Polar Ice Packs

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, 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.

The area of sea ice around the poles in winter is about 15,600,000 km2 (6,000,000 sq mi) either for the Antarctic or Arctic. However, whereas the northern cap is shrinking at a rate of about 3% per decade, the southern cap is expanding at a rate of 0.8% per decade. The amount melted each summer is affected by the different environments: the cold Antarctic pole is over land, which is bordered by sea ice in the freely circulating Southern Ocean. The summer ice cover is about 12% of the winter coverage in the Antarctic and 50% in the Arctic.

This animation shows the Arctic Ocean melt during the summer of 2011. NASA Cryosphere Program manager Tom Wagner in his video interpretation of the 2011 sea ice record. This animation shows a semi-transparent graph of the annual minimum area of Arctic sea ice in millions of square kilometers. The background shows the annual minimum sea ice concentration over the arctic for each year from 1979 through 2012.