Biggest threat to polar bears reconsidered

What presents a bigger risk to current polar bear populations: natural hazards that have already proven deadly or potential, yet-to-be-realized threats prophesied to occur due to human activities? That’s a perfect question for International Polar Bear Day.

Natural ice_snow variation and polar bears_model_PolarBearScienceFeb 20 2016

Dag Vongraven, chair of the IUCN Polar Bear Specialist Group, remarked last year that “until 2001, everything was fine.

Polar bear researchers thus assume that 2001 was the year climate change became the new over-hunting – but is it true? What are the relative harms presented by proven natural causes, potential human-caused threats, and predicted threats due to sea ice declines blamed on global warming? Considered objectively, is climate change really the single biggest threat to polar bear health and survival right now?

Polar bear specialists almost unanimously insist that predicted declines in summer sea ice due to rising CO2 emissions from fossil fuel use are now the biggest threat to polar bears, while other human-caused hazards are lesser evils (Derocher et al. 2013; Stirling and Derchoer 2012; Vongraven 2013; Vongraven et al. 2012).

However, they can only make that assertion by ignoring the devastating effects that natural variations in ice and snow conditions have had on polar bear population numbers in the recent past – especially those caused by cold winters.

Their conclusion is based on an assumption that before the declines of summer sea ice that occurred after 2001 (see graph below, from NOAA’s 2015 Arctic Report Card”),  Arctic sea ice provided a ‘stable’ habitat and therefore, population numbers never varied naturally – but it’s a major fallacy that’s already been exposed (Crockford 2015).

Sea ice anomaly_March vs Sept fig4.2-perovich

Their assumption is not true and they know it: the polar bear’s Arctic habitat of ice and snow has always varied on a regional basis and profoundly negative effects on polar bear health and population size have been documented.

Proven, potential and predicted threats

Proven threats: thick spring ice,thick snow cover, thin snow cover, winter/spring rain events.
Potential threats: biochemical contamination, oil spills, over-hunting, poaching.
Predicted threats: summer sea ice declines and increased spring rains due to global warming.

What we know about these threats:

Proven natural threats: marked population declines have occurred in at least two populations (S. Beaufort in 1974-1976, 1986, 1992, 2004-2006; WHB in the late 1980s-early 1990s, see Appendix below) due to thick spring ice and/or thick snow cover/thin snow cover, and although the populations later recovered, they may not have rebounded to previous levels (discussed in detail in Crockford 2015); there have not been winter/spring rain events affecting either denning polar bears or ringed seals reported since 1990 (Stirling and Derocher 2012).

Possible human-caused threats: contaminants present in E. Greenland bears but no definite ill effects have been demonstrated; no Arctic oil spills have yet occurred that have harmed seals or polar bears; over-hunting is controlled through harvest management in Canada, where 2/3 of polar bears live; poaching is an issue primarily where legal hunting has been forbidden (Russia).

Predicted human-caused threats: so far, there have been no proven population-wide ill-effects caused by summer sea ice declines due to global warming – on the contrary, Chukchi Sea bears are in good condition and reproducing well despite experiencing an extraordinarily long open-water season in 2007; similarly, Southern Beaufort Sea polar bear numbers began to recover in 2007, after the lowered survival wrought by thick spring ice in 2004-2006 and there was no reported devastation after the low summer ice of 2012, although the opposite should have occurred, if summer sea ice is as critical as the models suppose; Western Hudson Bay polar bear numbers suffered their most dramatic decline between 1989 and 1992 due to effects of thick spring ice and/or heavy snow conditions on ringed seal number but have been stable since 2004; Svalbard area bear numbers, expected to have been decimated by poor ice conditions in 2013, by 2015 were 42% higher than in 2004.

Starvation events of polar bears caused by thick ice and snow during the critical spring feeding period have been profound in both the Southern Beaufort (a decline of 25-50%) and Western Hudson Bay (Bromaghin et al. 2015; Crockford 2015; Lunn et al. 2016). In both cases, marked declines in survival occurred and population numbers plunged (Lunn et al. 2016). The devastation wrought by thick spring ice conditions in the Southern Beaufort from 2004-2006 demonstrate that such conditions can occur even if summer sea ice continues to decline; a prolonged winter cold spell lasting more than a few years, with or without heavy snow (such as from a major volcanic eruption, such as Mt. Pinatubo in 1992, or just a shift in local weather pattersn) – could spell local decimation of WHB polar bear numbers far exceeding the prophesied effects of manmade global warming.

The bottom line is this: Global warming won’t stop volcanoes from erupting, prolonged Arctic low temperatures from developing over the winter, or Arctic currents from driving multiyear ice against the shore of the Beaufort Sea, which means such natural events pose now the biggest and most immediate threats to polar bear health and survival.

P.S. Don’t miss the companion post to this one (25 February 2016):
Ten dire polar bear predictions that have failed as global population hits 20-31k

Appendix: Notes about Beaufort Sea thick spring ice

1. Ian Stirling (2002:68) had this to say about the reduced survival of polar bears due thick spring ice in the Beaufort Sea, that could also be said to apply to the similar events that took place in 2004-2006:

In the eastern Beaufort Sea, in years during and following heavy ice conditions in spring, we found a marked reduction in production of ringed seal pups and consequently in the natality of polar bears (Stirling and Lunn, 1997). The effect appeared to last for about three years, after which productivity of both seals and bears increased again. These clear and major reductions in productivity of ringed seals in relation to ice conditions occurred at decadal scale intervals in the mid-1970s and 1980s (Fig. 5) and, on the basis of less complete data, probably in the mid-1960s as well (Stirling et al., 1977b; Stirling and Lunn, 1997).” [my bold]

2. Below is an excerpt from a previous post on an incident of cannibalism that occurred in 1976 that’s pertinent because of the descriptions of the spring ice conditions for 1975 and 1976, and the effects these had on both ringed seals and polar bears.

A report of an incident of cannibalism in 1976 was made by biologist Jack Lentfer in his summary of polar bear research in Alaska for the IUCN Polar Bear Specialist Group’s 6th meeting (1976, pg. 187 and 192, “Polar bear management and research in Alaska 1974-76”). See the map below for locations of communities mentioned:

Both 1975 and 1976 were “heavy” ice years. Bears traveled south to the southern Chukchi Sea and northern Bering Sea with a movement of heavy ice south early in the winter of these years. Unusually high kills [of polar bears] occurred on St. Lawrence Island and near the villages of Wales and Shishmaref in areas where bears are seldom encountered.

Sea ice was heavier than usual and heavy ice extended further south than usual in 1975 and 1976. This may have caused a movement of ringed seals, the principal food of bears, to the west and south (Burns et al. 1976) and a similar movement of some bears. In both years significantly fewer seals were killed by bears in the Barrow, Alaska area than in years when ice had not been so heavy. Also, perhaps related to reduced availability of seals, the first instance of predation by an adult bear, a male, on cubs was noted.

Both cubs in a litter were nearly completely consumed on 13 April 1976.” [my bold]

3. Seal biologists John Burns and colleagues described the unusual nature of the movements of seals and bears in their quarterly report on research they were doing on the distribution and abundance of seals in 1974 and 1975 (Burns et al. 1975). They say (in part):

“Survey results substantiated that a major, short-term shift in density had indeed occurred. Density of seals in the Beaufort Sea was down 10 fold with a corresponding increase in the Chukchi Sea of between 10 and 15 fold.

…In my opinion, the overriding factor affecting ringed seal distribution is the distribution of favorable sea ice conditions. From past experience it was obvious the prevailing sea ice conditions in the Beaufort Sea were, by and large, unfavorable for breeding ringed seals whereas they were excellent in the Chukchi Sea.

The distribution of ringed seals directly affected the distribution of their most significant predator, the polar bear. [my bold]

Chukchi Beaufort locations_PolarBearScience_sm

As Burns and colleagues pointed out, the ice conditions of 1974-1976 in Alaska and western Canada were unusual. In their 2008 paper on Eastern Beaufort polar bears, Ian Stirling and colleagues said that prior to 2004-2006 (when there was heavy spring ice and some very thin bears), they had not seen similar development of shorefast pressure ridges since 1974 — even though they had worked there from 1971 to 1979 and from 1985 to 1987.

This suggests that conditions in the eastern Beaufort were worse in the spring of 1974 (with almost as bad conditions the following two years, see previous post here), but that in the Chukchi Sea and western Beaufort, the worst effects were evident in 1975 and 1976.

Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications 25(3):634-651.

Burns, J. J., Fay, F. H., and Shapiro, L.H.  1975. The relationships of marine mammal distributions, densities, and activities to sea ice conditions (Quarterly report for quarter ending September 30, 1975, projects #248 and 249), pp. 77-78 in Environmental Assessment of the Alaskan Continental Shelf, Principal Investiagors’ Reports. July-September 1975, Volume 1. NOAA, Environmental Research Laboratories, Boulder Colorado. [available online] pdf here.

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.

Derocher, A.E., Aars, J., Amstrup, S.C., Cutting, A., Lunn, N.J., Molnár, P.K., Obbard, M.E., Stirling, I., Thiemann, G.W., Vongraven, D., Wiig, Ø., and York, G. 2013. Rapid ecosystem change and polar bear conservation. Conservation Letters 6:368-375.

Lentfer 1976. Polar bear management and research in Alaska 1974-76. Pg. 187-197 in [Anonymous]. Polar Bears: Proceedings of the 6th meeting of the Polar Bear Specialists Group IUCN/SSC, 7 December, 1976, Morges, Switzerland. Gland, Switzerland and Cambridge UK, IUCN.

Lunn, N.J., Servanty, S., Regehr, E.V., Converse, S.J., Richardson, E. and Stirling, I. 2016. Demography of an apex predator at the edge of its range – impacts of changing sea ice on polar bears in Hudson Bay. Ecological Applications, in press. DOI: 10.1890/15-1256

Stirling, I., Richardson, E., Thiemann, G.W. and Derocher, A.E. 2008. Unusual predation attempts of polar bears on ringed seals in the southern Beaufort Sea: possible significance of changing spring ice conditions. Arctic 61:14-22.

Stirling, I. and Derocher, A.E. 2012. Effects of climate warming on polar bears: a review of the evidence. Global Change Biology 18:2694-2706. doi:10.1111/j.1365-2486.2012.02753.x

Vongraven, D., Aars, J., Amstrup, S., Atkinson, S.N., Belikov, S., Born, E.W., DeBruyn, T., Derocher, A.E., Durner, G., Gill, M., Lunn, N., Obbard, M., Omelak, J., Ovsyanikov, N., Peacock, E., Richardson, E., Sahanatien, V., Stirling, I., Wiig, Ø. 2012. A circumpolar monitoring framework for polar bears. Ursus 23 (sp2): 1-66.

Vongraven D. 2013. Circumpolar monitoring framework for polar bears. Presentation 5.1 at the International Polar Bear Forum, December 3-6, Moscow. Download pdf here; available online here:‎


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