The primary difference between the Arctic and Antarctica is geographical. The Arctic is an ocean, covered by a thin layer of perennial sea ice and surrounded by land ("Perennial" refers to the oldest and thickest sea ice). Antarctica, on the other hand, is a continent, covered by a very thick ice cap and surrounded by a rim of sea ice and the Southern Ocean.
The Arctic Ocean is very deep and closely linked with the climate systems around it, making it more sensitive to climate changes than Antarctica.
During the centuries of human exploration in the Arctic, sea ice covered the Arctic Ocean well year-round, up until recent decades. But satellite observations show that Arctic sea ice has been declining in extent*, thickness and volume since 1979.1 Average Arctic sea ice extent is at its lowest since 1850.