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I am aware that the data is not satellite data; but going back to the 1950s at least, ice extent was estimated by aerial survey. Going back further than that, ship and coastal surveys provide the data. How would you argue that such observations are so flawed that they do not provide a reasonably accurate indication of sea ice extent?

Here are some of those "observations":

The Arctic ocean is warming up, icebergs are growing scarcer and in some places the seals are finding the water too hot.... Reports from fishermen, seal hunters and explorers, he declared, all point to a radical change in climate conditions and hitherto unheard-of temperatures in the Arctic zone... Great masses of ice have been replaced by moraines of earth and stones, the report continued, while at many points well known glaciers have entirely disappeared. Very few seals and no white fish are found in the eastern Arctic, while vast shoals of herring and smelts, which have never before ventured so far north, are being encountered in the old seal fishing grounds. - Washington Post 11/2/1922 (This observation does not appear in your chart)

The United States and the Soviet Union are mounting large-scale investigations to determine why the Arctic climate is becoming more frigid, why parts of the Arctic sea ice have recently become ominously thicker and whether the extent of that ice cover contributes to the onset of ice ages. – New York Times - July 18, 1970 (This observation contradicts your chart)

The Oceanographic observations have, however, been even more interesting. Ice conditions were exceptional. In fact, so little ice has never been noted. The expedition all but established a record….Journal of the Royal Meteorological Society - January 1905 (This observation does not manifest itself in your chart)

Scientist says Arctic ice will wipe out Canada, Professor Gregory of Yale University stated that “another world ice-epoch is due.” He was the American representative to the Pan-Pacific Science Congress and warned that North America would disappear as far south as the Great Lakes, and huge parts of Asia and Europe would be “wiped out.” – Chicago Tribune August 9, 1923 (This observation does not manifest itself in your chart)

From 2003:

Dwindling Arctic Ice

Quote from page 3:

"When compared to longer- term, ground-based surface temperature data, the rate of warming in the Arctic from 1981 to 2001 is eight times larger than the rate of Arctic warming over the last 100 years. There have also been some remarkable seasonal changes. Arctic spring, summer, and autumn have each warmed, lengthening the seasons when sea ice melts by 10 to 17 days per decade. Temperatures increased on average by almost one and a quarter (1.22) degrees Celsius (C) per decade over sea ice in the Arctic summer."