” … polar bears’ numbers have not yet shown significant decline.
“However, escalating habitat losses anticipated due to global
warming and other pressures are expected to severely impact
bear populations …”
Public release date: 11-Mar-2008
University of Wisconsin-Madison
Contact: Eric DeWeaver
Arctic climate models playing key role in polar bear decision
MADISON – The pending federal decision about
whether to protect the polar bear as a threatened
species is as much about climate science as it is
about climate change.
The U.S. Fish and Wildlife Service (FWS) is
currently considering a proposal to list the
polar bear as a threatened species under the
Endangered Species Act, a proposal largely based
on anticipated habitat loss in a warming Arctic.
Climate models – mathematical representations of
the natural processes affecting climate –
factored heavily in the scientific information
requested by the FWS to guide its official
recommendation, which was due Jan. 9. While
scientists have used such models for decades,
their use in this decision demonstrates the
growing recognition of the value of modeling to
predict future climate conditions and inform
Eric DeWeaver, the physical climatologist on the
International Polar Bear Science Team and a
professor of atmospheric and oceanic sciences in
the University of Wisconsin-Madison, evaluated
existing climate models to identify those that
best represent observed changes in sea ice – a
crucial component of polar bear habitat – and
which are expected to best predict future
conditions in the Arctic.
His findings, detailed in a U.S. Geological
Survey report provided to the FWS, were applied
in subsequent reports to predict how Arctic sea
ice changes over the next 100 years will likely
affect polar bear populations.
These reports, available online at
formed the basis of the scientific guidance
requested by the FWS.
Climate models strive to represent the physical
laws that govern climate systems to forecast how
climate will respond to changes, such as
greenhouse gas increases. Due to the variability
of natural systems and the difficulty of
mathematically representing such complex systems,
all models contain some element of uncertainty,
“A climate model is not a crystal ball,” he says.
“It’s impossible to make a perfect representation
of climate… There are choices you make in model
development that lead to a range of model
behaviors. Often it is not possible to say that
one [model] is better than another.”
A discussion of the uncertainty inherent to
climate models sometimes creates the impression
that the models cannot provide useful
information, he says, which is absolutely not the
Instead, he likens climate modeling to other
predictive sciences like weather forecasting and
economics. While short-term predictions may
accurately pinpoint specifics, longer-scale
projections are expected to reveal bigger-picture
trends but fewer details.
For Arctic sea ice, the trend is clear, DeWeaver
says – all models point to widespread reductions
in sea ice in coming decades. What’s less certain
is how much melting to expect and how quickly.
Since each model represents climate in a slightly
different way, the exact degree of melting – and
timing of the first occurrence of an ice-free
Arctic – vary from model to model.
Far from being a drawback, these variations in
model output are “enormously helpful in
understanding a range of outcomes,” DeWeaver
says. “Having a multi-modal ensemble gives you a
way to boil things down to the essentials,”
identifying the most robust changes consistent
across several models.
Anticipated climate change has been a key element
of the polar bear equation throughout the entire
listing process, he says. Unlike most species
considered for federal protection, polar bears’
numbers have not yet shown significant decline.
However, escalating habitat losses anticipated
due to global warming and other pressures are
expected to severely impact bear populations in
the near future, according to the listing
petition filed by the advocacy group the Center
for Biological Diversity.
In the scientific reports filed with the FWS, the
climate models predict a loss of more than 40
percent of prime spring and summer polar bear
habitat by 2050, based on current rates of
greenhouse gas production. Polar bear biologists
believe these losses will lead to the demise of
more than 60 percent of the current population
within the next 50 years, with near-extinction
likely by the end of the century.
The application of climate science to this
decision is a win-win situation for both
scientists and policymakers, with the need for
information driving advances in basic scientific
knowledge and improved policy, DeWeaver says. “It
sets a precedent that yes, you can use models
[that include] uncertainty – and that’s good,” he
— Jill Sakai, (608) 262-9772, email@example.com