Sea Ice is frozen seawater that forms when ocean water loses enough heat for ice crystals to develop and consolidate at the ocean surface. It differs fundamentally from glaciers and ice sheets because it originates from seawater rather than accumulated terrestrial snowfall, and most sea ice remains floating on the ocean. Formation begins when the upper ocean cools to its freezing point, which is lower than that of freshwater because dissolved salts depress the freezing temperature.
Initial crystals can form as frazil ice in turbulent water, eventually concentrating into a surface cover known as grease ice and then more continuous forms. Sea ice contains both solid ice and a network of saline brine inclusions, with much of the salt excluded during freezing. Its physical properties therefore change substantially as temperature varies and brine drains from the ice. Sea ice can be seasonal, forming and disappearing within one year, or multiyear, surviving at least one summer and generally becoming thicker and less saline over time.
Sea ice develops through thermodynamic growth and mechanical deformation. During freezing, salt is preferentially rejected into the surrounding seawater, increasing the salinity and density of the water beneath the ice. This brine rejection can contribute to ocean mixing and dense water formation in polar regions. New ice generally contains more brine than older ice, while repeated freezing, drainage and melting modify its internal structure.
Snow accumulation on top of sea ice can insulate the ocean from cold atmospheric temperatures and also increase surface albedo. Wind, ocean currents and tides move ice and create areas of compression and divergence. Compression can produce ridges, while divergence can open leads and polynyas where exposed seawater remains in contact with cold air. Sea ice thickness therefore reflects not only air temperature but also ocean heat, snow cover, currents, wind stress and mechanical deformation.
Sea ice has major climatic importance because its reflective surface returns a substantial fraction of incoming solar radiation to space, particularly when covered by snow. When sea ice retreats, darker ocean water is exposed and absorbs more solar energy, producing an albedo feedback that can contribute to additional warming and further ice loss. The strength of this feedback varies with season, cloud conditions, surface properties and ice concentration.
Sea ice also insulates the relatively warm ocean from very cold polar air, reducing heat transfer from water to atmosphere when an ice cover is present. During open water conditions, ocean heat can escape more readily into the atmosphere. Sea ice therefore acts as both a reflective surface and a thermal barrier. Changes in its seasonal extent influence atmospheric and oceanic processes and can modify ecosystems, navigation conditions and regional climate patterns.
Arctic and Antarctic sea ice differ in geography, seasonality and dynamics. Arctic sea ice is largely confined within an ocean surrounded by continents, whereas Antarctic sea ice forms around a continent surrounded by the Southern Ocean and can spread substantially northward during winter. Arctic ice has historically included a significant proportion of multiyear ice, while Antarctic sea ice has generally been more seasonal and mobile.
Wind and ocean circulation exert strong controls on Antarctic ice distribution, while Arctic sea ice is strongly affected by oceanic heat transport, atmospheric circulation and the geometry of surrounding land. Regional trends also differ, and annual variability can be large, so individual years should not be treated as representative of long term behaviour. Sea ice extent is commonly defined using a concentration threshold derived from satellite observations, whereas sea ice area weights the actual ice concentration within each grid cell. These metrics are related but not identical.
Sea ice supports distinctive ecosystems and provides habitat for organisms ranging from microscopic algae to polar marine mammals. Algae can grow within and beneath sea ice, forming an important foundation for seasonal food webs. Ice associated organisms contribute to nutrient cycling and provide food for zooplankton and higher trophic levels. Sea ice also affects the timing and location of marine productivity by regulating light penetration, nutrient exchange and water column stratification.
Changes in ice duration, thickness and seasonal retreat can alter breeding, feeding and migration conditions for species adapted to particular ice regimes. Human activities are also influenced by sea ice through navigation, fisheries, resource operations and Arctic communities. Reduced or redistributed ice can change accessibility while simultaneously creating new hazards through thinner, more mobile and rapidly changing ice conditions.
Sea ice moves through a combination of wind forcing, ocean currents, internal stresses and interactions with coastlines and neighbouring ice. Individual floes can collide, rotate and fracture, while large areas can become compressed into thick ridges. Leads are elongated openings within the ice that expose ocean water, and polynyas are persistent or recurring open water areas surrounded by sea ice. These features can be important locations for ocean atmosphere heat exchange and biological activity.
Sea ice deformation can create pressure ridges whose keels extend below the ocean surface and may pose hazards to vessels or infrastructure. Ice drift observations are used to estimate transport pathways and investigate connections between regional sea ice conditions and broader atmospheric or oceanic circulation. Satellite radar is particularly useful because it can observe sea ice during darkness and through many cloud conditions, allowing year round monitoring.
Sea ice measurement commonly relies on satellite passive microwave sensors, synthetic aperture radar, optical imagery, aircraft observations and direct field measurements. Passive microwave instruments provide long records of concentration because microwave emission differs between ice and open water, although retrievals are affected by snow, melt ponds and atmospheric conditions. Optical sensors provide high spatial detail when daylight and cloud free conditions permit. Radar instruments can detect ice structures regardless of darkness and can help distinguish ice types and deformation features.
Buoys can measure drift, temperature, pressure and other local conditions. Ships and aircraft provide detailed observations that help validate satellite products. Scientists distinguish between sea ice extent, concentration, area, thickness, volume and age because these properties answer different questions and can change independently. A region may maintain similar extent while losing substantial thickness, making volume an important additional indicator.
Sea ice is distinct from icebergs, glaciers and ice shelves despite their shared frozen appearance. An iceberg is a piece of freshwater glacier ice that has broken away and floats in the ocean, whereas sea ice forms directly from seawater. An ice shelf is floating land based glacier ice connected to a grounded ice sheet or glacier system, whereas sea ice is ocean derived and generally forms as a seasonal or mobile surface layer. Sea ice can therefore be highly saline when newly formed, although much of the original brine is later expelled. Its presence affects ocean salinity, atmospheric exchange, ecosystems and climate. Understanding sea ice requires integration of cryospheric physics, oceanography, meteorology, marine ecology and remote sensing.
