Overview

Sea ice is ocean water that has frozen in place rather than ice that originates on land. It forms, thickens, moves, and melts within the marine environment and is a distinct material from an iceberg, a glacier or ice sheet, or an ice shelf. Sea ice is present seasonally and year-round at high latitudes in both the Arctic and the Antarctic. A concise definition is simply frozen ocean water, but its behavior and impacts are complex.

Formation and key properties

Sea ice begins as tiny platelets or frazil crystals in supercooled water. These crystals consolidate into a continuous layer (nilas or young ice), and with more freezing and snow load they can become thicker, saltier or fresher in discrete bands. Salinity, temperature and embedded brine pockets give sea ice different optical and mechanical properties than freshwater ice. When ice forms it rejects salt into the underlying water (brine rejection), which can influence local density and ocean circulation.

Types and structure

  • First-year ice: forms and melts within a single seasonal cycle.
  • Multi-year ice: survives one or more summers and tends to be thicker and less saline.
  • Floe, lead, ridge: sea ice is broken into floes that drift, with leads (open water) and pressure ridges formed by interaction and wind.

Ecological and climatic importance

Sea ice shapes polar ecosystems by providing habitat for algae, seals, polar bears and other species that depend on its seasonal rhythm. It strongly influences climate through surface reflectivity (albedo): extensive ice reflects more sunlight, while open water absorbs more heat. Changes in sea ice extent and thickness are closely watched because they affect regional weather, ocean circulation, and feedbacks in the Earth system.

People navigate, fish and work in polar waters where sea ice conditions matter for safety and infrastructure. Scientists have monitored sea ice with ships, aircraft and, since the late 20th century, routine satellites and automated buoys; these observations reveal seasonal cycles and longer-term variability. In recent decades the Arctic has shown notable reductions in summer ice cover, while Antarctic trends are more variable and influenced by regional winds and ocean processes.

Distinctions and notable facts

Sea ice differs from land-origin ice both in origin and behavior: an iceberg breaks from a glacier, whereas sea ice grows in the ocean. Its salt content and interaction with waves and currents make it dynamically different from freshwater lake ice. Because it changes rapidly with weather and climate, sea ice is a key indicator and driver of polar change and is of interest to scientists, coastal communities, and industry alike.