Arctic Sea Ice | mysimulator.uk

Two kinds of sea ice

Arctic sea ice follows a strong seasonal cycle, typically reaching its maximum extent around March and its minimum around September. But not all ice is the same age: first-year ice has frozen since the previous autumn and is relatively thin and salty, while multi-year ice has survived at least one summer melt season, losing much of its salt and growing thicker and mechanically tougher with each additional winter it survives.

Growing ice: a thermodynamic square root

Ice thickening follows essentially the same physics as permafrost's Stefan problem, just run in the opposite direction: cold air above draws heat out of the relatively warm ocean water below, freezing a new layer at the ice-water interface.

h ≈ sqrt(2 · k · ΔT · t / (ρ · L))     simplified ice-growth relation

h  = ice thickness
k  = thermal conductivity of ice
ΔT = temperature difference (freezing point − air temperature)
ρ  = density of ice, L = latent heat of fusion

h grows with sqrt(t) — thin ice grows fast, thick ice grows slowly

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The albedo feedback

Fresh sea ice and snow reflect a large share of incoming sunlight, typically 50-90% depending on how fresh and snow-covered it is, while open ocean water absorbs roughly 90% of the sunlight that hits it. That contrast sets up a powerful positive feedback: any summer that melts more ice exposes more dark water, that water absorbs more solar heat, the extra heat melts still more ice the following season, exposing even more dark water. This ice-albedo feedback is a major reason the Arctic has been warming two to four times faster than the global average, a pattern known as Arctic amplification.

Why old ice matters more than area alone

A single winter's maximum extent can look reassuringly stable even while the underlying ice pack is weakening, because extent alone doesn't say anything about thickness or age. The fraction of the pack made up of multi-year ice has declined steadily for decades; the remaining ice is increasingly thin, salty, first-year ice that is far more vulnerable to melting out completely in a single warm summer. That's why sea ice scientists track the September minimum and multi-year ice volume as the more meaningful long-term signals, rather than the winter maximum alone.

Frequently asked questions

Why does sea ice grow more slowly the thicker it gets?

The ice sheet itself insulates the cold air above from the relatively warm ocean water below, so a thicker layer means heat from the water has to conduct through more ice to escape and freeze the next increment - the same square-root-of-time slowdown seen in the Stefan problem for permafrost thaw.

What is the ice-albedo feedback?

Sea ice and snow reflect most incoming sunlight back to space, but the dark ocean water exposed when ice melts absorbs most of it instead. That extra absorbed heat warms the water and melts more ice, exposing more dark water - a positive feedback loop and a major reason the Arctic is warming faster than the rest of the planet.

Does winter sea ice area recovering each year mean the ice pack is healthy?

Not necessarily. Winter maximum extent can look relatively stable even as the pack thins, because the key long-term signal is the loss of thick multi-year ice that survived a previous summer. A pack made mostly of thin first-year ice is far more vulnerable to melting out completely.

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Arctic Sea Ice: Freeze, Retreat and the Albedo Feedback Loop