According to these guys, Arctic ice extent is correlated not with air temperatures so much as ocean currents
Arctic sea ice in longest decline seen over past 1,450 years: study – The team examined tree rings, ice cores from glaciers and lake and ocean sediments. To check the validity of their approach, scientists compared their calculations for the last couple of centuries with real-world observations from satellites, ship logs and other historical accounts.
They found that by the mid-1990s sea ice had fallen even further than in previous lows such as the so-called Medieval Warm Period between 800 and 1300.
Team members were also able to conclude that sea ice is influenced by more than just temperature. They discovered that ice actually shrank during what’s known as the Little Ice Age, a period between 1450 and 1850, due to relatively warm ocean waters moving north.
That’s what’s happening now, said Mr. Zdanowicz. (Canadian Press) [em added]