Historical and future development of Arctic sea ice:
The climate of the Earth has always been changing, in particular owing to natural variations of solar insulation, atmospheric composition, ocean circulation and changes in surface albedo.
In response to these changes, the sea-ice cover has varied substantially on geological time scales, with for example no sea ice whatsoever during warm periods beyond 50 million years ago, to very extensive sea-ice coverage during ice-age periods. Such variations can be obtained from drilling of ocean sediment cores, whose composition allows some inference about the sea-ice coverage throughout time.
For the recent geological period, the Quaternary, substantial sea-ice coverage has been prevailing for the past 800,000 years. This period was clearly dominated by year-round sea-ice coverage on the Arctic Ocean. The recent retreat of Arctic sea ice, as well as the possible further future retreat of the ice coverage are hence both extraordinary on very long time scales.