An “ice-free” Arctic Ocean, sometimes referred to as a “Blue Ocean Event”, is often defined as “having less than 1 million square kilometers of sea ice”, because it is very difficult to melt the thick ice around the Canadian Arctic Archipelago. The IPCC AR5 defines “nearly ice-free conditions” as a sea ice extent of less than 10⁶ km² for at least five consecutive years.

Intro:

Sea ice in the Arctic region has declined in recent decades in area and volume due to climate change. It has been melting more in summer than it refreezes in winter. Global warming, caused by greenhouse gas forcing is responsible for the decline in Arctic sea ice. The decline of sea ice in the Arctic has been accelerating during the early twenty‐first century, with a decline rate of 4.7% per decade (it has declined over 50% since the first satellite records). Summertime sea ice will likely cease to exist sometime during the 21st century.