Thesesympagic organisms form the base offood webs. Ice algae, in particular, are a critical food source for small invertebrates such ascopepods andamphipods, which are consumed by larger animals includingkrill, fish and seabirds.[49]
Life in sea ice must cope with extreme conditions. Temperatures inside the ice are below freezing, while brine channels are often saltier than seawater. For much of the year there is little or no sunlight, followed by months of continuous daylight in summer.[50] Many organisms have evolved special strategies to adapt, such as producing antifreeze compounds, going dormant until light and nutrients return, or timing growth to the summer season.[51][52]
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The ecology of sea ice is seasonal. In spring and summer, increasing light and melting ice stimulate algal growth, which is released into the water column. This seasonal pulse supports largephytoplankton blooms that fuel productivity across polar marine ecosystems. The timing and extent of sea ice melt therefore influences the availability of food for highertrophic levels.[53][54]
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In the Southern Ocean,Antarctic krill rely on sea ice algae during their juvenile stages, forming the foundation of food webs that support fish, penguins, seals, and whales.[55] In the Arctic, sea ice also hosts algae that sustain zooplankton which support fish, seals,walruses, andpolar bears.[56][57]
Sea ice also regulatesbiogeochemical processes. It stores and redistributes nutrients such as iron, and its seasonal melting influences ocean mixing and primary productivity. In this way, sea ice contributes to globalcarbon cycling and climate regulation.[58]
Declines in sea ice extent and duration due toclimate change pose significant ecological risks. Species that depend directly on sea ice for feeding, breeding, or resting are highly impacted. These include ringed seals and polar bears[59] in the Arctic, andEmperor[60] andAdélie penguins[61] in Antarctica. Indirect impacts cascade through food webs, threatening the productivity and stability of entire polar ecosystems.[62]
Otherelements andcompounds have been speculated to exist as oceans and seas on extraterrestrial planets. Scientists notably suspect the existence of "icebergs" of soliddiamond and corresponding seas of liquidcarbon on theice giants,Neptune andUranus. This is due to extreme pressure and heat at the core, that would turn carbon into asupercritical fluid.[63][64]
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Click to Download Monthly Ice Volume Data (text file, CSV file for Spreadsheets)
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