Some Aspects of Uncertainty in Predicting Sea Ice Thinning - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 60

Some Aspects of Uncertainty in Predicting Sea Ice Thinning

Cecilia M. Bitz

Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA

A high proportion of the uncertainty in the decline of Arctic sea ice thickness in
recent global climate models can be explained by the uncertainty in the ice thickness
in the late 20th century. Experiments with one model indicate that this sensitivity
to the mean state remains even when ice-albedo feedback is eliminated from the
model. The magnitude of ice-albedo feedback is quantified and found to be too
small to be a major source of uncertainty in thickness decline in climate models.
Instead, it is shown that the sea ice growth-thickness feedback in combination
with large biases in the sea ice thickness during the 20th century can easily give
rise to very large uncertainty in future thickness decline. Reducing biases in the
surface fluxes and better tuning the surface albedo would improve uncertainty in
both present and future prediction.

Some have claimed that a model's success at simulating
the observed mean climatology and recent trends is a metric
of model reliability for future forecasts [e.g., Stroeve et al. ,
2007]. Indeed, it has been shown that the mean state of sea
ice strongly influences trends in the volume of Arctic sea ice
in a given model. Gregory et al. [2002] found this to be true
in a global climate model when they noted that sea ice vol-
ume declined more rapidly in the early 21st century simula-
tion, when the sea ice was thicker and more extensive, and
the rate of decline slowed long before the ice disappeared.
The rate change could not be explained by a difference in
forcing. This result may seem surprising if one expects that
ice-albedo feedback increases as sea ice thins and therefore
might cause sea ice decay to accelerate in the 21st century
(e.g., see Holland et al. [2006a] and counter arguments by
Winton [this volume]).
Bitz and Roe [2004] explained Gregory et al. 's [2002]
results in terms of a strong negative feedback that depends
inversely on sea ice thickness. When subject to an increase
in downwelling longwave radiation, perennial sea ice melts
faster during the melt season, but it also tends to grow faster

1. INTRODUCTION

Large and rapid changes in the Arctic sea ice in the past
few decades have attracted attention to future sea ice predic-
tions in global climate models. Models are consulted to see
if future changes will continue at the current pace or if they
will accelerate or decelerate [e.g., Holland et al. , 2006b]. But
uncertainty (i.e., spread) in the 21st century sea ice predic-
tions in the models used for the Intergovernmental Panel on
Climate Change Fourth Assessment Report is considerable
[ Arzel et al. , 2006; Zhang and Walsh , 2006]. Understanding
the cause for this uncertainty could help scientists interpret
the results of current models, and may help reduce the uncer-
tainty in developing future models.