Major Antarctic glacier is 'past its tipping point'

Shanta Barley in New Scientist: A major Antarctic glacier has passed its tipping point, according to a new modelling study. After losing increasing amounts of ice over the past decades, it is poised to collapse in a catastrophe that could raise global sea levels by 24 centimetres. Pine Island glacier (PIG) is one of many at the fringes of the West Antarctic ice sheet. In 2004, satellite observations showed that it had started to thin, and that ice was flowing into the Amundsen Sea 25 per cent faster than it had 30 years before.

Now, the first study to model changes in the ice sheet in three dimensions shows that PIG has probably passed a critical "tipping point" and is irreversibly on track to lose 50 per cent of its ice in as little as 100 years, significantly raising global sea levels. The team that carried out the study admits their model can represent only a simplified version of the physics that govern changes in glaciers, but say that if anything, the model is optimistic and PIG will disappear faster than it projects.

Richard Katz of the University of Oxford and colleagues developed the model to explore whether the retreat of the "grounding line" – the undersea junction at which a floating ice shelf becomes an ice sheet grounded on the sea bed – could cause ice sheets to collapse.
Warm seas

Climate change is warming the Amundsen Sea, which is at the southern margin of the Pacific Ocean. As rising sea levels push the warm water beneath the ice shelves, it melts them from below, pushing the grounding line higher up the continental shelf.

By raising sea levels, and therefore the grounding line, in their model, Katz's team were able to find the point of no return beyond which the glacier would be unable to recover. That's because the Antarctic sea bed has a small lip in it: it rises slowly up the continental shelf, then makes a slight dip before rising again to the shoreline. The researchers found that as long as the grounding line is on the outer rise of the sea bed, before the lip, small changes in climate lead to correspondingly small changes in the glacier's ice volume….

A NASA shot of the Pine Island Glacier