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When was the earliest ice breakup year for Western Hudson Bay polar bears?

Was 1990 really the earliest ice breakup year for Western Hudson Bay polar bears – and why aren’t breakup dates derived from the same data identical? Two serious questions that need answering.

Sea ice extent Canada 2015 June 17 CIS

The suggestion a few weeks ago by Andrew Derocher that the unusual breakup pattern of sea ice breakup in Hudson Bay this year might set a new record reminded me how often I’ve questioned the claim that 1990 had the earliest breakup for WHB since 1971.

I call this date into question for two reasons: 1) sea ice maps and charts like those shown below suggest an early breakup in 1990 was not possible, by any definition of the term; 2) none of the research reports on WHB bears, for periods that included 1990, mention that breakup in 1990 was especially early, even though many commented (often at length) about the especially late breakup in 1992.

How much does this very early breakup in 1990 contribute to the apparent declining trend in breakup dates since 1979?1  Is the early breakup date in 1990 real, perhaps a bizarre consequence of analyzing the data square-by-square over a grid? Or, were there errors in 1990 sea ice data that eventually got corrected by sea ice folks but not by polar bear biologists?

UPDATE 17 June 2015 PDF HERE of this post, with higher resolution images, for those who want to explore the questions I’ve posed or share them.
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