Polar bears are not in peril due to recent sea ice changes

To counter the misleading ploy used by the Sunday Times — of implying polar bears are in peril because of recent changes in Arctic sea ice (Sunday Times & The Australian, 21/22 Sept. 2014 Arctic ice cap in a ‘death spiral’) — I’ll go over again why the polar bear as a species is not threatened by declines in summer sea ice or even winter ice that is predominantly “thin” (first year) ice.

Polar Peril_Arctic ice cap in a death spiral_SundayTimes_Sept 21 2014_21_NWS_20_POLAR_1096592k
Graphic above from the Sunday Times, September 21, 2014

Here is the ‘death spiral’ prediction made by Peter Wadhams, from The Australian version of the story (not paywalled):

“The Arctic ice cap is in a death spiral,” said Peter Wadhams, professor of ocean physics at Cambridge University. He has just returned from a research voyage in which the thickness of Arctic sea ice was measured by sending a mini-submarine under the floes.

“On average it was about 0.8m thick, compared with 5m when I first went in 1976,” he said.

“One or two warm summers could melt it away. The evidence suggests that September will be ice-free very soon, and that will increase so that within five years or so we could be seeing an ice-free Arctic for up to four months in the summer, and much thinner ice for the rest of the year too.”

Even if this implausible and dire-sounding prediction of Wadhams turns out to be true, does that mean the end for polar bears?

Well, no. I’ve covered this ground before, so here are some excerpts from previous posts:

Polar bears have not been harmed by sea ice declines in summer – the evidence Posted August 18 2013

[The quote below is from my final summary statement: “PBSG” is the Polar Bear Specialist Group;; note that the polar bear subpopulation regions listed in brackets are those where studies show evidence to support my general statement – see original post for links and references ]

PBSG biologists continue to make correlations of polar bear ‘success’ with duration and extent of open water in September (i.e., the seasonal sea ice minimum) but the total of all studies suggest these correlations are weak or nonexistent, as I’ve summarized below:

Less summer ice ≠ few bears (Davis Strait; S. Hudson Bay; Barents Sea; S. Beaufort; W. Hudson Bay).

[Less summer ice = fewer bears (W. Hudson Bay, from 1998-2004)]

Less summer ice ≠ “skinnier” bears (Chukchi Sea; S. Hudson Bay).

[Less summer ice = “skinnier” bears (Baffin Bay; W. Hudson Bay, up to 1998 & 2004 only; S. Beaufort, based on false correlations).

“Skinnier” bears ≠ fewer bears (S. Hudson Bay; S. Beaufort; Davis Strait).

[“Skinnier” bears = fewer bears (W. Hudson Bay, 1998-2004)].

Less summer ice ≠ lower cub survival (S. Hudson Bay; Chukchi Sea).

[Less summer ice = lower cub survival (S. Beaufort; W. Hudson Bay, up to 1992 only)].

Less summer ice ≠ more cannibalism & hybridization

Negative effects on polar bear populations may occur if Arctic ice-melt in the future advances to the point that mid-to-late spring sea ice is affected. However, virtually all evidence to date indicates that declines in summer sea ice have not harmed polar bears and this means that as far as polar bears are concerned, it does not matter how low the sea ice extent gets in September – whether it’s this September or a future one.

Record sea ice loss in 2007 had no effect on polar bears, Chukchi study confirms Posted Sept 15 2013

One aspect of the recently published study on Chukchi Sea polar bears (Rode et al.2014 [now in print] 2013; see here and here) has not been stressed enough: their finding that the differences in overall condition between bears in the Chukchi and Southern Beaufort Seas came down to disparities in spring feeding opportunities and therefore, the condition of spring sea ice.

The fact that spring — not summer — is the most critical period for polar bears is something I’ve pointed out before (see here and here, for example) but it’s worth repeating at this time of year, when all eyes are on the annual ice minimum. It is often treated as a given that the decline in extent of summer sea ice in the Arctic since 1979 has been detrimental to polar bears. However, this is an assumption that we can now say is not supported by scientific evidence (see summary of that evidence here).

The results published by Rode et al. (2014 2013) not only add further support to the conclusion that declines in summer sea ice have not harmed polar bears, but should put the matter to rest – unless new evidence to the contrary is produced.

Chukchi bears, the report tells us, had more food available in the spring than Southern Beaufort bears (see map below) and this was the primary reason that bears were doing very well in the Chukchi and not quite as well in the Southern Beaufort. And because the polar bears for this study were captured and measured in mid-March to early May, from 2008 to 2011, they reflect spring-time conditions for 2008-2011 as well as year-round conditions from 2007 through 2010.

This means that the annual low ice extent for 2007 (record-breaking at the time), in the fall before this study began, had no discernible negative effect on either Chukchi or Southern Beaufort polar bears – and neither did similarly low annual minimums in two of the three remaining years of the study (Fig 1).

Record low sea ice extent and what it means for polar bears…
Posted August 28 2012

In regards to sea ice, how ‘thin’ is ‘too thin’ for polar bears?

Newly-formed ice that is present in the fall starts out thin but quickly thickens due to the frigid temperatures. But how ‘thin’ is too thin for polar bears?

We hear Arctic scientists talking about ‘thin’ ice – especially, the perils of thin ice – but what does ‘thin’ ice really mean?

According to NSIDC:

new ice is < 10cm thick
young ice is 10-30cm thick (30cm is about 1 foot)
thin first year ice is 30-120cm (0.3-1.2m) thick
thick first year ice is 120-200cm (1.2-2.0m) thick or more.

So, technically speaking, ‘thin’ ice can be 1.2m (about 4 feet!) thick. Try to remember that the next time you hear or read the media or research spokespersons ranting about the dire state of the Arctic because of ‘thin ice.’

[See Canadian Ice Service definitions of ice thickness here]

Work on polar bears in the Canadian Arctic around Baffin Bay (Ferugson et al. 2000) has shown that the bears routinely hunt on ‘young ice’ (0.1-0.3m) and are known to use thick first year ice (1.2-2.0m) for over-wintering activities, including denning – and are as successful doing so as bears that have access to multiyear ice.

In the Barents Sea (around Svalbard, north of Norway), polar bears are known to den on first year ice (Mauritzen et al. 2001). In other areas that are dominated by 1st year ice (e.g. Southern Beaufort Sea, Chukchi Sea, Laptev Sea, and East Greenland), bears that den on stable annual ice or pack ice that is 1-2 years are common (Amstrup and Gardner 1994; Schliebe et al. 2008:1000).

In fact, it appears that while some polar bears make use of multiyear ice for denning where it is available in winter (Durner and Amstrup 1995:339; Ferguson et al. 2000:769), they rarely use it in summer in the areas that have been studied extensively (e.g. Durner and Amstrup 1995:340).

UPDATE September 22, 2014 See also this previous post:
Sea ice experts make astonishing admissions to polar bear specialists July 29, 2014

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