Tag Archives: snow

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?
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Snow depth over spring sea ice affects polar bear feeding success and ringed seal survival

Snow depth over sea ice in spring affects the hunting success of polar bears on ringed seal (Phoca hispida) pups, but the relationship is more complicated than you might think and there is less data on this phenomenon than you would believe.

Ringed seal lair_snow and ice thickness_PolarBearScience_sm

Regional snow depth in spring (April-May) varies naturally from year to year due to weather patterns driven in part by long-term climate cycles (like the Atlantic Multidecadal Oscillation, Pacific Decadal Oscillation, and the Arctic Oscillation).

This year, it was very cold in Eastern North America, with record-breaking snow fall in some areas. Snow depth was apparently greater than average over Hudson Bay sea ice this spring but was it deep enough to have impaired polar bear hunting success?

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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).

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