10 April 2009

Another one bites the dust

Another ice shelf bites the dust, to little surprise in the polar community. In this case, it's the Wordie ice shelf, and you can see a photo and a short write up at:

The reason I'm not surprised, and anybody who has been reading the science is not surprised, is that we knew years ago that warming was going to lead to the breakup of ice shelves. See, for example:

Rapid disintegration of the Wordie Ice Shelf in response to atmospheric warming, by C. S. M. Doake & D. G. Vaughan
. That was 1991. You do have to remember that glaciologists are used to things that move at glacial speeds.

The breakup was more specifically tied to atmospheric warming in a more recent (1996) paper by D. G. Vaughan & C. S. M. Doake Recent atmospheric warming and retreat of ice shelves on the Antarctic Peninsula. The authors cite the earliest suggestions of problems for Antarctic ice shelves in a warming world as Mercer's 1978 paper
West Antarctic ice sheet and CO2 greenhouse effect: a threat of disaster. Rather drastic title. But it serves to both establish that the collapse of these ice shelves due to CO2-induced climate change is an old issue, and to provide yet another example of a scientist in the 1970s who was not expecting 'another ice age' any time soon.

The scientific question left on ice shelf collapses on the Antarctic peninsula is not whether, but which one is going to go next, and how soon. For the ice shelves farther to the pole -- which are the big ones and of the greatest concern (the reason for the 'threat of disaster' in Mercer's title) for the future, we still have several scientific questions about whether, and what happens how fast if they do go.

Some folks are making comments which mix up sea ice and ice shelves. Please don't do that! See my note about ice types for the simple differences between the sorts of ice.

It's also been mentioned that the breakup of ice shelves can't change sea level. This isn't actually true. I'll take up the details in two notes to come. First, why it isn't true (you can get ahead by reading the Sea Level Change FAQ -- same process that means sea ice can change sea level applies to ice shelf ice). Second, why it is someone else published it first, even though I had the answer 10 years earlier. A bit of how science is done, and reminder that scientists are fallible.


Hank Roberts said...

Can I be lazy and ask if you have comments on some of the papers I noted at RC? Either here or there.

Some ice is a mix:

Gavin suggested at the top that the pattern of fracture is interesting and hoped for comments from the researchers.

A few other comments earlier in the same thread, and a pointer to:
http://www.uni-muenster.de/Physik.GP/Polargeophysik/Wilkins-Schelfeis.html in hope someone reading German will interpret.

Penguindreams said...

I'm afraid I don't have much to add. You've found some interesting papers, but if you're read them all, you know more than me. From your note over there (folks, do take a look at the pdf Hank links to)

Ok, interesting stuff out there on structure:


Structure of Eastern Antarctic Peninsula Ice Shelves and Ice Tongues from Synthetic Aperture Radar Imagery


Examination of synthetic aperture radar data collected over the southeastern Antarctic Peninsula shows that features sometimes mapped as ice shelves are more likely composed of numerous ice tongues interspersed within a matrix of fast ice and icebergs. The tongues are formed by the seaward extension of numerous small mountain glaciers that drain from the Antarctic Peninsula. Once afloat, the tongues intermingle with a matrix of fast ice and brash. Examination of 1997 Radarsat-1 image mosaics shows that southeastern Antarctic Peninsula composite-ice-shelves covered an area of about 3500 km2. Similar to ice tongues around the rest of Antarctica, these features are highly fragmented and likely to be susceptible to mechanical failure.

This is interesting in its own right. I didn't distinguish ice shelf and ice tongue back in doing my 'types of ice' note. For the most part, this is reasonable. They're both ice that is floating on the ocean being fed by ice on land. The only distinction being that the ice tongue is long and narrow.

From this paper, there is a distinction to be drawn between different ice shelves -- some ice shelves have as important components ice that froze from the ocean, sea ice, and which then froze on to land (fast ice, as in frozen fast) or ice tongues. Once enough such freezing has happened between a number of tongues, you've got an ice shelf of a novel sort. (Brash ice refers to a stage of development for sea ice. We've got a million of them, it seems.)

The mechanical weakness referred to comes about in two different ways. One part is, the sea ice is much thinner than the ice tongue. Second part is that piling up a bunch of rocks and freezing them together is much weaker than having a solid sheet of the material.

Consequently, this sort of ice shelf will be much more inclined to collapse. And, when it goes, it's likely to collapse much faster.

The major Antarctic ice shelves, on the other hand, are not of this sort. The Filchner-Ronne and Ross Ice Shelf are the very thick ice extruded from land type. They're also farther south, so in colder areas. This is important because the mechanical strength of ice decreases as the temperature rises. It is concerns about these two which occasioned Mercer's drastic title. There's more to the story about why they're a concern, but I'll take that up separately.