Global warming - impact on Antarctica

(March 1997)

Our major theme this month seems to be the weather. Not the usual annual change in seasons, but something rather longer-term. In Mid-March, Time magazine ran a piece on the possible risk that the West Antarctic Ice Shelf might break up, causing sea levels across the world to rise. Puzzled as to how you would tell west Antarctica from east Antarctica, we began to investigate the science behind the story.

The difference between east and west Antarctica is most obvious when you stand on the meridian of Greenwich, somewhere in Antarctica. From where you are, west to the International date line, is the part known as lesser or western Antarctica, while east of you, all the way around on the other side, is greater or eastern Antarctica.

If you sit somewhere south of Tahiti on the globe, and look at Antarctica, you can see west Antarctica nearest you, with a tail running up towards South America, the Antarctic Peninsula, like a spinal cord leading out of the brain, an effect that is enhanced by the cerebellum of east Antarctica, piled over the top. In our map (reference to the map on the CDROM), east Antarctica is on the right, and west Antarctica is on the left.

Now before we look at what is happening to the ice shelves and sea ice of Antarctica, we need to look at what happens to glaciers when the weather gets warmer, and that means we have to move a little north of Antarctica, to New Zealand. Glaciers are very sensitive to slight shifts in the temperature, so what we see in New Zealand will help us to understand what is happening further south.

When the Earth's average temperature rises, these global thermometers withdraw, pulling back into the mountains, and when it cools again, the glaciers descend into the valleys once more. The 127 glaciers of the Southern Alps of New Zealand have been shrinking since the end of the Little Ice Age, losing 25% of their area in the last century, and taking their fronts almost 100 metres back up the mountains, suggesting a warming of 0.6øC during that time.

This same warming has led to large pieces of the ice shelves around Antarctica breaking up. These ice shelves float on the sea, and surround the larger "grounded" masses of ice on and around the continent. In 1986, more than 11 000 square kilometers of the Larsen Ice Shelf and 11 500 square kilometers of the Filchner Ice Shelf broke off and floated out into the Weddell Sea, along with 1600 square kilometers of the Thwaites Iceberg Tongue.

This could be just a natural "calving", but the mass of ice released was equal to three or four years' accumulation of snow and ice across the whole of Antarctica. More than 1300 square kilometers of the Wordie Ice Shelf have broken away since 1966, and a 1993 Norwegian study revealed more icebergs than usual in southern waters. Then in February 1995, a 2600 square kilometer Iceberg calved away from the Larsen Ice Shelf on the Antarctic Peninsula.

There has certainly been a warming in the area, and this would necessarily push the mean annual air temperature isotherms around Antarctica further south. As the ice shelves only seem to be stable past the -4øC mean annual isotherm, this could explain why the northern ice shelves are breaking up: they are certainly the most vulnerable. In the past fifty years, temperatures on the Peninsula have risen by some 2.5øC, but it is unclear whether this is a sign of global warming, or just a normal variation.

The famous greenhouse disaster scenario, of course, has all the polar caps melting, flooding the oceans and drowning the rich coastal fringes where most of the world's agriculture happens. Is this what we can see beginning? Not necessarily. The shelf ice that is breaking away was already floating on the sea, and even Archimedes could tell us that we don't have to worry about floating ice. It melts into the sea, and leaves no trace of a rise. We needn't worry about the ice shelves melting and drowning us in our sleep.

The other ice that is already in the sea, but sitting massively on the bottom of a deep marine basin is another matter. And so is the land ice tied up in ice sheets that are some 4.3 km (about two and a half miles) thick, but close to the sea. If those ice deposits get away, it will be like a very fat person climbing into the bath.

The problem is that there is a large weakness across and through the rock that lies beneath the West Antarctic Ice Sheet (WAIS). This is no mere gap, but a rift that could be as serious as the San Andreas Fault or the Rift Valley of eastern Africa. It is an area of regular volcanic activity and perhaps more. Some geologists suspect that there is a plate boundary there, and the recent discovery of a volcano beneath the ice makes them even more certain that their suspicions are correct.

The volcano is rather muffled by the layer of ice, but what would happen if there was a major eruption? Could enough steam and water be released to make the grounded ice unstable? The evidence that other scientists have been gathering from ice cores in Greenland and Antarctica are telling us that Ice Ages may begin and end very quickly. If the climate warms and ice sublimes or evaporates, the blocks will get lighter, they will begin to rise, and sea water can start to flood in beneath the ice, "greasing the skids" and speeding the flow of ice to the edge, increasing the instability. Is that what rings the changes, something as unpredictable as a volcanic eruption?

The east Antarctic ice sheets would produce a sea level rise of some sixty metres (200 feet) if they melted, but that seems a remote chance right now. The WAIS, on the other hand, would only produce a rise of six metres. But even six metres would be catastrophic for large parts of the world. Capital cities like London, protected from high tides only by tidal barrages in the Thames, much of the world's best agricultural land, river estuaries all over the world, and every coral atoll, would all be threatened.

For now, the experts say that, while the WAIS is unstable, and probably contains 3.2 million cubic kilometers of ice, all resting in a deep marine basin, the ice is most likely to stay where it is, and the low-lying parts of the world remain safe.

So if the WAIS is not likely to be a problem, is the sea ice shrinkage also something we can ignore? The qualified answer is yes, but only if we don't appreciate penguins. Some penguins need sea ice so they can get access to the sea, both for the parents to feed, and for the young to launch themselves into the sea when they are ready to take on independence. As the sea ice breaks up earlier in the southern summer, so the chances rise that the new juveniles will be trapped, with no easy jetty of ice to take them clear of the shallows and waves of the shore.