Holding back the flood

What is happening to Antarctica’s ice shelves (9 minute read)

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At Concordia Station, on the Antarctic plateau, a temperature of -11.8 degrees Celsius constitutes a heatwave. Concordia, a joint French/ Italian research station, sits at an altitude of 3200 metres above sea level, which means that if it was in New Zealand it would be not far from the top of Mount Tasman (Horokoau), our second highest mountain. The altitude and proximity to the South Pole (it’s around 1600 kilometres away) mean that at this time of year the average temperature at Concordia is less than -55oC.

It's a similar picture at Russia’s Vostok Station, which is even higher than Concordia, and closer to the South Pole. In 1983, Vostok experienced the coldest temperature ever recorded in Antarctica, -89.2oC, and the March average temperature is below -60oC. But on the 18th of March, it recorded a temperature of -17.7oC. That’s still almost unimaginably cold to me, but by the standards of the Antarctic plateau, it’s unseasonably warm.

When unusual temperatures are recorded in such a cold place, it’s tempting to blame global warming. But Antarctica is a big place – nearly twice the size of Australia – and a couple of unseasonable temperature records don’t necessarily signal a wider trend. So, I set out to learn more about what is going on in Antarctica.

Most of the time when we see a map of Antarctica, it sits at the bottom of the world, and we don’t get a sense of its shape. But if we look at it on a globe, it appears to have two connecting parts. One part looks a bit like a map of Australia, standing on one end. The other part is a shape resembling an animal head with a horn (at least that’s what I see – if you think it looks like something else, let me know). The part that looks like Australia is East Antarctica, and that is where Concordia and Vostok Stations are, as well as the South Pole. The other side is West Antarctica, and the horn is the Antarctic Peninsula.

Image credit: Leonello Calvetti/ Getty Images

In terms of climate, East Antarctica and West Antarctica are different. The Antarctic Peninsula extends further north than anywhere else in Antarctica, crossing the Antarctic circle. It’s the warmest part of the continent, and also the only place to have flowering plants. But they’re not the kinds of flowers you can see in your garden – Antarctica’s flowering plants are tiny and inconspicuous. They cling to life in some of the ice free areas on the Antarctic Pensinsula, as well as some of the sub-Antarctic islands further to the north.

Close-up of Antarctic hairgrass and pearlwort (Image credit: Gerald Corsi/ Getty Images)

Understanding Antarctic temperature trends turns out to be tricky. The Antarctic Peninsula gives a good illustration of the challenges. From 1950 until 2000, it was one of the most rapidly warming places on earth, warming by around 2.8oC. But then something strange happened – it began to cool at a similar rate to the warming of the previous decades. The reason for the cooling is that the easterly winds which blow around the Southern Ocean increased in intensity. The cooling prompted some debate, but recent evidence suggests it is a temporary fluctuation, not a long-term trend. Despite the period of cooling, the area is still warmer than it was when records began.

Other parts of West Antarctica are warming too, but the picture is less clear for East Antarctica. For much of the 20th century, the South Pole appeared to be cooling, but over the last 30 years, it has begun to heat up. Mostly, these changes appear to be linked to natural climate cycles and not human-induced climate change.

Air temperatures, however, tell only part of the story. The oceans around Antarctica are warming too. Again, some of this can be linked to natural fluctuations, but there is increasing evidence that some of the temperature changes are caused by climate change.

Antarctica is a long way away, though. Should we be concerned about what climate change may be doing in the Antarctic?

It turns out that temperature changes in the Antarctic have the potential to affect all of us, especially those who live close to sea level. There’s a huge amount of water locked up as ice in the Antarctic, and some of that ice is melting, with the potential to cause sea level rise.

The risks can be most clearly seen in the Antarctic Peninsula. Since the late 1990s, several ice shelves have collapsed, sometimes in dramatic fashion. To understand why ice shelves are important, and why they are collapsing, it’s useful to know a few basic points about them. An ice shelf is a mass of ice that extends from a glacier. A glacier is a river of ice, formed by the accumulation of snow over many years. Like rivers, they flow downhill, although they flow very slowly, usually only a couple of hundred metres a year. There’s a great video showing glacier flow using time lapse photography here.

When glaciers reach the sea, the mass of ice floats like an ice cube in a drink, forming either an ice tongue or an ice shelf. The distinction between the two is largely a matter of shape, with ice tongues being long and narrow, and ice shelves wider. Ice shelves are also often fed by a number of separate glaciers. Ice tongues and ice shelves are important because they stabilise glaciers, reducing the rate at which they flow. Without the ice tongues and ice shelves holding them back, glaciers flow more rapidly, adding more water into the ocean and contributing to sea level rise.

So why are ice shelves collapsing? There are several different factors that contribute to the collapse of ice shelves. One factor relates to water temperature – if warmer ocean water comes into contact with the base of the ice shelf, it causes the ice to melt. Another factor relates to air temperature. When air temperatures warm, pools of water can form on the surface, eventually melting through the ice to form fractures. For some ice shelves, it is water temperatures which are more important in causing melting, while for others, it is air temperatures.

Since warmer ocean and air temperatures contribute to ice shelves collapsing, the collapses seen on the Antarctic Peninsula could be an effect of climate change. On the other hand, perhaps we are noticing ice shelves collapsing because there are more people studying Antarctica. Research has suggested that this is partly true – most of the ice shelves that have collapsed in the last 35 years have collapsed previously within the preceding few thousand years, sometimes more than once. But one ice shelf which collapsed, Larsen B, had been stable for at least the last 10,000 years.

Larsen B made headlines around the world when more than 3000 square kilometres of ice shelf collapsed. Scientists were shocked by the speed at which it collapsed, turning from a solid ice shelf to a mass of icebergs in the space of a month. It’s a worrying sign when an ice shelf which has been stable for so long suddenly disintegrates.

However, the ice shelf collapses seen on the Antarctic Peninsula aren’t contributing to global sea level rise. The ice shelves themselves are already floating on the ocean. When they melt, they don’t change the sea level, just as the water level in a glass of drink doesn’t rise when an ice cube in the glass melts. And the glaciers held back by ice shelves like Larsen B are not large enough to make much change to global sea levels.

But West Antarctica and East Antarctica are home to some glaciers of staggering size. One of the largest is Thwaites glacier in West Antarctica, the widest on earth at 120 kilometres across. Thwaites glacier has doubled the speed at which it flows in the last 30 years. The increased flow from the Thwaites glacier is already putting more water into the ocean. So far, the average global sea level has risen by more than 20 centimetres since 1880. And it’s thought that about 4% of that increase is the result of water melting from the Thwaites glacier. This doesn’t sound like much, but it could soon be contributing much more.

Part of the Thwaites glacier is held back by an ice shelf.  Scientists have been studying the ice shelf, and have found that the shape of the sea floor funnels warmer water underneath. As a result, the ice shelf is unstable, and it is predicted to collapse, perhaps within the next five years. When it collapses, more of the Thwaites glacier will end up in the ocean.

Thwaites is just a single glacier. But it’s one of several glaciers connected to the West Antarctic ice sheet. Much of West Antarctica’s land is actually below sea level. The only reason we think of it as “land” is because it is covered with a layer of ice, in some cases more than 2 kilometres thick. Scientists have a hypothesis that the whole ice sheet could be vulnerable to collapsing, particularly if the ice shelves surrounding it collapse. That won’t be happening any time soon, as long as we can keep the global temperature rise below 2oC. But if we don’t, we could be facing the frightening scenario of losing the whole ice sheet. If that happens, it could raise global sea levels by 1.5 metres, or more.

But what about East Antarctica, where Concordia station is located? There, the picture is not so clear. We don’t have enough historical temperature records to know whether the heatwave seen at Concordia and Vostok stations is just a natural anomaly or human-induced climate change. But it’s certainly worrying. Within the last couple of days, an ice shelf to the north-west of Concordia has collapsed. As ice shelves go, it’s not a particularly large one, less than half the size of Larsen B. Nor is the glacier behind it large, so on its own, it won’t make much difference to sea levels. Nonetheless, it’s a reminder that we need to know more about what is going on in this most remote part of the world.

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