The long-term survival of polar bears is threatened by loss of sea-ice due to global warming, new study confirms

Péter Molnár

Assistant Professor, University of Toronto Scarborough

The article is accurate and summarizes our findings very well. I only have one extremely minor comment, which doesn’t affect the overall accuracy of this article. In the second paragraph, the author writes that “the loss of sea ice would force the animals onto land and away from their food supplies for longer periods”. In some cases (e.g. the Beaufort Sea populations), the animals may choose not to go on land but instead follow the ice as it retreats over the Arctic Basin. As these are deep, dark, unproductive waters, the effect is therefore the same as going on land (food-deprivation), and this omission does not affect the overall accuracy of the article.

Andrew Derocher

Professor, University of Alberta

The statement of “nearly extinct” is consistent with the findings of Hamilton et al. (2014) which stated “Under business-as-usual climate projections, polar bears may face starvation and reproductive failure across the entire Archipelago by the year 2100.”^[8] Further, the study concluded with “Nevertheless, by 2100 all regions of the study area may cross the critical point-of-no-return, putting the persistence of the CAA [Canadian Arctic Archipelago] polar bear populations in jeopardy.” This study examined the Canadian Arctic Archipelago populations where sea ice is expected to persist the longest.

Castro de la Guardia et al. (2013) examined sea ice in Hudson Bay and that study stated “Our predictions suggest that the habitat of polar bears in WH [Western Hudson Bay] will deteriorate in the 21st century. Ice predictions in A1B [medium greenhouse gas emissions] and A2 [high greenhouse gas emissions] suggest that the polar bear population may struggle to persist after ca. 2050. Predictions under B1 [low greenhouse gas emissions] suggest that reducing GHG emissions could allow polar bears to persist in WH throughout the 21^st century.”^[9]

Thus, the Molnár et al. study is supported by both Hamilton et al. (2014) and Castro de la Guardia et al. (2013)^[1,8,9]. The time frame and approach in these two studies is similar to that of Molnár et al. and use a fasting duration limit relative to sea ice loss estimation.

Regehr et al. (2016) stated “The estimated probabilities that reductions in the mean global population size of polar bears will be greater than 30%, 50% and 80% over three generations (35–41 years) were 0.71 (range 0.20–0.95), 0.07 (range 0–0.35) and less than 0.01 (range 0 – 0.02), respectively.”^[10] Given that Molnár et al. consider a much longer timeframe and that it considers a severe climate change scenario (RCP 8.5) as the upper bound, the study is consistent with Regehr et al. (2016). They also examined a lower climate change scenario (RCP 4.5), which shows impacts on polar bears that would be consistent with projected changes noted by Regehr et al. (2016).^[1,10]

The statements about extended fasting, shorter feeding period, and traveling more, are consistent with the broader literature addressed in the review by Stirling and Derocher (2012) and both empirical and projection studies by Stirling et al. (1999), Lunn et al. (2016), Pilfold et al. (2016), and Durner et al. (2017)^[11,5,12,13].

In summary, I find that the statement is consistent with the published work being addressed and that the statement overall is consistent with the broader peer-reviewed literature on polar bears.

Ian Stirling

Adjunct Professor, University of Alberta

Overall, I would say Molnar et al. 2020 should be regarded as scientifically correct, quantitative in terms of the data analyzed, and analytically supported^[1].

I started and maintained the long-term monitoring of the polar bear population in Western Hudson Bay (WH). My early results were clear enough by the late 1990s for the analyses to be the first to confirm the negative effects of climate warming on polar bears. WH is the longest continuous data base on the effects of climate warming on polar bear abundance, survival, and reproduction anywhere in the Arctic. When I initially started the long-term monitoring of polar bears there it was because I wanted to learn more about the amount of natural variation inherent in the polar marine ecosystem because of its obvious importance to conservation practises. I had already seen the results of large scale interannual changes in the ecosystem on polar bears and ringed seals in the Arctic (Beaufort Sea) and on Weddell seals in the Antarctic and those experiences were enough to convince me that understanding more about natural fluctuations in the ecosystem was something we really needed to know a lot more about. It was only by the mid-1990s (the period identified by Molnar) that we began to realize from our data that something else was going on besides possible natural cycles and, because we had a long-term database, and the ability to continue the monitoring, that we were able to identify and confirm what was happening because of climate warming.

I think it is fair to say that the physiological parameters with respect to reproduction and body condition, etc. measured in WH could be reasonably applied to polar bears elsewhere. What such focused models projecting into the future do not take into account overall though of course is that the whole arctic marine ecosystem (and the interrelationships of all species) is changing because of climate warming and the speed of such changes will likely accelerate as time progresses so exactly what might happen specifically to polar bears as an icon species becomes more difficult to predict precisely the farther out we get. That said, the modelling approach remains invaluable for what it clearly indicates is coming our way soon, regardless.

I think a major strength of the Molnar paper is that it correctly assesses the populations (for which there are sufficient data) independently rather than as a single world population^[1]. That appears to be the major weakness in the Hamilton et al. (2014) paper, which comes to the same conclusion as Molnar (see their Conclusions/Significance statement) but is based mainly only on bears in the Canadian Arctic Archipelago and treats the subpopulations resident there as a single population and not as several independent populations^[8]. Still, the fact that these two papers basically come to a more or less identical conclusion, via different routes and levels of analysis is interesting.

While I think the Molnar paper is quite correct overall, I think it overlooks two important ecological situations which don’t weaken their overall conclusions but could have given them a bit more ecological depth of understanding of some important qualifications for some populations and might have been important to include, though the space restrictions in journals like Nature might preclude doing so.

In my opinion, a common flaw in almost all (if not all) the big overview papers that predict bears having problems loss of ice because of climate warming and then becoming extinct, or nearly so, by some date is that they tend to make a 1:1 relationship between the area of ice and its decline and the subsequent decline in the bears for nutritional reasons. However, ecologically, the relationship is much more complicated and geographically variable than a simple 1:1 relationship. Ringed seals, which are the major food source of polar bears throughout their range are reliant on having hard wind blown snow over the breathing holes of the adult females in their prime breeding habitat for giving birth to their young^[14]. With the warmer weather and rain that will appear in most areas, probably at significantly increased levels well before 2100, the devastating decline in ringed seal reproduction and survival will have an immediate negative effect on the bears’ nutrition, survival, and reproduction^[15]. We already know that reproductive failure of ringed seals (as seen on decadal scale intervals in the southern Beaufort Sea^[16]) causes immediate and very strong negative effects on reproduction and survival of young polar bears as well as lowering the body condition in adults^[17]. If such levels of loss of ringed seal pups is significant because of climate for several years in some populations, the negative effects on the bears there will be significant.

Also, there are major differences in the availability of different prey species for polar bears in different populations^[18]. In particular, the role of some of these prey species to provide carrion in some populations that could help a smaller number of bears survive for a protracted period has been overlooked. For example, the potential for walrus populations (which will be less negatively impacted by climate warming in some areas) to provide a long-term potential food source, at predictable locations, for both predation and scavenging in several areas. Similarly, where whale carcasses resulting from natural mortality occur in predictable locations, they may provide a long-term food source for bears in areas such as Chukotka^[19].