Collapse of polar bear snow dens – Stirling and Derocher’s anecdotal evidence

In their summary of facts supposedly supporting the premise that global warming is already having an impact on polar bear populations (discussed previously here, here, and here), biologists Ian Stirling and Andrew Derocher (2012:2700) include a list of incidents of warm winter weather and/or “rain on snow events” that have led to the collapse of polar bear maternity dens and ringed seal birthing lairs.

Stirling and Derocher state that both polar bears and ringed seals (their primary prey) have a demonstrated

“high vulnerability …to increased mortatily resulting from warm temperatures and rain. Such rain on snow events are predicted to increase as the climate warms in the Arctic (Hansen et al. 2011).”[my bold]

However, their so-called evidence for polar bears and ringed seals having a proven vulnerability to these events comes not from scientific studies but what they admit outright are anecdotal reports.

They describe four incidents, including one case of a maternity den collapse (involving a 6 yr old, probably first-time mother and two, 3-4 week old cubs) in the southern Beaufort in 1989, apparently caused by a bit of warm weather followed by heavy snow in late January (a picture of the dead bear is included, see below, just so we don’t forget that a bear died).

From Stirling and Derocher (2012:2700), evidence that a bear died.

They also describe a rain event in early March 1990 (but no den collapses) in western Hudson Bay (“we found no evidence of dead bears the following summer”). The other incidents are about ringed seals: a rain and warm weather event in early April 1979 in southeastern Baffin Island that resulted in “increased predation mortality of ringed seal pups” by polar bears and Arctic foxes, and another warm period, also in southeastern Baffin Island, in late April 1990, that reportedly generated a three-fold increase in polar bear predation success on newborn ringed seal pups because the snow birthing lairs (dens) were easier to break or had melted and collapsed.

So, all of the four reported warm weather incidents reported as anecdotal evidence in this 2012 paper occurred in 1990 or before. From this, I conclude that since 1990, there has not been a single more recent case of either polar bear den collapse due to warm weather or any more recent cases of increased predation on ringed seal pups because of snow lair weakness or collapse – despite the fact that apparently, ‘climate warming’ is continuing to cause a multitude of bad things to happen in the Arctic, including “progressive unidirectional changes to sea ice distribution” (Stirling and Derocher 2012:2704).

Most incredibly, the grand total of known incidents of polar bear maternity dens collapsing due to warm weather and/or rain and killing pregnant females with cubs, is exactly one (1) – in more than 33 years of Arctic research. And even in that one case, the cause of death was determined months after the fact (in June 1989) – it was not witnessed firsthand. In Stirling and Derocher’s words (2012:2700), the den collapse was:

“likely triggered by the combined effects of unusually warm weather followed by heavy snowfall in late January, apparently crushing the adult female and her two cubs (Fig. 6 [the photo of the dead bear]).” [my bold]

In fact, Peter Clarkson and Doug Irish (Clarkson and Irish 1991:84), who originally found the dead bears, had to surmise which weather events (which they call ‘climatic’ events) might have caused the collapse: these included a reported rise of temperatures to +2⁰C in the area on one day (17 December 1988), about a month prior to the presumed time of collapse, some reported high winds on 12 January 1989 (that may have caused drifting snow) and a major snow storm on 26-28 January 1989. They guessed that this combination of events, coupled with the inexperience of the young mother bear, might have caused the den to collapse – but they did not know for sure.

However, both polar bear researchers and advocates are hyping this single event from January 1989 as a “phenomenon” or listing it as one of several “documented” threats to polar bears from global warming that have already transpired. Notice the definitive, and often plural, way this supposed evidence is presented:

1) Polar Bears International, “Climate Change” page:

“Warm spring weather leads to increased rainfall causing dens to collapse.”

2) Derocher, Lunn and Stirling (2004:166-67) (“Polar bears in a warming climate”).

”In those populations where females den in snow, significant changes in the distribution and timing of snowfall may alter when suitable snow is available, both in the autumn and in the spring. Insufficient snow will preclude den construction or result in use of poor sites where the roof may collapse…. An additional concern specific to female polar bears in dens with altricial cubs is the possibility that rain might become more frequent in late winter and cause the snow cover over dens to collapse and suffocate the occupants Clarkson and Irish, 1991). [my bold]

3) WWF “Polar bears at risk” (2002:6), a pdf document.

“Rain in the late winter can cause maternity dens to collapse before females and cubs have departed, thus exposing occupants to the elements and to predators. Such rains also destroy the denning habitat of ringed seals, the polar bears’ primary prey.” [my bold]

4) National Resource Defense Council (NRDC), “Polar bear SOS: the bear is sliding toward extinction, imperiled by climate change and trophy hunters”

“The polar bear has been sending us a desperate S.O.S. There have been documented reports …of snowy dens collapsing on newborn cubs and their mothers from unseasonable rains….” Robert F. Kennedy Jr., NRDC Senior Attorney. [my bold]

5) Stirling and Derocher (2012:2700) (“Polar bears in a warming climate”). See above, an entire section entitled “Warm weather and unseasonal rain,” with photo of dead bears.

The question is, are anecdotal reports appropriate scientific evidence to use in this way? Without a proper scientific study, are polar bear biologists justified in using one den collapse as “documented” evidence of global warming? Is admitting that evidence presented is anecdotal, but then going ahead and including it in a list presented as scientific evidence in support of an argument, an appropriate scientific approach?

As a trained biologist, now that I understand the facts of the ‘evidence’ presented by Stirling and Derocher, I think it is more reasonable to conclude that maternity den or birth lair collapses have been rare over the last 30 years or so, are not increasing, and appear to be due to unpredictable weather events coming at a bad time for polar bears and ringed seals – and do not constitute demonstrated effects of global warming.

Another day, I’ll discuss the anecdotal evidence for the supposed increase in cannibalism in polar bears that is apparently caused by global warming, which is a similar kind of story.

References
Clarkson, P.L. and Irish, D. 1991. Den collapse kills female polar bear and two newborn cubs. Arctic 44:83-84. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/1522 pdf here.

Derocher, A.E., Lunn, N.J. and I. Stirling. 2004. Polar bears in a warming climate. Integrative and Comparative Biology 44:163-176. http://intl-icb.oxfordjournals.org/content/44/2/163.full pdf here (free)

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