Even though it’s in graph form only, we finally have an update on sea ice freeze-up dates for Western Hudson Bay for 2016-2020 (but not breakup dates).
This graphed data published by Miller et al. 2022 extends by five years that published in 2017 by Castro de la Guardia and colleagues, which contained graphed data for breakup and freeze-up dates from 1979-2015 (with exact dates for 2005-2008 only).
It confirms a statement I made last month, that between 2016 and 2021 “there has been only one ‘late’ freeze-up year (2016)–but five very early ones.” Of course, 2021 was not included in this new dataset, so that would be “four very early ones” up to 2020.
Here it is, as part of Figure 2. The freeze-up data is upper left (a):
Here is that tiny panel (a) on its own, enlarged (a screencap from the pdf), showing the trend line from1991-2020:
Extracting numbers off graphs is time-consuming and often inaccurate, but apparently polar bear specialists don’t care: that is all the public and their colleagues are being given.
In the lists below, the first number is the year, the second number is the Julian day of the year (e.g. 313), and the third is the calendar date of the corresponding Julian day from the Miller paper vs. the de al Guardia paper. For the last five years, I’ve added my own assessment made at the time, and because this is a screencap, here are the links for my estimates: for 2020 (when bears were killing seals on offshore sea ice by 31 Oct), 2019, 2018, 2017, 2016. I didn’t transcribe all of the pre-2005 dates, only a select few.
Most of entries in the period of overlap between the two datasets are either identical or vary by only 1-3 days. However, those in bold vary by almost a week or more (e.g. 2008, 2011), even though there is no explanation in the Miller paper regarding why that would be so:
As you can see, the earliest freeze-up year was 1993 (although 1991, 1986, and 1978 were almost that early) and the latest apparently in 2016 (although 2009, 1998, and 1981 were almost as late depending on which dataset you look at).
Given the potential error rate of 2-3 days either way, freeze-up was as early in 2020 as it had been in 1978, 1979, 1986, 1991, and 1993 (the earliest on record); freeze-up dates in 2017, 2018, and 2019 were the same as the average in the 1980s (de la Guardia et al. 2017).
Overall, Miller and colleagues found no temporal trend in sea ice or departure dates of polar bears from shore between 1991 and 2020, and perhaps counter-intuitively, that bears departed for the ice earlier in years when freeze-up was earlier.
This means WH sea ice coverage in the fall has not been ‘steadily declining’ over the last 30 years and polar bear have not been departing for the ice later and later in the season over that period, as many imply.
However, although the authors collected long-overdue data on body condition of females with cubs and independent juveniles, these values are reported only as a ‘vulnerability index’ which is impossible to compare with raw data collected in the 1970s and 1980s.
While these indices indicate that the body condition of females with cubs-of-the-year (but not those with yearling cubs) “declined over the last 30 years“, it is impossible to say by how much compared to detailed studies done prior to the 1990s (eg. Derocher and Stirling 1992, 1995; Ramsay and Stirling 1988) or to those used to justify classifying polar bears as ‘threatened’ on the US Endangered Species List (Regehr et al. 2007).
In other words, body weight data–so critical to the argument that the health of WH polar bears is declining due to sea ice loss–is still being withheld.
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/
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.
Derocher, A.E. and Stirling, I. 1995. Temporal variation in reproduction and body mass of polar bears in western Hudson Bay. Canadian Journal of Zoology 73:1657-1665. http://www.nrcresearchpress.com/doi/abs/10.1139/z95-197
Miller, E.N., Lunn, N.J., McGeachy, D., and Derocher, A.E. 2022. Autumn migration phenology of polar bears (Ursus maritimus) in Hudson Bay, Canada. Polar Biology 45:1023-1034.
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
Regehr, E.V., Lunn, N.J., Amstrup, S.C. & Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management 71: 2673-2683. Paywalled, subscription required. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract