We have precise data for Arctic sea ice extent from satellite observations over about the last 30+ years, and a pretty good estimate for the last century or more from ship and aircraft observations. The data show clearly that Arctic sea ice has declined dramatically over the last few decades, a decline the like of which has certainly not been seen for over 100 years prior to that. In addition, a recent survey of available proxy information — not a reconstruction, but a thorough review of the evidence — tells us that the modern decline in Arctic sea ice is “unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.”
Now there’s even more evidence of the shocking state of the Arctic sea ice. New research reports an actual reconstruction of the extent of Arctic sea ice over the last 1,450 years (Kinnard et al. 2011, Nature, 479, 509-512, doi:10.1038/nature10581).