The shrinking of the summertime Arctic Ocean icecap in recent decades has raised the question of when the last time the Arctic was ice-free. It turns out this is a hard question to answer due to the fact that Arctic sea ice undergoes a little melting every summer from top (weather) and bottom (unfrozen ocean) which leaves a poor record relative to the Antarctic and Greenland icecaps. The most accepted theory is that the North Pole has been capped in ice continuously or nearly continuously for 2.7 million years, since the beginning of the present Wisconsin Ice Age.
During the past 2.7 million years, there have been many periods of glacial advances in which ice has moved southward into the mid-latitudes and then retreated back to the Polar region during relatively brief interglacials. The land-based and much colder Antarctic icecap is thought to be around 34 million years old
