What Is the Trajectory of Arctic Sea Ice? - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 180

increase in surface air temperature led to the disappearance
of sea ice in August and September. Also in 1979, Kellogg
[1979, p. 85] wrote, “There are good reasons to believe that
the Arctic Ocean may have just two stable states, a largely
frozen-over one (as at present) and an ice-free one.” What
is new today compared to 1979 is the sustained downward
trend in Arctic sea ice, capped by record-shattering losses in
August and September 2007. Using data from the national
Snow and Ice Data Center [ Fetterer et al. , 2002], we find
that the best linear fit of September sea ice extent versus year
(1979-2006) leaves residuals with standard deviation 4.2 ×
10 5 km 2 , in terms of which the September 2007 sea ice extent
falls 4 standard deviations below the trend line. This is the
type of abrupt loss of summer sea ice simulated by CCSM3
starting in the year 2024 [ Holland et al. , 2006b, this vol-
ume], and it continues the pattern of observed ice extent de-
clining faster than model predictions [ Stroeve et al. , 2007].
One could argue that Arctic sea ice is now entering a new re-
gime. In any case, it seems likely that increasing greenhouse
gases, driving increasing air temperatures, will eventually
lead to summers without sea ice in the Arctic Ocean. The in-
terplay between the internal dynamics of the climate system
and the external forcing will determine the extent to which
this outcome is achieved sooner rather than later. Whether
or not CCSM3 or the real climate system actually has a “tip-
ping point” is still unknown.
Several extensions of this work are possible. One could
divide the Arctic Ocean and peripheral seas into regions, and
construct and analyze the trajectory of sea ice in each region.
One could add a third category of ice thickness to the present
thin ice and thick ice (e.g., medium thick ice), thereby cap-
turing more of the variance. A more interesting extension
would be to construct a simplified physical model of the
evolution of thin ice and thick ice, rather than an empirical
model. This was proposed by Stern et al. [1995], based on
the framework of Thorndike et al. [1975] and Hibler [1980].
One could then analyze the stability of the physical model
and attempt to attribute the observed changes in sea ice to
changes in the external forcing and to the influence of inter-
nal dynamics.

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Acknowledgments. We thank j. Zhang for providing the PIO-
MAS model output, and we acknowledge the computational facili-
ties of the national Center for Atmospheric Research (nCAR) for
the CCSM3 model output. We thank the Editor, Eric DeWeaver,
for his helpful comments. This work was supported by the nASA
Cryospheric Sciences Program (grant nnG06GA84G) (H.S. and
R.L.) and the NSF Office of Polar Programs (grant 0454843) (C.B.
and P.H.).