Using CCSM3 to investigate future abrupt Arctic sea ice change

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Using CCSM3 to investigate future abrupt Arctic
sea ice change

• Marika Holland
• NCAR

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an abrupt climate change occurs when the climate
system is forced to cross some threshold,
triggering a transition to a new state at a rate
faster than the cause.
A mechanism that might lead to abrupt climate
change would need to have the following
characteristics A trigger or, alternatively,
a chaotic perturbation, with either one
causing a threshold crossing (something that
initiates the event). An amplifier and
globalizer to intensify and spread the
influence of small or local changes. A source
of persistence, allowing the altered climate
state to last ...
From Abrupt Climate Change Inevitable Surprises
(2002)
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Role of sea ice as an amplifier
VAvariable albedo FAfixed albedo
(From Hall, 2004)
(DJF SAT)

• Surface albedo feedback amplifies climate
  perturbations
• Models have been used to explore/quantify these
  effects.

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Observed Arctic Conditions
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The observed Arctic sea ice
Fowler, 2003
Sea ice concentration
June 6, 2005
(Perovich, 2000)
(NSIDC, 2005)
Observed thickness Laxon et al., 2003
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• Observations indicate large changes in Arctic
  summer sea ice cover

Sept Ice Extent
Trend 7.7 per decade
1980
2000
From Stroeve et al., 2005
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Suggestions that ice has thinned
Ice draft change 1990s minus (1958-1976)
Rothrock et al., 1999
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Indications that Arctic Ocean is warming

• Pulse-like warming events entering and tracked
  around the Arctic
• General warming of the Atlantic layer

Polyakov et al., 2005
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North Atlantic Oscillation Positive Phase
From University of Reading webpage
JFM NAO Index 1950-1992
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Timeseries of JFM NAO Index
Maybe it is not all the NAO/AO?
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Have led to suggestions that
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Future ProjectionsWhat can models tell us?
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Future climate scenarios

• Relatively gradual forcing.
• Relatively gradual response in global air
  temperature

Meehl et al, 2005
Wigley, 2000
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September Sea Ice Conditions

• Gradual forcing results in abrupt Sept ice
  transitions
• Extent from 80 to 20 coverage in 10 years.
• Winter maximum shows
• Smaller, gradual decreases
• Largely due to decreases in the north
  atlantic/pacific

Abrupt transition
Observations Simulated 5-year running mean
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Forcing of the Abrupt Change
Dynamic

• Change thermodynamically driven
• Dynamics plays a small stabilizing role

Thermodynamic
Change in ice area over melt season
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Processes contributing to abrupt change
OW formation per cm ice melt
March Arctic Avg Ice Thickness (m)

• Increased efficiency of OW production for a given
  ice melt rate
• As ice thins, vertical melting is more efficient
  at producing open water
• Relationship with ice thickness is non-linear

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Processes contributing to abrupt change Albedo
Feedback
cm/day

• Increases in basal melt occur during transitions
• Driven in part by increases in solar radiation
  absorbed in the ocean as the ice recedes

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Processes contributing to abrupt
changeIncreasing ocean heat transport to the
Arctic
Increases in ocean heat transport occur during
the abrupt transition. Contributes to increased
melting and provides a trigger for the event.
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Arctic Ocean Circulation Changes
Evidence that ocean circulation changes are
related to changing ice/ocean freshwater exchange
(Bitz et al., 2006)
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Both trend and shorter-timescale variations in
OHT appear important
OHT natural variations partially wind
driven. Correlated to an NAO-type pattern in SLP
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Mechanisms Driving Abrupt Transition

• Transition to a more vulnerable state
• thinning of the ice cover
• A Trigger
• rapid increases in ocean heat transport.
• Other natural variations could potentially play
  the same triggering role?
• Positive feedbacks that accelerate the retreat
• Surface albedo feedback
• OHT feedbacks associated with changing ice
  conditions

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Effects of transition on atmospheric conditions

• Winter air temperature increases rapidly during
  abrupt ice change, with a gt5C warming in 10 yrs
• Precipitation shows general increasing trend with
  largest rate of change over abrupt ice event

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Projections of Near-surface Permafrost
Courtesy of Dave Lawrence, NCAR (Lawrence and
Slater, 2005)
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Some Cautions in Using Models to Examine these
(and other) issues
Models provide a powerful tool for examining
climate feedbacks, mechanisms, etc but
Biases in simulated control state can affect
feedback strength Uncertainties in model
physics/response Acknowledgement that model
physics matters for simulated feedbacks
Ethical Considerations
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ITD Influence on Albedo Feedback
ITD (5 cat) 1 cat. 1cat tuned
Strength of albedo feedback in climate change
runs
(Holland et al., 2006)

• Model physics influences simulated feedbacks
• Getting the processes by which sea ice amplifies
  a climate signal right can be important for our
  ability to simulate abrupt change

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Feedbacks contribute to Arctic amplification
But, that amplification varies considerably among
models
(Holland and Bitz, 2003)
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Sea ice in fully coupled GCMs
IPCC AR4 1980-1999 ice thickness Red line marks
observed extent
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Aspects of the Models Internal Variability
Model Standard Deviation
Model 1 1.93
Model 2 1.90
Model 3 1.72
Model 4 1.68
Model 5 0.42
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Summary

• Sea ice is an effective amplifier of climate
  perturbations
• due to surface albedo changes
• due to ice/ocean/atm exchange processes
• CCSM3 simulates abrupt transitions in the future
  ice cover
• preconditioning (thinning)
• trigger (ocean heat transport changes)
• positive feedbacks (surface albedo oht changes)
• Models provide a useful tool for exploring the
  mechanisms that result in simulated rapid climate
  transitions
• Never completely trust the tool
• comparisons to other models sensitivity tests
  digging into the feedbacks, etc. can increase
  confidence in simulated processes

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Role of sea ice as an amplifier
SAT Difference
SST
SST
Reduced Ice
LGM
From Li et al., 2005
Insulating effect of sea ice contributes to large
atmospheric response to sea ice changes. Models
are a useful tool to quantify these impacts.
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Role of sea ice for abrupt transitions in a
paleoclimate context?
GISP2, Greenland
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Temperature (?C)
-40
d18O (per mil SMOW)
-50
-60
x1000 years ago
(slide courtesy of Carrie Morrill)
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Simulated abrupt transitions in sea ice
abrupt forcing (freshwater hosing) can result in
abrupt ice changes
Sea ice change
SAT Change
(From Vellinga and Wood, 2002 Vellinga et al,
2002)

• Sea ice changes amplify climate response
• Global teleconnections can result
• Longevity of these changes are an issue

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SAT Change at end of 21st century
From A1B scenario
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• Processes Involving ice/ocean FW exchange
• In warmer climate, increased ice growth due to
  loss of insulating ice cover results in
• Increased ocean ventilation
• Ocean circulation changes
• Transient response

Change in Ice growth rates at 2XCO2
cm
Change in Ocean Circulation
Change in Ideal Age at 2XCO2
Change in Ideal age at 2XCO2
Yr 40-60
From Bitz et al., 2006
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How common are abrupt transitions?
September Ice Extent
Obs
Simulated 5yr running mean
Abrupt transition
Transitions defined as years when ice loss
exceeds 0.5 million km2 in a year
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How common are forcing mechanisms?
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How common are effects?
Lagged composites relative to initiation of
abrupt sea-ice retreat event
Arctic Land Area
Courtesy of David Lawrence, NCAR
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• Increased Arctic Ocean heat transport occurs even
  while the Atlantic MOC weakens

20th Century
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Do other models have abrupt transitions?
Some do
Data from IPCC AR4 Archive at PCMDI
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Climate models as a useful tool for addressing ACC

• As a tool to flesh out/test hypotheses or
  processes
• How is a climate signal amplified by sea ice
  interactions
• What processes influence threshold behavior in
  the sea ice
• How does the control climate state modify the
  persistence of anomalies
• How are teleconnections between high latitudes
  and tropics realized

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Precipitation Changes
2040-2049 minus 1990-1999

• Precipitation generally increases over the
  20th-21st centuries
• Rate of increase is largest during the abrupt sea
  ice transition

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OHT and polar amplification
Change in poleward ocean heat transport at 2XCO2
conditions
DOHT
Both control state and change in OHT are
correlated to polar amplification
(From Holland and Bitz, 2003)
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Importance of sea ice state for location of
warming

• Models with more extensive ice cover obtain
  warming at lower latitudes
• The location of warming can modify the influence
  of changes on remote locations

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Importance of sea ice state for the magnitude of
polar amplification
(From Holland and Bitz, 2003)

• Magnitude of polar amplification is related to
  initial ice thickness
• With thinner initial ice, melting translates more
  directly into open water formation and consequent
  albedo changes

Complicates paleoclimate issues since control
state not as well known
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Does it matter?

• Sea ice is an important amplifier in the system
• When a change is made in a coupled model, often
  the most dramatic response is in the ice covered
  regions
• This often occurs for changes that are not polar
  specific - e.g. diurnal cycle stuff.
• Getting the processes by which sea ice amplifies
  a climate signal right can be quite important
  for our ability to simulate abrupt change
• These will likely include ice/ocean and
  ice/atmosphere interactions (ice growth/brine
  rejection - how it changes - seasonally, etc.
  how changes influence the ocean
• Threshold behavior of the ice cover - examples
  on/off of the initial ice growth (freezing temp)
  perennial to seasonal ice cover perennial to
  seasonal snow cover