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survive the melt season in order to result in a large ice extent
change. The lower mean ice extent state from which these
increases occur is also likely important. More specifically,
with smaller summer ice cover and a larger distance between
the ice edge and coastline, there is a larger potential for ice
edge expansion.
While our analysis and additional sensitivity simulations
suggest little predictive capability for these abrupt ice loss
events from a knowledge of prior ice and ocean conditions, we
can clearly identify some factors that are necessary for them
to occur. Probably the most critical is that the ice cover is ad-
equately thin. In the IPCC-AR4 CCSM3 simulations this thin-
ning occurs in response to rising greenhouse gas concentrations
and as such, abrupt ice loss events are not present in the 20th
century integrations. Another necessary factor is that there is
ample natural or intrinsic variability in the modeled system to
initiate these events. Finally, large positive feedbacks (such as
the surface albedo feedback) are required to accelerate the ice
loss. Simulations from various climate models undoubtedly
vary in all of these aspects of their simulations, which is likely
why they also differ in their simulation of abrupt ice loss.
The thin ice conditions that are necessary for the simu-
lated abrupt ice loss to occur may have some practical im-
plications for understanding observed ice loss. While the
large-scale time-varying aspects of Arctic ice thickness are
difficult to observe, other related variables (such as ice age)
are more easily monitored. There are indications that the
Arctic ice age has decreased considerably [ Maslanik et al .,
2007], which suggests that the Arctic sea ice may well be
primed for the type of rapid ice loss simulated by CCSM3.
Indeed, the conditions in the summer of 2007 and the syn-
optic (likely “natural”) atmospheric variability implicated
in the 2007 ice loss [ Stroeve et al ., 2008] have some broad
similarities to the processes responsible for simulated rapid
ice loss in the CCSM3 modeled system. It is possible that the
Arctic is currently undergoing rapid sea ice loss not unlike
the abrupt ice loss events simulated by CCSM3 for later this
century. However, we will only be able to determine this
after several more years of observations.
(NCAR). Additional simulations were performed by CRIePI using
the earth Simulator through the international research consortium
of CRIePI, NCAR, and LANL under the Project for Sustainable
Coexistence of Human Nature and the earth of the Japanese Minis-
try of education, Culture, Sports, Science and Technology.
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Acknowledgments. M.M. Holland would like to acknowledge
NASA grant NNG06GB26G. C.M. Bitz was supported by NSF OPP-
0454843. B. Tremblay was supported by the Natural Sciences and
engineering Research Council of Canada Discovery Grant Program
and by the National Science Foundation under grant OPP-0230325
from the Office of Polar Programs and grant ARC-05-20496 from
the Arctic Science Program. D.A. Bailey was supported by NSF
OPP-0612388. We thank eric DeWeaver, Bill Chapman, and an
anonymous reviewer for helpful comments that led to improve-
ments in this manuscript. We also thank the community of scien-
tists involved in developing CCSM. Computational facilities have
been provided by the National Center for Atmospheric Research
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