How Sea Ice Works

Every year, the ocean around Antarctica freezes into a white skirt larger than the continent itself. On the fourteenth of September 2026 it probably reached its widest: about seventeen point six million square kilometres.

NASA and the National Snow and Ice Data Center reported that this was the third-lowest winter peak in the satellite record. At the other pole, Arctic ice shrank to a summer low tied for tenth-lowest.

What is this frozen lid on the sea? And why did Antarctic ice, which grew a little for decades, suddenly start to shrink?

Sea ice is not an iceberg and not a glacier. It is the sea itself, frozen: salt water that has turned solid and floats on the ocean surface.

Fresh water freezes at zero degrees Celsius. Dissolved salt gets in the way of the water molecules locking together, so ordinary seawater must cool to about minus one point eight degrees before it freezes.

Press Cool the sea and watch the first crystals appear at the surface, where the water loses heat to the freezing air.

Freezing starts with tiny crystals called frazil. In calm water they freeze into a thin, flexible sheet called nilas. In rough water they thicken into a soupy slush known as grease ice.

In rough water, the slush is pushed into round cakes that bump into each other and build raised rims. These pancakes can grow to several metres across and more than half a metre thick.

Then they freeze together into a sheet, and the sheet keeps growing from below as more seawater freezes onto its underside. Press Calm water and Rough water to compare.

Here is the surprise. Ice crystals have no room for salt. As seawater freezes, most of the salt is pushed out into tiny pockets of very salty water called brine.

Over time the brine drains out. Young sea ice can hold about ten parts per thousand of salt; old ice only one to three. With less brine weakening it, ice that survives the summer becomes much stronger.

The salt that leaves the ice ends up in the water just below it, which becomes saltier, colder and denser, so it sinks. Press Freeze more to speed it up.

Near Antarctica, where strong winds keep patches of water open and ice forms fast, this sinking water helps make Antarctic Bottom Water, the densest and coldest water in the global ocean.

Ice that grows through one Arctic winter, first-year ice, reaches about one and a half to two metres. Ice that survives one or more summers becomes older, multiyear ice, and thickens year after year to about three metres. Multiyear ice is strong enough to be a serious obstacle even for icebreakers.

Wind and currents also push floes together. The ice crumples into pressure ridges, with keels that usually reach ten to twenty-five metres below the surface, and sometimes fifty.

On average, ridges probably hold about forty percent of the Arctic's sea ice mass, and more near the coasts.

The two poles are opposites. The Arctic is an ocean almost enclosed by land. Its ice grows to about fifteen or sixteen million square kilometres in late winter and typically shrinks to about five or six million in September; recent years have gone lower, like this year's four point six.

Antarctica is the reverse: a continent surrounded by open ocean. With no land in the way, winter ice spreads far north, into warmer waters where much of it melts each summer.

This year it fell to about two point six million square kilometres in February, then grew almost sevenfold by September. Press Season to run the year.

Sea ice matters far beyond the poles because of its colour. The share of sunlight a surface reflects is called its albedo.

Open ocean is dark: it reflects only a small fraction of sunlight and absorbs the rest as heat. Bare sea ice can reflect up to sixty percent. Fresh snow on top can push that to about ninety.

So a white sea throws most of the summer sunshine back to space, and a dark sea soaks it up. Press the buttons to compare the surfaces.

Now link the two. A warm summer melts some ice and exposes dark water. The water absorbs more sunlight, warms up, and melts more ice.

Melt ponds on the ice do the same on a small scale. Scientists call this the ice–albedo feedback: a positive feedback, where positive means self-reinforcing, not good. Press Warm summer to start the loop.

It is one reason the Arctic is warming about twice as fast as most other regions, an effect called polar amplification.

Most of the polar ocean is dark for months and covered in cloud, so how do we measure the ice? With satellites that listen for microwaves.

Ice and water give off different amounts of natural microwave energy, and microwaves pass through clouds and need no daylight. This passive microwave record began with NASA's Nimbus-7 satellite in late 1978.

Later satellites have carried it on, building a long, continuous record of both poles from a chain of spacecraft, nearly forty-eight years so far.

In the Arctic, the record tells a clear story. In September 2012 the ice shrank to three point four one million square kilometres, nearly half the average of 1979 to 2000. The previous record, set in 2007, had been four point one seven million.

That is still the lowest on record. According to NSIDC, the twenty lowest Arctic minimums in the satellite era have all happened in the last twenty years.

This year's four point six million ties for tenth-lowest. One year can buck the trend; the long record is what scientists watch.

Antarctica has been harder to read. From 1979 to 2014 its sea ice grew slightly, by about one percent a decade, and set record winter highs in 2012, 2013 and 2014.

Then, from 2016, it fell sharply. In 2023 Antarctica set both its lowest summer minimum and its lowest winter maximum on record, and 2026 brought the third-lowest maximum.

Scientists are still working out whether this is a lasting shift or part of the region's large natural swings.

One common worry is that melting sea ice will flood coastlines. It will not, at least not directly. Fill a glass to the brim with water and ice, and wait. Press Melt floating ice to try it.

As the ice melts, the glass does not overflow. Floating ice already pushes aside its own weight of water, so its meltwater simply fills the space it took up. That is Archimedes' principle, and it works almost exactly the same way in the Arctic Ocean.

Ice on land is different: when glaciers and ice sheets melt, new water pours into the sea. Sea ice matters for a different reason: its mirror.

The Arctic is a region where Earth loses heat overall: it is dark for months, and in summer its snow and ice reflect much of the sunlight.

The Intergovernmental Panel on Climate Change, in its sixth assessment, says the Arctic is likely to be practically free of sea ice in September at least once before 2050.

Practically free means very little ice is left, not none at all.

So sea ice is the ocean breathing with the seasons: salt water that freezes below minus one point eight, squeezes out its salt, and spreads a white mirror across the poles.

Next September, the satellites will measure it again. In the Arctic, will it set a new low? In the Antarctic, will the ice recover, or keep falling? Watch the numbers.