It’s like pulling teeth, getting up-to-date information on breakup dates of Hudson Bay sea ice. You’d think with the importance of this seasonal event to the polar bears of Hudson Bay (some of which, we’re told, face the most perilous future of all polar bears worldwide), we’d get a press release every summer alerting us to the precise date of sea ice breakup and the subsequent arrival of the bears onshore. I’m thinking of something similar to the dispatches we get from the National Snow and Ice Data Center (NSIDC) when Arctic sea ice reaches its yearly maximum (e.g. March 2012) and its yearly minimum extent (e.g. September 2012).
Sadly, this is not the case.
So I was intrigued to see that a new paper just out in the Journal of Animal Ecology, by Seth Cherry (a Ph.D. student of Andrew Derocher) and colleagues, dealt with Hudson Bay breakup dates. I was hoping for some data beyond 2007, which has been the limit of information provided so far by polar bear biologists (see previous posts here and here). Unfortunately, because the methods for determining breakup dates in this paper are so different from previous ones (more detail below), the new data (1991-2009) can’t be compared to earlier studies that go back to 1979. But there is some good news.
Although you wouldn’t know it by the author’s conclusion, the results of the study confirm for this region my previously stated contention that polar bears need spring and early summer ice (March through June) for gorging on young, fat seals and documented declines in sea ice have rarely impinged on that critical feeding period – by which I meant, bears have seldom, if ever, been forced off the ice of Hudson Bay as early as June.
The study also confirms that there has not been any kind of spectacular retreat of sea ice breakup dates – coming earlier and earlier in the season – over the last 19 years and that polar bears did not arrive on shore in 2009 until very late – approximately 22 August – the same date they came ashore in 1992.
Below are two figures from the paper: one (the map) necessary to understand the new “Cherry method” of calculating breakup dates for Hudson Bay (no disrespect intended) and the other (the graph), which presents the data collected. The table with my converted breakup dates is below them. A few quotes from the study and some comments on it follows.
UPDATE (March 21 2013; 6:12 PM PST). A typo in the table has been fixed (2001 is Jun 21 not Jul 21) and the post amended accordingly.
UPDATE 2 (July 7, 2013) The Cherry et al. paper is now in print, the references have been updated accordingly.
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