Arctic Sea Ice Decline: Introduction - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 2

located with the largest sea ice loss in the Chukchi and East
Siberian seas. On the other hand, a strong role for insola-
tion as a positive feedback is not in dispute. Perovich et al .
[2008] find a 500% increase in January to September solar
heat input to the Beaufort Sea compared to the 979—2005
climatology. They further determine that the large increase
is due to the large area of low-albedo ocean surface exposed
by the dramatic sea ice retreat. Accompanying the increased
heat uptake, they report a sixfold increase in bottom melt
measured by an ice mass buoy in the Beaufort Sea. Their
results thus document the classical sea ice-albedo feedback,
presumably initiated by wind-driven opening of the ice pack.
The modeling study of Zhang et al . [2008] concludes that
70% of the 2007 loss anomaly was due to amplified melting
while 30% resulted from ice motion.
In these studies, the meteorology of 2007 is generally
given less prominence than the vulnerability of the 2007 sea
ice cover. Maslanik et al. and Nghiem et al. document the
long-term change from multiyear sea ice to younger floes
which are thinner and more prone to breakup and melting.
Overland et al. and Kay et al. also question the novelty of
the 2007 meteorological conditions. They relate the offshore
winds and sunny skies of 2007 to a surface high over the
western Arctic Ocean, a rare but not unprecedented occur-
rence. Four years with comparable high pressure can be seen
in the 50-year record shown by Overland et al. (their Figure
11), while Kay et al. find four additional years with sunnier
skies than 2007. The older, thicker ice in these earlier years
was not dramatically affected by the adverse meteorology.
The contribution of greenhouse warming in producing
sharp, single-year declines is not easily quantified, since
warming favors these events indirectly as it helps precondi-
tion the ice to a thinner state (e.g., Overland et al.). However,
Stroeve et al . [2007] point out the consistency of the 2007
event with the periods of rapid loss found by Holland et al .
[2006] in global warming simulations. Stroeve et al. note in
particular the similarity between the March 2007 thickness
estimates of Maslanik et al. and the mean Arctic thickness in
simulations analyzed by Holland et al. The analysis of Hol-
land et al. is expressed in terms of a three-part conceptual
framework in which ice is first preconditioned for rapid loss
by decades of thinning, after which loss is “triggered” by nat-
ural variability and then amplified by the sea ice-albedo feed-
back. The “preconditioning, trigger, feedback” framework
was developed by Lindsay and Zhang [2005] to account
for the observed sea ice decline from 988 to 2003, and the
same framework was invoked in the Zhang et al. study of the
2007 event. Thus, while the 2007 loss was unprecedented,
descriptions of it are quite consistent with descriptions of
the longer-term Arctic losses of the recent past and the rapid
declines found in simulations of future Arctic change.

3. PAST THE TIPPING POINT?

Has the Arctic sea ice passed a tipping point? This is per-
haps the most consistently asked question in news accounts
about the 2007 and 2008 losses. Understandably, respond-
ents to the question have not voiced much hope for reversal:
“It's hard to see how the system may come back (I. Rigor,
quoted by Kizzia [2008])”; “I'm much more open to the
idea that we might have passed a point where it's becoming
essentially irreversible” (J. M. Wallace, quoted by Revkin
[2007]); “It's tipping now. We're seeing it happen now” (M.
Serreze quoted by Borenstein and Joling [2008]). No doubt,
the tipping point terminology aptly captures the precipitous
loss of 2007 and lack of recovery in 2008. But questions re-
main as to how literally the tipping language should be taken.
In a formal sense, tipping refers to a sudden and irreversible
transition between two stable states of a system (e.g., right
side up versus overturned), occurring as the system crosses
some threshold value of a key parameter (e.g., angle to the
local vertical). The scientific challenge would then be to find
and characterize the stable states and threshold values of the
Arctic sea ice system.
The idea of an unstable transition between ice-covered
and ice-free Arctic states has a long history (see references
of Winton and Merryfield et al . [this volume]), and such be-
havior does occur in simple energy balance models with dif-
fusive heat transfer (the “small ice cap instability” of North
[984]). However, unstable transitions are somewhat elusive
in global climate models, as Winton [this volume] shows.
The rapid loss events in simulations of the Community
Climate System Model (CCSM) shown by Holland et al .
[2006] are commonly compared to the recent Arctic losses,
yet threshold values for sea ice cover and thickness were
not found for the CCSM events. Instead, the authors argue
that rapid loss can occur through the superposition of natural
variability and a steady downward trend (further analysis of
these simulations is given by Holland et al . [this volume],
Merrifield et al . [this volume], Stern et al . [this volume],
and Gorodetskaya and Tremblay [this volume]. The lack
of identifiable thresholds in CCSM is significant, since the
yearly sea ice losses during CCSM rapid declines are larger
than the 2007 loss observed by Holland et al., despite the
absence of easily identifiable tipping points.
The primary motivation for claims of a tipping point
comes from the destabilizing effect of the sea ice-albedo
feedback. No doubt this is a strong feedback, but there is
some subtlety in assessing its strength. Gorodetskaya and
Tremblay point out that the effect of sea ice removal is miti-
gated by the cloudiness of the Arctic in summer, and note
that the presence of sea ice reduces the top-of-atmosphere
albedo by only 10 to 20%, despite the large albedo contrast