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Outcome of seawater as it freezes

Sea ice arises as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (as does fresh water 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 regions: the Arctic ice pack of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean. Polar packs undergo a significant yearly cycling in surface extent, a natural process upon which depends the Arctic ecology, 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 icebergs, which are chunks of ice shelves or glaciers that calve into the ocean. Depending on location, sea ice expanses may also incorporate 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 can be classified according to whether or not it is attached (or frozen) to the shoreline (or between shoals or to grounded icebergs). If attached, it is called landfast ice, or more often, fast ice (from fastened). 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 to drift ice,

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 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 .

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, The thickness of old sea ice typically ranges from 2 to 4 m. 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.

While fast 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. 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 (frazil ice) 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. 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, which led to the discovery of Vize Island.

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 has been shown to influence glacier flow and potentially the stability of the Antarctic ice sheet. 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.

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 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 extent gradually increased in the period of satellite observations, which began in 1979, until a rapid decline in southern hemisphere spring of 2016.

Sea ice provides an ecosystem for various polar species, particularly the polar bear, 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) 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 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. A decline of seasonal sea ice puts the survival of Arctic species such as ringed seals and polar bears at risk.

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 and corresponding seas of liquid carbon on the ice giants, Neptune and Uranus. This is due to extreme pressure and heat at the core, that would turn carbon into a supercritical fluid.