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Polar bears of western Hudson Bay and climate change: Are warming spring air temperatures the “ultimate” survival control factor?

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Long-term warming of late spring (April–June) air temperatures has been proposed by Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, J., 1999. Long-term trends in the population ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52, 294–306] as the “ultimate” factor causing earlier sea-ice break-up around western Hudson Bay (WH) that has, in turn, led to the poorer physical and reproductive characteristics of polar bears occupying this region. Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar bears in a warming climate. Integr. Comp. Biol. 44, 163–176] expanded the discussion to the whole circumpolar Arctic and concluded that polar bears will unlikely survive as a species should the computer-predicted scenarios for total disappearance of sea-ice in the Arctic come true. We found that spring air temperatures around the Hudson Bay basin for the past 70 years (1932–2002) show no significant warming trend and are more likely identified with the large-amplitude, natural climatic variability that is characteristic of the Arctic. Any role of external forcing by anthropogenic greenhouse gases remains difficult to identify. We argue, therefore, that the extrapolation of polar bear disappearance is highly premature. Climate models are simply not skilful for the projection of regional sea-ice changes in Hudson Bay or the whole Arctic. Alternative factors, such as increased human–bear interaction, must be taken into account in a more realistic study and explanation of the population ecology of WH polar bears. Both scientific papers and public discussion that continue to fail to recognize the inherent complexity in the adaptive interaction of polar bears with both human and nature will not likely offer any useful, science-based, preservation and management strategies for the species.
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Polar bears of western Hudson Bay and climate change:
Are warming spring air temperatures the ‘‘ultimate’’
survival control factor?
M.G. Dyck
a,
*, W. Soon
b,
**, R.K. Baydack
c
, D.R. Legates
d
, S. Baliunas
b
,
T.F. Ball
e
, L.O. Hancock
f
a
Environmental Technology Program, Nunavut Arctic College, Box 600, Iqaluit, Nunavut X0A 0H0, Canada
b
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
c
Clayton H. Riddell Faculty of Environment, Earth, and Resources, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
d
Center for Climatic Research, University of Delaware, Newark, Delaware 19716, USA
e
Climate and Environment Consultant, Victoria, British Columbia, Canada
f
MSN H-5-503, 1818 H Street, NW, Washington, DC 20433, USA
ecological complexity xxx (2007) xxx–xxx
article info
Article history:
Received 1 March 2007
Accepted 2 March 2007
Keywords:
Polar bear
Climate change
Hudson Bay
Extinction
abstract
Long-term warming of late spring (April–June) air temperatures has been proposed by
Stirling et al. [Stirling, I., Lunn, N.J., Iacozza, J., 1999. Long-term trends in the population
ecology of polar bears in western Hudson Bay in relation to climatic change. Arctic 52, 294–
306] as the ‘‘ultimate’’ factor causing earlier sea-ice break-up around western Hudson Bay
(WH) that has, in turn, led to the poorer physical and reproductive characteristics of polar
bears occupying this region. Derocher et al. [Derocher, A.E., Lunn, N.J., Stirling, I., 2004. Polar
bears in a warming climate. Integr. Comp. Biol. 44, 163–176] expanded the discussion to the
whole circumpolar Arctic and concluded that polar bears will unlikely survive as a species
should the computer-predicted scenarios for total disappearance of sea-ice in the Arctic
come true. We found that spring air temperatures around the Hudson Bay basin for the past
70 years (1932–2002) show no significant warming trend and are more likely identified with
the large-amplitude, natural climatic variability that is characteristic of the Arctic. Any role
of external forcing by anthropogenic greenhouse gases remains difficult to identify. We
argue, therefore, that the extrapolation of polar bear disappearance is highly premature.
Climate models are simply not skilful for the projection of regional sea-ice changes in
Hudson Bay or the whole Arctic. Alternative factors, such as increased human–bear inter-
action, must be taken into account in a more realistic study and explanation of the
population ecology of WH polar bears. Both scientific papers and public discussion that
continue to fail to recognize the inherent complexity in the adaptive interaction of polar
bears with both human and nature will not likely offer any useful, science-based, preserva-
tion and management strategies for the species.
#2007 Elsevier B.V. All rights reserved.
*Corresponding author. Present address: Department of Environment, Government of Nunavut, Box 209, Igloolik X0A 0L0, Canada.
E-mail addresses: mdyck@gov.nu.ca (M.G. Dyck), wsoon@cfa.harvard.edu (W. Soon).
** Corresponding author. Tel.: +1 617 495 7488.
E-mail addresses: mdyck@gov.nu.ca (M.G. Dyck), wsoon@cfa.harvard.edu (W. Soon).
ECOCOM-105; No of Pages 12
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journal homepage: http://www.elsevier.com/locate/ecocom
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doi:10.1016/j.ecocom.2007.03.002
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
1. Introduction
Polar bears (Ursus maritimus) are charismatic megafauna that
symbolize the Arctic. They play an important cultural,
spiritual, mystical, and traditional role in the lives of Canadian
Inuit through hunting and subsequent sharing of meat and
fur. Additionally, Inuit-guided sport hunts provide important
revenue for the economically challenged communities (Lee
and Taylor, 1994). The latest research findings suggest,
however, that this multi-purpose natural resource faces
threats from climatic change and environmental stress
(Stirling and Derocher, 1993; Stirling et al., 1999;World Wide
Fund for Nature, 2002;Derocher et al., 2004) or from simply
unsustainable harvests by human hunters (see recent discus-
sion in Taylor et al., 2005).
Unfortunately, polar bears and their shrinking ice habitat
are commonly used rhetoric to argue for the possible severity
of climate change and global warming to the general public
(cf., Washington Post, 2005). The polar bears that are most
often cited are a specific population that inhabits the south-
western Hudson Bay coast—1 of 14 polar bear populations
found in Canada (Derocher et al., 1998; Taylor et al., 2001). The
area they occupy encompasses almost the southernmost
extent of the species (only the southern Hudson Bay polar bear
population reaches farther south; Derocher et al., 1998).
Population stresses have been observed, which has led to
the proposition that an earlier break-up of Hudson Bay ice (and
an associated increase in spring air temperatures) is the cause
of decreases in reproduction, subadult survival, and body
mass of some of these bears (Stirling and Derocher, 1993;
Stirling et al., 1999). A long-term warming trend of spring
atmospheric temperatures was proposed, though not shown
directly,
1
to be ‘‘the ultimate factor’’ (Stirling et al., 1999,p.
294). As a result, it is commonly believed that climatic changes
(or ‘‘global warming’’) are the predominant factors leading to
adverse conditions for the polar bear populations, although
other factors have been acknowledged (e.g., density-depen-
dent population responses; Derocher and Stirling, 1992).
We argue thatthere are several relatedstress factors that can
explain the observed patterns in polar bear population ecology.
Global warming may indeed have an effect on the polar bears of
western Hudson Bay (WH) but it must be assessed in a more
realistic framework that considers all the likely stress factors
and their cumulative impacts. In such a context, it is difficult to
isolate one factor of predominant severity and,consequently, it
is simply not prudent to overstate the certainty of any single
factor. As emphasized in Li (2004) and Loehle (2004), a full
scientific understanding of an issue as complex as the
population ecology of polar bears must necessarily requires
the combined assessment of both the natural and social
systems rooted in the problem rather than consideration of
either component in isolation (i.e., warmer spring air tempera-
tures and related sea-ice conditions in WH).
In the next two sections, we examine some of the potential
nonclimatic causes of decreased reproduction, offspring survi-
val, and body masses, including repeated bear–human inter-
actions, food availability and competition. We then consider
climatic factors by examining available surface air temperature
records and ice dynamics in the Hudson Bay basin. Finally we
synthesize these findings to critically evaluate the forecasts of
polar bear extinction in relation to model projected scenariosof
global warming by Derocher et al. (2004).
2. Human–polar bear interactions in western
Hudson Bay
Western Hudson Bay polar bears have a long history of
interactions and confrontations with humans. Stirling et al.
(1977) discusses interactions between humans and WH polar
bears from Churchill at dump sites, in town, and adjacent
town areas. Over the years, the three main sources of bear–
human interactions for the WH bears are activities related to
(a) scientific research, (b) tourism, and (c) the Polar Bear Alert
Program.
Research activities for the WH area began in 1966, and
continue today as a long-term ecological monitoring project in
which over 80% of the bear population is marked (Stirling et al.,
1977; Lunn et al., 2002). The majority of this field work has been
carried out by the Canadian Wildlife Service (CWS), although
universities also conduct research on polar bears in the area.
Many bears are captured, marked, and eventually recaptured,
sometimes within the same year, over a number of years (e.g.,
Calvert et al., 1991a,b, 1995a,b, 1998). For example, from 1977 to
1995, an estimated total of 2772 bears were captured (Derocher
and Stirling, 1995, their Tables 2 and 3; Lunn et al., 1997a, their
Tables 2 and 3), with a minimum (i.e., since not all captures are
clearly reported in publications and conflicting information
exists) of about 1100 recaptures (recapture rates of between 52
and 90%; mean number of bears captured/year between 1977
and 1995 is about 145 bears; see summary total of columns 2
and 3 in Table 1). If one considers that the WH population
estimate then was between 700 and 1200 bears (Amstrup and
Wiig, 1991; Wiig et al., 1995), and about 15–30% of the
population was captured and recaptured due to high fidelity
to locations along the coast (Derocher and Stirling, 1990a,b), it
is very likely that many bears were/are exposed to capture
activities on a repeated basis.
An assumption most frequently made by researchers is that
their work (i.e., capturing and handling wildlife repeatedly) has
no significant effect on fitness, behaviour or survival of the
wildlife species in question (Seber, 1973; Lehner, 1979). Long-
term trends of handling polar bears were suggested by Ramsay
and Stirling (1986) and included the possible effects on females
with cubs. Although their study did not find any statistically
significant results, the trends they presented indicated that
females may suffer from handling by being displaced from
feeding sites,possibly resulting in lowered body mass. Note that
female polar bearbody mass is positively related to cub survival
(Derocher and Stirling, 1996, 1998a). If females lose body mass
due to handling, cubs willbe adversely affected in their survival
rates. Also, most polar bear capture work occurs either on
family groups in spring as they emerge from their dens, or
during the ice-free period while bears are distributed along the
southwestern shore of Hudson Bay—times when the bears are
either stressed due to lactation (Arnould, 1990)orundergoa
fasting period while living off their stored fat reserves (Watts
1
Stirling et al. (1999) relied on the mean air temperature results
of Skinner et al. (1998).
ecological complexity xxx (2007) xxx–xxx2
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
and Hansen, 1987). While the handling effect study of Ramsay
and Stirling (1986) covered only 1967–1984, we suggest an
additional analysis of capture–recapture data for handling
effects that extends their time period to the present.
Almost concurrently with research activities at WH, some
of the bears in the WH population are exposed to tourists and
tourism activities during the fall. Since about 1980, polar bear
viewing from large customized vehicles has been practiced
near the town of Churchill. Polar bears leave the ice during
June/July and slowly migrate north to the shores of Hudson
Bay (approximately 35 km east of Churchill) where they
congregate and wait the early freeze-up of the Bay, usually
during November. Tour companies transport visitors into the
congregation area (approximate coordinates are: 588450Nto
588480N, and 938380Wto938500W) during October/November to
view the bears (Dyck, 2001). Although the viewing period is
short, usually between 1 October and 15 November, it is very
intense, with about 6000 tourists and 15 large tundra vehicles
per day in the area (Dyck and Baydack, 2006). Baiting,
harassment and chasing of bears have been documented to
occur (Watts and Ratson, 1989; Herrero and Herrero, 1997). The
Polar Bear Technical Committee has expressed concern over
these activities, suggesting that harassment of bears during
this time of the year might be very stressful due to their fasting
(Calvert et al., 1998). In the first baseline study conducted in the
area to address tundra vehicle behaviour and vigilance (i.e., a
motor act that corresponds to a head lift interrupting the
ongoing activity) of resting polar bears, Dyck and Baydack
(2004) found significant increases in vigilance behaviour of
resting male polar bears in the presence of vehicles. The
authors speculated that increased vigilance could lead to
increased heart rates and metabolic activity, subsequently
adding other factors that possibly contribute to the negative
energy balance of bears while on land.
Another bear–human interaction occurs in the form of the
Polar Bear Alert Program (PBAP) at Churchill. The Manitoba
provincial management agency initiated the program in 1969
to protect local residents from bears, and vice versa (Kearney,
1989). The area around the town is patrolled, and bears that
enter certain zones will either be deterred, captured, handled,
or destroyed. From its inception up to 2000, an average of 48
bears per year (a total of 1547 bears) have been handled
(Kearney, 1989; Calvert et al., 1991b, 1995b; Lunn et al., 1998; for
a detailed PBAP description, see Kearney, 1989). Handling
procedures are similar to those during research activities, and
effects can be assumed to be similar.
Considering CWS-related research activities and the PBAP
activities between 1977 and 1995, a total of 3558 bears (not
including university-research handled bears) have been
handled (last column in Table 1). This is about three times
greater than the actual estimated WH population of 1100
(Derocher and Stirling, 1992), indicating that all bears are, on
average, subject to repeated handling. Moreover, these
activities occur when bears are either fasting or leaving their
dens and are already energetically stressed. It is plausible that
these repeated bear–human interactions have adversely
stressed the bears over the past 30 years.
3. Food availability and competition
Between 1978 and 1990, the WH polar bear population was
estimated to be around 1100 bears (Derocher and Stirling, 1992).
Derocher and Stirling (1995) estimated the mean size of the
population between 1978 and 1992 to be around 1000 bears. Up
to 1997, the population did not change significantly, and was
estimated to be around 1200 bears (Lunn et al., 1997a;Fig.6in
Stirling et al., 1999). When published yearly population
estimates from Derocher and Stirling (1995) and Lunn et al.
(1997a) areexamined, several tendencies are apparent. First, the
Derocher and Stirling (1995) data for 1977–1992 show an
increasing trend (F=4.16, p=0.06, r
2
= 0.23), although that
trend is not statistically significant. Second, the Lunn et al.
(1997a) data from 1984 to 1995 indicate a stable population
(F=0.71,p=0.42,r
2
= 0.07). When both data sets are combined
(i.e., the Derocher and Stirling (1995) data from 1977 to 1992 and
the Lunn et al. (1997a) data for 1993–1995), a significant increase
in the population size is implied (F=6.40, p=0.02, r
2
=0.27).
Most recently, however, it was noted that the population since
1995 has been declining to ‘‘less than 950 in 2004’’ (IUCN/Polar
Bear Specialist Group, 2005). We clarify that the published
estimate by Lunn et al. (1997a), combining Churchill and Cape
Tatnam studyarea (both in WH) datasets, gives a 1995 WH polar
bear population of 1233 with a 95% confidence interval that
ranges from 823 to 1643 bears, so the actual confidence in the
‘‘decline’’of the WH polar bear populationin 2004, relativeto the
1995 values, is difficult to confirm.
Given these long-term data on population estimates and
responses, itis possible that density-dependent processeshave
been imprinted in the observed records of polar bears at WH. It
Table 1 – Captures of polar bears for research (males and
females), for the Polar Bear Alert Program (PBAP), and
total polar bear captures per year from 1977 to 1995
Year Males
a
Females
a
PBAP
b
Total captures/
year
1977 53 34 32 119
1978 29 26 16 71
1979 15 10 27 52
1980 20 29 18 67
1981 32 36 27 95
1982 68 42 32 142
1983 95 95 92 282
1984 96 63 18 177
1985 95 59 76 230
1986 84 53 26 163
1987 115 149 30 294
1988 140 152 35 327
1989 168 163 51 382
1990 107 92 64 263
1991 86 68 18 172
1992 57 74 54 185
1993 42 54 58 154
1994 63 64 79 206
1995 86 58 33 177
Total 1451 1321 786 3558
Mean 76 69 41 187
a
Derocher and Stirling (1995); Tables 2 and 3, and Lunn et al.
(1997a); Tables 2 and 3; whenever data were conflicting in their
tables, we used the greater number for each gender/year.
b
Kearney (1989),Calvert et al. (1991b, 1995b) and Lunn et al. (1998).
ecological complexity xxx (2007) xxx–xxx 3
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
is important, however, to recognize the great difficulties in
demonstrating density dependence in population studies (e.g.,
Ray and Hastings, 1996; Mayor and Schaefer, 2005), among
which is the sensitivity of the phenomenon on spatial scale
covered by the population sampling techniques (e.g., Taylor
et al., 2001). We concur with Derocher and Stirling (1995) and
Stirling et al. (2004) that the WH population was at least stable
during the 1984–1995 period (and likely up to 1997; see Stirling
et al., 1999, their Fig. 6). Prior to that the WH population was
hunted heavily, which led to hunting restrictions (Stirling et al.,
1977; Derocher and Stirling, 1995). After the population
recovered, and then increased, bear body mass, reproductive
parameters, cub survival, and growth declined (Derocher and
Stirling, 1992, 1998b). Derocher and Stirling (1992, 1995, 1998b)
considered whether these responses reflect density-dependent
population control mechanisms. They discarded them either
because no accurate population estimates for WH existed, or no
change in population size was detected. Typically, density-
dependent responses, similar to those exhibited by WH polar
bears, are detected in increasing populations (Eberhardt and
Siniff, 1977; Fowler, 1990). By contrast, however, individuals ofa
populationnear carrying capacity(given that the WH population
remained relatively stable for so long)can also exhibit traits that
were observed for this polar bear population, namely poorer
physical condition, lower survivorship, and lower rates of
reproduction (Kie et al., 1980, 2003; Stewart et al., 2005). It is
possible that the WH population has been stable for so long
because carrying capacity has been reached, and intraspecific
competition increased with increasing polar bear density,
resulting in the documented responses.
It is important to notethat the southern half of Hudson Bay is
shared between polar bear populations of WH and southern
Hudson Bay (SH) (Derocher et al., 1998). Polar bears of SH have
exhibited better body condition as compared to their WH
counterparts (Stirling et al., 1999, 2004) but prolonged ice
conditions in that area seem not to be the explanation because
Fig. 1 (a) Climatological winter (the average of December, January and February), spring (the average of March, April and May),
summer (the average of June, July and August) and fall (the average of September, October and November) surface air
temperatures of Churchill, Manitoba, which are assumed to be representative of temperatures around the western Hudson
Bay from 1932 through 2002. (b) Late spring (defined as the average of April, May and June, following the discussion in Stirling
et al., 1999; top panel) and fall (bottom panel) temperatures with statistically insignificant (i.e., with p> 0.05; again chosen in
order to follow discussion in Stirling et al., 1999) trend lines (dotted) fitted through the 1932–2002 interval. The dashed trend
line fitted through 1981–1999 verifies the late spring warming episode noted by Stirling et al. (1999) for that limited period.
ecological complexity xxx (2007) xxx–xxx4
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
recent updated analysis by Gagnon and Gough (2005a)
suggested tendencies toward earlier ice break-up (hence
shorter overall duration of sea-ice cover) in James Bay and
along the southern shore of Hudson Bay. Population estimates,
which have been conducted almost entirely via aerial surveys,
indicate an increasing trend for this SH population from 1963 to
1996 (i.e., see Table 2 and Fig. 4c of Stirlinget al., 2004). Although
both populations are recognized as independent (e.g., Derocher
and Stirling, 1990a,b; Kolenosky et al., 1992; Taylor et al., 2001),
possible overlap can occur on the sea-ice. If population density
for SH has been increasing, whereas food supply has been
insufficient due to increased competition, then some SH bears
may have expanded their hunting forays, leading to competi-
tion for food with WH bears. Yet there has not been a drastic
decline in the WH population detected. One reason may be that
the bears have learned to hunt seals during the ice-free period
along the shores in tidal flats. This phenomenon has been
observed for several years at Churchill in the polar bearviewing
area (Dyck, personal observations).
Data on the bear food supply is needed to draw more clear
conclusions about the interplay between population densities
and worsening physical attributes of polar bears. The main
prey of polar bears are ringed (Phoca hispida) and bearded seals
(Erignathus barbatus)(Stirling and Archibald, 1977; Smith,
1980), but seal population data are too limited at present to
resolve this issue (Lunn et al., 1997b).
4. Air temperature and climate variability
around Hudson Bay
Fig. 1a shows the surface air temperature records
2
of nearby
Churchill, Manitoba (assumed here to be representative of
WH) from 1932 to 2002 for the four climatological seasons. The
large interannual variability of the seasonal temperatures
suggests that establishing a meaningful long-term trend in
any of these relatively short records would be difficult and that
a trend determination, especially over short periods, will be
highly sensitive to the time interval considered (e.g., Pielke
et al., 2002; Cohen and Barlow, 2005). Fig. 1b attests that no
statistically significant warming trend (dotted trend lines
fitted over the full records in Fig. 1b) can be confirmed for
either the late spring (defined here as the average of April, May
and June, following discussion in Stirling et al., 1999) or fall
seasons when the full record from 1932 through 2002 is
considered. Thus, the hypothesis that a warming trend is the
principal causative agent for the supposed earlier spring melt
and later fall freeze of the sea-ice around WH cannot be
confirmed. Further, that the temperature trend is not
statistically different from zero indicates it is not obviously
forced by anthropogenic greenhouse gases as commonly
assumed and extrapolated to suggest implications for polar
bear ecology in future scenarios of climate change. Such
extrapolations remain premature at best.
An apparent tendency towards late spring warming can be
derived by examining the period from 1981 to 1999, illustrated
by the dashed trend curve in Fig. 1b. Clearly, the choice of end
points is very influential on the results. The trend fails to
persist when data through 2002 are included and we make no
inferences 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)
for the limited period from 1981 to 1999, the longer record
reveals a fuller range of air temperature variability that argues
against assuming a persistent warming trend.
Gough et al. (2004) recently identified snow depth as the
primary governing parameter for the interannual variability of
winter sea-ice thickness in Hudson Bay because of its direct
insulating effect on ice surfaces. By contrast, the concurrent
winter or previous summer air temperatures yield only weak
statistical correlations with ice thickness. Detailed high-
resolution modelling efforts by Saucier et al. (2004) that
considers tides, river runoff and daily meteorological forcing,
found tidal mixing to be critically important for ice-ocean
circulation within, and hence the regional climate of, the
Hudson Bay basin.
We further examined records of winter and spring air
temperatures at Frobisher Bay (now called Iqaluit, Nunavut) by
the Hudson Strait and the respective winter and spring Arctic
Oscillation (AO) circulation indices
3
(Fig. 2)tobetter
2
Our data source is 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/.
Churchill and Frobisher Bay data shown here are from the
7-station- and 5-station-merged records, respectively. Missing
Churchill temperatures from NASA GISS database for 1993–1996
were replaced by data points from Churchill Airport given by
CLIMVIS Global Summary of the day available from the U.S.
National Climatic Data Center.
3
Arctic Oscillation (AO) is a natural, planetary-scale pattern or
mode of atmospheric circulation variability that is characterized
by a seasaw of the air mass anomaly between the Arctic basin and
the midlatitude zonal ring centered at about 458N. A 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. A 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. Because of the relatively larger variability and stron-
ger coupling of stratospheric and tropospheric air circulation
during the cold season, AO is mainly a winter phenomenon How-
ever, AO has been demonstrated to be relevant to temperature and
precipitation fields 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 complete
tutorials. Although there have been several suggestions that the
post-1969 or post-1989 AO index remained in an ‘unusual’, highly
positive phase as a result of forcing by anthropogenic carbon
dioxide, the current generation of climate models and modelling
efforts are not sufficiently mature to confirm or refute such a
proposal (Soon et al., 2001; Soon and Baliunas, 2003). Furthermore,
it has been pointed out that AO index has been mostly neutral or
negative in the most recent 9 years (1996–2004) despite the notable
high-positive AO phase during the 1989–1995 interval earlier (e.g.,
Cohen and Barlow, 2005; Soon, 2005). Cohen and Barlow (2005)
argued that even though the AO may contribute to regional warm-
ing in the Arctic and even the Northern Hemisphere for a parti-
cular period, but the pattern and magnitude of temperature signal
induced by AO are physically quite different from the large-scale
features produced by global warming trend in the last 30 years,
thus disallowing any direct attribution of AO to radiative forcing
by anthropogenic greenhouse gases.
ecological complexity xxx (2007) xxx–xxx 5
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
characterize the regional pattern of air temperature variability.
Fig. 2 shows two important points. First, note the rather
strong cooling trend (at a rate of about 0.4 8Cperdecade
since the 1950s) for the winter and spring temperatures of
Frobisher Bay. Regional differences in the pattern of the
temperature variability, especially on the multidecadal
timescale, are large. This pattern of large temperature
gradients between the southwestern and northeastern
corners of the Hudson Bay oceanic basin has been well
noted by Ball (1995),Catchpole (1995) and Skinner et al.
(1998)—these authors also provided a comprehensive dis-
cussion on climate regimes around WH, including a broad,
historical perspective on the range of natural variabilities.
Among other things this indicates that a hypothesis of late
spring warming negatively affecting the WH polar bear
population ecology cannot be universally extended to other
locations.
The second point of Fig. 2 is that the air temperature and
climatic conditions around the Hudson Strait and Hudson Bay
areas have a close association with the AO circulation index.
The correlations shown in Fig. 2 are statistically significant,
with AO variability explaining up to 20–50% of the interannual
temperature variances at Frobisher Bay.
To examine the link between the AO index and Frobisher
Bay air temperatures, both series were regressed on a matrix
of monthly dummy variables to remove fixed seasonal
effects. The residuals of these regressions (denoted AO
r
and FR
r
) were then tested in a vector autoregression to
determine leading patterns of Granger causality (see Hamil-
ton, 1994, Chapter 11). While AO
r
shows a significant Granger-
causal pattern on FR
r
no such pattern exists in the other
direction. This means the current value of the AO index
significantly improves forecasts of monthly Frobisher Bay air
temperatures, but the current air temperature does not
improve forecasts of the AO. Finally, FR
r
was regressed on its
first two lags, AO
r
and the first three lags of AO
r
to remove
serial correlation in the mean. After a trend term that was
insignificant was removed, the r
2
from this regression was
0.39 (with an adj-r
2
of 0.38). A Wald test of the joint AO
r
terms
yielded a chi-square (d.f. = 4) statistic of 235.6. A p-value on
the hypothesis of no influence of the current and lagged AO
anomalies on the current monthly temperature anomaly is
less than 0.00001.
The AO circulation index appears to be physically
relevant for two reasons. First, from an examination of
the statistics of sea level pressure and sea-ice motion from
the 1979 to 1998 data collected by the International Arctic
Buoy Programme, Rigor et al. (2002) confirmed that the AO
circulation pattern can explain at least part of the thinning
sea-ice trend observed over the Arctic Ocean. Polyakov and
Johnson (2000) and Polyakov et al. (2003a) further empha-
sized the importance of the relative phasing of the decadal
and multidecadal (i.e., 50–80 years) oscillatory modes of
Arctic atmospheric circulation variability in explaining the
recent Arctic sea-ice areal extent and thickness trends.
Rigor et al. (2002) clarified that instead of assuming that the
warming trend in surface air temperature caused the sea-ice
to thin, it is the AO-induced circulation pattern that
produces the tendencies for sea-ice to thin and sea-ice
area to retreat (see further discussion on regional sea-ice
trends and mechanisms in Zhang et al., 2000; Kimura and
Wakatsuchi, 2001; Polyakov et al., 2003b; So
¨derkvist and
Bjo
¨rk, 2004). In turn, it was the changes in sea-ice that
caused the air temperature to warm because of an
increasing heat flux from the interface with the ice-free
ocean. Beyond atmospheric AO, Shimada et al. (2006)
recently documented and highlighted the key role played
by the inflows of warm Pacific summer water through the
Bering Straits in causing the large sea-ice areal reduction in
the Arctic that began in the late 1990s. Thus, such a complex
physical picture connecting oceanic and atmospheric
processes with sea-ice variability is dramatically different
from Stirling et al. (1999)’s suggestion in which warm spring
air temperature is considered to be the ultimate cause for
Fig. 2 – Statistically significant (i.e., p0.001) correlation
between temperature (solid) at Frobisher Bay (now as
Iqaluit), 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 a
reversed scale 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 by the respective AO
indices. Both the standard Pearson’s rand non-parametric
Kendall’s twere computed, and statistical significance of
the results are established based on both statistical
measures.
ecological complexity xxx (2007) xxx–xxx6
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
the earlier spring sea-ice break-up
4
and poorer conditions of
polar bears.
The second reason to discuss the AO index is related to a
recent finding that climatic change effects associated with the
AO index are propagated through two trophic levels within a
high-arctic ecosystem (Aanes et al., 2002). From the statistical
analyses of the 1987–1998 growth series of Cassiope tetragona
(Lapland Cassiope) and the 1978–1998 abundance series of an
introduced Svalbard reindeer (Rangifer tarandus platyrhynchus)
population near Broggerhalvoya, on the NW coast of Svalbard,
Aanes et al. (2002) found that high positive values of the AO
index are associated with decreased plant growth and
reindeer population growth rate. Thus, the reindeer popula-
tion 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 reverse 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 useful climatic variable for further examina-
tion of the dynamic of trophic interactions under various
settings of the arctic ecosystem.
It must also be asked whether natural climate oscillations
as those described above – reducing sea-ice cover and
changing the freeze-and-thaw cycles that affect the food
sources of polar bears at higher latitudes – are really as
detrimental to biodiversity as suggested. These changes may
create more polynyas, which are productive oases in the ice
(Stirling, 1997), or increase marine productivity overall (Fortier
et al., 1996; Rysgaard et al., 1999; Hansen et al., 2003) primarily
because of the modulation of the food web of the lower trophic
levels by freshwater-limiting and light-limiting processes.
Bears do not feed year-round, but do feed during late spring
when seal pups are abundant. More fat deposits may 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
for polar bears. This scenario could be very likely because polar
bears evolved from brown bears (Kurte
´n, 1964). Alternatively,
a supplementary feeding strategy could evolve where berries
and vegetation are consumed in higher frequencies during the
ice-free period, as has been observed for bears of Hudson Bay
(Russell, 1975; Derocher et al., 1993).
5. Extrapolating polar bear populations
In light of these considerations we do not consider it a sound
methodology to assume that local air temperature trends
adequately explain WH population conditions and that
extrapolating WH results generates predictions for polar bears
and their habitat over the circumpolar Arctic (e.g., Stirling and
Derocher, 1993;World Wide Fund for Nature 2002;Derocher
et al., 2004). We take particular exception to the suggestion by
Derocher et al. (2004, p. 163) that polar bears will not likely
survive ‘‘as a species’’
5
if several computer-generated scenar-
ios of air temperature-driven disappearance of sea-ice ‘‘by the
middle of the present century’’ come true. The conjecture
seems errant for two reasons. First, most climate models
predict a complete disappearance of sea-ice over the central
Arctic for only the late summer (i.e., September) while the
whole Hudson Bay is always ice-free during this time
regardless of the forcing by anthropogenic greenhouse gases
(see for example Figs. 8 and 9 in Johannessen et al., 2004).
Second, in the cited climate model projections, sea-ice at the
Hudson Bay for the late winter or early spring (i.e., March) was
never predicted to completely disappear by the end of this
century, even under scenarios that posit greenhouse gas
accumulations at rates considerably faster than currently or
historically observed. In a recent multi-model study of climate
projection in the Hudson Bay region, Gagnon and Gough
(2005b, p. 291) concluded that ‘‘Hudson Bay is expected to
remain completely ice covered in those five models by the end
of this century for at least part of the year.’’
It should also be noted that Gough et al. (2004) had earlier
reported that the observed thickening of sea-ice cover during
the last few decades on the western coast of Hudson Bay was
4
It should be noted that the tendency or trend for earlier spring
sea ice break-up in WH from 1979 to 1998 pointed out by Stirling
et al. (1999) is not statistically significant (with p= 0.07) under the
authors’ own criterion and admission. Houser and Gough (2003)
was also unable to demonstrate statistical significance in the
trend of timing of the spring sea ice retreat at the Hudson Strait
over the full interval from 1971 through 1999; although they
suggest that an earlier spring ice retreat or break-up seems clear
for the data starting 1990. We argue that this new tendency may be
related to the sustained positive phase for the AO circulation
index since 1989 till 1995 or so (see footnote 3) and it remains
to be confirmed if that the AO index might remain in that trend of
high positive values or the AO variability might undergoes a shift
toward the low (negative) AO-value phase as in the 1950s and
1960s. Updated results shown by Gagnon and Gough (2005a) on
trends in the timing of ice break-up, although now able to claim
‘‘statistical significance’’ under rigorous statistical testing for
James Bay and western half of Hudson Bay [though it should be
noted that in several records, threshold p-value of less than 0.10,
instead of the threshold of 0.05 adopted for example by Stirling
et al. (1999), is now used to claim significance], point out that
detecting surface air temperature trends is still sensitive to the
time interval of data records (see e.g., Cohen and Barlow, 2005).
Another real concern is the definition of spring ice break-up and
autumn freeze-up where we are not sure if the criterion of 50% ice
cover for the onset of melting and freezing seasons has been
optimized for the understanding of polar bear population ecology
(see Rigor et al., 2000 for other suggestions and threshold criteria),
In general we wish to discourage the over reliance on statistical
confidence that bypasses clear physical arguments or hypotheses
(see e.g., Wunsch, 1999).
5
However, it should not be too surprising to find somewhat
contradictory or more restrictive statements by these same
authors from what we faithfully quoted about polar bears facing
extinction in the Arctic by Derocher et al. (2004). For example, Dr.
Ian Stirling was quoted in WWF (2002) to have said that ‘‘For every
week earlier that break-up occurs in the Hudson Bay, bears will
come ashore roughly 10 kg lighter and thus in poorer condition.
With reproductive success tied closely to body condition, if tem-
peratures continue to rise in response to increases in greenhouse
gas emissions and the sea ice melts for longer periods, polar bear
numbers will be reduced in the southern portions of their range
and may even become locally [emphasis added] extinct.’’ (p. 5).
ecological complexity xxx (2007) xxx–xxx 7
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temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
in direct contradiction to the thinning ice scenario that is
posited by warming due to an enhanced CO
2
atmosphere.
Under these CO
2
-warming scenarios, the models predicted not
only an earlier spring break-up of sea-ice but also later fall
freeze-up at Hudson Bay (Gagnon and Gough, 2005b). Available
observations from 1971 to 2003, by contrast, do not show any
tendency for a later freeze-up of ice especially at WH or
southwestern Hudson Bay (Stirling et al., 1999; Gagnon and
Gough, 2005a). Further to the north, Melling (2002, pp. 2–18),in
his study of sea-ice around the northern Canadian Arctic
archipelago, concluded that ‘‘[i]nterannual fluctuations in
late-summer ice coverage obscure any evidence of trend [in
the Sverdrup Basin]. A decadal cycle contributes variability to
the times series of both total and multiyear ice concentrations.
Because the reputedly extreme conditions of 1998 are similar
to occurrences in 1962 and 1971, there is little basis on which to
view them as evidence for anthropogenic change.’’
We therefore conclude that it is highly premature to argue
for the extinction of polar bear across the circumpolar Arctic
within this century as incorrectly suggested in Derocher et al.
(2004).
Finally, we wish to encourage a renewed archaeological
search for information related to polar bear population
ecology from 1760 to 1820, when historical evidence (based
on early thermometers at trading posts of Churchill Factory
and York Factory) suggests that the climatic regimes at WH
had shifted from temperate to arctic conditions (see Ball,
1995; Catchpole, 1995). Ball (1983, 1986) documented large
changes and abrupt shifts in both floral (i.e., treeline
boundary between the boreal forest and the tundra) and
fauna (i.e., migration of wild geese) ecosystem responses of
the Hudson Bay region that occurred naturally as a
consequence of the varying mean locations of the Arctic
Front (Bryson, 1966). Ball (1995) suggested that the three
consecutive decades from 1770 to 1800 at York Factory
consisted of very wet and variable winter conditions
oscillating between extremes of heavy snow versus almost
snow-free conditions, which made the thriving of wildlife
populations difficult. Heavy late winter rains, for example,
have been proposed as a cause of the collapse of maternity
dens, suffocating the occupants (Stirling and Derocher,
1993). Excessive snowfall was noted to alter oxygen flux
through the snow layer of maternity dens and could
negatively impacting survival rates of young altricial cubs
that need to be nursed for 3 months before they are able to
leave the den with their mothers (Derocher et al., 2004). The
records compiled by Ball and Kingsley (1984) suggested an
interval with a relatively warm late spring (April–May–June)
at York Factory of about 2.9 8C for 1779, 1780, and 1782 (no
data for 1781) when monthly air temperature readings were
available from the Hudson Bay’s Company and Royal
Society’s archives. These data may be applied to assess
the resiliency of polar bears under adverse climate condi-
tions. The latest research by Scott and Stirling (2002) have
successfully dated, through sophisticated timing and finger-
printing techniques of dendro-sciences, polar bear maternity
dens and dens activities inland from the coast of WH, south
of Churchill and north of York Factory, since at least 1795,
while reports of polar bears have been recorded at least since
1619. These authors concluded that ‘‘there does not appear
to be a relationship between climate trends and the rates of
den disturbance during the overall 1850–1993 period’’ and
that ‘‘changes in the frequency and pattern of disturbances
at den sites may be related to the pattern of hunting and
trading of hides at York Factory during the 19th and early
20th century’’ (p. 163). Thus, the reality of human activity
impacting population ecology of polar bears at WH is clear
while empirical evidence for polar bear resiliency under
extended records of weather extremes and a wide range of
climatic conditions may be stronger than previously
thought.
6. Conclusions
The interactions among sea-ice, atmospheric and oceanic
circulations, and air and sea temperatures are complex and
our understanding of these issues in the Arctic context is
limited. We suggest that large interannual variability, which
we view as stochastic in nature (e.g., Wunsch, 1999),
dominates the climatic changes in WH. Improved under-
standing of polar bear resiliency and adaptive strategy to
climatic changes must consider human–bear interactions,
natural population dynamics, and the dominant compo-
nents of variability of the Arctic ice, ocean and atmosphere
that operate naturally on decadal to multidecadal time-
scales (Vinje, 2001; Polyakov et al., 2003a,b; Soon, 2005). The
clear evidence for strong regional differences in the spatial
pattern of historical climate change around the Hudson Bay
region add a layer of uncertainty to the task of explaining
empirical evidence. It is certainly premature, if not impos-
sible, to tie recent regional climatic variability in this part of
central Canada to anthropogenic greenhouse gases and,
further, to extrapolate species-level conditions on this basis.
These complex interactions of man-made and natural
factors will ultimately bring about particular ecosystem
responses (perhaps yet unintelligible to us) but we find that
late spring air temperature has not emerged as a decisive
causal factor or reliable predictor. Such a complexity within
the Hudson Bay’s ecosystem clearly challenges the useful-
ness of the original proposal in considering polar bears as
indicators of climatic warming made by Stirling and
Derocher (1993).
The broad claim for the sea-ice to be ‘‘gone by the middle
of the present century’’ could be both misleading and
confusing in that existing model predictions are for the
complete disappearance of late summer, rather than spring,
sea-ice over the central Arctic ocean. Climate models
actually expected Hudson Bay to be fully covered with
sea-ice at least part of the year (including early spring) even
under rather extreme forcing assumptions by involving
rapid increases in anthropogenic greenhouse gases by the
end of this century. This is why extrapolation studies
arguing for severe negative impacts of polar bears under a
global warming scenario are neither scientifically convincing
nor appropriate.
The fate of the charismatic polar bear population is of
considerable public concern, and rightly so. Science can best
contribute to the goals of conservation by providing the most
accurate possible understanding of the factors affecting the
ecological complexity xxx (2007) xxx–xxx8
ECOCOM-105; No of Pages 12
Please cite this article in press as: Dyck, M.G. et al., Polar bears of western Hudson Bay and climate change: Are warming spring air
temperatures the ‘‘ultimate’’ survival control factor?, Ecol. Complex. (2007), doi:10.1016/j.ecocom.2007.03.002
population ecology of these impressive animals. Our concern
in this paper is that if attention is inappropriately confined to a
single mechanism, namely greenhouse warming, opportu-
nities to understand other relevant mechanisms behind
changes in bear population and health parameters may be
lost in the process. It is also abundantly clear that relying on
such a strict single-variable-driven scenarios of global warm-
ing by increasing atmospheric carbon dioxide and related
melting sea-ice in discussing an issue as complex as the
population and well being of polar bears runs counter to the
underlying realities and challenges of ecological complexity
that emphasizes at least the six co-dimensions of spatial,
temporal, structural, process, behavioural and geometric
complexities (as e.g., outlined in viewpoints of Li, 2004; Loehle,
2004; Cadenasso et al., 2006).
Therefore, we believe it is premature to make the ‘‘one-
dimensional’’ predictions about how climate change may
affect polar bears in general and there is no ground for
raising public alarm about any imminent extinction of
Arctic polar bears. The multiple known and likely stresses
interact dynamically and may contribute in an additive
fashion to negative effects on polar bears. To quantify the
severity of these stress co-factors, however, is very difficult,
if not almost impossible, with current limitations on data.
Areas of research we would particularly encourage include
archaeological investigations, improved data on prey popu-
lation dynamics, and examination of lower trophic levels to
provide 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 climatic and meteorological signals through the coupled
ecosystems of the Arctic land and sea that promises only
the undeniable complexity of multi-trophic level interac-
tions (Fortier et al., 1996; Steinke et al., 2002; Hansen et al.,
2003).
Acknowledgements
We thank our colleagues (especially those sharing our
concerns for the well beings of polar bears) for important
conversations and lessons throughout the years about this
topic. We further thank R. McKitrick for performing the
Granger causality tests on statistical associations shown in
Fig. 2 of this paper and other substantial contributions. We are
grateful for the constructive comments on earlier versions of
the manuscript from S. Polischuk, S.-L. Han, and A. Derocher,
which were critical for the improvement of the final version.
(The open review on a 2002–2003 version of our manuscript by
A. Derocher is available at http://cfa-www.harvard.edu/
wsoon/polarbearclimate05-d/.) All views and conclusions
are strictly our own and do not reflect upon any of those
acknowledged (especially A. Derocher) or any institutions with
whom we are affiliated.
M. Dyck and W. Soon initiated this scientific study around
2002–2003 without seeking research fundings and both have
contributed equally. W. Soon’s effort for the completion of this
paper was partially supported by grants from the Charles G.
Koch Charitable Foundation, American Petroleum Institute,
and Exxon-Mobil Corporation. The views expressed herein are
solely of the authors and are independent of sources providing
support.
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