If summer ice was critical for S. Beaufort polar bears, 2012 would have decimated them

Did we hear a huge hue and cry in 2013 about starving polar bears and low cub survival in the Southern Beaufort Sea? No, we did not. Despite the record-breaking low summer sea ice extent the year before (2012), and despite the fact that USGS biologists were putting collars on polar bear females there the spring of 2013 (Rode et al. 2014), we heard not a peep about a polar bear catastrophe in the Southern Beaufort. Odd, isn’t it?

Sea ice extent 2012 Southern Beaufort_PolarBearScience

Several polar bear biologists and sea ice experts were busy late last fall suggesting to the media that a decline in polar bear numbers in the Southern Beaufort was due to declines in summer sea ice, which they blamed on global warming (see quotes below and earlier discussions here, here and here). However, they made no mention of the fact that the record-breaking September ice extent in 2012 did not seem to have any noticeable effect on polar bear health or survival in 2013.

Sea ice maps from the National Snow and Ice Data Center (NSIDC) tell most of the story about what the media were, and were not, told about summer sea ice in the Southern Beaufort between 2001 and 2013. 

Background
According to the IUCN Polar Bear Specialist Group’s latest assessment (2013) for the Southern Beaufort (SB):

The SB subpopulation is currently considered to be declining due to a negative trend in sea ice conditions, particularly over the continental shelf, resulting from the continuing effects of climate warming.” [my bold]

What the media was told
Lead author Jeffrey Bromaghin, a research statistician for the United States Geological Survey, was interviewed by the Huffington Post (18 November 2014) about his team’s paper (Bromaghin et al. 2015 now in print 2014) that was published last fall about the latest population count of Southern Beaufort polar bears.

We suspect the primary cause of reduced survival and population decline was starvation, due to climate-induced sea ice conditions that reduced access to seals,” Bromaghin told HuffPost. “During the lengthening open water period, most bears in the southern Beaufort Sea stay on the remnant ice far from shore where few seals are thought to occur.” Essentially, the bears were forced farther away from their food source.” [my bold]

And Steve Amstrup (formerly USGS, now spokesperson for Polar Bears International, known for its “save our sea ice” donation slogan), a co-author of the Bromaghin et al. paper, was interviewed for a 17 November CBC news story:

Because of the shrinking ice in the Beaufort, which is warming faster than any other sea in the world, the team expected to find dropping populations.

“They projected dramatic population declines,” Amstrup said.

“It’s apparent that those declines actually occurred.”

The researchers found that bear numbers had fallen almost in half by the middle of the decade — “really harsh years for bears.”

Between 2003 and 2007, only two of 80 cubs in one of the paper’s datasets survived beyond infancy. Overall numbers of bears fell to about 900. … [Amstrup said]… “In the case of the Beaufort, we can probably expect continued declines.” [my bold]

[Yes, well – except that the authors only analyzed data up to 2010, and according to a later US Fish & Wildlife survey, by the fall of 2012 numbers were higher than they had been in a decade even though ice extent had hit a record-breaking low that summer. Also, in the paper itself, the authors acknowledged that the decline in survival experienced in the mid-2000s was due to heavy spring sea ice]

And finally, in another story (17 November 2014, The Telegram), sea ice expert Mark Serreze (director of the National Snow and Ice Data Center in Colorado) was asked to weigh in on the apparent decline in polar bear numbers:

“And 2007 was “a wake-up call” for scientists, he [Serreze] said, because sea ice shrank to a low scientists had not expected or seen before. Sea ice levels dropped even lower in 2012 and have recovered a tad since.

There is definitely a relationship here between what’s happening to the bears and what’s happening to the ice,” said Serreze, who wasn’t part of the study.” [my bold]

[If so, why weren’t Southern Beaufort polar bears decimated by the record-breaking low in 2012? Did that not occur to him?]

What the sea ice maps tell us
Figure 1 below shows the September minimum ice extent compared to the average extent for September for 2012 (the record-breaking annual minimum) and 2007 (the second-lowest). As you can see, there is a bit of difference but not much (click to enlarge) — I show this because for most years previous to 2007, only the average for September is available from the NSIDC archive.

Sea ice extent 2012 vs 2007 comparison Southern Beaufort

Figure 1. September minimum ice extent 2012 compared to the average September extent for 2012, and the same for 2007. Southern Beaufort polar bear subpopulation area is marked in yellow along the shoreline. Click to enlarge.

The map below (Figure 2) shows the differences in September averages for 2002 through 2007, years covered by the Bromaghin et al. 2015 2014 paper. Note that 2007 was when polar bear survival began to recover, even though it had the lowest summer ice extent seen (so far) since 1979.

Sea ice extent 2002-2007 comparison Southern Beaufort

Figure 2. Differences in September averages for 2002 through 2007, years covered by the Bromaghin et al. 2014 2015 paper. Notes about polar bear abundance and survival are from that paper. Click to enlarge.

Figure 3 (below) shows the length of the open water period in 2012. Note that by the end of July, the ice was receding fast (and was well offshore by 13 August, see inset map), and lasted until early November (inset map for October shows that by 6 November, the ice had almost enveloped the coast). It was the longest open-water period seen in the Southern Beaufort since at least 1979.

Sea ice extent 2012 season by season Southern Beaufort

Figure 3. The length of the open water period in 2012, from July to early November. By the end of July, the ice was receding fast; it was well offshore by 13 August, see inset map). By then end of October (lower right), the ice was still well offshore but fast encroaching, as the inset map for 6 November shows). Click to enlarge.

In 2007 (shown in Figure 4), the length of the open-water season was clearly not as long as in 2012. But note that in 2007, by the time that there was ice against the Southern Beaufort coast in early November (lower right insert map), there was still open water in the Chukchi Sea (where polar bears were found to be in good condition and reproducing well).

Sea ice extent 2007 season by season Southern Beaufort

Figure 4 The length of the open water period in 2007. Note that by the end of July, the ice was well offshore and was only just forming onshore by end of October. Note that by November, there was still open water in the Chukchi Sea. Click to enlarge.

Conclusions
In short, the reason why polar bears in the Southern Beaufort Sea were not decimated by the extraordinarily long open-water seasons in 2012 is because summer ice extent matters very little to polar bear health and survival. It’s the same reason that Chukchi Sea bears are doing so well despite some of the most extensive declines in summer sea ice of all subpopulations — spring ice conditions are what matter to polar bears.

And as the Bromaghin et al. paper has shown yet again (which I’ve said before) — thick ice in spring is what causes polar bears to starve.

Beaufort Sea pressure ridges_Spring 1949 wikipedia sm

References
Bromaghin, J.F., McDonald, T.L., Stirling, I., Derocher, A.E., Richardson, E.S., Rehehr, E.V., Douglas, D.C., Durner, G.M., Atwood, T. and Amstrup, S.C. 2015 2014 in press. Polar bear population dynamics in the southern Beaufort Sea during a period of sea ice decline. Ecological Applications 25(3):634-651. http://www.esajournals.org/doi/abs/10.1890/14-1129.1 Open Access.

Rode, K. D., Pagano, A.M., Bromaghin, J.F., Atwood, T.C., Durner, G.M. and Simac K.S. 2014. in press. Effects of capturing and collaring on polar bears: Findings from long-term research on the southern Beaufort population. Wildlife Research 41(4):311-322. [paywalled]
http://www.publish.csiro.au/nid/144/paper/WR13225.htm Supplementary Info: 10.1071/WR13225_AC Pdf here.

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