From Polar Bear Science
Susan Crockford
A new collaboration by sea ice and polar bear specialists that predicts a catastrophic future for polar bears in Hudson Bay (Stroeve et al. 2024) can be dismissed as yet another bit of utterly useless fearmongering for two reasons: 1) it’s a model projection that uses widely discredited SSP5-8.5 “business as usual” climate scenarios for its predictions; and 2) it’s based on the false premise that Western and Southern Hudson Bay polar bears have already suffered harm from reduced sea ice blamed on fossil fuel-caused global warming.
The fact that recently-deceased Ian Stirling was a prominent co-author should come as no surprise: his irrational promotion of the idea that future “climate warming” could doom polar bears to near-extinction – even after recording and publishing evidence to the contrary – will go down in history as an appalling violation of scientific principles.
Adding to the dubious validity of the paper: lead author Julienne Stroeve’s 2007 paper predicting summer sea ice decline by 2050 was proven wrong by actual data by the time it was published (Stroeve et al. 2007, 2014) and a more recent update failed to foresee the recent 17-year stall in decline. And co-author Steve Ferguson, a seal biologists, rashly stated in 2016 that Hudson Bay could be ice-free in winter as early as 2021 [which, needless to say, never came close to fruition].
I’d say if Southern Hudson Bay polar bears might be extirpated as soon as 2030, as the paper’s co-author Alex Crawford suggests, the global temperature and ice melt had better get a move on: a survey showed the SH population was thriving in 2021 and Hudson Bay sea ice hasn’t hit any kind of death spiral in the three years since.
The paper
A newly-released paper by Julienne Stroeve and colleagues attempted to predict future sea ice coverage of Hudson Bay based on sea ice thickness in order to correlate it with future polar bear survival. The paper is called, “Ice-free period too long for Southern and Western Hudson Bay polar bear populations if global warming exceeds 1.6 to 2.60C.”
Since no one else appears to have attempted this method of sea ice projection, it remains to be seen if it will prove any more accurate than previous failed methods based on ice extent by the same lead author (e.g. Stroeve et al. 2007, 2014).
The authors admit that getting accurate sea ice thickness readings from satellite data and other sources is difficult at small scales, especially during the shoulder seasons of ice melt (early summer) and ice formation (autumn), when determining whether or not the supposedly critical 10cm of ice is present or not (i.e. enough to support an adult male bear).
Their decision to include primarily satellite-generated estimates of snow depth over sea ice in their predictive model — a critical metric for ringed seals in spring (when newborn pups are predated on by polar bears) — only adds to the potential inaccuracy of the entire endeavour. Sea ice thickness during the winter and snow depth over ice were variable factors Ian Stirling couldn’t account for in the 1980s when he was trying to figure out why cub survival was so low and bears were coming off the ice in such poor condition.
It’s questionable whether these snow and sea ice thickness data are accurate enough for the intended purpose and unfortunately, there is no way to double-check them since there are very few “ground-truth” measurements available for Hudson Bay.
Even more concerning is the basic premise of this paper which states that polar bears in Western and Southern Hudson Bay are already showing negative impacts of a longer-than-usual open water season. In short, the conclusion stated in the abstract is based on a lie: “…with longer ice-free periods already substantially impacting recruitment [not true], extirpation for polar bears in this region may already be inevitable.”
First of all, the change in the length of the open water season since 1979 happened as a “step-change” around 1998. After about 2000, the ice-free period was about 3 weeks longer than it had been before the turn of the century (Castro de la Guardia et al. 2017) but hasn’t changed since then. The implication that there has been a steady, year-after-year increase in the length of the ice-free season since 1979 in Western Hudson Bay is deliberate obfuscation.
In addition, the authors blatantly misrepresented data published in 2022 about aerial surveys conducted in 2021 for Western and Southern Hudson Bay polar bears, which I discussed previously here (Atkinson et al. 2022; Northrup et al. 2022; see also Crockford 2024). In other words, there has been no major change in sea ice coverage since at least 2001 and no change in polar bear population sizes in Western and Southern Hudson Bay since at least 2011 (and probably since 2004), so the premise of the model is false.
Moreover, for reasons that have never been adequately explained, the 1980s saw weights of polar bears and cub survival decline, with a marked increase in the loss of whole litters over what had been documented in the 1960s and 1970s (Calvert et al. 1986:19, 24; Derocher and Stirling 1992, 1995; Calvert et al. 1986:19, 24; Ramsay and Stirling 1988; Stirling and Lunn 1997).
It’s only when you start your WH data at 1980 – so that you don’t have to compare it to the two previous decades – that 1980 looks like the good old days for polar bears (Stirling and Derocher 2012; Stirling et al. 1999) and makes it appear that the step-change in the ice-free period (IFP) had a big impact.
The erroneous premise of the paper is used to justify using the time polar bears spend onshore as the only metric to correlate with sea ice thickness projections. From the Methods section: “To calibrate the IFP to the polar bear fasting period (i.e., the period of time polar bears spend onshore), we use the onshore/offshore dates for polar bears reported in Fig. 2 of Cherry et al. (2013).”
Why would they use only onshore/offshore dates from a paper using data from only 1991-1997 and 2004-2009 when more recent dates are available from on-going but unpublished tracking studies conducted by Andrew Derocher and his students?
Perhaps it has something to do with the inconvenient fact that, contrary to predictions, the ice-free periods for several of the last few years have been as short as they were in the 1980s: especially 2020 and 2022 but also 2017, 2018 and 2019.
Final Thoughts
We have a paper that uses a false premise to predict the possible extirpation of SH bears by 2030 at the earliest, based on the most pessimistic predictions of global temperature increases melting Hudson Bay sea ice, based on similarly pessimistic models of fossil fuel emissions, from which the authors conclude that fossil fuel use must be curtailed.
No discussion about SH or WH bears moving north if conditions indeed deteriorate, like they did in the Barents Sea: just death and population decline, oh my.
However, the final conclusions of this paper are revealing (note IFP = ice-free period):
“While it is difficult to provide a hard-limit of IFP before extirpation of WHB or SHB polar bear populations occurs, confronted with these threats, proactive measures are imperative.”
In other words, the authors can’t be sure when, or even if this catastrophe will happen, but they think we absolutely must rearrange society to reduce fossil fuel use just in case.
As I said, another utterly useless modelling paper. It’s certainly not science.
PS: No reports of polar bears onshore yet in WH or near Churchill despite the reduced ice on Hudson Bay caused by wind, so it looks like the movement of bears onshore will not be early again this year.
References
Atkinson, S.N., Boulanger, J., Campbell, M., Trim, V. Ware, J., and Roberto-Charron, A. 2022. 2021 Aerial survey of the Western Hudson Bay polar bear subpopulation. Final report to the Government of Nunavut, 16 November 2022. pdf here.
Calvert, W., Stirling, I., Schweinsburg, R.E., Lee, L.J., Kolenosky, G.B., Shoesmith, M., Smith, B., Crete, M. and Luttich, S. 1986. Polar bear management in Canada 1982-84. In: Polar Bears: Proceedings of the 9th meeting of the Polar Bear Specialists Group IUCN/SSC, 9-11 August, 1985, Edmonton, Canada. Anonymous (eds). Gland, Switzerland and Cambridge UK, IUCN. pg. 19-34.
Castro de la Guardia, L., Myers, P.G., Derocher, A.E., Lunn, N.J., Terwisscha van Scheltinga, A.D. 2017. Sea ice cycle in western Hudson Bay, Canada, from a polar bear perspective. Marine Ecology Progress Series 564: 225–233. http://www.int-res.com/abstracts/meps/v564/p225-233/
Cherry, S.G., Derocher, A.E., Thiemann, G.W., Lunn, N.J. 2013. Migration phenology and seasonal fidelity of an Arctic marine predator in relation to sea ice dynamics. Journal of Animal Ecology 82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract
Crockford, S.J. 2024. State of the Polar Bear 2023. Briefing Paper 67. Global Warming Policy Foundation, London. Download pdf here.
Derocher, A.E. and Stirling, I. 1992. The population dynamics of polar bears in western Hudson Bay. pg. 1150-1159 in D. R. McCullough and R. H. Barrett, eds. Wildlife 2001: Populations. Elsevier Sci. Publ., London, U.K. See abstract below:
Abstract: Reproductive output of polar bears in western Hudson Bay declined through the 1980’s from higher levels in the 1960’s and 1970’s. Age of first reproduction increased slightly and the rate of litter production declined from 0.45 to 0.35 litters/female/year over the study, indicating that the reproductive interval had increased. Recruitment of cubs to autumn decreased from 0.71 to 0.53 cubs/female/year. Cub mortality increased from the early to late 1980’s. Litter size did not show any significant trend or significant annual variation due to an increase in loss of the whole litter. Mean body weights of females with cubs in the spring and autumn declined significantly. Weights of cubs in the spring did not decline, although weights of both female and male cubs declined over the study. The population is approximately 60% female, possibly due to the sex-biased harvest. Although estimates of population size are not available from the whole period over which we have weight and reproductive data, the changes in reproduction, weight, and cub mortality are consistent with the predictions of a densitydependent response to increasing population size. [my bold]
Derocher, A.E. and Stirling, I. 1995. Temporal variation in reproduction and body mass of polar bears in western Hudson Bay. Canadian Journal of Zoology73:1657-1665. http://www.nrcresearchpress.com/doi/abs/10.1139/z95-197
Northrup, J.M., Howe, E., Lunn, N., Middel, K., Obbard, M.E., Ross, T., Szor, G., Walton, L., and Ware, J. 2022. Southern Hudson Bay polar bear subpopulation aerial survey report. Final report to Ontario Ministry of Natural Resources, 29 November 2022, pdf here.
Ramsay, M.A. and Stirling, I. 1988. Reproductive biology and ecology of female polar bears (Ursus maritimus). Journal of Zoology London 214:601-624. http://onlinelibrary.wiley.com/doi/10.1111/j.1469-7998.1988.tb03762.x/abstract
Stirling, I. and Lunn, N.J. 1997. Environmental fluctuations in arctic marine ecosystems as reflected by variability in reproduction of polar bears and ringed seals. In Ecology of Arctic Environments, Woodin, S.J. and Marquiss, M. (eds), pg. 167-181. Blackwell Science, UK.
Stirling, I., Lunn, N.J. and Iacozza, J. 1999. Long-term trends in the population ecology of polar bears in Western Hudson Bay in relation to climate change. Arctic 52:294-306. pdf here.
Stirling, I., Schweinsburg, R.E., Kolenasky, G.B., Juniper, I., Robertson, R.J., and Luttich, S. 1980. Proceedings of the 7th meeting of the Polar Bear Specialists Group IUCN/SSC, 30 January-1 February, 1979, Copenhagen, Denmark. Gland, Switzerland and Cambridge UK, IUCN., pg. 45-53.http://pbsg.npolar.no/en/meetings/ pdf of except here.
Stroeve, J., Crawford, A., Ferguson, S., Stirling, I., Archer, L., York, G., Babb, D. and Mallet, R. 2024. Ice-free period too long for Southern and Western Hudson Bay polar bear populations if global warming exceeds 1.6 to 2.60 C. Nature Communications Earth & Environment 5:296 [open access] https://doi.org/10.1038/s43247-024-01430-7
Stroeve, J., Holland, M.M., Meier, W., Scambos, T. and Serreze, M. 2007. Arctic sea ice decline: Faster than forecast. Geophysical Research Letters 34:L09501. https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2007GL029703
Stroeve, J.C., Markus, T., Boisert, L., et al. 2014. Changes in Arctic melt season and implications for sea ice loss. Geophysical Research Letters 10.1002/2013GL058951. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2013GL058951
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