Replicating ODP1.2 sea‑ice loss via albedo modification in the Isca aquaplanet GCM

Determine whether, in the Isca idealised aquaplanet general circulation model with a slab ocean and thermodynamic sea‑ice, adjusting the prescribed sea‑ice albedo parameter α_ice within the range α_ice ≥ 0.1 (equal to the ice‑free ocean albedo) can reproduce the degree of Northern Hemisphere sea‑ice area loss achieved in the ODP1.2 experiment that increases longwave optical depth by a factor of 1.2.

Background

The study assesses methods for imposing sea‑ice loss in an idealised Isca aquaplanet GCM, comparing freezing point modification (which isolates the true effect without added energy), nudging (which adds heat to remove ice), and albedo modification (which darkens sea‑ice). The authors target sea‑ice area (SIA) from three climate‑change experiments (ODP1.05, ODP1.1, ODP1.2), which progressively increase longwave optical depth (ODP) to simulate warming.

While albedo modification successfully matches SIA for lower warming cases (e.g., ODP1.05 and ODP1.1) using α_ice values between 0.5 and 0.1, the authors report an inability to reproduce the larger sea‑ice loss in ODP1.2 with any α_ice ≥ 0.1, where 0.1 is the albedo of ice‑free ocean. This raises a specific unresolved question about whether sea‑ice loss at the ODP1.2 level can be emulated solely through albedo reduction within physically plausible bounds in this model configuration.