Sea ice

Sea ice forms as seawater freezes. Because ice is less dense than water, it floats on the ocean's surface (just like fresh water ice). 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.

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. 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.

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 (as in 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 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); and giant – more than 10 km (6.2 mi). 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.

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.