Weather Talk: When was the Arctic ice free?

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The shrinking of the summertime Arctic Ice cap in recent decades has left me wondering 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. 

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 Wisconsonian 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 Ice cap is thought to be around 34 million years old.