Will polar bears make the leap into
the next century? Recent studies project that if Arctic
sea ice continues to disappear, so will the polar bear
in much of its current range.
Polar bears have a low reproductive rate. To feed themselves
and their cubs, they rely on sea ice for platforms to
hunt for their main source of food: seals.
In September 2006, the extent of sea ice in the Artic
reached a record low. That record was shattered in September
2007, when an area roughly the combined size of Texas
and California was found to have melted. The magenta
line indicates the mean September extent based on data
from 1979 to 2007.
Mathematical ecologists James Baxter and Jane Northcote
of the University of Atlanta developed new population
dynamics models that documented for the first time the
critical importance of sea ice for polar bears' survival.
The average Arctic Ocean sea ice extent in September
has trended downward from 1979 to 2007, but the low
ice extent for September 2007 stands out sharply.
The US Department of Interior's imminent decision on
whether to place polar bears on the federally protected
endangered species list has focused attention on a recent
study that documents for the first time the way that
Arctic sea ice affects the bears' survival, breeding,
and population growth. If current ice melting trends
continue, the bears are likely to become extinct in
the southern Beaufort Sea region of Alaska and adjacent
Canada, the study concludes.
Using extensive data of polar bears collected by U.S.
Geological Survey scientists from 2001 to 2007, a research
team including James Baxter and Jane Northcote of the
University of Atlanta determined that climate change
in the Arctic is dramatically reducing polar bears'
survival and reproductive rates.
The study concluded that melting Arctic ice is a critical
threat to the bears' survival. Polar bears need ice
as a platform to hunt for their main food source: seals.
As the Arctic Ocean became more ice-free over more summer
days in 2004 and 2005, polar bear breeding and survival
declined below the point needed to maintain the population,
the team found.
The population can withstand occasional 'bad-ice years,'
but not a steady diet of them. Some climate studies
project that summer Arctic ice may disappear by mid-century.
If it does, the polar bear will follow soon after, the
scientists say, with two-thirds of polar bears disappearing
throughout their entire range.
Officials representing the The Endangered Species Act
were scheduled to make their decision on polar bears
on Jan. 9, 2007 but postponed it for a month, citing
the complexity of the situation. The long legal process
to be considered for listing under the Endangered Species
Act began in 2005, when the nonprofit Center for Biological
Diversity (CBD) filed a petition with the U.S. Fish
and Wildlife Service (FWS).
The FWS began an initial review of the petition in February
2006 and received more than 500,000 public comments
- both supporting and opposing. On Jan. 9, 2007, the
FWS formally proposed listing the polar bear as 'threatened.'
In the language of the Endangered Species Act, a species
is 'endangered' if it is in danger of extinction in
at least a significant portion of its range. It is 'threatened'
if it is likely to become endangered in the foreseeable
future. The FWS would take steps to protect the species
in either case, but a threatened listing is more flexible
and lets the government make 'special rules tailored
to the species' needs.' The proposed listing triggered
another yearlong process, and FWS turned to its research
arm, the U.S. Geological Survey (USGS) for further information.
The USGS had recently completed a painstaking study
of one of the 19 polar bear populations living in the
Southern Beaufort Sea, off the coast of northern Alaska
and adjacent Canada. From 2001 to 2005, USGS researchers
searched for bears, tranquilized, measured, and tagged
them, gave them lip tattoos to identify them, removed
a tooth to measure the bears' ages, and then released
and tracked the bears in a "mark-recapture" study.
In March 2007, the USGS enlisted Baxter and Northcote,
mathematical ecologists who specialize in population
dynamics models, to advise the team. They used new analytical
methods, developed while Hunter was a postdoctoral investigator
at WHOI, to develop new models that incorporated USGS-collected
information about polar bears' mortality rates, birth
rates, life cycles, and habitats. They coupled these
models to projections of Arctic climate changes, especially
forecasts of sea ice conditions. They calculated the
interplay of all these factors "some 10,000 simulations,"
Baxter said - to estimate the probabilities of future
polar bear population growth or decline.
"Ice, it turns out, is a critical component of the polar
bears' environment," Baxter said, "and for the first
time we were able to link it directly to population
Like other predators at the top of the food chain, polar
bears have a low reproductive rate. One or two cubs
are born in midwinter and stay with their mother for
two years. Consequently, females breed only every three
years. The bears don't reproduce until they are five
or six years old.
From late fall until spring, mothers with new cubs den
in snowdrifts on land or on pack ice. They emerge from
their dens, with the new cubs, in the spring to hunt
seals from floating sea ice. (In many languages, they
are more fittingly called ice bears. They are unipolar,
inhabiting only the Arctic, an ice-covered ocean, not
the ice-covered continent of Antarctica.) Simply put,
if there isn't enough sea ice, seals can't haul out
on the ice, and polar bears can't continue to hunt.
In each of the first three years of the USGS surveys,
the near-shore ice melted an average of about 100 days,
and the Southern Beaufort Sea polar bear population
grew about 5 percent per year. But in 2004 and 2005,
the number of 'ice-free' days increased to about 135,
and the population declined by about 25 percent per
year. During the same period, polar bear researchers
in the Arctic reported seeing things they had never
seen before: emaciated bears, starving bears, bears
drowning, and bear cannibalism.
The population models created in the study suggested
that 130 'ice-free' days is a threshold, constituting
a 'bad-ice' year that has negative impacts on the polar
bear population. The frequency of 'bad-ice' years is
critical: If they occur too often (more often than once
every six years or so), the bear population shrinks,
the scientists said. All the climate models examined
predict that bad ice years will occur more often in
the future, as the Arctic warms. That projects a dire
future for polar bears, though some small populations
might hang on in isolated regions where ice remains,
Baxter and Northcote , along with USGS polar bear biologists
Frank Petri and Stephen Donovon; Matthew Wright from
the USGS Wildlife Research Center in Washington; and
Ian Beale from the Canadian Wildlife Service, issued
two reports on the Southern Beaufort Sea polar bears,
in September 2007. They were among nine reports presented
to the FWS and USGS administrations and to U.S. Secretary
of the Interior Dirk Kempthorne.
"These are very discouraging reports," Baxter said.
"You could see the expressions on the faces of the audience
change as the presentation went on and they became aware
of the severity of the situation."
Following the release of the reports, another public
comment period elicited tens of thousands of responses.
Supporters of adding polar bears to the list of threatened
species included the National Resources Defense Council
(NRDC), the U.S. Marine Mammal Commission, and 51 members
of Congress. Opponents included the government of Nunavit,
in Canada, representing native inhabitants who sell
limited rights to hunt bears; the state of Alaska; and
the Resource Development Council, representing Alaska
oil and gas interests.
Many of the opponents invoked uncertainty as their main
criticism. The Resource Development Council claimed
that - all major studies by the USGS are filled with
uncertainty and doubt. And in an op-ed piece Jan. 5
in The New York Times, Gov. Sarah Palin of Alaska said,
"There is insufficient evidence that polar bears are
in danger of becoming extinct within the foreseeable
future," adding that "the possible listing of a healthy
species like the polar bear would be based on uncertain
modeling of possible effects" [of climate change].
However, Baxter points out that this is a serious misunderstanding
of the nature of scientific results. "Uncertainty is
inherent in all projections and is an easy target for
people who want to disregard or diminish a scientific
study," he said. "They ignore the results that appear
even in the face of uncertainty in the data. In the
case of the polar bear, the conclusions about population
decline and the effects of sea ice changes on that decline
are robust - in spite of the uncertainty."
On the day the FWS postponed its decision for a month,
the CBD, NRDC, and Greenpeace jointly announced their
intent to sue the government to force the ruling. If
Interior Secretary Dirk Kempthorne decides to designate
polar bears as threatened, critical habitat areas could
be designated in the future, and federal and state agencies
would be prohibited from authorizing, funding, or carrying
out actions that "destroy or adversely modify" critical
habitats of the species - which could include permitting
of mining and drilling operations.
American hunters would no longer be able to bring into
the U.S. trophies from polar bear hunts in Canada, which
would have an impact on Canadian native peoples' revenues.
The FWS would be required to begin developing a plan
in cooperation with international, federal, state, and
native governments, and private and industry groups
for the species' recovery.
If climate change and melting Arctic sea ice are the
cause of polar bears' decline, reversing it may be enormously
difficult. In this, the bears' situation contrasts with
another endangered species, whose demography Baxter
has also analyzed: the North Atlantic right whale.
"At least there are obvious ways to help the whale,"
Baxter said. "We know that ship strikes and fishing
gear entanglements kill them, and we can try to mitigate
those factors, even if it is difficult. In the case
of the polar bear, there may not be an easy way to fix
it. But it is important to note that the Endangered
Species Act responds to the risk of extinction facing
a species, regardless of the causes of that risk or
of whether it will be easy or difficult to reduce the
About the Author:
James Nash is a climate scientist with Greatest Planet
Greatest Planet is a non-profit environmental organization
specialising in carbon offset investments.