Tag Archives: Vibe

Claim of range-contraction of polar bears due to declines in summer sea ice doesn’t hold up

Have polar bears suffered a contraction of their historical range due to recent declines in summer sea ice? Buried in a recent journal article lies such a claim, one I can’t recall having seen before. That makes it worth close examination.

Figure 2. A drawing of polar bears on St. Matthew Island that accompanied the May 1, 1875 Harper’s Weekly Journal of Civilization article written by Henry Elliot. See here.

A drawing of polar bears on St. Matthew Island that accompanied the May 1, 1875 Harper’s Weekly Journal of Civilization article written by Henry Elliot. See here.

The assertion appears in the introduction of a recently published paper that got a lot of attention online (“Implications of the Circumpolar Genetic Structure of Polar Bears for Their Conservation in a Rapidly Warming Arctic” by Peacock and colleagues (2015), discussed previously here, news coverage here and here).

Here is how the authors put it:

There is already evidence of change in the contemporary distribution of polar bears. For example, polar bears, once common in Newfoundland [29], are now seen there only infrequently and in small numbers. Similarly, polar bears once regularly summered on St. Lawrence and St. Matthew islands in the Bering Sea [30–32]. Now they are irregularly observed in the Bering Sea and do not spend summers on St. Matthew Island. Although these changes in polar bear distribution may also have been related to overharvest, the recent reductions in the extent of sea-ice due would prevent current and regular use of these areas.[my emphasis]

There are three main reasons the claim doesn’t hold up to scrutiny:
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Why is it that every decade, Eastern Beaufort sea ice gets really thick?

I’ve written before about the incidents of starving polar bears in the eastern portion of the Southern Beaufort Sea (here, here, and here). For two or three years every decade since the 1960s, shorefast ice in the Eastern Beaufort (Fig. 1) has become too thick and compressed in the spring for ringed seals to maintain their breathing holes, so most or all of them presumably go elsewhere — as seals did in Greenland when ice got too thick there (Vibe 1965). With few or no seal pups born during March and April in thick ice years, some bears had a hard time finding enough food: starving bears and dying cubs were the result.

Figure 1. Eastern portion of the southern Beaufort Sea.  The communities of Tuktoyatuk (locally known as ‘Tuk’), and Sachs Harbour on southern Banks Island, have been useful starting points for polar bear research because they are accessible by plane via the larger community of Inuvik The light blue portions, e.g. along western Banks Island and the Eastern Beaufort/Yukon mainland coast, indicate shallow continental shelf areas (20 km wide in places) where extensive shorefast ice develops every winter. Main map from Beaufort Sea Partnership, inset map from Wikipedia.

Figure 1. ‘Eastern Beaufort’ (yellow square) polar bear study region.
The communities of Tuktoyatuk (locally known as ‘Tuk’), and Sachs Harbour on southern Banks Island have been used as base camps for polar bear research because they are accessible by plane via the larger community of Inuvik.
The light blue portions along western Banks Island and the Eastern Beaufort/Yukon mainland coast indicate shallow continental shelf areas (20 km wide in places) where extensive shorefast ice develops every winter.
Main map from Beaufort Sea Partnership, inset map from Wikipedia.

I’ve been trying to get my head around why this would happen in the Eastern Beaufort. Once or twice – maybe – but several times every decade? What on earth drives such a process?

So, I did some reading (actually, quite a lot of reading) and have what appears to be at least a partial answer.

All indications are that the occasional development of exceptionally thick spring ice in the Eastern Beaufort is the result of an entirely natural, cyclical phenomenon. However, some polar bear biologists are attempting to blame the latest episode (but not earlier ones) on increased amounts of open water in the Chukchi Sea during fall of the early 2000s. That doesn’t seem a plausible explanation to me, given the history of the sea ice in this region. Have a look.

Figure 2. Beaufort sea pressure ridges, spring 1949. Courtesy Wikipedia (from NOAA “At the ends of the Earth” image collection #corp1014).

Figure 2. Beaufort sea pressure ridges, spring 1949. Courtesy Wikipedia (from NOAA’s “At the ends of the Earth” image collection #corp1014).

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