Thick spring ice due to natural causes is currently the single biggest threat to polar bears. Not declining summer sea ice – thick spring ice. That could change in the future but right now, the evidence supports that statement.
Polar bear deaths due to cyclical changes in Arctic sea ice thickness in the spring have continued despite rising CO2 emissions and declining summer sea ice extent (last major incident, 2004-2006): there is no reason to expect this will not continue. Unwarranted attention on summer ice extent has deflected attention from this major cause of local polar bear population decline.
Sea ice models do not address past or future changes in spring ice thickness and predictive models of polar bear survival blame all population declines on summer sea ice declines despite strong evidence to the contrary (Crockford 2015: The Arctic Fallacy).
Thick spring ice near shore drives seals to give birth elsewhere because they cannot maintain their breathing holes in the ice (below). This leaves mothers emerging from onshore dens with newborn cubs (above) with nothing to eat at a time when they desperately need food: cubs die quickly, mothers more slowly. Young bears on their own for the first time also die at higher rates than usual.
While some bears die, an unknown portion of the population size change is due to bears moving out of the region to find better hunting.
Despite the hype over September sea ice declines in recent years, the reality is that since 1973, when wanton over-hunting was halted, thick spring ice due to natural causes has been the single biggest threat to polar bears. Over the last 40 years or so, marked polar bear population declines have virtually always been associated with thick spring ice that reduced local ringed seal prey, although in some areas (like Hudson Bay) thick snow on top of sea ice have produced a similar result some years.
Thick spring sea ice conditions have occurred repeatedly in the Southern Beaufort (where numbers may have declined up to 50%, most recently in 2004-2006, but also in 1974-76) and occasionally in Hudson Bay. Historically, similar conditions have been noted in East Greenland.
Fortunately, when sea ice returns to normal, numbers have largely rebounded but where they have not, it’s possible some bears moved permanently out of the area. Evidence going back hundreds of years suggests this kind of population size fluctuation has always occurred and likely always will.
Despite these incidents, global polar bear numbers are higher than they have been in more than 50 years, although exact figures are still frustratingly uncertain after 40 years of research effort (newest estimate, by IUCN Red List 2015: 20,000-31,000).
Claimed negative effects of chemical pollution on polar bears has been circumstantial and inconclusive: no evidence of actual harm to polar bears has been demonstrated.
The same is the case for poaching. Some Russian authorities claim poaching is a big problem for polar bears in Russia but no evidence to support this assertion has been provided to the public, as far as I am aware (such as records of arrests or confiscated material).
Meanwhile, current polar bear habitat is pretty much on par for this time of year. According to NSIDC for the month of December 2015, ice extent was within two standard deviations of the 30 year average (1981-2010): higher in some areas than usual, lower in others. In the ice extent map for December 4, 2015 below (NSIDC), all of that ice (except for the Sea of Okhotsk and the Baltic) is polar bear habitat. Just as it should be.
References cited (others cited in previous blog posts linked to above)
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.