Sea ice and a polar bear in the Fram Strait (Arctic Circle).
Sea ice is frozen seawater that floats on the ocean's surface. It forms in the winter (in both the Arctic and Antarctic) and retreats in the summer, but does not completely disappear.
Ice crystals form on the surface of the ocean. They expel salt, which increases the water salinity below them. This cold salty water is dense, and it can sink to the ocean floor, whence it flows back towards the equator.
Sea ice has an important influence on polar environments (e.g. weather, regional climate and ocean circulation). The sea ice layer restricts wind and wave movement near coastlines, which reduces coastal erosion and protects ice shelves.
It also creates an insulating “cap” on the ocean, reducing evaporation and heat loss to the atmosphere. Because of this, the weather in ice-covered areas tends to be colder and drier than in areas not covered by ice.
When the ice melts in the summer, it releases nutrients into the water. This stimulates the growth of phytoplankton, which are at the centre of the marine food web. The melting ice exposes the ocean water to sunlight, which encourages photosynthesis in plankton.
When ice freezes and the saltier, denser water sinks, it mixes the water column and brings nutrients to the surface. The sea ice is a habitat for animals including polar bears, seals, penguins and Arctic foxes.
The influence of sea ice is global. The white surface reflects more sunlight back into space than the water does (i.e. the ice has a higher albedo). When it begins to melt, it exposes more dark water to the sun, and this water absorbs more sunlight. The sun-warmed water then melts more ice, creating a self-reinforcing cycle. Over several years, this ice-albedo feedback can influence global climate.












