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Loss of summer sea ice need not be irreversible at fixed external forcing. Open water can release excess heat in autumn, and thin ice grows faster than thick ice during winter because it offers less resistance to thermal conduction. Continued climate change can nevertheless shift the long-term ice cover downward. A one-summer perturbation and persistent forcing are different physical experiments.

A pressure ridge is a pile of fractured sea ice formed by horizontal convergence. Ice displaced upward forms a sail; ice displaced downward forms a keel. Newly formed ridges contain angular blocks and pores, and freezing, melting and further deformation alter their strength and shape.

Relative tangential motion across a fracture in sea ice can break and pile blocks along the fracture. Local convergence at rough contacts makes ridging possible even when the large-scale deformation is mainly shear.

The thermodynamics of sea ice couples conductive heat transport, radiative and turbulent surface fluxes, basal ocean heat and latent heat of freezing and melting.

Global mean sea level can rise through changes in ocean volume at fixed mass and through additions of ocean mass. Local relative sea level additionally depends on ocean circulation, gravity and vertical motion of the land. Floating sea ice has already displaced seawater, so its melt has a much smaller direct sea-level effect than transfer of grounded land ice into the ocean.

A sensible-heat polynya is maintained by ocean heat delivered through currents, mixing or upwelling, sufficient to inhibit local sea ice growth. Mechanical export and sensible heating may act together, so this classification identifies a dominant mechanism rather than an exclusive geographical class.

At fixed boundary temperatures, a snow cover decreases winter conductive heat loss through the resistance $h_s/k_s+h_i/k_i$ and inhibits sea ice growth. Its high albedo and persistence into summer reduce ablation. Therefore annual equilibrium thickness has competing snow sensitivities. The near cancellation at modest snow depth and thickening with persistent summer snow in the original Maykut-Untersteiner model are forcing-dependent responses, not a universal monotonically decreasing snow-thickness relation.

Wave attenuation is a reduction of a chosen wave component as it travels through sea ice. An amplitude coefficient $\alpha$ corresponds to an energy coefficient $2\alpha$. Wave scattering at a planar interface can remove energy from the forward wave without dissipating total energy; viscous dissipation, collisions and internal ice losses irreversibly convert mechanical energy to heat.

Wave fetch is the distance over which wind acts on an uninterrupted water surface to generate waves. Growth also depends on wind speed and duration. A polynya can provide a local fetch inside otherwise extensive sea ice.