Kilometres of ice have built up over Antarctica. From today's perspective, the vast sheet of ice and snow looks like an eternal feature. Yet the part of Antarctica that lies west of the Transantarctic Mountains is largely below sea level today, and its ice sheet has probably collapsed at least partly during past warm episodes. The West Antarctic ice sheet has therefore been flagged as a possible location of exceptional climatic sensitivity where a large mass of land-based ice may be lost to the ocean in response to relatively moderate changes in climate.

According to the fourth assessment report from the Intergovernmental Panel on Climate Change, global sea level would rise by about 5 m if the West Antarctic ice sheet were to collapse. However, Jonathan Bamber, of the University of Bristol, and colleagues have estimated the potential rise in global mean sea level from the disintegration of this ice sheet at only about 3.3 m (Science 324, 901–903; 2009).

They base this value on a detailed reassessment of the volume of potentially vulnerable ice: ice resting on bedrock below sea level (the brown colours in the image) that slopes downwards inland. According to the so-called marine ice-sheet instability hypothesis, land ice under these conditions can be subject to rapid and irreversible removal if the buttressing ice shelves — such as those holding the West Antarctic ice sheet in place — disintegrate.

Credit: © 2009 AAAS

A sea-level rise from West Antarctic ice-sheet collapse would not be globally uniform. Mainly because of the ice mass's gravitational pull on the surrounding oceans, Bamber and colleagues project regional sea-level rise to be about 25% above the global mean (or about 4 m in absolute terms) along the US Pacific and Atlantic coasts. In contrast, coasts at the tip of South America would only be affected by about half the global mean rise.

But even with this lower estimate, a potential disintegration of the West Antarctic ice sheet would affect millions of people around the world who live in low-lying areas.