Polar ice packs are large areas of pack ice formed from seawater in the Earth's polar regions, known as polar ice caps: the Arctic ice pack (or Arctic ice cap) of the Arctic Ocean and the Antarctic ice pack of the Southern Ocean, fringing the Antarctic ice sheet. Polar packs significantly change their size during seasonal changes of the year. However, underlying this seasonal variation, there is an underlying trend of melting as part of a more general process of Arctic shrinkage.
In spring and summer, when melting occurs, the margins of the sea ice retreat. The vast bulk of the world's sea ice forms in the Arctic ocean and the Southern Ocean, around Antarctica. The Antarctic ice cover is highly seasonal, with very little 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 meter (3.28 ft) thick. The Arctic Ocean is very different, being a polar sea surrounded by land rather than to a polar continent surrounded by sea, and its ice shows less seasonal variation. Currently 28% of Arctic basin sea ice is multi-year ice,[1] thicker than seasonal: up to 3–4 meters (9.8–13.1 ft) thick over large areas, with ridges up to 20 meters (65.6 ft) thick.