Predicting Arctic Sea Ice Retreat

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Predicting Arctic Sea Ice Retreat by Cecilia
Bitz Atmospheric Sciences University of Washington
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Arctic Summer 2007
Crew members of the USCG Healy, August 2007.
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Sept 15, 2007 ice extent Median ice edge in pink
Siberia
Canada
extent above 15 coverage
figure from NSIDC
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New York Times, 9 days ago
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Low
High
Sea level pressure Summer 2007
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-20
Cloud Cover Summer 2007 Deviation from mean
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8F
Surface Air Temperature Summer 2007 Deviation from
mean
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Solar heating trend 1979-2005
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September Arctic Sea Ice Extent, 1979-2007
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September 1979 sea ice extent and successive
September record lows September 2007 monthly
average will fall somewhere between the 9/1 and
9/16 images pictured below
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1980s
1990s

• Increased ice advection away from the Russian
  coast.
• Faster export of sea ice from the pole to Fram
  Strait.

(Rigor et al. 2002)
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1980s
1990s
(Rigor et al. 2002)
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Sea ice has strong postive feedback in summer -
reflectivity (albedo) Stronger negative feedback
in winter - conduction/growth
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Sea Ice in the Climate System
Repartitions shortwave radiation in the climate
system Moderator of air-sea heat and moisture
exchange Freshwater storage and transport Brine
rejection and its influence on water-mass
formation, thermohaline circulation, etc.
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• The very best sea ice models in global climate
  models
• Continuum fluid-like mechanics
• Viscous-plastic rheology with elliptical yield
  curve (isotropic and scale independent)
• Subgrid-scale parameterization for ice-thickness
  distribution (pdf)
• Account for internal melt around brine pockets
• Two-stream, multiple-scattering radiative transfer

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Upcoming series of slides summarize results from
global climate models used in the
Intergovernmental Panel on Climate
Change 2007 Forcing (greenhouse gases and
aerosols) vary as estimated from 21st century
observations In the future, forcing is estimated
according to economic/political scenarios. SRES
A1B is a moderate scenario
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21st century warming SRES A1B
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Relevant papers Arctic sea ice decline Faster
than forecast, Stroeve et al 2007 Future abrupt
reductions in the summer Arctic sea ice, Holland
et al 2006
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0.6 Correlation coefficient for linear trend and
mean 1990-2020
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Simulations with the CCSM3 Community Climate
System Model version 3 resolution 2.8 deg in
atmosphere and land 0.5-1 deg in ocean and sea ice
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A1B Scenario with CCSM3
September Ice Extent in one ensemble member
106 km2
Holland, Bitz, and Tremblay, 2006
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more days
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Ocean Transport Absorbed Sunlight
1) Increase in absorbed shortwave is lead by 2)
Increase in Ocean Heat Transport through Fram
Strait Two strong positive feedbacks?
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Excursions from Ensemble Mean in 106 km2
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Feedback analysis applied to Earths temperature
Climate sensitivity equilibrium change in
global mean temperature, ?T due to the
reduction in outgoing terrestrial (or longwave)
radiation, ?R that would result from 2XCO2
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Planetary Energy Balance
S/4
(S/4)a
R
S (1-a) / 4 R
Atmosphere
Earth

• S Solar constant
• Albedo (or reflectivity)
• R Outgoing Terrestrial (longwave) Radiation

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For a blackbody Earth-like planet
R s TE4 TE 255K ?o (4sTeq3)-1
?To ?o ?R 1.2 K
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?T as a function of latitude, global mean is 2.6
ºC
For all feedbacks G 2.6/1.2 2.2 f0.54
G and f are functions of latitude too
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Considering individual physical processes
net feedback
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?T as a function of latitude
For only sea ice albedo feedback G 2.6/2.0
1.3 f0.23
All feedbacks 2.6 C
No sea ice albedo feedback 2.0 C
G and f are functions of latitude too
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Same thing for sea ice H ice thickness
f net feedback
gain
BUT ?o is fundamentally nonlinear!
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Simulated Present Day Equilibrium Ice Thickness,
H
Equilibrium runs are computed without a
dynamical ocean model!
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Equilibrium Ice Thickness Change for all
Feedbacks, ?H
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No Ice-Albedo Feedback - ?H0
all Feedbacks - ?H
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feedback factor f0.33 (on average)
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Uncertainty in climate sensitivity
Spread in f can give a very long tail in ?T -
Roe and Baker (2007) soon to appear in Science
Especially as f approaches 1
f isnt that close to 1 for ice-albedo feedback
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If f is normally distributed with f 0.65 and
sf0.1
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f 0.33 and sf0.1 for ?H
f 0.65 and sf0.1 for ?T
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Initial Thickness - H
No Ice-Albedo Feedback - ?H0
The no-feedback, or reference, ?H mirrors H!

• Ultimately stems from insulating effect of sea
  ice, which depends on 1/H
• Thin ice is a poor insulator, so it can grow fast
• Thick ice is a good insulator, so it grows slowly

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Knowing H is key for predicting ?H, rather than
knowing f to very high accuracy
Bitz and Roe (2003) showed that ?o a b
H2 Here I have shown that f0.3 for sea ice
albedo feedback
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Summary September 2007 Arctic sea ice cover was
20 lower than any time in the satellite era. The
cause was from anomalous high pressure, warm air
advection, low cloud cover, and ice
transport. Sea ice in climate models can be
complex. Generally the models appear to either
have too little variability and/or trend, though
two models are consistent with 1979-2006
observations. Rapid retreat appears to be larve
variability on top of a trend, not an
instability. Sea Ice albedo feedback causes sea
ice thickness to decrease about 50-100 faster.
Although positive, the feedback is not enough to
cause much uncertainty in thickness prediction,
instead errors are probably more a function of
error in the mean state.
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GAIN from adding Ocean Circulation Feedback - ?H
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Next series of slides present the governing
equations for state of the art sea ice model used
for climate studies (i.e., appropriate for basin
scale or larger and for full seasonal cycle or
longer) Specialty models exists with greater or
lesser detail. Some cannot meet the spatial or
temporal requirements. Others may, but have not
yet been implemented in climate models (to my
knowledge).
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1st Governing Equations
Ice thickness distribution g(x,y,h,t) evolution
equation from Thorndike et al. (1975)
A PDF of ice thickness h in a region, such as a
grid cell
g(h)dh
h
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1 2 3 4
5

1. Lagrangian time derivative of g following
   parcel
2. Convergence of parcel
3. Y Mechanical redistribution
4. Ice growth/melt results in advection of g in
   thickness space
5. L Reduction of g from lateral melt

h ice thickness u ice velocity ƒ growth
rate
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Y Mechanical redistribution
g(h)dh
h
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Advection in thickness space from growth
g(H)dH
g(h)dh
H
h
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2nd Governing Equation
Conservation of momentum, see for example Hibler
(1979)
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Impact of Arctic Oscillation Residence Time of
Sea Ice On Arctic Ocean
Low AO (1980s)
High AO (1990s)

• Increased ice advection away from the Russian
  coast.
• Faster export of sea ice from the pole to Fram
  Strait.

(Rigor et al. 2002)
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2007 First Time Ice-Free Area
(http//NSIDC.ORG)