Sea ice
Sea ice forms as seawater freezes. Because ice is less dense than liquid water, it floats on the ocean's surface. 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 regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern 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 for microorganisms that form the base of unique food webs. The presence or absence of sea ice also shapes navigation routes, regional weather, and global ocean 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 the albedo effect. Sea ice also insulates the ocean below, limiting the transfer of heat, water vapor, and gases such as carbon dioxide between the sea and the atmosphere.
Satellite records have shown a marked decline in Arctic sea ice extent and thickness in recent decades, a trend linked to global climate 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 from icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice may contain embedded icebergs.
Features and types
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.Physical properties
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. 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%. Air volume fraction during ice growth period is typically around 1–2 %, but may substantially increase upon ice warming. Air volume of sea ice in can be as high as 15 % in summer and 4 % in autumn. Both brine and air volumes influence sea-ice density values, which are typically around 840–910 kg/m3 for first-year ice. 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. 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.Fast ice versus drift (or pack) ice
Sea ice can be classified according to whether or not it is attached to the shoreline. If attached, it is called landfast ice, or more often, fast ice. 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 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 of 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 by 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, slush, or shuga. Other terms, such as grease ice and pancake ice, 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 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 or ridge.
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 but does not survive the spring and summer months. The thickness of this ice typically ranges from to. First-year ice may be further divided into thin, medium and thick.Old sea ice
Old sea ice is sea ice that has survived at least one melting season. 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. 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. Multi-year ice is much more common in the Arctic than it is in the Antarctic. 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 and polynyas
and polynyas 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.Dynamics and cycles
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 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 to slightly below the freezing point, at which time tiny ice platelets form. With time, this process leads to a mushy surface layer, known as grease ice. Frazil ice formation may also be started by snowfall, rather than supercooling. Waves and wind then act to compress these ice particles into larger plates, of several meters in diameter, called pancake ice. These float on the ocean 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 around Antarctica.