Sea ice is often classified in accordance with its age. One-year ice melts during the summer. Perennial ice (i.e. multi-year ice) has survived at least one summer without melting completely. All ice in the Baltic Sea is of the one-year variety, while in the oceans, both one-year and perennial ice occurs. There is considerable regional variety in the thickness of sea ice.

In the Arctic Ocean, the ice coverage varies from 3 000 000 – 17 000 000 km². Most of the ice in the European parts of the Arctic Ocean is of the one-year variety and its thickness is less than two meters. Most of the ice in the areas around the North Pole is of the perennial type and its thickness varies between two and three meters. Thickest ice, between four and five meters or even more, occurs in the Lincoln Sea, northwest of Greenland.

In the Antarctic Ocean, the ice covered area varies from less than 2 000 000 to 20 000 000 km². It is at its largest in September. Thickness of the ice is typically less than one meter, but can be even 2 meters.

Sea ice is one of the most important factors regulating climate in cold areas. Ice reflects most of the incoming solar radiation back into the space: for ice covered by dry frozen snow the rate can be as much as 90 per cent, while for open sea, the figure is less than 10 per cent. Ice also absorbs a huge amount of energy and water, and that´s why it ha s a great impact on the energy budget of the atmosphere and the sea.