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Sea ice
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Sea ice

Sea ice is formed from ocean water that freezes. Because the oceans are salty, this occurs at about -1.8 oC. Sea ice itself is largely fresh, since the ocean salt, by a process called brine rejection, is expelled from the forming and consolidating ice. The resulting highly saline (and hence dense) water is an important influence on the ocean overturning circulation.

The vast bulk of the world's sea ice forms in the Arctic ocean and the oceans around Antarctica. The Antarctic ice cover is highly seasonal, with very little (find figures) ice in the austral summer, expanding to an area roughly equal to that of Antarctica in winter. Consequently, most Antarctic sea ice is first year ice, up to 1 m thick. The situation in the Arctic is very different (a polar sea surrounded by land, as opposed to a polar continent surrounded by sea) and the seasonal variation much less, consequently much Arctic sea ice is multi-year ice, and thicker: up to 3-4 m thick over large areas, with ridges up to 20 m thick.

Sea ice has an important effect on the heat balance of the polar oceans, since it acts to insulate the (relatively) warm ocean from the much colder air above, thus reducing heat loss from the oceans. Especially when covered with snow, sea ice has a high albedo - about 0.8 - and thus the ice also affects the absorption of sunlight at the surface. The sea ice cycle is also an important source of dense (saline) "bottom water". While freezing, water rejects its salt content (leaving pure ice) and the remaining surface, made dense by the extra salinity sinks, leading to the productions of dense water masses, such as Antarctic Bottom Water. This production of dense water is a factor in maintaining the thermohaline circulation, and the accurate representation of these processes is an additional difficulty to climate modelling.

Reliable measurements of sea ice edge begin with the satellite era in the late 1970's using SSMI. Measurements since then indicate a downward trend in Arctic ice area and an insignificant (upwards?) trend in Antarctic ice.

Sea ice may be contrasted with icebergs, which are chunks of ice shelves or glaciers that calve into the ocean.

The picture below shows the cycle of sea ice in both hemispheres (blue = northern, black = southern; units square meters).