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Science Progress (2008), 91(2), 151 173 doi: 10.3184/003685008X324506
Effects of climate change on polar
bears
ØYSTEIN WIIGa, JON AARSb and ERIK W. BORNc
ABSTRACT
In this article, we review the effects on polar bears of global warming that have
already been observed, and try to evaluate what may happen to the polar bears
in the future. Many researchers have predicted a wide range of impacts of
climate change on polar bear demography and conditions. A predicted major
reduction in sea ice habitat will reduce the availability of ice associated seals,
the main prey of polar bears, and a loss and fragmentation of polar bear
habitat will ultimately lead to large future reductions in most subpopulations. It
is likely that polar bears will be lost from many areas where they are common
today and also that the total population will change into a few more distinctly
isolated populations.
Keywords: climate change, Arctic, polar bear
1. Introduction
The polar bear was a terrestrial mammal that became dependent on
sea ice. Polar bears are perfectly adapted to live on the Arctic sea
ice where they spend nearly all their life. Those that stay on land for
longer periods typically still depend upon sea ice for hunting.
During a brief period in spring polar bears feed intensively on
seals to replenish the fuel stores that allow them to sustain a long
dark winter. Habitat loss and fragmentation is likely the largest
threat to the species today1. The rapid decrease of sea ice due to
global warming poses a severe threat to polar bears.
a
Natural History Museum, University of Oslo, PO Box 1172 Blindern,
0318 Oslo, Norway. E-mail:[e-mail]
b
Norwegian Polar Institute, 9296 Tromsł, Norway
c
The Greenland Institute of Natural Resources, P.O. Box 570,
DK-3900 Nuuk, Greenland
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The polar bear is at the top of the Arctic food chain and is
generally regarded as a token for a pristine Arctic environment.
Global climate change is one of the major environmental challenges
of our time. According to the International Panel of Climate Change
(IPCC), the average global temperature is predicted to increase by
1.1  6.4 C during the 21st century. The temperatures will likely rise
at a particularly high rate in the Arctic. Some of the future scenarios
predict that the Arctic will be free of sea ice in summer by the end
of this century2.
If this happens, it will have a profound impact on polar bears and
the ice-associated ecosystems that they depend on. It has been
suggested that polar bears may go extinct in a future warmer Arctic
if the summer ice disappears3,4.
A key to the understanding of how global warming may affect
polar bears is identifying how less sea ice will influence their biology
and ecology3. It is obvious that survival and reproduction of polar
bears will decline with reduced availability of sea ice, and the
challenge is to understand and predict how seriously the different
subpopulations will be affected.
In this article, we review effects to polar bears of global warming
that have already been observed in certain areas. We also try to
evaluate what may happen to the polar bears in the future. However,
to allow for an understanding of why polar bears are particularly
vulnerable, we first briefly describe how polar bears function.
2. The biology of polar bears
2.1 Distribution and movement
From an evolutionary perspective the polar bear  or Ursus mari-
timus,   the sea bear   is a young species that evolved from brown
bear ancestors in the Bering Strait region probably 250,000
400,000 years ago5 7, when they adapted to a life on the Arctic
sea ice. Brown bears and polar bears are closely related and they are
able to produce fertile crossbreds in zoos8. The first crossbred in the
wild between these two bear species was found in 2005 in Arctic
Canada (http://www.msnbc.msn.com/id/12738644).
Polar bears have a circumpolar distribution and are confined to
Arctic and sub-Arctic ice covered seas, especially to areas with
annual ice cover over continental shelves. Such areas are highly
productive which is why the main preys of polar bears occur
there  in particular, the ringed seal and the bearded seal. Polar
bears are distributed over a large latitudinal gradient from the James
152 Øystein Wiig, Jon Aars and Erik W. Born
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Fig. 1 Distribution of polar bear subpopulations in the Arctic. SA, southern
Beaufort Sea; NB, northern Beaufort Sea; VM, Viscount Melville Sound; NW,
Norwegian Bay; LS, Lancaster Sound; MC, M Clintock Channel; GB, Gulf of
Boothia; FB, Foxe Basin; WH, western Hudson Bay; SH, southern Hudson Bay;
KB, Kane Basin; BB, Baffin Bay; DD, Davis Strait. Taken with permission from the
International Union for Conservation of Nature and Natural Resources (ref. 8).
Bay in the Canadian Arctic at 51 N to the North Pole. They are not
evenly distributed, but are found in more or less isolated subpopula-
tions that each is treated as a separate management unit. A 19th
subpopulation may inhabit the Arctic Ocean proper but this has not
been documented (Figure 1)9. While the number of polar bears in
some subpopulations in, for example, Canada, West Greenland and
the Barents Sea are well documented, that of several others remains
less well known. Therefore, it is not possible to give an accurate
estimate of the world population of polar bears, although the range
is thought to be of the order of 20 000  25 0009.
Polar bears rely heavily on sea ice as a platform for travelling,
hunting, mating and resting10, but in most areas use terrestrial
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habitat for denning. Polar bears are wide-ranging and highly mobile
animals. In general, their movement is governed by the annual
dynamics of the sea ice.
When the ice retreats north in the summer the bears must either
follow the ice or go on land and wait until the sea ice returns in the
autumn. The pattern varies between subpopulations. In Svalbard, for
example, some bears rest on land but most bears follow the ice
north in the Barents Sea11,12. In the western Hudson Bay, all bears
stay on land during the summer13.
The home ranges of female polar bears tracked with satellite
transmitters have ranged from ca 1,000 to ca 600,000 km2 12,14. In
general, polar bears that inhabit active and highly dynamic offshore
drift ice have larger home ranges than do bears inhabiting stable
land-fast ice.
Apparently, male polar bears have the same overall movement
pattern as females15,16. We assume that cubs learn the movement
pattern from their mother. Polar bears typically also show fidelity to
den areas17.
2.2 Feeding
Polar bears mainly forage at the ice covered sea and in most areas
secures the major bulk of food during a relatively brief period
during spring. They mainly prey on ringed seals and bearded seals.
In certain areas, the latter seems especially to play a role in the late
spring and early summer. However, occasionally also belugas,
narwhals, walrus, other seals, birds and fish are taken18  20.
The spring period is very important to polar bears. During this
period they feed intensively on the sea ice where they take ringed
seals and their pups in birth lairs. Newly weaned pups and other age
classes of ringed seals are preyed upon when they rest on the ice.
An   average  polar bear of about 200 kg needs about 2 kg of
blubber per day to keep going21. It is their hunting success in spring
and early summer that gives them the body reserves they need to
survive throughout the rest of the year20.
2.3 Reproduction
The polar bear is a sexually dimorphic species with males about
twice the mass of females22. The breeding season occurs in March
through May. The fertilized egg does not attach to the uterine
mucosa until some time in the autumn when the direct development
of the foetus begins (delayed implantation). At that time the
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pregnant female enters a den. Usually the den is dug in the snow on
a steep hill side on land not far from the sea, but in southern
Beaufort Sea the females also den on the pack ice23. In some
Canadian areas, dens are made in the soil relatively far inland, and
extended by accumulated snow later in winter24.
The polar bear females give birth to one or two (sometimes three)
cubs between late November and early January. The cubs weigh
only about 500  700 g at birth and are blind.
From then on they are raised on the mother s fat reserves
suckling her highly energy-rich milk. Polar bear milk may contain
close to 50% fat25. Pregnant females fast during the long denning
period in winter. In the western Hudson Bay in Canada, pregnant
females that go on land in June and enters a den in fall may not
have eaten for 8  9 months when they emerge from the den next
spring26. The mother bear must therefore be able to reach the sea
ice quickly after den emergence in order to catch seals and replenish
her energy reserves. The mother and the cubs leave the den usually
in April at which time the cubs weigh 10  12 kg. Female polar
bears normally keep offspring for 2.5 years and therefore breed
every third year27,28. This prolonged mother  offspring association
results in a skewed sex ratio during the mating season with two to
three adult males available for every oestrous female29. Male polar
bears thus compete fiercely for access to oestrous females and can
suffer severe injuries during the fights30.
2.4 Mortality
Mortality rate of cubs vary among subpopulations but is generally
high. At Svalbard only about 34% survived until weaning at about
2.5 years of age28 whereas values about twice as high have been
recorded during studies in Canada and Alaska9,31,32. In western
Hudson Bay, the cub survival during the first year was no more
than 44%33. Small cubs may die from starvation, hypothermia,
drowning, or because they are killed by adult male polar bears34,35.
Weaned young have hard years as subadults. Bears that seek
human settlements or camps are often subadult bears that have
problems with finding food on their own. Bears older than four
years generally have a high yearly survival (495%)32. The
maximum lifespan of wild roaming polar bears is about 30 years36.
Very high levels of some lipophilic anthropogenic pollutants
(persistent organic pollutants: POPs) are found in polar bears, in
particular in the subpopulations inhabiting East Greenland,
Svalbard, Franz Josef Land and the Kara Sea region37. Several
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studies suggest that these contaminants may negatively impact
hormone levels, and impair the immune system38,39.
Such health related effects are again likely to affect survival and
reproduction, although it is difficult to say how large this effect is
on the more polluted subpopulations today.
Polar bears are hunted for subsistence purposes by indigenous
peoples living in Alaska, Canada and Greenland, where on an
annual basis about 800 polar bears are killed amounting to 3  4% of
the estimated total world population (20 000  25 000). Polar bears
are completely protected in Russia and in Norway9,40. However,
poaching is substantial in some areas in Russia, but the number of
bears taken is unknown41.
3. Arctic climate change
3.1 Past changes
Over millions of years the Arctic has experienced climatic condition
that has varied from one extreme to another. The last major build up
of glaciation started about 3 million years ago (Myr BP) at
Greenland4. During the Quaternary period (1.6 Myr ago) the
climate has varied between glacial (cold) and interglacial (warm)
periods. The last interglacial lasted between 130 and 107 thousand
years ago (kyr BP). The last glacial ended 12 kyr before the present.
Climate conditions during the last interglacial are generally
considered to be comparable to, or slightly warmer than, present
day conditions4. There are even indications that the Bering Strait
winter sea ice limit was 800 km farther north than today and that
during some summers the Arctic Ocean may have been ice free42.
However, the question of an ice-free Arctic during the last
interglacial is uncertain, and others claim that there is no paleocli-
mate evidence for an ice-free Arctic during the last 800 kyr43.
During the last glaciation the global ice extent reached its
maximum 24  21 kyr BP. After the termination of the last ice
age, the Arctic has experienced a fluctuating climate. Overall, the
Arctic went into a period with warmer and moister climate than
before. There are some indications of a cold period between 8 and
5 kyr BP based on the distribution of marine mammals44. The
evidence is not unambiguous and there seems to have been large
regional differences. The same applies to the so called   Medieval
Warm Period  which occurred between the 9th and the mid 15th
century and was restricted to the areas in and around the North
Atlantic. A period referred to as   the Little Ice Age  between 1550
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and 1900 was probably the coldest period after the last glaciation4.
Overall, air temperatures in the Arctic have increased since the end
of the 19th century45. Although most pronounced in winter and
spring, all seasons have had an increase in temperature over the last
decades.
The change in temperature during the 20th century and into this
century has had profound effect on the polar bear prime habitat, the
Arctic sea ice. The amount of sea ice varies through the year: from
about 14 million km2 in March to about 7 million km2 in
September46. Since 1978 passive microwave sensors on satellites
has made it possible to detect overall trends in the extent of sea
ice47. During the period 1979 to 2006 the annual sea ice area in the
Arctic decreased by about 3.2%ydecade. The decrease was larger in
summer (Septemberź8.9%ydecade) than in winter (Marchź
2.5%ydecade)45. There are regional differences in this pattern. In
the Greenland Sea and the Barents Sea, the ice has decreased by
10.6% and 6.0%ydecade, respectively, since 1979. Corresponding
figures for Baffin Bay and Hudson Bay are 8.6% and 5.0%ydecade,
respectively. The Canadian Archipelago seems to be least affected
with a decrease of 0.4%ydecade45. The sea ice has also become
thinner in areas where it has not disappeared4. The Arctic average
reduction is about 10  15% with particular areas showing up to
40% reduction between the 1960s and the late 1990s.
A factor of great importance to polar bears is the timing of break
up of the sea ice during the spring. For example, in the Hudson Bay
and the Baffin Bay areas, ice break-up during spring has advanced
by ca 3 weeks since 197948. Such shifts have also been reported
from other areas. There is a scientific debate on the causes of the
shrinkage of the Arctic sea ice. There is some support of the idea
that it is a part of a natural fluctuation in the polar climate while
others claim it is a result of global warming due to increased levels
of greenhouse gases4. The 2007 meeting of the IPCC concluded
that most of the observed increase in the global average temperature
since the mid-20th century is very likely due to an increase in the
anthropogenic greenhouse gas concentrations2.
3.2 Projected changes
The projected changes in climate including Arctic sea ice conditions
are based on different climate models4. These models project a
decline of roughly 10  50% in annual average sea ice extent by
2100. The loss of summer ice will be larger. The average over five
different models is more than 50% reduction and some models
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indicate a nearly complete disappearance of summer ice in the
Arctic by 2100. There are regional differences, however. It is likely
that the Barents Sea and the Bering Sea will be ice free in the
summer by 20504. The projections of the decrease in the volume of
sea ice are greater than that of its extent meaning that the ice also
progressively will become thinner45.
Taken into account that there is no palaeoclimate evidence for an
ice-free Arctic during the last 800 kyr43, it could be that polar bears
will encounter the period with least habitat available ever during
their existence as a species within a century. Already in the near
future, climate impact on sea ice and the associated ecosystem will
thus likely impose the largest challenge to polar bears so far, given
that the general predictions hold of how climate change will impact
the sea ice in arctic regions.
4. Effects of climate change on polar bears
During their evolutionary history polar bears have experienced
several periods of warming and cooling in the Arctic. During the
last several hundreds millennia they have survived interglacial
periods with temperatures as high as or higher than present and
also glacial periods with extreme cooling of the northern hemi-
sphere. However, the present rapid decrease of the sea ice and
changes in timing of sea-ice breakup will have great influence on
the ecology, distribution and numbers of polar bears4.
4.1 Documented past effects
Several researchers have summarized the assumed effects of climate
change on polar bears3,49  51. In the following, we give some
examples of such effects.
4.1.1 Distribution and movement
Clearly, the distribution of polar bears has been influenced by
climate variation since the later part of Mid Pleistocene (ca
300 kyr BP until now). Due to scarcity of fossil finds little is
known about the distribution of polar bears in early times. Fossil
finds from the London area in the UK dated to 40  50 kyr BP5, and
southern Scandinavia 13  10 kyr BP52 show that polar bears have
ranged much further south during the last glaciation than nowadays.
Information on changes during the last decades is substantial. In
Alaska, there has been an increased use of coastal habitats by polar
158 Øystein Wiig, Jon Aars and Erik W. Born
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bears from the Southern Beaufort Sea and the Chukchi subpopula-
tions53. The bears in this area typically move from the offshore pack
ice to the nearshore environment in the autumn to take advantage of
higher productivity and availability of seals. In this region, the
autumn freeze up has been delayed 1  2 months during the last 10 
15 years and the number of interactions between   land locked 
polar bears and humans have increased. Such a shift in distribution
is reported for the same period also in the northern coast of
Chukotka and on Wrangel Island in the eastern Russian Arctic54
and in the eastern Canadian Arctic48.
Since the early 1990s, Inuit living on the eastern coast of Baffin
Island, eastern Canada, have experienced an increased occurrence of
polar bears near the shore and on land concomitant with a reduction
in sea ice. The observations were interpreted to reflect an increase in
the population of polar bears in Baffin Bay48. Similarly, the Inuit
living in northwestern Greenland on the opposite side of Baffin Bay
were interviewed about polar bears and polar bear hunting in 2006.
Similar to their Canadian neighbours they had generally experi-
enced an increased near-shore occurrence of bears reflected in an
increase in catches since the early 1990s. Most of the Greenlandic
hunters thought that the population had increased whereas others
suggested that the change in distribution was related to the marked
decrease in sea ice that they themselves had witnessed55.
However, an estimate of population size from the late 1990s56
and information on high catches taken by Canadian and
Greenlandic hunters from the early 1990s until the present indicate
that the Baffin Bay population has decreased and is currently
depleted due to overexploitation9,48. This suggests that the increased
number of observations near-shore reflects a change in distribution
of polar bears likely due to change in the ice conditions rather than
an increase in the population48.
4.1.2 Feeding
In a study of temporal and spatial variation in the diets of polar
bears across the Canadian Arctic, it was found that in western
Hudson Bay the diet shifted in relation to timing of ice break up in
the spring during the 1990s57. The amount of ringed seals decreased
in the diet of both sexes and the amount of harbour seals and to
some extent bearded seals increased. It was not clear whether this
was a result of decreased availability of ringed seals, an increased
availability of harbour and bearded seals, or both.
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In 1998, the summer ice of the Chukchi Sea covered 25% less
area than during the minimum for the previous 35 years. A
substantial proportion of the ice edge was north of the continental
shelf over water that was too deep for walruses to feed58. During
such years an unusually high numbers of walruses use the terrestrial
haulout of Wrangel Island, Russia. Predation by polar bears was
high, especially on the young of the year walruses59.
Along the Alaskan coast of the southern Beaufort sea, the
number of polar bears that seek up bowhead carcasses from
subsistence whaling activity has been studied53. During the
period 2002  2004 an increased number of polar bears were seen
feeding at the carcasses. This may be a result of the increased use
of coastal habitat in the region due to reduction of the pack ice.
The bears may be more dependent of alternative prey in order to
survive the longer summer season60.
4.1.3 Reproduction
Nowadays the southern limit of polar bears is in the Hudson Bay in
Canada. The subpopulation of polar bears living in the western
Hudson Bay has been intensively studied by Canadian researchers
since the late 1970s26. Their studies indicate that the population has
already been negatively affected by the climate changes48. The
situation in the western Hudson Bay subpopulation exemplifies
what likely will happen to polar bears in other areas of the Arctic.
Studies in the western Hudson Bay have indicated that if the
body mass of an adult female bear gets lower than a   critical
body mass  of ca 189 kg in the autumn, she will be unable to
reproduce successfully26. In this area, a significant positive
relationship was found between the timing of sea ice break up
in the spring and the body condition of adult polar (i.e. the later
the ice break-up occurs the heavier the bears become) bears as
well as their ability to produce and raise offspring61. In the early
to mid-1980s, the fecundity of female polar bears in western
Hudson Bay was the highest recorded for polar bears anywhere.
In some of those years, females successfully weaned up to about
40% of their cubs at 1.5 years of age instead of the normal 2.5
years62. Through the late 1980s and the 1990s a steady decline in
body condition of both males and females has been documented.
During the same period the fecundity decreased. In the 1980s,
the mean weight of pregnant female polar bears in western
Hudson Bay in the autumn was 283 kg. Up until 1992 an
annual reduction in average body mass of 4.7 kg for pregnant
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females was recorded63. Assuming a continued loss of body mass
at the same rate, they predicted that most females will be below
the   critical body mass  for successful reproduction by 20123.
In the neighbouring subpopulation of polar bears living in
southern Hudson Bay, the body condition of bears in all age and
sex classes declined from 1984  1986 until 2000  2005. Decreases
in condition were most dramatic in pregnant females and subadult
bears. The reason for the change is unclear but is likely a
combination of factors that include later freeze up, earlier break-
up, unusual spring rain events and less snow may affect body
condition of polar bears in this population64.
Female polar bears show a high fidelity to general denning
areas17. It is important for females that they are able to reach the
denning area in autumn either by swimming or by walking on the
ice. In the autumn, there is a large inter-annual variation in ice
extent and distance between the ice edge and the den sites. We have
observed that the number of maternity dens at the island of Hopen
at Svalbard, the southernmost of the denning islands in this
archipelago, varied between 0 and 35 during 1994  2001. The
number of dens on Hopen was strongly negatively correlated to
the date of sea ice arrival at the island the previous autumn. In the
fall of 1999 when the ice did not arrive until close to Christmas, the
bears were not able to reach the island at all and there were no dens
there (Derocher et al., unpublished).
We do not know if those female bears that presumably would
have denned at Hopen had the ice cover been better in a particular
year, skipped a year or chose to den in other areas with better ice
condition that year. Nevertheless, the example indicates that if the
extent of sea ice decreases significantly in the future, certain areas
with suitable denning habitat will no longer be available to pregnant
female bears.
In northern Alaska, a high proportion of maternity dens are found
on drifting multiyear ice several hundred kilometres north of the
coast23. These dens drifted up to 1000 km during the denning
period and the mothers appeared to be able to successfully raise
cubs. The proportion of dens on the pack ice has now decreased
from about 62% in 1985  1994 to 37% in 1998  200465. It was
concluded that the denning distribution has changed in response to
reductions in stable old ice, increases in unconsolidated ice, and
lengthening of the melt season which has reduced the availability
and quality of pack ice denning habitat. At the same time a decline
in physical stature and survival of cubs in the area has been
observed66.
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4.1.4 Mortality
Warming may result in an increase of unusual rain events during the
winter and spring negatively affecting ringed seals and polar
bears67. An instance has been documented where rain during
spring caused the roof of a polar bear den to collapse killing the
mother and her cubs3.
The sea ice has decreased markedly in the Beaufort Sea during
the last decades47. During aerial surveys for marine mammals in the
Beaufort Sea in the late summer and autumn 1987  2004 a
particularly high number of swimming bears were observed
during 2004  a year with minimal ice cover. That same year,
four bears that presumably drowned in the open water between the
coast and the offshore pack ice during a strong storm were seen. It
was speculated that mortalities due to offshore swimming during
late-ice (or mild-ice) years may be an important source of mortality
that will increase in the future68.
In 2004, three confirmed instances of intraspecific predation and
cannibalism in polar bears were observed in the Beaufort Sea69.
This is exceptional since only two other similar instances have been
observed in the area during 35 years of relatively intense research. It
was hypothesized that such killings may reflect nutritional stress
related to the longer ice-free seasons that have occurred in recent
years in the Beaufort Sea.
The decrease in sea ice and earlier spring break-up has caused
polar bear populations to decline. Between 1987 and 2004 the
subpopulation of polar bears in western Hudson Bay has declined
by ca 22% from an estimated 1194 to an estimated 93570. Total
survival of prime adult male and female polar bears (age 5  19
years) was stable over the study period whereas survival of juvenile,
subadult and senescent polar bears was correlated with spring sea
ice break-up date which was ca 3 weeks earlier in 1984 than in
2004. This has caused the feeding time in spring to become shorter
and bears have started the fasting season on land in a leaner
condition than before.
A similar pattern was documented from the southern Beaufort
Sea where survival rates and body mass of cubs decreased
significantly from 1967 to 200666. The authors believed this was
due to a decreased physical condition of the mother. The survival
rate of older bears was also found to have decreased in recent years
as well as the size of adult males. All these effects are believed to
reflect nutritional stress in the population due to the changes in the
ice conditions in the southern Beaufort Sea.
162 Øystein Wiig, Jon Aars and Erik W. Born
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Global warming could affect the human caused mortality in polar
bears. More bears close to land will increase the availability of polar
bears to hunters. Furthermore. the earlier spring break-up and later
autumn freeze-up has resulted in a longer boating season in recent
years. This allows for an effective search for polar bears from boat
during a short period of time as opposed to the traditional sled trips
during spring. In Northeast Greenland (i.e. between about 69 N and
about 76 N), the fraction of polar bears that was shot from a boat as
opposed to during sled trips in spring increased from about 5% of
all bears killed in 1983  199171 to about 30% in 1994  199972. A
similar increase in the fraction of polar bears being taken from a
boat has been seen in northwestern Greenland during the last 10 
15 years (Born unpublished data).
However, this climate-related increase in hunting effort has been
mitigated by the introduction of quotas for the catch of polar bears
in Greenland in 2006.
An increase in polar bears occurring near or on land has resulted in
an increase in human  polar bear interactions. This has resulted in an
increase in number of polar bears killed in self defence. For example,
in Alaska the number of bears harvested for safety reasons has
increased from about three per year in the early 1990s to about 10 per
year 10 years later53.
4.2 Predicted effects
The effects of global warming on polar bears are likely to be several
and depend on the scenario and region under consideration. Some
effects will be direct whereas others will be indirect and therefore
likely more cryptic. Direct effects include the loss of sea ice habitat
whereas indirect effects include regional shifts in prey availability
and prey type. Both direct and indirect effects will affect geographic
range and movement, nutritional status, reproductive success and
survival. To polar bears and its major prey sea, ice is a critical
habitat the extent and quality of which have basic influence on their
life history and ecology3,50.
It has been suggested that if sufficient ecological changes occur,
polar bears may become extinct73. In 2005, the IUCN Polar Bear
Specialist Group concluded that during the next 35  50 years one
may expect more than a 30% decrease of the total polar bear
population of the world due to a reduction in ice cover9,74. In 2006,
the polar bear was listed as a threatened species on the IUCN Red
List75. In 2007, the US Fish and Wildlife Service calling on external
experts in climate change and polar bears evaluated the status of the
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polar bear in relation to the US   Endangered Species Act  and
proposed that the polar bear should be protected as a threatened
species (http://alaska.fws.gov/fisheries/mmm/polarbear/issues.htm).
The polar bear was listed as   threatened  in May 2008.
4.2.1 Distribution and movement
Under the scenarios of climate warming sea ice cover is expected to
retreat further into the Arctic basin, to break up earlier in the spring
and to freeze-up later in the autumn and to become thinner and
more mobile4. Due to their dependence on sea ice, polar bears will
probably not be found outside areas with winter ice in the future.
Inevitably, this will lead to a considerable decrease in their range.
It is likely that the Barents Sea and most of the East Greenland
Sea will be free of ice even in winter by 20804. If so, polar bears
will probably be extinct from this area. In the Siberian Seas where
the ice cover is expected to decrease markedly and to be distributed
north of the shelf76, the access to breeding areas like Severnaya
Zemlya and New Siberian Island will be more difficult for polar
bears. The future situation of the seas of the North American Arctic
is more uncertain since many aspects (e.g. presence of fast ice,
strong seasonality, complex water mass structure, through flow) of
these regions are not included in the current global climate models4.
It is, however, likely that most of the Bering Sea will be ice free
before the end of this century and consequently the distribution of
polar bears in the area will shrink.
At present. the majority of polar bears are found in Canada9. The
Canadian Archipelago is a large and complex shelf area for which it
is very difficult to draw conclusions regarding effects of global
warming4,77. However, the decrease in sea ice during 1979  2004
has been a minimal 0.4% per decade45 and it is likely that polar
bears living in the Canadian High Arctic will be least affected by
global warming. Large uncertainty also exists for the projected
changes in sea ice in the Labrador Sea.
The impact of climate warming will certainly vary among
subpopulations3. Polar bears on sea ice may spend most of the
year moving against the direction of the ice drift, to keep contact
with the preferred habitat78. The energetic cost of locomotion for
polar bears is twice that expected of a mammal of that body size79.
Hence, movements are energetically expensive. Increased energy
use could result in declining survival and reproductive rates. This is
a situation polar bear might encounter in areas like the Barents sea,
164 Øystein Wiig, Jon Aars and Erik W. Born
Black plate (165,1)
Davis Strait, Baffin Bay, northern and southern Beaufort Sea where
polar bears inhabit moving pack ice for an extended period of time.
Earlier spring break-up of ice could separate traditional den sites
of the polar bears from spring feeding areas. Young cubs are not
likely to survive for long in water close to freezing temperature as
they, in contrast to adults, have little subcutaneous fat34. Females
might thus in many areas be prevented from reaching hunting areas
with their cubs in bad ice years, this might be the first serious effect
lowering population growth rate in many places.
With an earlier ice break up in spring and later freeze up in the
fall the polar bears must spend more time on land in areas where the
drift ice does not persist during the open water period as, for
example, in western Hudson Bay, the Baffin Bay and Davis Strait48.
Even though studies so far have shown that polar bears of prime
age seems to tolerate shorter hunting seasons, the negative effect on
reproduction and survival of young animals could in many areas
lower the intrinsic population growth rate to less than 1, and if so,
ultimately lead to local extinctions (unless immigration from other
areas can counter the effect).
Overall, the decrease in the extent of sea ice will lead to a
decrease in the range of polar bears and likely their distribution will
be confined to areas where sea ice will remain for longer into the
future. This is to high latitude areas like the coast of northern
Greenland and areas with relatively stable continental climate like
the Canadian Archipelago80.
4.2.2 Feeding
Current predation patterns are essential for understanding the effects
of climate change on polar bears51. Disappearance of the ice will have
a profound negative effect on the pagophilic (ice-loving) seal
species  ringed seals, harp seals, hooded seals, spotted seals and
ribbon seals (Kovacs and Lydersen, accompanying article). Their
distribution and numbers will decline and therefore the polar bears
will have less of their   traditional  food available3,81. We suggest
that the polar bear and its main prey the ringed seal both evolution-
arily, behaviourally and ecologically are so intimately connected that
a substantial decrease in the abundant and widely distributed ringed
seals cannot readily be compensated for by switching to more locally
distributed and less numerous alternative prey. Ultimately this will
lead to a decrease in the number of polar bears.
However, as a part of their normal predatory life style polar bears
exhibit some degree of ecological and behavioural plasticity. Other
www.scilet.com Effects of climate change on polar bears 165
Black plate (166,1)
marine mammal species than ice breeding seals are to a lesser extent
hunted by polar bears. Also carcasses, fish and sea birds are known
to be eaten by polar bears. They have occasionally been observed to
be able to catch seals in open water. Some bears even hunt birds82,
and some may sustain summer at the colonies of sea birds feeding
on birds and egg19 or may start fishing83. Given both the
opportunistic nature of polar bears, and what seems to be a range
of hunting and feeding strategies beside their main adaptation as
expert seal hunters, it seems that local populations might be able to
compensate for some of the loss of ringed seals as food by using
other food sources.
4.2.3 Reproduction
In northern Alaska, the proportion of dens on the pack ice has
decreased in response to changes in ice condition. If the ice
conditions in the area continue to decline it was anticipated that
the number of bears denning on land will increase until the open
gap between the ice edge and the coast is too wide for the pregnant
females feeding in pack ice in summer to reach the coast in autumn
for den entry. A future reduced quality and quantity of pack ice
denning habitat and reduced access to coastal denning regions will
likely decrease the growth rate of this subpopulation65.
In the Hudson Bay in Canada, pregnant female polar bears spend
up to 8 months on land fasting. If the sea ice breaks up earlier in the
spring in the future, females will have less time to eat in spring and
must go on land much leaner than earlier. Their ability to bear
young will be reduced and the production of young will certainly
decrease3, a continuation of the trend already seen today.
A further increase in temperature and precipitation may directly
affect the ability of females to construct proper and safe dens in
traditional denning areas3. The snow fall has decreased47 and will
decrease to some extent in the future whereas the total precipitation
will increase4. The changes are most pronounced in late autumn and
in spring4, the former an important period for den site selection for
polar bears and the latter the period when females with cubs leave
the dens and heavily depend on fast access to prey. Furthermore,
rain-on-snow events in the Arctic are projected to increase in the
future84. Hence, the snow layer may not be of sufficient depth to
allow for the construction of a den and heavy rain in denning areas
might cause the den to collapse leading to increased mortality of
females and cubs.
166 Øystein Wiig, Jon Aars and Erik W. Born
Black plate (167,1)
4.2.4 Mortality
Survival rates are likely to be lower than today for polar bears in
areas with shorter hunting seasons in a future warming Arctic. More
unstable weather during the denning period may lead to collapse of
maternity dens. Cubs may die from exposure and thermal stress or
from being crushed under the collapsed roof. Furthermore, newly
emerged cubs that must follow their mother over longer stretches of
open water in order to get to suitable habitat are likely to suffer
from increased mortality from fatigue and drowning.
Less food will lead to starvation and death in some cases. All age
classes and both sexes may experience this but it is most likely that
young inexperienced polar bears and weak and old bears will die
from starvation as observed at present in the western Hudson Bay.
If bears are forced to spend more time on land with an increased
density along the coast during the open water period one may
expect a temporary increase in intraspecific predation, infanticide
and cannibalism. More bears on land and near human habitation
will lead to an increase in human bear conflicts and more bears
being shot in self defence.
Climate change associated with ice and precipitation and with the
function of ecosystems have the greatest potential to alter contami-
nant pathways and exposure85. The pathway of mercury to the Arctic
is not well known but it is believed to be through a climate-sensitive
process. The melting of permafrost may provide a mechanism by
which mercury trapped in organic carbon in the ground is released
and transported into the Arctic Seas by larger rivers.
Also organochlorines are transported to the Arctic through atmo-
spheric systems as well as by larger rivers and sea currents which
may be altered by climate change. When polar bears are under
nutritional stress caused by climate change, lipophilic pollutants
and their metabolites will be released in the blood stream. The
effect of this is uncertain but it has been suggested that on the long
term this may lead to increased mortality as well as decreased
reproductive success86,87 in areas like in East Greenland, the
Barents Sea and the Kara Sea.
4.3 Management implications
Polar bears have been hunted through centuries. They were totally
protected in Soviet Union in 1953. During the 1960s the five polar
bear nations negotiated the Agreement on the Conservation of Polar
Bears and their Habitat that was signed in Oslo, Norway, in 197340.
www.scilet.com Effects of climate change on polar bears 167
Black plate (168,1)
Based on the Agreement polar bears were also protected in Norway
(Svalbard) but could still be legally hunted in Greenland, Canada
and USA (Alaska). Today the hunting is regulated by quota systems
in each of these three countries9. Sixteen of the 19 subpopulations
generally recognized are hunted while three are protected (in
Norway and Russia).
If the ice pattern changes as predicted the movement pattern and
distribution of polar bears will change and probably will lead to
change in delineation between subpopulations. The number of bears
in the subpopulations will also change. It is important that such
changes are continuously monitored and that changes are incorpo-
rated into management plans in order to secure a continuous
sustainable harvest.
5. Conclusion
Many researchers have predicted a vide range of impacts of climate
change on polar bear demography and condition. In the western
Hudson Bay, a clear link has been established between climate
warming, reduced ice presence, and observed declines in polar bear
physical and reproductive parameters61. These effects have been
followed by a decrease in population size of more than 20% during
the last 20 years70. A predicted major reduction in sea ice habitat
will reduce the availability of ice associated seals, the main prey of
polar bears, and a loss and fragmentation of polar bear habitat will
ultimately lead to large future reductions in most subpopulations. It
is likely that polar bears will be lost from many areas where they
are common today and also that the total population will change
into a few more distinctly isolated populations. Given that polar
bears are rather flexible animals that opportunistically can switch to
alternative prey, it is very hard to predict how well they might cope
as a species in future. However, it is obvious that a major loss of
habitat will have a large impact and drastically change the distribu-
tion and number of polar bears.
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