More than two-thirds of the Earth's surface is covered by sea and an average of five per cent of the seas is covered by ice. In the northern sea areas, the ice cover is at its largest between February and March when an average of 15 million square kilometres are covered by ice.
Sea ice is often classified in accordance with its age. One-year ice melts during the summer. Perennial 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. 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 metres. Most of the ice in the areas around the North Pole is of the perennial type and its thickness varies between two and three metres. Thickest ice (between four and five metres or even more) occurs in the Lincoln Sea, northwest of Greenland.
In the Antarctic Ocean, the area covered by ice is at its largest in September and the thickness of the ice is typically less than one metre.
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.