The only thing more frightening than an advancing glacier may be one that is shrinking and raising sea levels round the world

Both the north pole and the south pole are situated in the middle of huge ice deserts which are melting around the edges under the influence of human activity. The difference that matters between them is that the ice of the Arctic floats: if it melted nothing much would happen to aggregate sea levels. The ice of Antarctica, like that of Greenland, rests on land. If it all were to melt, as it has done in the far distant past, sea levels could rise by as much as 60 metres. That is most unlikely to happen. What is possible, though, is that the smaller portion of the continent, west Antarctica, which is divided from the rest by a mountain range, could lose much of its ice. Even that would be catastrophic. A significant retreat in west Antarctica, as seems to be already under way, could raise sea levels by between one and three metres by the end of this century. Children now alive will see that happen across their lifetimes. That is what is meant by the urgency of global warming.

This week saw the publication of fresh research showing that the glaciers of west Antarctica are retreating faster than they were at the end of the last ice age, when water levels also rose significantly. The ice sheet is not one homogeneous mass, but a collection of glaciers all moving slowly but inexorably towards the sea. Their retreat is happening underwater, and invisibly, as the ocean erodes the foot of the glacier, known as the “ground line”, where its contact with the sea floor ends. Beyond that point, long tongues of ice stick out into the ocean, providing the coastline that we can see and map. But the capacity of the ice sheet to lock up water depends on the position of the ground line. As that retreats, invisibly, the sea level rises and the whole of the ice sheet grows less stable, something which makes further sea rise still more likely.

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Read More The Guardian view on Antarctica: the worrying retreat of the ice | Editorial

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