The Arctic ice pack or arctic ice cap is an ice shield covering the Arctic Ocean. It changes during the year. Usually, the ice melts in spring and summer, and increases during autumn and winter. The minimum cover is around mid-September. Summer ice cover in the Arctic is about half of winter cover. Some of the ice survives from one year to the next. Currently, 28% of Arctic basin sea ice is multi-year ice. Multi-year ice is thicker than seasonal ice: up to thick over large areas, with ridges up to thick. As well as the regular seasonal cycle there has been an underlying trend of declining sea ice in the Arctic in recent decades.