Early sea ice breakup in W Hudson Bay caused by “record breaking” warmth in 2023 but not 2015?

According to Polar Bears International, the “3rd-earliest” breakup date for Western Hudson Bay was caused by a “record breaking” heat wave in May. Western Hudson Bay sea ice hit the 30% coverage threshold used by PBI to define “breakup” on 17 June this year, prompting speculation about potential future impacts on polar bear survival should breakup come even earlier.

This year’s break-up date of June 17 is the 3rd earliest in the 45 years of satellite-based sea ice data from Western Hudson Bay, after 2015 and 2003.” [Flavio Lehner, PBI]

17 June 2023 is day 168 on the Julian calendar used to graph the data in the image included in the PBI essay (see copy below). However, the data point for 2003 is about three days earlier, on day 166 (14 June) and the point for 2015 is on day 152 (1 June).

If “record-breaking” heat caused this year’s early ice retreat, what caused the ice to retreat more than two weeks earlier in 2015? May was warm that year along the west coast as well but obviously not “record-breaking” warmth, because the records were broken this year. In fact, whatever warmth that occurred only affected ice melt in the western sector, while very thick ice over the rest of the bay resisted melt and allowed bears to stay out many weeks later than usual.

Sea ice breakup in 2023

Closest in my archive to 17 June is 19 June (but 15 June isn’t much different along the western portion so it probably didn’t change much between the 17th and 19th). Note how much ice is missing in the eastern bay as well.

Sea ice breakup in 2015

Compare the above to 1 June 2015 (below), when the same 30% ice coverage threshold was apparently reached. Note this is 30% of the Western Hudson Bay region in particular, not Hudson Bay in general. In fact, the ice melt pattern in 2015 was quite unusual, with lots of open water in the western sector but thick, solid ice everywhere else that resisted melt. Despite early melt in Western Hudson Bay, at August 13, only 1992 had more ice remaining in the bay overall.

At 20 July 2015, more than half of collared Southern Hudson Bay bears were reported still far out on the ice. On the same date (see tweet below), polar bear specialist Andrew Derocher noted that only 2 out of 9 collared bears had come ashore (one way over in Hudson Strait), when the prediction based on 30% WH ice coverage should have seen them all onshore by 30 June (Cherry et al. 2013):

According to Cherry et al. (2013):

“Throughout the study, bears arrived ashore a mean of 28.3 day (S.E. = 1.8) after 30% ice cover.”

Thus, according to Cherry’s criteria, this year we should expect most bears to be onshore in WH by 15 July. However, as of 27 June, only 6 of Andrew Derocher’s tagged bears were onshore and the rest (27 of them) were still out on low concentration sea ice (see map below). Several of them are way out in the middle of the bay. Time will tell whether they will all head to shore over the next two weeks or if some will linger a few weeks longer on bits of remnant ice, as seems to be the usual pattern in recent years.

All this suggests that the 30% ice coverage metric may not be an especially good predictor of polar bear behaviour. That’s not just my opinion: one of Derocher’s students undertook an analysis that came to a similar conclusion.

Her more recent paper (Castro de la Guardia et al. 2017) found that a threshold of 50% ice coverage over WH was the best predictor of when polar bears come ashore in the summer:

The most suited definition of breakup was the 50% ice concentration. There was a high positive correlation (r2 > 0.9, p = 0.07) between migration onshore and breakup from 2006 to 2009, with polar bears migrating onshore approximately 20 d after breakup.” [Castro de la Guardia et al. 2017, pg. 230, my emphasis]

If this more recent study, which uses more data and a 50% ice coverage threshold to define breakup, is the best predictor of when polar bears come ashore, why are PBI representatives using a 30% threshold? They don’t say, or even mention that another method has ever been used to assess when polar bears come ashore. Nor do they mention that in general, since 2015 at least, some bears simply haven’t been behaving as expected, and have been staying out on highly degraded ice late into summer. Odd, that.

Problem Bears Onshore

The first real Churchill Polar Bear Alert Report came out (for week 2 of the summer season, 19-25 June), with only a few bears onshore close to town requiring encouragement to hang out further afield.


Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/

Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract

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