The Sea Ice-Albedo Feedback in a Warming Climate: Albedos from Today and Reflections on Tomorrow Don Perovich 1 and Tom Grenfell 2 1 Cold Regions Research and Engineering Laboratory, 2 University of Washington

+ Melting + + Lower albedo Absorbed sunlight

Ice-albedo feedback: seasonal evolution JulyAugust 7September AprilMay 17June 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 properties June 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. Large-scale albedo Combine albedos with fractional areas

Albedo evolution 5 phases – timing is critical Snow warming Pond evolution Fall freezeup Pond formation Snow melt

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

Bottom ablation and solar heating In the past source of heat was sunlight deposited in leads SHEBA 1998 Heat 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