Scientists have been measuring sea ice very carefully since 1979. Prior to that, there are estimates that are of varying degrees of usefulness. I know for a fact that many New England lighthouses were attached to land by winter-long ice in places that have not had sea ice in any living person’s memory, and there are similar bits and pieces of historical data suggesting that sea ice was once much more extensive in the Northern Hemisphere than at present.
Since 1979 there have been three years in which Arctic sea ice reached a rather alarming minimum size prior to reforming. We are in one of those years now.
Daily Arctic sea ice extent on September 10, 2010 was 4.76 million square kilometers (1.84 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole. (From the NCIDC)
On September 10, 2010 sea ice extent dropped to 4.76 million square kilometers (1.84 million square miles). This appears to have been the lowest extent of the year; sea ice has now begun its annual cycle of growth.
The 2010 minimum ice extent is the third-lowest recorded since 1979.
This year is the third time since data have been collected that ice is measured at less than 5×106 square kilometers. The other times have all been within the last four years. In other words, the typical extent of Arctic sea ice is now much smaller than it was previously, a strong indicator and effect of global warming.
More details are here.