Tag Archives: Arctic ice extent

Summer sea ice melt and polar bear maternity dens

The yearly sea ice minimum extent is almost upon us, which has recently been the seasonal signal for excitable biologists and their activist groupies to resume their breathless rants about what sea ice loss could mean for polar bears.

Polar bear den_CreditUSFWS_labeled

Never mind that the summer minimum extent reached in September, no matter how low it goes, is pretty much irrelevant to polar bear health and survival. As I’ve discussed before, what’s really important is the presence of not-too-thick ice during the spring, so they can catch lots of young seals and put on lots of fat.

But to a lesser degree, the extent at mid-to-late summer is important because this is when pregnant females that prefer to make their maternity dens on shore are looking for good places to spend the winter.

So the topic for today is this: how much does the extent of ice at the height of summer dictate where polar bear females make their winter dens?
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September sea ice ballyhoo and why it doesn’t matter to polar bears

The end of September sea ice summary from the US National Snow and Ice Data Center (NSIDC) finally became available late last week (October 3, “A better year for the cryosphere”).

The summary figure NSIDC provides are the average ice extent for the month (not the maximum achieved at the end of the month), which are compared to previous years.

[There has been considerable ice growth since the end of September (updated daily here].

Here is why the September extent doesn’t matter to polar bears: it is the extent in June that is important to polar bear survival. June is the end of the critical spring feeding period for polar bears (see previous post here) – healthy bears eat more seals over a shorter period of time from March to June than any other time of year. After the end of June, most bears have enough fat to survive a fast of 4 months or more.

In contrast to September – when many bears are taking a time-out on shore – ice extent for June over the last 30 years or so provided an extensive hunting platform for polar bears throughout the Arctic. To show you how extensive, I’ve constructed a composite of ice maps from selected years (Fig.1, below).

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Good news for polar bears: no early breakup of W. Hudson Bay sea ice this year

The sea ice chart provided by the Canadian Ice Service (Fig. 1 below) shows a lot of ice still present in Hudson Bay today, the last day of June, 2013.

Figure 1. Sea ice extent in Canada, June 30, 2013. From the Canadian Ice Service.

Figure 1. Sea ice extent in Canada, June 30, 2013. From the Canadian Ice Service.

This means we have long passed the point when breakup of the sea ice in Western Hudson Bay (30% ice concentration) could be considered ‘early.’ See Table 1 below for previous breakup dates (1991-2009) and previous post here for more details.

 Table 1. Breakup dates calculated for Western Hudson Bay, 1991-2009, using a new method described by Cherry et al. (in press). More details in previous post here.


Table 1. Breakup dates calculated for Western Hudson Bay, 1991-2009, using the new method described by Cherry et al. (2013, in press). More details in previous post here.

Since most polar bears don’t leave the ice until almost a month after the official breakup date is declared, it means that even if breakup for Western Hudson Bay occurs within the next few days, most polar bears would not start their summer fast until the beginning of August.

Regardless of when polar bear biologists decide that breakup has occurred, one thing is now clear — this will not be an early breakup year for Western Hudson Bay. That’s good news for polar bears.

And what about the rest of the Arctic? You’ll see from Fig. 2 below that as of yesterday (June 29), there was still ice present in each of the 19 polar bear subpopulation regions — more good news for polar bears.

Figure 2. Sea ice extent worldwide vs. polar bear subpopulations at June 29, 2013. On this date, there was still sea ice present in every one of the 19 subpopulation regions. Map on the left from US National Snow and Ice Data Center (NSIDC “MASIE”) here; map on the right from the Polar Bear Specialist Group, with labels added. Click to enlarge

Figure 2. Sea ice extent worldwide vs. polar bear subpopulations at June 29, 2013. On this date, there was still sea ice present in every one of the 19 subpopulation regions. Map on the left from US National Snow and Ice Data Center (NSIDC “MASIE”) here; map on the right from the Polar Bear Specialist Group, with labels added. Click to enlarge

References
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(4):912-921.
http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract