Sea Ice–Albedo Feedback and Nonlinear Arctic Climate Change - Arctic Sea Ice Decline: Observations, Projections, Mechanisms, and Implications - page 108

Sea Ice-Albedo Feedback and Nonlinear Arctic Climate Change

Michael Winton

Geophysical Fluid Dynamics Laboratory, NOAA, Princeton, New Jersey, USA

The potential for sea ice-albedo feedback to give rise to nonlinear climate change
in the Arctic Ocean region, defined as a nonlinear relationship between polar and
global temperature change or, equivalently, a time-varying polar amplification,
is explored in the Intergovernmental Panel on Climate Change climate models.
Five models supplying Special Report on Emissions Scenario A1B ensembles for
the 21st century are examined, and very linear relationships are found between
polar and global temperatures (indicating linear polar region climate change) and
between polar temperature and albedo (the potential source of nonlinearity). Two
of the climate models have Arctic Ocean simulations that become annually sea
ice-free under the stronger CO 2 increase to quadrupling forcing. Both of these runs
show increases in polar amplification at polar temperatures above −5°C, and one
exhibits heat budget changes that are consistent with the small ice cap instability
of simple energy balance models. Both models show linear warming up to a polar
temperature of −5°C, well above the disappearance of their September ice covers
at about −9°C. Below −5°C, effective annual surface albedo decreases smoothly as
reductions move, progressively, to earlier parts of the sunlit period. Atmospheric
heat transport exerts a strong cooling effect during the transition to annually ice-
free conditions, counteracting the albedo change. Specialized experiments with
atmosphere-only and coupled models show that the main damping mechanism for
sea ice region surface temperature is reduced upward heat flux through the adjacent
ice-free oceans resulting in reduced atmospheric heat transport into the region.

1. INTROduCTION

the climatic impact of the Arctic sea ice cover (see review
by Royer et al . [1990]). Although climate models show that
global temperature change is mainly linear in climate forc-
ing over a broad range [ Manabe and Stouffer 1994; Hansen
et al ., 2005], the nonlinear relationship between ice albedo
and temperature may introduce local nonlinearity. Simple
diffusive energy balance models, that represent this relation-
ship with a step function, produce an abrupt disappearance
of polar ice as the global climate gradually warms [ North ,
1984]. The phenomenon is known as the small ice cap in-
stability (SICI) as it disallows polar ice caps smaller than
a certain critical size related to heat diffusion and radiative
damping parameters. Thorndike [1992] coupled an atmo-
spheric energy balance model to a simple analytical model

The speculation that Arctic climate has nonlinear behav-
iors associated with sea ice albedo feedback has deep roots
in climatology [ Brooks , 1949; Donn and Ewing , 1968]. En-
ergy balance models (EBMs) were used to study ice albedo
effects starting in the late 1960s, and one of the first uses of
atmospheric global climate models (GCMs) was to explore