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?
I offer the following references for your examination. If you have problems with the methodologies, explain what they are.
Global Warming and Northern Hemisphere Sea Ice Extent
The key reference is 13, by Chapman and Walsh. Here's their Web site:
Arctic Climate Research at the University of Illinois
The data is online and Dr. Chapman invites anyone with questions about it to contact him by email. Why don't you email him and discuss how the data quality of the observational archive compares with the more recent satellite data?
Here is the documentation page for the sea ice data. I recommend reading the "Expert User Guidance" section:
Walsh and Chapman Northern Hemisphere Sea Ice Data Set
dangus, there are a couple of nice papers out there reporting that increased Antarctic sea ice is a consequence of global warming. I've posted the links to them before and I'm a tad lazy today. Google works to find them.
Try an example: The warmest year in recent history was 1934. 1931, 1938 and 1939 were in the top ten. However this cannot be discerned in the ground-based ice extent observations shown in your chart. 1953, the seventh warmest year...had the greatest sea ice extent, according to your chart!
Either the chart is flawed, or its association with average temperatures is flawed.
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 Great masses of ice have been replaced by moraines of earth and stones, the report continued, while at many points well known glaciers have entirely disappeared.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)