Polar bears and climate change?
M. G. Dyck 1, 2 and W. Soon3
1
Natural Resources Institute, University of Manitoba, Winnipeg, Manitoba R3T 2N2 Canada
2
Correspondence address:
Department of Sustainable Development
Wildlife Division
Box 1000, Station 1170
Iqaluit, Nunavut
X0A 0A0 Canada
Ph: (867) 975-5943
Fx: (867) 975-5981
Email: MDYCK@GOV.NU.CA
3
Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, 02138, USA
1
Polar bears (Ursus maritimus) are charismatic megafauna and the symbol of the Arctic.
For Canadian Inuit people, polar bears play an important cultural and traditional role in their lives
through hunting activities and the sharing of meat. Additionally, Inuit-guided sport hunts provide
crucial revenue for the otherwise economically-challenged communities (Lee and Taylor 1994).
However, if one believes latest research findings, this multi-purpose natural resource faces threats
by climatic change (Stirling and Derocher 1993; Stirling et al. 1999; World Wide Fund for
Nature 2002).
Long-term ecological studies on polar bears of western Hudson Bay (WH) indicated that
this population has been exhibiting a decrease in female body mass, litter sizes, and cub survival
(Derocher and Stirling 1992; 1995). It was suggested that the proximate factor for the decline in
these physical and reproductive parameters between 1981 through 1998 was a change in freeze
and thaw cycles of Hudson Bay causing bears to reach the shore in progressively poorer
condition. A long-term warming trend of spring atmospheric temperatures was proposed, though
not shown directly1, to be the ultimate factor (Stirling et al. 1999). The WH results were used and
applied to generate general predictions for polar bears and their habitat (Stirling and Derocher
1993; World Wide Fund for Nature 2002).
Figure 1a shows the surface air temperature records of Churchill, Manitoba2 (assumed
here to be representative of WH) for four climatological seasons from 1932-2002. The large
interannual variability of the seasonal temperatures suggests that confirming a long-term trend in
any of these relatively short records will be difficult and the trend determination will be
sensitively dependent on the time interval considered. Figure 1b attests that no statistically
significant temperature trend can be confirmed for either the spring (defined specifically here as
1
Stirling et al. (1999) inferred from the mean air temperature results of Skinner et al. (1998).
2
averages of April, May and June to follow discussion in Stirling et al. 1999) or fall seasons when
the full records from 1932 through 2002 or 1932 through 1992 were considered. A clear tendency
towards spring warming did occurred from 1981 to about 1999 (marked by the dashed trend
curve in Figure 1b) but that tendency did not persist nor any systematics observed with data
updated through 2002 (yet we make no further confident inference about any concurrent
ecological responses). Thus, although our independent results for temperature change and
variability over the WH do not contradict Stirling et al. (1999), but the full range of temperature
variability we observed argues for a less dominating role of spring temperature as the explanatory
variable of the population ecology of polar bears.
Figure 2 shows the records of winter and spring temperatures at Frobisher Bay (Iqaluit,
Nunavut) and their respective winter and spring Arctic Oscillation (AO) circulation indices3 with
two important points. First note the rather strong cooling trend (at a rate of about 0.4ºC per
decade since the 1950s) for the winter and spring temperatures of Frobisher Bay by the Hudson
Strait. So regional differences in the pattern of the temperature variability, especially on
multidecadal time-scale, are large. This pattern of large temperature gradient between the
southwestern and northeastern corners of the Hudson Bay oceanic basin has been previously
noted by Skinner et al. (1998)―whom had also provided a brief but comprehensive discussion on
2
Our data source is based on the quality-controlled version of records from the NASA Goddard Institute for Space
Studies web site: http://www.giss.nasa.gov/data/update/gistemp/station_data/.
3
Arctic Oscillation (AO) is a natural, planetary-scale pattern or mode of atmospheric circulation variability that is
characterized by a seasaw of air mass anomaly between the Arctic basin and the midlatitude zonal ring centered at
about 45º N. High (positive) AO value is defined as lower-than-normal atmospheric pressure over the Arctic and
colder stratosphere which are associated with strong subpolar westerlies while the low (negative) AO value
represents higher-than-normal Arctic atmospheric pressure, less cold polar stratosphere and weak subpolar
westerlies. The AO index is available from http://horizon.atmos.colostate.edu/ao/Data/index.html. Although the AO
is mainly a winter phenomenon because of the relatively larger variability and stronger coupling of stratospheric and
tropospheric air circulation during the cold season, the relevance of AO has been demonstrated in other seasons as
well (Gong and Ho 2003; Kryjov 2002; Overland et al. 2002). Please see Wallace (2000), Baldwin (2001) and
Thompson and Wallace (2001) for the complete tutorials. Although there have been several attempts to suggest that
the post-1969 or post-1989 tendency of the AO index to remain in the high positive AO phase is a result of the
3
climate regimes around the western Hudson Bay. Therefore the proposal of a warming spring
temperatures, regardless if the fact may holds for any particular time period, for the polar bear
population ecology cannot be universally applied elsewhere at other locations. The second point
of Figure 2 is that the temperature and climatic conditions around the Hudson Strait and Hudson
Bay areas have close association with the AO circulation index. The correlations shown in Figure
2 are highly significant with the AO variability explaining up to 20 to 50% of the temperature
variances at Frobisher Bay.
Why the labor of introducing and discussing the AO circulation index? Two reasons.
First, from the examination of the statistics of sea level pressure and sea ice motion from the
1979-1998 data collected by the International Arctic Buoy Programme, Rigor et al. (2002)
confirmed that AO circulation pattern can explain at least part of the thinning sea ice trend
observed over the Arctic Ocean. But Rigor et al. (2002) clarified that instead of assuming the
warming trend in the surface air temperature to cause the thinning of sea ice, it is the AO
circulation pattern that produces the sea ice thinning tendency. In turn, it was the thinning of sea
ice that caused the air temperature to warm because of increasing heat flux from the interface of
ice-free ocean. Thus this physical picture is indeed different from Stirling et al. (1999)’s proposal
in which warm spring air temperature is considered to be the ultimate root cause for the earlier
spring sea ice breakup and poorer polar bear conditions. The second reason for our discussion of
the AO circulation index is related to a recent finding of the AO index-climatic change effects
being propagated through two trophic levels within a high-arctic ecosystem (Aanes et al. 2002).
From the statistical analyses of the1987-1998 growth series of Cassiope tetragona (Lapland
Cassiope) and the 1978-1998 abundance series of an introduced Svalbard reindeer population
forcing by anthropogenic carbon dioxide, but the current generation of climate models and modelling efforts are
simply too immature to confirm or refute such a proposal (Soon et al. 2001; Soon and Baliunas 2003).
4
near Broggerhalvoya, NW coast of Svalbard, Aanes et al. (2002) found that high positive values
of AO index are associated with decreased plant growth and reindeer population growth rate.
Thus, the reindeer population at Svalbard, through the mediation of the climate modulated effects
on plant growth, is plausibly connected to climate through a bottom-up sequence. But Aanes et
al. (2002) noted that the bottom-up scenario may be density-dependent in that at higher reindeer
densities, a top-down sequence of trophic interaction is becoming more important in which
grazing has a dominating influence on the forage species and plant communities. The AO index
is thus promising as a robust and reliable climatic variable for further examination of the dynamic
of trophic interactions under various settings of the arctic ecosystem.
Science evolves through new research and critical thinking. In this paper, we examine
peer-reviewed literature related to polar bears and the climate change conclusion. We believe that
some published results provide room for alternative explanations why bears appear to be in
poorer physical condition.
Perhaps the effects of climate change, reducing sea-ice cover, and changes in freeze and
thaw cycles on polar bears at higher latitudes might not be as detrimental as predicted. These
changes might create more polynyas, which are productive oases in the ice (Stirling 1997), or
increase marine productivity overall. Bears do not feed year-round, but during late spring when
seal pups are abundant. More fat deposits might be accumulated during this time, and a “true
hibernation state” like black (U. americanus) and brown bears (U. arctos) could become an
evolutionary strategy for the remainder of the year, as polar bears evolved from brown bears
(Kurtén 1964). Alternatively, a supplementary feeding strategy could evolve where berries and
vegetation are consumed during the ice-free period, as has been observed for bears of Hudson
Bay (Russell 1975; Derocher et al. 1993).
5
From 1981 through 1998, the WH population estimates were considered relatively
constant at about 900 – 1200 individuals (Lunn et al. 1997a; Stirling et al. 1999). Various studies
conducted on population dynamics, growth, and reproduction during this period found declining
female reproductive parameters (e.g., Derocher and Stirling 1992, 1995; Stirling and Derocher
1993; Stirling et al. 1999), as well as decreasing female body length (Atkinson et al. 1996;
Derocher and Stirling 1998). Whenever female body condition is poor, cub survival and litter size
are negatively affected as these parameters are closely related (Derocher and Stirling 1996;
Atkinson and Ramsay 1996). These declining trends were consistent with density-dependent
responses, where lowered food abundance and increased intraspecific competition were
suggested as causes (see Derocher and Stirling 1992 for details). Perhaps what is being observed
for WH polar bears is caused by density-dependent factors (but see Derocher and Taylor 1994)
rather than by climate change. How population dynamics of ringed (Phoca hispida) and bearded
seals (Erignathus barbatus), polar bears’ main prey items (Stirling and Archibald 1977; Smith
1980), have changed during this period is unclear, as data are very limited (Lunn et al. 1997b).
We caution the interpretation of results whenever body mass of polar bears was estimated
by chest girth using a general equation (i.e., Kolenosky et al. 1989). Polar bears span a range of
body mass over two orders in magnitude, depending on season, sex, age, and reproductive class
(Stirling et al. 1977; Watts and Hansen 1987; Kolenosky et al. 1989). They also cycle between
periods of hyperphagia and fasting, where body fat constitutes as much as 50% or as little as 10%
of their total body mass, respectively (Cattet 1990; Pond and Ramsay 1992; Pond et al. 1992;
Atkinson et al. 1996). This indicates that polar bears are highly variable in body mass, and that
morphometry is neither accurate nor precise to estimate body mass. Not general, but rather
6
population specific equations for adult males, females, and subadults should be employed (Cattet
et al. 1997).
Given that the uncertain state of our understanding on the complex interaction among sea
ice, atmospheric and oceanic circulations, and air and sea temperatures, we suggest that large
interannual variability, which is largely stochastic in nature, dominates the climatic changes that
will ultimately bring about particular ecosystem responses (perhaps yet unintelligible to us) but
spring temperature is not necessarily a decisive predictor. Given the alternative interpretations
regarding the decline of reproductive parameters of female polar bears of WH, we believe it is
premature to make further-reaching predictions about how climate change may affect polar bears
in general. It might be beneficial to re-examine existing data on temporal variation of growth for
body length of females of other polar bear populations for comparison. In addition, specific
population equations could be employed for body masses of WH bears. If what is being observed
in WH is truly a density-dependent response, then long-term ecological studies for the Svalbard
polar bear population should continue. The Svalbard population is currently the only studied
population with no ongoing harvest, and is thought to have recovered from over-harvesting
(Prestrud and Stirling 1994; Wiig 1998). Possible density-dependent effects could be assessed by
studying and comparing these two populations. In addition, we encourage research, conducted
simultaneously with polar bear studies, that examines lower trophic levels to aid in providing
more insight into the proximate effects of climate change on Arctic species. We further suggest
that the AO circulation index may be useful in tracking the propagation of any climatic signals
through the coupled ecosystems of the Arctic land and sea that promises only the undeniable
reality of multi-trophic level interactions (Steinke et al. 2002).
7
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Figure Captions
Figure 1: (a) Climatological Winter (as averages of December, January and February), Spring (as
averages of March, April and May), Summer (as averages of June, July and August) and Fall (as
averages of September, October and November) surface air temperatures of Churchill, Manitoba
(assumed to be representative of temperatures around the western Hudson Bay) from 1932
through 2002. Four-year gaps between 1993 and 1996 are missing or incomplete data. (b) Spring
(defined specifically as averages of April, May and June to follow the discussion in Stirling et al.
1999; top panel) and Fall (bottom panel) temperatures with statistically insignificant (i.e., with p
> 0.05) trend lines (dotted) fitted through the 1932-2002 and 1932-1992 intervals. Dashed trend
line fitted through 1981-1999 verifies the spring warming episode noted by Stirling et al. (1999).
Figure 2: Statistically significant (i.e., p << 0.001) correlation between temperature (solid) at
Frobisher Bay, Nunavut and the Arctic Oscillation (AO) index (dotted) for Winter (as averages of
January, February and March; top panel) and Spring (as averages of March, April and May;
bottom panel). The axis for AO indices has reverse scales such that high positive AO values
mean colder temperatures at Frobisher Bay. 52% and 20% of the variance of the winter and
spring temperatures for the 1943-2002 interval are explained the respective AO indices. Both the
standard Pearson’s r and non-parametric Kendall’s τ were computed and statistical significance
of the results are established based on both statistics.
12