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Sea Ice: definition.

Sea ice forms at the ocean surface once the surface temperature drops to the freezing point during fall and winter. The freezing point for salty ocean water is about 29oF (-2oC), slightly colder than it is for fresh water (32oF, 0oC). When sea ice forms, a lot of the salt is expelled from the ice crystal structure, but the ice still ends up being slightly salty (about 1% salt, compared with about 3.5% salt in the ocean). This is distinct from the ice of {ice shelves}, which originally formed from snow falling on land, and so are completely fresh. Sea ice can be up to 20 feet thick, and is thickest in areas where the ice is “land-fast” and stays there year-round. Around Antarctica, sea ice is more usually 1-6 feet thick and can be cut through by icebreakers, although ridges can form which are much thicker and prevent any ships from getting through it.

Who cares? The formation of sea ice plays a major role in ocean circulation. As sea ice forms in the Antarctic and Arctic, the salt that is rejected from the ice is added to the underlying ocean, making it denser. The water then falls down to the ocean abyss, where it can move towards the equator. Dense, cold water from Antarctica can be traced along the sea bed all the way to the northern hemisphere. This “bottom” water is replaced by the poleward flow of warmer upper-ocean waters, in what has been called “The Great Conveyor Belt”. This is a major mechanism by which heat is moved from the equator to the poles, making the Earth habitable over a much larger region than would otherwise be possible. Sea ice has two other properties that are important to people who model climate. First, it is a great reflector: when ice is present, especially ice with snow on top of it, most of the heat in sunlight bounces off the ice/snow and returns to space, whereas ice-free water absorbs most of the heat. Second, ice is a great insulator: when ice is present, the rate of exchange of heat and gases (e.g., CO2) between the atmosphere and the ocean is greatly reduced. Thus, sea ice plays an important role in modeling how the ocean and atmosphere might change in the future.