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The Sea Ice-Albedo Feedback in a Warming Climate: Albedos from Today
and Reflections on Tomorrow
Don Perovich1 and Tom Grenfell21Cold Regions Research and Engineering Laboratory, 2University of Washington
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incident
reflectedAlbedo
Melting
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Lower albedoAbsorbedsunlight
Ice-albedo feedback: seasonal evolution
July August 7 September
April May 17 June 10
Surface types: snow, bare ice, melt ponds, and leads
Albedo evolution of surface types
Snow, leads, bare ice constant – ponds always changing
Surface physical propertiesJune 17 August 7
Regenerating surface scattering layer Deepening ponds
• Estimate as a composite of surface types• determine the albedos of types• determine the relative areas (A)• albedo () is linear combination
•S=snow, i=, p=pond and w=water.
wwppiiss AAAA
Large-scale albedo
Combine albedos with fractional areas
Albedo evolution
5 phases – timing is critical
Snow warming
Pond evolution
Fall freezeup
Pon
d fo
rmat
ion
Sno
w m
elt
General form
Know form of albedo evolution
Modest changes
Adjust “wavelength” and “amplitude”
What if …Multiyear
Not a minor adjustment – it’s a new regime
First year
big changes
First year ice – albedo evolution
Decrease is rapid
First year ice – albedo evolution
Decrease is rapid, sometimes is reversed
First year ice – albedo evolution
Decrease is rapid, sometimes is reversed, sometimes oscillates
First year ice – albedo evolution
24 June
15 June
18 June
• Tremendous variability• Multiple evolutionary paths
• Flooded• Drained• Superposed
• Need to understand physical properties• Role of melt, topography, and permeability
Big change in physical evolution – big change in albedo evolution
Deformed first year ice
Who knows? Interconnections cause cascading changes
Changes are happening now
Average bottom ablation from 100 thickness gauges
Bottom ablation measured at SHEBA - 1998
Bottom ablation and solar heating
In the past source of heat was sunlight deposited in leads
SHEBA 1998Heat used in bottom ablation
Solar heating
Transmittance through leads not sufficient
Solar heating
Leads, melt ponds and bare ice all contribute
1975 versus 1998
Ice is thinner, transmission to ocean is greater
• No change in bare ice albedo• Decrease in pond albedo• Increase in bare ice transmittance• Major increase in pond transmittance
• Simple optical model• “Standard” ice conditions
• 3.0 m vs. 1.5 m• 2.5 m vs. 1.0 m
Summary
Integration and synthesis are key
Current state•Good understanding•Five distinct phases in albedo evolution•Summer is the interesting time•Melt ponds are major uncertainty
Modest changes•Fundamentals are the same•Can adapt present treatment•Change “wavelength / amplitude” of albedo cycle
Major changes•All bets are off•Can speculate, but not extrapolate•Probably accelerated feedback•Difficulty is that changes are interrelated