October 20, 2008
Dr. Crockford begins with a summary followed by the introduction to her study. Full report with illustrations, footnotes and references are linked following this summary.
by Dr. Susan Crockford Summary for Policy Makers
Globally, more than one third of the nineteen subpopulations of polar bears are increasing or stable, while the remaining third have insufficient data available; its status in the central Arctic Basin, the largest of the nineteen designated regions, is completely unknown.
All but one of the declining subpopulations listed in 2006 are threatened by over-harvesting, not reduced sea ice.
The offshore sea ice that lies well north of the pack ice edge has not been surveyed in any comprehensive fashion for polar bears OR ringed seals, their primary prey: we simply do not know how many bears or seals live deep in the polar pack, although several studies suggest that the number of ringed seals living and breeding well offshore must be substantial.
Virtually the only areas studied in any detail, for polar bears and ringed seals, are the neashore areas of Hudson Bay (Canada), the Beaufort Sea (shared by the USA and Canada) and Svalbald (Norway). Some studies have also been done in Canada's Davis Strait/Baffin Bay and the Canadian High Arctic. There is only about thirty years worth of data for even the most intensely studied populations.
Most of what we know about polar bear biology is based on the western Hudson Bay population but since this is an anomalous population, Hudson Bay bears are not a good proxy for ALL polar bears.
Among polar bear females, pelagic-dwelling bears live in drifting sea ice year round, while nearshore-dwelling females inhabit shorefast ice year round. Both pelagic-dwelling and nearshore-
dwelling individuals of both sexes are known in all subpopulations studied and each type behaves differently to reduced seasonal ice.
Polar bears routinely hunt on new ice < 30 cm thick and are quite capable of utilizing thick first year ice (> 120 cm thick or more.) for over-wintering and denning. They do not require thick multi-year ice.
As offshore sea ice over deep water is suitable for polar cod, it should be suitable for ringed seals and polar bears also; ringed seal and polar bear habitat probably includes ice over water of all depths.
The polar bear survived two major warming periods over the last 11,000 years, the first of which saw temperatures rise rapidly to at least 2.50C higher than present and there is no evidence that Arctic sea ice disappeared entirely during those times or that any ice-dependent species became extinct.
It is highly unlikely that in the future, polar bears would move to land in response to reduced sea ice, although a few might: it is far more likely that most bears would stay out on the sea ice that lies well offshore - as in the past, bears will die on the ice or at sea, leaving no evidence of their existence.
Sea ice thickness assessments in the huge Arctic Basin region are estimates based on very few actual measurements that have been extrapolated for use in various climate models. Satellite data on sea ice extent have only been available for 30 years: this is insufficient for assessing long-term trends.
The computer model results, presented in 2007, which forecast dramatic declines in polar bear numbers based primarily on predicted reductions in seasonal sea ice thickness and extent, have not yet been tested against even a single years worth of independent data.