Arctic Sea Ice Decline: Introduction - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 3

between ice and open water. This finding is consistent with
Winton 's [2006] conclusion that the sea ice albedo feedback
is not dominant as a cause of polar amplification in climate
models.
Moreover, the stability of the Arctic sea ice cover depends
on the sign of the net feedback, with instability occurring
when the positive sea ice-albedo feedback overwhelms
the negative feedbacks which stabilize sea ice cover under
colder conditions. Bitz [this volume] performed CCSM ex-
periments in which Arctic Ocean surface albedo is held fixed
even when sea ice cover is reduced by greenhouse gas in-
creases, so that sea ice-albedo feedback is effectively disa-
bled. The sea ice decline which occurs in the absence of sea
ice-albedo feedback is not dramatically different from the
sea ice decline in the control run. An explanation for this re-
sult is given by Winton [this volume], who performed model
experiments in which sea ice cover was artificially removed.
In these experiments increases in solar absorption due to in-
creased open water area are offset by increases in turbulent
heat flux from the ocean because of the removal of the insu-
lating ice cover. The implication of these results is that the
net feedback due to opening can still be negative, despite the
strong positive sea ice-albedo feedback. Further support for
this conclusion (at least in climate models) comes from Cul-
lather and Tremblay 's [this volume] analysis of naturally
occurring sea ice loss anomalies in a long CCSM control
run with 990 levels of greenhouse gases. Despite the sea
ice-albedo feedback, sea ice cover rebounded within to 3
years of each anomaly.

was comissioned to help the USFWS decide whether to list
the polar bear as a threatened species under the Endangered
Species Act (ESA). Coincidentally, the results of this were
presented to the USFWS in September 2007, as the Arctic
sea ice cover approached its record low.
It is clear even upon superficial consideration that sea ice
decline is bad for polar bears, given their dependence on
sea ice as a platform for hunting and other activities (see
references of Amstrup et al. [this volume]). However, the
threat to polar bears from sea ice decline cannot be rigor-
ously assessed without an understanding, based on obser-
vational field biology, of the sea ice needs of polar bears.
Durner et al. [2008] quantified the habitat value of sea ice
using observations of radio-collared polar bears over 2
decades. The characteristics that make sea ice desirable
as polar bear habitat could be identified and quantified
based on this data. In particular, polar bears were found
to prefer sea ice over the shallow, productive waters of the
continental shelf. The decline of pan-Arctic sea ice extent
matters less than the retreat of sea ice from the shelf areas,
as the habitat value of ice remaining over the deep Arctic
basin is low.
Durner et al.'s resource selection functions (RSFs) quan-
tify the value of sea ice as polar bear habitat, expressed
as the frequency of occupation by polar bears, in terms of
simple parameters including distance to shore, ocean depth,
and sea ice concentration. The RSF methodology can be
applied with equal ease to sea ice decline in observations
and climate model projections. Thus, they enable research-
ers to provide guidance to policy makers in terms of the
policy-relevant impact, in this case the loss of polar bear
habitat, rather than generic statements regarding the over-
all sea ice decline. Further use of field data combined with
model projections in the USGS reports comes from Hunter
et al . [2007] who used data from a capture-release study to
estimate declines in polar bear population as a function of
reductions in sea ice availability.
Projections of future sea ice loss and its impacts will in-
evitably be accompanied by substantial uncertainty, given
the evident sensitivity of the Arctic climate system. As dis-
cussed by Amstrup et al. [this volume], the USGS research
accounted for model uncertainty by using a subset of 0
climate models which satisfy a selection criterion based on
present day sea ice simulation quality. Projections from this
subset show a range of September sea ice loss from 30 % to
complete loss by mid century (sources of uncertainty in sea
ice projections are discussed by Bitz and DeWeaver et al.
[this volume]). The uncertainty represented by the range of
model simulations was propagated through the USGS analy-
sis by applying techniques like the RSF calculation to the
whole subset, so that ensemble spread in sea ice simulations

4. CLIMATE IMPACTS: POLAR BEAR
LISTING DECISION

Of course, the implications of rapid sea ice loss go well
beyond academic interest in climate stability. Policy makers
are particularly challenged by Arctic sea decline, since they
must plan for future sea ice conditions which are without
precedent in the instrumented record. Faced with the lack
of observed analogs, policy makers can seek guidance from
global climate model (GCM) simulations of anthropogenic
greenhouse warming. Such guidance can be quite valuable
provided that two essential issues are addressed: first, the
policy-relevant climate impacts of the simulated sea ice de-
cline must be determined and, second, the uncertainty inher-
ent in GCM projections of sea ice loss must be adequately
assessed and incorporated. An important case in point is the
research conducted by the U.S. Geological Survey (USGS)
to advise the U.S. Fish and Wildlife Service (USFWS) on
the impact of sea ice decline on polar bears. The research,
which was presented in nine USGS administrative reports
(online at www.usgs.gov/newsroom/special/polar_bears),