Antarctica is, as we all know, a continent of ice. But the ice isn’t just the ice caps and glaciers on the continent itself – it extends off the coast in all directions as the Southern Ocean seasonally freezes over. This fringe of frozen sea gives the penguins a place to escape their predators and the seals a safe place to raise their young; it shelters a number of fascinating ecosystems in the fertile waters beneath it; it offers a platform from which scientists can study their submarine subjects more or less directly.
It also provides a very convenient way of getting to places which would be cut off by water otherwise, but before one can travel on the sea ice, one needs to understand it. The practical skills of sea ice safety are predicated on an understanding of how the natural system works.
Sea ice forms when the air temperature drops below -2°C/28.4°F, the freezing point of salt water. Obviously Antarctica gets well below that in the winter, so the sea ice spreads over much of the surrounding ocean and freezes to a substantial depth. There are other forces acting upon the ice than temperature: currents, wind, storms, and the activities of humans and animals can break it up, and it often breaks up a few times before finally freezing solid for the winter. (The winter before I arrived was particularly bad for sea ice formation, which resulted in much wariness about its condition come summer, and significantly impacted some regular operations.)
When summer comes, and the air and water warm up again, the sea ice gradually decreases and weakens to the point where it breaks up and heads out to sea, returning open water to the shores of McMurdo Sound and starting the cycle over again. How much of the ocean freezes, and how much of that frozen ice breaks away the following summer, fluctuates year on year.
The ice attached to the solid rock of the shore, or to the face of an ice shelf, is called ‘fast ice’ (think of the phrase ‘hold fast’ rather than an indication of speed). When it breaks up and floats freely on the ocean, that’s a floe.
The floes drift out to sea and join the pack ice, a belt of broken-up seasonal ice and icebergs that rings the continent before gradually melting into the warmer ocean further north.
From sea level, it’s hard to tell what’s happening with the ice more than a mile or so away, but sometimes the sky gives you a clue. Just as water and ice reflect what’s around them, clouds do too, albeit in a more diffuse way. A cloud over open water will look significantly darker than one over ice, because ice is white and open water nearly black. In illustration form, from a high vantage point, it looks like this:
In the real world, this is what a water sky looks like from the vantage point you’re most likely to see it. In this case it’s reflecting the open water at Cape Royds, 20 miles away!
Throughout the summer, the edge of the ice eats closer and closer to land, accelerating as the ocean and air warm up. Often the ice reaches a tipping point and the last of it goes out in a rush. Last year it went out all the way to the ice shelf, which it does not often do, in just a few days. Luckily someone caught it on video:
So: the sea ice, as the name suggests, rests on the surface of the sea. But the sea is not a static thing! The tides raise and lower it twice a day. The sea ice is not a rigid lid; it neither suppresses the tides nor rises uniformly with them, but breathes almost like a skin, thanks to the mechanics of the tide crack.
A strip of ice stays firmly attached to the land – this is the ‘ice foot.’ The constant flexing of the ice in response to the changing water levels makes a permanent crack some distance out from the land. There is a little more ice, then another crack, or two, and these joints allow the ice to rise and fall with the tide. Here is a time lapse of that exact thing happening:
Seals love the tide crack because it affords them a dependable breathing hole and often a means by which to climb out onto the surface of the ice. Wherever you see seals you know there will be a crack somewhere nearby. These seals appear to be enjoying the tide crack around Glacier Tongue, looking across McMurdo Sound by way of Tent Island.
We visited a tide crack closer to home on my sea ice training: this one is at the foot of Arrival Heights. As you can tell, it is low tide. The great boulders of ice on the slope had been thrown up in one of the winter’s storms, which will give you an idea why the ice took so long to form.
There is a LOT more ice on Antarctica than just the seasonal ice, however. Most of the permanent ice is in the polar ice cap, miles thick in places, which slowly flows down to the coast in great glaciers. In some cases these spread out over the surface of the sea, forming an ice sheet. The Ross Ice Sheet, for example, is the size of France and over a thousand feet thick on average. It is floating on water – which, as it is liquid, is above -2°C* – so the nearer it gets to the open ocean the more it melts away, though it is always exponentially thicker than the sea ice. As this great sheet of ice is subjected to warmth and weather, it grows unstable, and bits of it break off and float away. These are the icebergs. Every ice sheet in Antarctica produces icebergs, as do most of the glaciers: the important thing is that icebergs are not frozen ocean water but chunks of the polar ice cap that have broken off into the sea.
Because icebergs are so much more massive than the sea ice, their lower reaches can be subject to deeper ocean currents. These can move them around quite independently of the sea ice, which is powerless against such a monster even if it appears to be frozen solidly in. Icebergs have been seen ploughing their way through pack ice that would stop a ship.
At the boundary between the sea ice and the ice shelf, the constantly blowing southerly winds have deposited a snowdrift, so from the surface, it looks like one gradual slope, and you’d never guess you were climbing from thin ice maybe a few years old to a fixture of several centuries.
Now you know about sea ice! Next time I will teach you all about cracks and how not to fall into one, and then you will be all set to whizz over that thin skin of ice between you and the dark cold depths of the sea.
*There are conditions in which liquid water can be below -2°C, but for the sake of discussing sea ice generally, it is simpler to ignore the rare exceptions in the interest of clarity.