seaice

Sea ice refers to frozen ocean water that forms in polar and subpolar regions and floats on the surface of the ocean. It forms when seawater freezes and expands during cold conditions, growing in winter and melting in summer. Unlike land ice, sea ice is not grounded and can drift with winds and currents, influencing ocean circulation and regional climate.

Sea ice is commonly described by age and thickness. First-year ice forms during a single winter and is relatively thin, while multiyear ice survives one or more summers and tends to be thicker and more resilient. Thickness can range from a few tenths of a meter for new ice to several meters for older, compacted ice. Brine pockets and channels within the ice create a complex, porous structure that affects salinity and heat exchange with the ocean.

The seasonal cycle and geographic distribution differ between the Arctic and Antarctic. The Arctic Ocean contains the most extensive sea ice, centering near the North Pole, while Antarctic sea ice surrounds the continent and nearby seas in the Southern Ocean. The Arctic shows a clear long-term decline in extent and thickness in recent decades, whereas Antarctic sea ice has exhibited more variable behavior with a weaker, less consistent trend.

Measurement and monitoring rely heavily on satellite passive microwave observations since the late 1970s, complemented by airborne and ship-based observations and, for thickness, radar and submarine data in some periods. Sea ice affects climate through high albedo, acts as an insulating barrier between ocean and atmosphere, and supports polar ecosystems, including habitat for seals and, in the Arctic, polar bears. It also influences human activities, such as shipping and resource management.