It’s like pulling teeth, getting up-to-date information on breakup dates of Hudson Bay sea ice. You’d think with the importance of this seasonal event to the polar bears of Hudson Bay (some of which, we’re told, face the most perilous future of all polar bears worldwide), we’d get a press release every summer alerting us to the precise date of sea ice breakup and the subsequent arrival of the bears onshore. I’m thinking of something similar to the dispatches we get from the National Snow and Ice Data Center (NSIDC) when Arctic sea ice reaches its yearly maximum (e.g. March 2012) and its yearly minimum extent (e.g. September 2012).
Sadly, this is not the case.
So I was intrigued to see that a new paper just out in the Journal of Animal Ecology, by Seth Cherry (a Ph.D. student of Andrew Derocher) and colleagues, dealt with Hudson Bay breakup dates. I was hoping for some data beyond 2007, which has been the limit of information provided so far by polar bear biologists (see previous posts here and here). Unfortunately, because the methods for determining breakup dates in this paper are so different from previous ones (more detail below), the new data (1991-2009) can’t be compared to earlier studies that go back to 1979. But there is some good news.
Although you wouldn’t know it by the author’s conclusion, the results of the study confirm for this region my previously stated contention that polar bears need spring and early summer ice (March through June) for gorging on young, fat seals and documented declines in sea ice have rarely impinged on that critical feeding period – by which I meant, bears have seldom, if ever, been forced off the ice of Hudson Bay as early as June.
The study also confirms that there has not been any kind of spectacular retreat of sea ice breakup dates – coming earlier and earlier in the season – over the last 19 years and that polar bears did not arrive on shore in 2009 until very late – approximately 22 August – the same date they came ashore in 1992.
Below are two figures from the paper: one (the map) necessary to understand the new “Cherry method” of calculating breakup dates for Hudson Bay (no disrespect intended) and the other (the graph), which presents the data collected. The table with my converted breakup dates is below them. A few quotes from the study and some comments on it follows.
UPDATE (March 21 2013; 6:12 PM PST). A typo in the table has been fixed (2001 is Jun 21 not Jul 21) and the post amended accordingly.
UPDATE 2 (July 7, 2013) The Cherry et al. paper is now in print, the references have been updated accordingly.
For this study, the researchers put satellite collars on a relatively small sample of adult female polar bears between 1991-1997 and 2004-2009 (none in 1998-2004) to see when they came ashore in summer and when they went back out on the ice in the fall. They tested models to see what concentration of sea ice best predicted when polar bears came ashore and when they left.
Figure 1. This is figure 1 from Cherry et al. (2013, in press). The authors said they used “daily sea ice concentrations approximated from daily Special Sensor Microwave/Imager (SSM/I) passive microwave data obtained from the National Snow and Ice Data Center (Boulder, Colorado). Sea ice concentrations were analysed at a resolution of 25 × 25 km cells. Daily sea ice concentrations were determined using the mean concentration value from all pixels within analysis-specific regions of interest. …Ice concentrations for these analyses were derived from an area within a 95% minimum convex polygon (MCP) determined from on-ice polar bear locations collected throughout the entire study (Fig. 1).”
No wonder only polar bear biologists calculate breakup dates.
Figure 2. This is figure 2 from Cherry et al (2013, in press). The original caption reads: “Dates of sea ice concentrations that best corresponded to dates ashore during break-up and departure from shore dates during freeze-up for collared polar bears in western Hudson Bay. Dates ashore and departure from shore dates are shown as annual means for all collared polar bears.” Note that the bottom line shows breakup dates (open triangles) and the line above it (filled triangles), the date collared polar bears arrived on shore. Note two things: 1) arrival on shore in 2009 appears to be as late as 1992, approximately Julian date 234; 2) there is no discernible trend in breakup dates over the 19 years of the study.
When I calculated the breakup dates, shown in the table below, leap years were accounted for but the dates may still be off by a day or two because of having to read the Julian dates off a graph. However, as you can see, June 17 appears to be the earliest date of breakup between 1991 and 2009, and July 30 was the latest. Only 8 9 out of 19 breakup dates occurred in June and none of those fell in the first half of the month.
The authors explain how their new method of calculating breakup dates differed from the old one:
Previously, annual mean arrival ashore for polar bears in western Hudson Bay was found to be correlated with the timing of 50% ice concentration in regions approximated by the Western Hudson Bay population management zone (Stirling, Lunn & Iacozza 1999). However, in this study we tested a range of sea ice concentration increments and accounted for the rate of ice disappearance to determine a suite of factors that best predicted the timing of polar bear migration on and off land. Using these multivariate methods that included the rate of ice disappearance and higher resolution (SSM/I) daily sea ice data, we found that polar bear dates ashore were best correlated with the timing of 30% sea ice concentration during break-up. We additionally determined that timing of 10% ice concentration during freeze-up, when considered with rate of ice appearance, was correlated with polar bear departure from shore dates (Fig. 2)…Throughout the study, bears arrived ashore a mean of 28.3 day (S.E. = 1.8) after 30% ice cover (Fig. 2)… Collared polar bears departed from shore an average of 2.5 days, S.E. = 0.7, after 10% freeze-up (Fig. 2). [my bold]
So, the data show that polar bears do not leave the ice until almost a month after the breakup date (average 28.3 days). So, for the earliest breakup date recorded (17 June), the polar bears they studied did not start their summer fast until the middle of July. But in 2009, as the graph (Fig. 2) shows, the bears stayed on the ice about two weeks longer than average and did not go ashore until the 22nd of August.
The authors also make this proud statement:
These results represent the first quantitative evidence of the relationship between sea ice concentrations and polar bear departure from onshore refugia anywhere in the Arctic. [my bold]
Well, it seems pretty obvious to me that there is a very close relationship between when sea ice develops in the fall and when polar bears are able to use it as a hunting and traveling platform. But now they have numbers and are very proud of it.
And why do they need numbers? For the models, of course – models don’t work without numbers! Ultimately, the whole point of the study (for them) seems to be that it gives them the ability to make “better” crystal-ball predictions about how much danger polar bears could be exposed to, some time in the future, because of global warming.
However, we don’t have to accept the conclusions the authors make about what is important about their study – we have the data they generated. The data confirm that there has not been any kind of spectacular retreat of sea ice breakup dates – coming earlier and earlier in the season – over the last 19 years. It also confirms that polar bears did not arrive on shore in 2009 until very late in the season – approximately 22 August – the same date they came ashore in 1992.
The massive June 1991 eruption of Mount Pinatubo, is said to have caused extra cold conditions in the Arctic the winter of 1991-1992 (Chambellant et al. 2012) – but what caused the cold of 2009? Could it have been natural variation?
References
Chambellant, M., Stirling, I., Gough, W.A. and Ferguson, S.H. 2012. Temporal variations in Hudson Bay ringed seal (Phoca hispida) life-history parameters in relation to environment. Journal of Mammalogy 93:267-281.
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(4):912-921.
http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract
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