Skip to content

Polar Bears Have Not Been Harmed By Sea Ice Declines In Summer

|
Susan Crockford, Polar Bear Science

Polar bear numbers overall have increased, despite the appearance of a ‘stable’ global population since 2001 and significant declines in Arctic sea ice coverage in summer. 

The polar bear biologists and professional activists of the IUCN Polar Bear Specialist Group (PBSG) continue to insist that since 1979 increasingly smaller amounts of Arctic sea ice left at the end of summer (the September ice minimum) have already caused harm to polar bears. They contend that global warming due to CO2 from fossil fuels (“climate warming” in their lexicon) is the cause of this decline in summer ice.

In a recent (2012) paper published in the journal Global Change Biology(“Effects of climate warming on polar bears: a review of the evidence”), long-time Canadian PBSG  members Ian Stirling and Andrew Derocher (both of University of Alberta) summarized their position this way:

“Climate warming is causing unidirectional changes to annual patterns of sea ice distribution, structure, and freeze-up. We summarize evidence that documents how loss of sea ice, the primary habitat of polar bears (Ursus maritimus), negatively affects their long-term survival”

I’ve spent the last year examining their evidence of on-going harm, but in addition, I’ve looked at the evidence (much of it not mentioned in the Stirling and Derocher paper1) that polar bears have either not been harmed by less sea ice in summer or have thrived in spite of it.

This is a summary of my findings. I’ve provided links to my original essays on individual topics, which are fully referenced and illustrated. You are encouraged to consult them for complete details. This synopsis (pdf with links preserved, pdf with links as footnotes) complements and updates a previous summary, “Ten good reasons not to worry about polar bears” (pdf with links preserved; pdf with a foreward by Dr. Matt Ridley, with links as footnotes).

1) Polar bear numbers overall have increased, despite the appearance of a ‘stable’ global population since 2001 and significant declines in Arctic sea ice coverage in summer.  The global population estimate generated by the PBSG (20,000-25,000 bears, as of July 2013) has been virtually unchanged since 2001, giving the impression that declines in some subpopulations have been offset by increases in others. However, adjustments to a few subpopulation estimates (primarily changing estimates that had formerly been included in the total to zero (0), without adjusting the total downward) suggest that the actual numbers must have increased by 2650 to 5700 bears since 2001. These increases in total polar bear numbers have occurred despite declines in the amount of sea ice remaining in September (Fig. 1 below) and a trend to earlier dates for the summer breakup of sea ice in regions such as Hudson Bay and Davis Strait.

 Figure 1. The graph that has some people worried that Arctic sea ice is in a “death spiral” - it shows the decline in extent of Arctic sea ice (in million kilometers squared) in September each year (the yearly minimum), based on satellite records since 1979. Note that the scale on the left axis does not go to zero. Courtesy the National Snow and Ice Data Center (NSIDC). See Fig. 3 for another perspective.
Figure 1. The graph that has some people worried that Arctic sea ice is in a “death spiral” – it shows the decline in extent of Arctic sea ice (in million kilometers squared) in September each year (the yearly minimum), based on satellite records since 1979. Note that the scale on the left axis does not go to zero. Courtesy the National Snow and Ice Data Center (NSIDC). See Fig. 3 for another perspective.

2) Davis Strait polar bear numbers have increased, despite declines in body condition and summer sea ice. Recent research on the Davis Strait subpopulation (Fig. 2) reported that declines in body condition (relative fatness) have occurred since 1978, yet sea ice declines did not begin until the 1990s. Despite the decline in body condition, the number of polar bears in Davis Strait increased significantly. In fact, the density of bears increased to such an extent thatresearchers could not determine whether the high density of bears or the declining sea ice, or both, was responsible for the decline in body condition. A similar phenomenon has been described in the Barents Sea, where polar bear population numbers appear to have stabilized (or are increasing very slowly) after a rapid post-1970s increase. Andrew Derocher, based on work in the Barents Sea between 1988 and 2002,concluded that he could not tell if the smaller litter sizes and increased number of adult bears were due to the high density of bears or changes in sea ice. The Western Hudson Bay subpopulation (see 5 below) also shows signs that stabilization following a rapid population increase might have occurred.

This is a major concern: if polar bear researchers can’t tell the good news (effects of population increases) from the bad news (effects of declining sea ice) from the information they collect on populations, it means that none of it is definitive for determining if global warming is harming polar bears.

3) Baffin Bay polar bears have experienced modest declines in body condition since 1980 and during that period, breakup of sea ice advanced by 2-3 weeks. However, it is not clear whether the sea ice changes can be blamed for the decline in body fatness or notbecause the population estimate has not been updated since 1997.

Full story