A5 Loss and Alteration of Sea Ice

Topic

Arctic sea ice has lost about forty percent of its minimum summer extent since 1979, with a loss trend of about thirteen percent per decade at the September minimum. Unlike glacial ice, Arctic sea ice does not directly contribute to sea level when it is lost, because it was already floating on the ocean before it melted, but the loss of Arctic sea ice triggers first-order climate feedbacks and reorganizes both regional and global atmospheric circulation.

The central physics behind the loss of Arctic sea ice involves the energy balance at the surface of the Arctic sea ice: the net heat flux at that surface determines both the rate at which Arctic sea ice grows and the rate at which it melts. The growth of Arctic sea ice at its base is increasingly slower as the already formed ice itself accumulates, because that already formed ice increasingly insulates the base of the Arctic sea ice from the cold air at the surface and reduces the flow of heat that the base can continue to lose. Melting on the surface of Arctic sea ice, on the other hand, responds directly to the sensible heat and latent heat that the air delivers to that surface.

Ice-albedo feedback is the dominant amplifying mechanism of Arctic sea ice loss: the ice-free ocean reflects only about seven percent of the solar radiation it receives, while fresh snow-covered Arctic sea ice reflects up to eighty-five percent of that same solar radiation. When Arctic sea ice is lost, dark water is exposed that absorbs much more solar radiation than the Arctic sea ice did, and that increased absorption of solar radiation warms the ocean and melts even more Arctic sea ice, closing a self-reinforcing feedback loop.

Arctic sea ice not only grows, melts and reflects solar radiation: it also drifts, and this drift of Arctic sea ice responds to four different forces: the force of the wind on the surface of the Arctic sea ice, the force of the ocean current that runs under that same Arctic sea ice, the force that results from the rotation of the Earth, and the internal tension that the Arctic sea ice itself exerts on itself when different blocks of ice collide or compress together. A standard rheological model represents Arctic sea ice as a material that behaves viscously under small stresses, while the ice yields and flows slowly, but that behaves plastically under higher stresses, resisting deformation up to a limit and yielding suddenly once that limit is exceeded, and this standard rheological model is the usual reference for simulating the dynamics of the entire Arctic sea ice package.

The expected behavior of Arctic sea ice includes a summer sea ice-free Arctic before 2050 under moderate emission scenarios, and that near-total loss of summer Arctic sea ice has direct impacts on the polar jet and mid-latitude weather patterns, the same reorganization of regional and global atmospheric circulation that this description began with.

ID:2654