The Role of Natural Versus Forced Change in Future Rapid Summer Arctic Ice Loss - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 129

The Role of Natural Versus Forced Change in Future Rapid
Summer Arctic Ice Loss

Marika M. Holland, 1 Cecilia M. Bitz, 2 L.-Bruno Tremblay, 3 and David A. Bailey 1

Climate model simulations from the Community Climate System Model, version
3 (CCSM3) suggest that Arctic sea ice could undergo rapid September ice retreat
in the 21st century. A previous study indicated that this results from a thinning of
sea ice to more vulnerable conditions, a “kick” in the form of pulse-like increases
in ocean heat transport and positive feedbacks that accelerate the retreat. Here we
further examine the factors affecting these events, including the role of natural
versus forced change and the possibility of threshold-like behavior in the simulated
sea ice cover. We find little indication that a critical sea ice state is reached that
then leads to rapid ice loss. Instead, our results suggest that the rapid ice loss events
result from anthropogenic change reinforced by growing intrinsic variability. The
natural variability in summer ice extent increases in the 21st century because of
the thinning ice cover. As the ice thins, large regions can easily melt out, resulting
in considerable ice extent variations. The important role of natural variability in
the simulated rapid ice loss is such that we find little capability for predicting these
events based on a knowledge of prior ice and ocean conditions. This is supported
by results from sensitivity simulations initialized several years prior to an event,
which exhibit little predictive skill.

1. INTRODUCTION are present in the atmosphere, ocean, sea ice and terrestrial

components (e.g., see Serreze et al . [2000] and Overland et
al . [2004] for reviews). One of the most striking of these
changes is the recent decrease in the summer Arctic sea ice
cover. Over the satellite record (1979-2007), a decrease
of 0.7 million km 2 per decade in the September ice cover
has occurred, amounting to a loss of almost 30% of the ice
cover in 29 years. This change is not linear over the 1979
to present record but instead has shown accelerated retreat
in more recent years [ Serreze et al ., 2003; Stroeve et al .,
2005]. The recent minimum for 2007 [ Stroeve et al ., 2008]
was particularly dramatic, being about 1.3 million km 2 lower
than the previous 2005 September record of 5.6 million km 2
[ Fetterer et al ., 2002]. This represents a 3 standard deviation
excursion from the linear trend. Additionally, the August
mean ice extent surpassed the September minimum in 2005.
While the sea ice change is largest in summer, a decrease in
Arctic ice cover since 1979 is present in all months [ Serreze
et al ., 2007].

In recent years, the Arctic has undergone dramatic changes
throughout the system that are consistent with increasing
anthropogenic forcing combined with large natural variabil-
ity [ Serreze et al ., 2007]. These changes are widespread and

1 National Center for Atmospheric Research, Boulder, Colorado,
USA.
2 Department of Atmospheric Sciences, University of Washington,
Seattle, Washington, USA.
3 Ocean and Atmosphere Sciences, McGill University, Montreal,
Quebec, Canada.