ice

ice, the solid phase of water, which, unusually, is lighter than liquid water. The molecules in ice crystals are arranged in a lattice and are more spaced out than in the liquid. The freezing point of sea water is about -1.9 °C (28.5 °F), depressed below 0 °C (32 °F) by the dissolved salts. It takes 80 calories to melt a gram of ice, enough heat to heat up the gram of liquid water to 80 °C (176 °F); this is termed the latent heat of ice formation. For the sea to freeze, it not only has to be extremely cold but also very windy to keep areas of water open and prevent a skin of ice from insulating the water surface from further cooling. Areas of open water are know as polynyas and are important for marine mammals like beluga whales and walruses, giving them access to air. When ice forms, the salts the water contained are left in solution, making the unfrozen water saltier. Because ice takes time to melt, it can drift well into warmer seas before finally melting. As it melts it dilutes the surface sea water, so the formation and melting of ice is one of the processes that modifies the salinity of surface sea water.

Frozen sea water forms pack ice. In the Arctic, the pack ice tends to last for up to five years. This multi-year ice gets deformed into pressure ridges by the surface currents and the wind. The pack ice moves with the drift of the surface current, whereas icebergs, with five-sixths of their volume below the surface, move with the deeper currents. A large tabular berg in the Southern Ocean can be as much as 200–300 metres (655–985 ft) deep. The deeper currents flow at an angle to the surface drift, because of the effects of Coriolis force generating a spiral profile to the currents known as an Ekman spiral, so big bergs often plough across pack ice.

Icebergs have a different origin: they are spawned from glaciers in the Arctic, and from the ice shelf in the Southern Ocean, where at any one time there may be up to 300,000 icebergs. In 2000 a massive berg 290 × 37 kilometres (180 × 23 mls.), roughly the size of Jamaica, broke off the ice shelf in the Ross Sea, devastating the food chain for penguins and cutting them off from their rookeries. Such a large berg will persist for several years. Fortunately, the currents of the Southern Ocean keep such navigational hazards well to the south. However, 1–2% of the 40,000 icebergs spawned annually from the glaciers of Greenland are carried well south down the east coast of Canada to Newfoundland and beyond, presenting a potent threat to shipping; the Titanic was sunk by one. Nowadays, satellites are used to track these icebergs and alerts are issued by the Canadian Coastguard as initiated by the International Convention for the Safety of Life at Sea (SOLAS). It is estimated that the ice in most Atlantic icebergs was formed at least 15,000 years ago, so is uncontaminated by industrial pollutants, one of today's environmental issues. Various uses have been suggested for icebergs: during the Second World War (1939–45) Admiral Mountbatten suggested one be converted into a floating airstrip; and it has been proposed to tow bergs to the arid coasts of Chile and south-west Africa where they can be used to supply fresh water.

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M. V. Angel