Sea ice — crewcabinrelvlt

The cycle

Autumn. Air cools. Surface water reaches its freezing point, a little below zero because of the salt. Crystals form. Frazil, then grease ice, a slick that dulls the waves. Then nilas: a thin, dark, elastic sheet. Wind breaks it into pancakes, rafts them, ridges them. Snow settles on top and insulates, so growth slows even as the air gets colder.

Late winter. Maximum area. Ice fills the central basin, the shelf seas, the bays, and spills south into the Bering Sea, the Sea of Okhotsk, Baffin Bay, the Barents Sea. The sun returns to an almost entirely white ocean.

Spring. Snow melts first. Ponds form on the surface, dark against white. They absorb sunlight and eat downward. Leads open along cracks and the pack begins to move. Summer. Retreat from the coasts, break-up along the margins, drift in the wind. Early autumn. Minimum area. Then the whole thing starts again.

Two moments define the year for anyone watching: the late-winter high and the late-summer low. The low is falling further, decade on decade. Freeze-up comes later. The season of open water is longer at both ends.

Two kinds of ice

First-year ice. Formed this winter. Thinner, saltier, flatter. Melts out easily when summer comes. Multiyear ice. Has survived at least one melt season. Thicker, because it has had more winters to grow and has been ridged by pressure. Fresher, because brine drains out over time. Harder for ships, harder for the sun.

The old ice is the pack's memory. As it goes, each summer starts from scratch with a young, thin pack that offers little resistance. The shift from a mostly multiyear Arctic to a mostly first-year Arctic is the underlying story; the falling extent is its visible surface.

Trait	First-year ice	Multiyear ice
Age	One winter	Two or more winters
Thickness	Thin to moderate	Thick, heavily ridged
Salinity	Higher	Lower; brine drained
Surface	Flat, saline snow	Hummocked, weathered
Summer survival	Low	High, but falling
Where found	Shelf seas, margins, increasingly everywhere	North of Greenland and the Canadian Arctic Archipelago

Feedbacks

Albedo. White ice reflects most sunlight. Open water absorbs most of it. Less ice, more heat into the ocean, later freeze-up, thinner ice next spring, earlier melt. The loop runs on its own once started.

Ocean heat. Warm, salty Atlantic water enters through the Barents Sea and Fram Strait. It normally sits below a cold, fresh surface layer. Where that layering weakens, heat reaches the ice from beneath. The Barents Sea is the clearest case: the ice edge retreats north and the sea behaves more like the Atlantic than the Arctic. Researchers call the process Atlantification.

Waves. Open water gives wind a fetch. Waves flex and fracture the ice edge, pushing it back further. Cloud and moisture. More open water in autumn means more evaporation, more low cloud, and a blanket that holds heat in as the dark returns.

Not every loop runs one way. Thin ice grows faster than thick ice in winter, a stabilising effect. Snow on ice slows melt. Wind can compact the pack and make it look larger than its volume warrants. The balance still tips towards loss.

Where the ice moves

Two great motions. The Transpolar Drift carries ice formed on the Siberian shelves across the pole and out through Fram Strait, between Greenland and Svalbard, into the Atlantic. The Beaufort Gyre circulates ice clockwise north of Alaska and Canada. Ice that once survived several loops of the gyre now melts on the warm southern arc.

Pressure from both systems piles the thickest ice against the north coast of Greenland and the islands of the Canadian Arctic Archipelago. Expect this to be the last summer ice to go. It is already the last place where multiyear ice is common.

Ice-free: what it means

Not zero ice. Practically ice-free: a late-summer pack reduced to remnants along northern Greenland and the archipelago. Winter ice returns. The ocean still freezes when the sun goes down for the season.

What changes. Open water across the central basin in summer. A transpolar shipping window. A hunting platform for polar bears that vanishes earlier and returns later. Coasts exposed to waves and storm surge for longer. Heat and moisture leaving the ocean into the atmosphere in autumn. In short: the Arctic Ocean becomes seasonally ice-covered, in the way Hudson Bay already is, freezing every winter and clearing every summer.

Watching

Extent is tracked daily from satellite. Thickness is harder; radar and laser altimetry give it, with uncertainty over snow depth. Age is tracked by following parcels of ice from year to year.