Thursday, January 14, 2010

Major Antarctic glacier at risk of major collapse - is 'past its tipping point'

One of the risks from the warming climate is that the glaciers on Greenland and the Antarctic would melt enough to collapse into the ocean. The melting of sea ice like the Arctic doesn't itself contribute to sea level rise. But if ice that's on land (a glacier) drops into the ocean it will rise ocean levels. Rising ocean levels have already been observed around the world but if a significant chunk of glaciers were to drop into the ocean it's thought to rise ocean levels dramatically. Coastal regions such as the place where I'm writing this article (in Silicon Valley) stand to be swamped by rising oceans.

Pine Island glacier (PIG) is one of many at the fringes of the West Antarctic ice sheet. A new modeling study has identified this glacier as being past a "tipping point" that will cause a 50% collapse of its ice into the ocean within the next 100 years. 100 years? What's the worry? Sigh, humans with their short time span of thinking. It means our children will have to deal with dramatically rising ocean levels.

This "tipping point" is their estimate of the required conditions for the glacier to collapse into the ocean. These glaciers have been there for a long time and obviously conditions change from year to year without the glaciers collapsing. But some antarctic glaciers have collapsed suddenly indicating that other glaciers could also collapse suddenly. A group of scientists have identified a geological feature in the Antarctic that, combined with already rising sea levels, will cause a sudden collapse sometime in the not-so-distant future.

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.

According to their models this glacier passed the tipping point in 1996.. quite a while ago. It's already too late. The model also says a neighboring glacier has also passed its tipping point. The article on New Scientists closes with an admonishment that the model is a conservative one which ignores some important physics, and that the real rate of retreatment may be faster than the model suggests, which would mean glacier collapse a lot sooner than 100 years in the future.

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