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Sea ice forms as seawater freezes. Because ice is less density 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. (2025). 9781119027898, John Wiley & Sons, Inc.. ISBN 9781119027898 Much of the world's sea ice is enclosed within the polar ice packs in the Earth's : 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 Sympagic ecology that form the base of unique food webs. The presence or absence of sea ice also shapes navigation routes, regional weather, and global Ocean current. 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 , which are chunks of ice shelf or that Ice calving into the ocean. Depending on location, sea ice may contain embedded icebergs.
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).
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 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 supercooling 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 , 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.
Level ice is sea ice that has not been affected by deformation and is therefore relatively flat.
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 of Ice shelf has been shown to influence glacier flow and potentially the stability of the Antarctic ice sheet.. These freshwater ponds are separated from the salty sea below and around it, until breaks in the ice merge the two.]]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, on the ice surface during the melt season lower the albedo 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 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.
September Arctic sea ice extent is currently decreasing at about 12% per decade, compared to the 1981-2010 average. 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. In September 2012 Arctic 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. A new second-lowest extent was later set in 2020. Predictions of when the first "ice free" Arctic summer might occur vary but are anticipated by mid-century (2035-2067).
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.
Sea ice also influences global Ocean current. 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 drive thermohaline circulation, a global "conveyor belt" of ocean currents that redistributes heat across the planet.National Snow and Ice Data Center. "Thermohaline circulation and sea ice". Accessed 8 September 2025.
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. Loss of sea ice lowers the albedo of polar regions, amplifying warming and accelerating the melt of the Greenland and Antarctic Ice sheet, which contributes substantially to sea level rise. Reduced sea ice alters ocean circulation and wave activity, which can enhance the erosion of coastal Ice shelf and Glacier.
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 the Atlantic Meridional Overturning Circulation. These changes alter the transport of heat and nutrients, with consequences for marine ecosystems as well as regional and global climate patterns.
Many global climate models (GCMs) have sea ice implemented in their numerical simulation scheme in order to capture the ice–albedo feedback correctly. Examples include:
The Coupled 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 for process study.
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. 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.
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 large phytoplankton blooms that fuel productivity across polar marine ecosystems. The timing and extent of sea ice melt therefore influences the availability of food for higher Trophic level. (2025). 9783030374723, Springer International Publishing. ISBN 9783030374723
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. In the Arctic, sea ice also hosts algae that sustain zooplankton which support fish, seals, Walrus, and Polar bear.
Sea ice also regulates Biogeochemistry 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 global Carbon cycle and climate regulation.
Declines in sea ice extent and duration due to climate 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 in the Arctic, and Emperor penguin and Adélie penguins in Antarctica. Indirect impacts cascade through food webs, threatening the productivity and stability of entire polar ecosystems.
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