Arctic Sea Ice Decline: Introduction - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 4

leads to ensemble spread in polar bear outcomes. However,
Amstrup et al. note that these projections may be overly op-
timistic, given Stroeve et al .'s [2007] finding that real-world
Arctic sea ice has declined at almost twice the rate found
in model simulations of the recent past. The USGS efforts
culminated in Amstrup et al.'s synthesis report, which uses
a Bayesian framework to assess the probability of decline
in polar bear population based on consideration of sea ice
decline and other factors. Despite the uncertainties of the re-
search, none of the outcomes were favorable for polar bears;
in effect, they run the gamut from bad to extremely bad.
In May 2008 the polar bear was listed as a threatened spe-
cies under ESA, after considerable delay. It is clear from
the final rule [ U.S. Fish and Wildlife Service , 2008] that
the policy makers understood and considered the scientific
guidance. Consideration of the science is also evident from
the announcement of the decision (www.doi.gov/secretary/
speeches/08405_speech.html), which included a prominent
display of Stroeve et al .'s [200X] work on observed and sim-
ulated sea ice trends, and maps of Arctic sea ice showing
the change in coverage by old (at least 5 years) and new
(less than 5 years old) ice, apparently from the drift model
of Rigor and Wallace [2004]. But while the effort to pro-
vide scientific input for the listing decision was successful
in some sense, it remains to be seen if the listing will have
any direct effect on the status of the polar bear (see analysis
of Revkin [2008]).

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5. CONCLUSION

The events of 2007 and 2008 highlight the need for im-
proved understanding of sea ice sensitivity and the impacts
of sea ice decline. Perhaps, if we are fortunate, our under-
standing of the Arctic sea ice and climate system can evolve
fast enough to keep pace with the changes occurring there.

Acknowledgments .The author's research is supported by the Of-
fice of Science (BER), U.S. Department of Energy, grant DE-FG02-
03ER63604. I thank Cecilia Bitz, Steven Amstrup and members of
the NCAR Polar Climate Working Group for helpful conversations.
I am indebted to the chapter authors for contributing their best work
to this monograph.

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