We are seeing one of the extremes in Hudson Bay sea ice variability this year, not only in extent but in distribution of ice. Ice coverage on Hudson Bay this year at 28 July was twice what it was in 2009, the last “late” ice breakup year for which detailed ice maps are available (409 vs. 204 thousand km2), according to NSIDC MASIE ice maps. Canadian Ice Service data show 2015 coverage for the week of 30 July was the highest since 1992.
The odd pattern of ice distribution presents a conundrum. Have a look at the maps and graphs below.
The amount of sea ice left on Hudson Bay this year is comparable overall to the “good old days” (before every early breakup year was blamed on man-made global warming) but the distribution is very different. The ice is primarily in Southern Hudson Bay (SHB) polar bear territory, as shown in the NSIDC sea ice map below for 28 July, where places that bears come ashore are marked in yellow (Obbard and Middel 2012) – the ice was thick enough last week to require an icebreaker to reach the community of Inukjuak. At 20 July, 5 out of 9 collared SHB females were still on the ice near the Belcher Islands.
I think it’s still too early to say for sure whether polar bears have drawn benefit from the longer ice coverage or if the conditions indicate that the ice was too thick for optimal hunting during the critical spring feeding period from April-June. I’d say the odds could be about even. Photos of WHB bears ashore so far look to be in good shape (e.g. here and here), so the hope is that these bears are representative of the populations at large.
We know from past research that thicker-than-usual ice conditions can and do occur on Hudson Bay – thick spring ice was reported to be the cause of a decline in WHB ringed seal numbers and polar bear survival in 1991 (Crockford 2015). But the odd distribution of ice melt this year suggests that thick ice conditions were perhaps confined to the eastern portion of the bay in 2015.
Early clearance of ice in the NW portion of the bay means that the official breakup date for WHB for 2015 (the one that polar bear biologists use to correlate with survival and body condition) probably will be quite early.
In contrast, in 2009 – even though it was a late breakup year – ice distribution at 28 July over Hudson Bay showed the more typical pattern, with ice spread out over territory used by both Western (WHB) and Southern Hudson Bay polar bears on the southwestern portion of the bay, below:
Because of the way the ice was distributed in 2009, WHB polar bears with collars came ashore at about the same times as they did in 1992, when the official breakup date was much later.
Canadian Ice Service (CIS) departure from normal at 27 July 2015, latest available (blue means more ice than average, very dark blue is a lot more than normal):
Sea ice concentration at 30 July 2015, CIS (click to enlarge):
Graphs for 30 July 2015 sea ice coverage (CIS, click to enlarge), for Hudson Bay, Davis Strait and Foxe Basin:
In contrast, when you look at Northwest Hudson Bay (see the red insert map on the graph below) for 11 June (7 weeks ago), ice coverage over that portion of the bay in 2015 was about the same as it was in 1990 (which apparently had the earliest breakup date for WHB since 1979):
Note that the region of NW Hudson Bay defined by the Canadian Ice Service in the above graph is not the same as the WHB polar bear subpopulation region: it has quite different boundaries. And also note that overall ice coverage in 1990 at 30 July (first of the graphs above, for WHB) was not particularly low – it wasn’t as high as it is this year (>20%), but it was still > 10% (cf. 1991 had almost no ice remaining at 30 July). So despite the early ice clearance of NW Hudson Bay in 1990, overall ice coverage was not particularly low later in the season.
And as I’ve pointed out before, none of the papers that correlate WHB breakup dates to polar bear condition and/or survival mention 1990 as the worst year ever, in spite of its very early official breakup date.
So, here is the conundrum: breakup of NW Hudson Bay was as early this year as it was in 1990 (earliest WHB breakup year on record) but ice coverage over the entire bay for the week of 30 July was the highest since 1992 (latest WHB breakup year on record). What that means for WHB and SHB polar bears remains to be seen.
Crockford, S.J. 2015. “The Arctic Fallacy: sea ice stability and the polar bear.” GWPF Briefing 16. The Global Warming Policy Foundation, London. Pdf here.
Obbard, M.E., Middel, K.R., Stapleton, S., Thibault, I., Brodeur, V. and Jutras, C. 2013. Estimating abundance of the Southern Hudson Bay polar bears subpopulation using aerial surveys, 2011 and 2012. Ontario Ministry of Natural Resources, Wildlife Research and Monitoring Section, Science and Research Branch, Wildlife Research Series 2013-01. Peterborough, Ontario. Pdf here.