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Is breakup imminent for Hudson Bay sea ice? Probably not, but this year more than ever, it will depend on how you define it. Despite a large patch of open water in western Hudson Bay (CIS chart above, for 1 June), there is still more than 70% sea ice coverage over the entire bay as of this week, when you use the standard breakup definition of 50% ice coverage (Fig. 1).  Ice remaining over the bay is mostly 90% or greater, as the chart above shows – which means there is still a lot of polar bear hunting habitat remaining.

Figure 1. Sea ice coverage over Hudson Bay, as a percentage, for the week of 4 June, 1971-2015. Click to enlarge.

The interconnected region of Hudson Bay, Hudson Strait, and southern Davis Strait (Fig. 2), what the Canadian Ice Service calls “Regional Hudson Bay,” is only slightly below average for the week of 4 June.

Figure 2. Regional Hudson Bay, week of 4 June. Click to enlarge.

Since ice concentration is factored into breakup date calculation, a record-early breakup is simply not possible, since the previous record date (2 June, for 1990) has already passed. It might be an earlier than average breakup year but not very early, based on the 50% coverage definition (Fig. 3, below). This year, because of the unusual pattern of breakup of Hudson Bay ice, it will be critical for polar bears which definition of breakup is used – the old, 50% method (adopted because it’s what sea ice professionals used) or the newest one, which was determined to be most relevant to WHB polar bears (Cherry et al. 2013).

UPDATE 6 June 2015: I’ve added the forecast for ice conditions over the summer for North America (which for these folks includes June because it’s aimed at temperate NA, polar bear folks call June the end of spring), provided by the Canadian Ice Service: “Seasonal outlook for North American Arctic Waters issued by the North American Ice Service on 2 June 2015” [points of potential interest marked] The sea ice forecast (Table 1) for southwestern Hudson Bay (where most western and southern Hudson Bay polar bears come ashore) is for complete ice melt by 1 Aug, eight days later than the earliest date over the period 1968-2013. Time will tell if that’s what happens.
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