The game changer that many feared is now unfolding in Antarctica. The Pine Island Glacier (PIG) is poised to suffer catastrophic collapse according to a new study by Richard Katz of the University of Oxford. The largest of the Western Antarctic Ice Sheet (WAIS), PIG has long been the focus of worried scientific attention. Joe Romm gave an exhaustive summary in August 2009 of the importance and tenuousness of PIG here.
 

But now with this study, it seems Mr. Katz has identified PIG’s tipping point, and unfortunately, it seems, we’ve past it. 

 

As reported in New Scientist by Shanta Barley, the tipping point mechanism is a lip in the continental shelf. An extended excerpt to explain:

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.

But as soon as the grounding line moves over the lip and starts to move down into the dip in the sea bed, the situation changes critically. “Once the grounding line passes the crest, a small change in the climate causes a rapid and irreversible loss of ice,” says Katz.

So when will the grounding line pass the crest you ask? Try 1996!
Katz’ study estimates a 50% collapse by 2100 contributing about 24cm to sea rise. But by his own estimation they are being conservative and underestimating the potential melting.
Ominously, Barley reports:

This assumes that the grounding line does eventually stabilise, after much of PIG is gone. In reality, PIG could disappear entirely, says Hindmarsh. “If Thwaite’s glacier, which sits alongside PIG, also retreats, PIG’s grounding line could retreat even further back to a second crest, causing sea levels to rise by 52 centimetres.” The model suggests Thwaite’s glacier has also passed its tipping point.

Not good.

Link:  http://theenergycollective.com/TheEnergyCollective/56558