What Is Sea Ice?

Sea ice is frozen seawater that forms at the ocean surface when air and surface water temperatures fall below the seawater freezing point of approximately -1.8 degrees Celsius, a threshold lower than freshwater's because dissolved salt suppresses the freezing temperature. It is distinct from glaciers and icebergs, which originate from snow accumulation and ice sheet calving on land; sea ice forms and melts in place on the ocean surface. Sea ice is a defining feature of the polar oceans, covering several million square kilometers in both hemispheres on a seasonal basis, and it plays an outsized role in the global climate system through its influence on ocean-atmosphere heat exchange, surface reflectivity, and ocean circulation.

The study of sea ice intersects physical oceanography, glaciology, atmospheric science, and remote sensing. Satellite-based observation programs have tracked sea ice extent continuously since 1979, producing the primary record used to quantify long-term changes in polar sea ice coverage.

Formation and Physical Properties

Sea ice formation begins with the nucleation of small needle-like ice crystals called frazil, which accumulate at the sea surface to produce a slushy layer termed grease ice. In calm conditions, grease ice consolidates into thin, flexible sheets known as nilas. In stormy seas, wave action breaks forming ice into rounded disks called pancake ice, whose raised edges develop where wind and waves force the disks together. As ice thickens and multiple seasons pass without melting, brine pockets trapped within the ice lattice gradually drain, producing multiyear ice that is substantially fresher and structurally stronger than first-year ice. The National Snow and Ice Data Center science of sea ice resource describes this evolution from frazil through nilas to consolidated pack ice as a function of temperature, wind stress, and ocean turbulence.

Albedo and Climate Feedback

Sea ice exerts a strong influence on the planetary energy budget through the ice-albedo feedback mechanism. Open ocean water absorbs roughly 94 percent of incident solar radiation, reflecting only about 6 percent. The NSIDC guide to why sea ice matters notes that sea ice reflects 50 to 70 percent of incoming solar energy, and snow-covered sea ice reflects up to 90 percent. As sea ice retreats, dark open water is exposed, absorbing additional solar energy and warming the surface ocean, which in turn inhibits ice formation and further reduces ice extent. This positive feedback amplifies Arctic warming relative to lower latitudes, a phenomenon observed in the instrumental record as Arctic amplification. Sea ice formation also drives thermohaline circulation: when seawater freezes, brine is expelled into the surrounding ocean, increasing local salinity and density, causing surface water to sink and feed the deep meridional overturning circulation that redistributes heat globally.

Arctic and Antarctic Sea Ice

The two polar regions exhibit contrasting sea ice dynamics. The Arctic Ocean is a nearly enclosed basin surrounded by continental land masses, which confines sea ice and allows multiyear ice to accumulate over successive winters. Arctic sea ice extent reached a record minimum in September 2012 and has shown a consistent long-term decline since satellite records began. The Southern Ocean surrounding Antarctica, by contrast, is open water through which sea ice expands and contracts freely each year. Antarctic sea ice is predominantly seasonal first-year ice, and its interannual variability is larger than that of the Arctic. NOAA's Climate.gov global sea level and climate monitoring resources document the accelerating loss of land ice that accompanies reduced sea ice, connecting polar cryosphere change to global sea level trends.

Applications

Sea ice research has applications across a wide range of scientific and operational fields, including:

• Climate modeling and prediction of polar and global temperature trends
• Navigation route planning for Arctic shipping and icebreaker operations
• Wildlife and ecosystem monitoring in polar marine habitats
• Radar and microwave remote sensing algorithm development for satellite ice classification
• Ocean circulation modeling and thermohaline transport studies