In the last few days, ice coverage on Western Hudson Bay finally dropped below the 30% level that now defines ‘breakup’ for polar bears: a few bears near Churchill started to come ashore late last week but most will stay on the ice until the end of July. That means breakup this year was unofficially July 8th, a week later than average (July 1) for the third year in a row.
Don’t’ tell that to the folks at Polar Bears International, though, because they’re busy telling people that the ice-free season for Western Hudson Bay bears is now longer than it was before the 1990s. What they mean is that the overall trend is toward early breakup dates.
But what they don’t admit is that over the last 44 years, breakup was a full two weeks earlier than average for Western Hudson Bay only six times and only three of those early breakups occurred within the last 13 years. See the calculations below and see what you think.
The latest, most accurate method of calculating sea ice breakup for Western Hudson Bay (WHB) that’s relevant for polar bears gives an average breakup date of July 1, based on data for 1991 to 2009.
That method is described in Cherry et al. 2013 (see Table 1 and Fig. 1 below, described in full here). In short, breakup was defined by the study as the date in spring when ice coverage reaches 30% over Western Hudson Bay and freeze-up, the date at 10% coverage in the fall.
The map below shows the ice area defined by Cherry and colleagues to calculate the ice coverage levels for Western Hudson Bay as an insert, with that approximate area overlaid (dark pink line, divided approximately in half to show the ~50% level) onto the daily sea ice map for July 8 generated by the Canadian Ice Service (CIS). This looks to me to show approximately 30% ice within the area defined as WHB — compare it to the map above for July 7, when there was clearly much more ice than that. That means we can roughly call breakup this year for July 8.
Breakup occurred after July 1st last year as well.
I’ve had to make my own approximation of recent breakup dates for Western Hudson Bay (2010-2013), because an update of breakup dates after 2009 has not been published using the new method.1
I discovered that you can get a good approximation of when the 30% coverage level for the WHB portion of the bay occurs by looking at CIS data for percentage coverage for the whole of Hudson Bay for the week of July 2. CIS can calculate those figures back to 1971 (Fig. 4 below, from the CIS website here).
Comparing Fig. 4 to Table 1 (breakup dates calculated from Cherry et al.) shows that Hudson Bay ice coverage as a whole was probably close to 20% by the time that WHB coverage is reduced to 30%, because of the peculiar way the ice melts in this basin. In Fig. 4, years when the graph shows levels ~20% or less for the week of July 2 for the entire Hudson Bay agree well with ‘early’ Western Hudson Bay breakup dates shown in Table 1 (on or around 17 June): 1998, 1999, 2001, and 2006.
For more recent years, it appears that 2010 and 2011 were also earlier than average. According to one source, in 2010 most bears were off the ice by mid-July – back-calculating using the Cherry et al. method suggests breakup of the ice may have been as early as 1999.
So, until we have additional published breakup dates using the Cherry et al. method, it appears we can use the 20% coverage of ice remaining on Hudson Bay as a whole (for the week of July 2), as a proxy for approximate breakup dates for Western Hudson Bay: years when breakup dates are ~2 weeks earlier than average show ~20% ice coverage or less on this graph, while years when breakup dates are average (July 1) or later, had more than 20% ice coverage for the week of July 2.
This means over the last 44 years, breakup dates were two weeks earlier than average in six years only. Since 2009 (the last year of published breakup dates), two years have experienced breakup dates that were ~2 weeks earlier than average, while breakup dates for the last three years (2012-2014) have been average or later.
Three of those early breakup years (1998, 1999 and 2001) occurred during the portion of the last mark-recapture study of WHB bears (1998-2004) that registered a 22% decline in numbers (Obbard et al. 2010:58; Regher et al. 2007). That might have made a difference, don’t you think?
If “breakup” was July 8th this year, based on sea ice coverage, how long will it take for all bears to leave the ice?
According to the research conducted by Seth Cherry and colleagues:
“Throughout the study, bears arrived ashore a mean of 28.3 day (S.E. = 1.8) after 30% ice cover.”
Presumably, this means the date when all their collared bears were on shore, although this is not explicitly stated. So, because bears don’t all come ashore at the same time, we can expect a wide range of dates for individual bears to leave the ice, depending on their location during the melt sequence.
For example, bears that are further north (around Churchill), may leave the ice early as it melts around them, rather than move south with the ice and continue hunting (see Figs. 2 and 5 for the location of Churchill relative to the remnant ice).
This means bears leaving the ice around Churchill early in the season may face a longer ice-free period than others but in many cases, that’s due to their own choice and/or poor decision-making, not because the melting ice absolutely forced them ashore early.
Over at his new and improved Polarbearalley blog, Kelsey Eliasson says a few bears were spotted coming ashore at Churchill on July 3.
“While some bears are ashore early, others are still out on the ice riding it further south. I would say that mothers and cubs start coming ashore in Wapusk National Park next week (maybe July 9th or 10th?) and the last bears ride it out until late July. However, the bulk of this population will end up far south…” [my bold – see Fig. 5 for location of the park]
Kelsey adds later:
“Each year, for about a week a good number of bears can be spotted swimming ashore in July. These are bears that either by choice, accident or simply geography have ended up on the Churchill side of the last sea ice and decided to swim ashore instead of heading back out into Hudson Bay to search of the dwindling pack ice.”
This also means that people living in (or visiting) Churchill always see bears ashore earlier than the bulk of the population actually comes ashore, perhaps giving the impression that the official date of breakup (as it relates to Western Hudson Bay polar bears) is earlier in the season than it actually is (see PBI reports here and here).
In conclusion, I’ve come up with three issues that have not been addressed about Western Hudson Bay ice breakup and its effect on polar bears:
• Why do we still not have published up-to-date breakup and freeze-up data for Western Hudson Bay for 2010-2013? This could be published every year as a short research note in one of the Arctic journals – if breakup dates are so critical for WHB polar bears, why isn’t this done so that we can all see the data?
• Where is the published data, using the new “Cherry” method (30% coverage for WHB breakup/10% coverage for freeze-up), that presents the number of ice-free days experienced by WHB bears for 1991-2009 or later? Number of ice-free days is a metric bandied about by journalists, NGOs, and polar bear biologists alike but the reality is, there is no published source for these numbers.
• Is an overall trend in breakup dates since 1979 really the same as highly variable breakup dates since 1992, as it relates to polar bear health? In other words, has the pattern we have seen in Western Hudson Bay (1-2 years when breakup was ~2 weeks early, followed by 2-3 years of average or late breakup dates, i.e., Fig. 4) really had the same effect on polar bear health over the long term as it would have if breakup dates had come relentlessly earlier and earlier over time (as implied by the statistical trend line and interpreted as “now two weeks earlier than 1980”)? Polar bear biologists imply the answer is yes – that these two phenomenon would have the same effect. But I would like to know if it’s really true or not. Are bears that have had 2-3 years to recover from a year or two of early breakup conditions really more likely to starve or lose their cubs the next time breakup comes early (because they have experienced an early breakup before), as assumed?
Things to think about and ask a polar bear biologist, if you have the chance.
Footnote: No breakup dates prior to 1991 have been back-calculated using this new method, which means that there is only 19 years worth of published Western Hudson Bay breakup data relevant for polar bears. Note this also means that papers using the old method of defining breakup for Western Hudson Bay, when the ice falls to 50% ice coverage (e.g. Stirling, Lunn & Iacozza 1999; Stirling and Parkinson 2006; Gagnon and Gough 2005; Scott and Marshall 2010) are now out of date for polar bear research.
Therefore, comments and/or research based on those older papers (e.g. Derocher et al. 2013; Stirling and Derocher 2012; Robbins et al. 2012) are no longer valid, including calculations of the ice-free period as well as trends in breakup and freeze-up dates.
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 Ecology82:912-921. http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12050/abstract
Derocher, A.E., Aars, J., Amstrup, S.C., Cutting, A., Lunn, N.J., Molnár, P.K., Obbard, M.E., Stirling, I., Thiemann, G.W., Vongraven, D., Wiig, Ø., and York, G. 2013. Rapid ecosystem change and polar bear conservation. Conservation Letters 6:368-375. http://onlinelibrary.wiley.com/doi/10.1111/conl.12009/abstract
Gagnon, A.S. and Gough, W.A. 2005. Trends in the dates of ice freeze-up and breakup over Hudson Bay, Canada. Arctic 58: 370-382. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/451
Regehr, E.V., Lunn, N.J., Amstrup, S.C., and Stirling, I. 2007. Effects of earlier sea ice breakup on survival and population size of polar bears in Western Hudson Bay. Journal of Wildlife Management71: 2673-2683. http://onlinelibrary.wiley.com/doi/10.2193/2006-180/abstract
Robbins, C.T., Lopez-Alfaro, C., Rode, K.D., Tøien, Ø., and Nelson, O.L. 2012. Hibernation and seasonal fasting in bears: the energetic costs and consequences for polar bears. Journal of Mammalogy 93(6):1493-1503. http://www.asmjournals.org/doi/abs/10.1644/11-MAMM-A-406.1
Scott, J.B.T. and Marshall, G.J. 2010. A step-change in the date of sea-ice breakup in western Hudson Bay. Arctic 63:155-164. Available here (open access) http://arctic.synergiesprairies.ca/arctic/index.php/arctic/issue/view/55
Stirling, I. and Derocher, A.E. 2012. Effects of climate warming on polar bears: a review of the evidence. Global Change Biology 18:2694-2706. doi:10.1111/j.1365-2486.2012.02753.x
Stirling, I., Lunn, N.J., Iacozza, J., Elliott, C., and Obbard, M. 2004. Polar bear distribution and abundance on the southwestern Hudson Bay coast during open water season, in relation to population trends and annual ice patterns. Arctic 57:15-26. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/479/509</a
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. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/article/view/935/960
Stirling, I. and Parkinson, C.L. 2006. Possible effects of climate warming on selected populations of polar bears (Ursus maritimus) in the Canadian Arctic. Arctic 59:261-275. http://arctic.synergiesprairies.ca/arctic/index.php/arctic/issue/view/16.