Sea Ice

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Frozen seawater, which is a different substance from the glacial ice of an ice sheet: it forms at the ocean surface at about -1.8 degrees Celsius, expels most of its salt as it freezes, and floats because it is already part of the ocean, so its melting does not by itself raise sea level. It is not a lid but a habitat with structure - a brine-channel network inside the ice supports algae, the underside is a grazing surface, the top carries snow that animals dig into, and the leads and cracks are breathing and hunting geometry. The Arctic and Antarctic ice behave differently in kind: Arctic ice is confined by land and can survive several summers, so a substantial multi-year component once existed, while Antarctic ice is unbounded to the north, spreads freely each winter and almost entirely melts away each summer. Continuous satellite passive-microwave monitoring of both since the late 1970s is the reason the two hemispheres can be compared at all.