environmentalresearchweb blog

« Green pricing issues - how long can a few carry all? | Main | Energy efficiency, conservation and “sacrifice” revisited »

Listening to Glaciers

The technology for finding out more about glaciers keeps getting more diverse. In addition to bouncing laser beams and radar waves off them to find out about changes in their shape, and to finding out about changes in their mass by weighing them, an intriguing recent innovation is to listen to them.

At glaciers that terminate in tidewater, the noise of great chunks of ice cracking off and collapsing into the water inspires awe if you happen to be within earshot, but I am thinking here of noises that are detectable not by the human ear but rather by seismographs. Most earthquake waves are sound waves, that is, more or less rapid fluctuations of pressure. Except that the seismographs are picking up waves that have travelled through the earth rather than the air, seismic waves are not different fundamentally from those to which our ears are sensitive. But the study of icequakes is still in its infancy.

Icequakes are fluctuations of pressure that originate in sudden motions of glacier ice, not of the rocky earth. They have been known for quite some time, but their interest and potential were first highlighted by Göran Ekström and colleagues (here, but you can’t get back to this page from there for some reason), who filtered the records from seismic observatories and identified several hundred long-period (that is, rumbly) events that did not look like ordinary earthquakes. Of the 71 found in areas usually regarded as seismically quiet, 46 were from glaciers, and of these 42 were from fast-flowing outlet glaciers in southern Greenland.

People sat up and took notice. Since the Ekström report in 2003, at least three different ways in which glaciers can make a noise have been documented. Their original suggestion can be interpreted as stick-slip motion at a patchily-frozen glacier bed. The ice lurches jerkily downstream, and some of the energy thus released finds its way to the seismic observatories, or to specially-deployed arrays of seismometers in the neighbourhood of the icequake.

Abrupt failure at the propagating tip of a water-filled crevasse can do essentially the same thing. This is an explanation favoured by Shad O’Neel and Tad Pfeffer, and is interesting because such failures seem to be possible precursors of even bigger events due to the calving of icebergs.

That brings us to the third mechanism, the calving itself, and to a recent analysis by J.A. Rial and colleagues that may be the most interesting of all. These authors studied Jakobshavn Glacier in west Greenland, a major ice-sheet outlet of which the terminus is falling to pieces dramatically. The rumblings they observed are consistent in many ways with earlier explanations, and in particular with the idea that things start with an iceberg breaking off the end of the glacier. But the rumblings go on for tens of minutes, and end with a large “culminating event” that can often be pinpointed to a part of the glacier margin 10-12 km upstream from the calving front. Evidently the loss of back-pressure due to loss of the iceberg leads to abrupt release of stress about half an hour later, at the frozen contact between the glacier and its valley wall.

This concern with the physics of how glaciers lose icebergs may strike you as finicky. What makes it worthwhile for the authors, and for their readers, is given away when they write that this sort of pattern suggests “a highly repeatable process of local glacier dynamics currently unknown to us”. In other words, something genuinely new is awaiting efforts to make sense of it.

What is more, it offers the prospect of finally being able to measure the calving rate. Seismic measurement of calving rates is not around the corner, but it would clear away one of the major obstacles to quantifying the mass balance of large tidewater glaciers. The rate at which the terminus advances or retreats is not the main part of the problem. Rather, we need to know the rate at which ice is arriving at and discharging from the terminus. At present the best method, at least for regular monitoring, is to watch the icebergs falling off and guess at their sizes. Listening to them might turn out to be a better idea.

Posted by Graham Cogley on September 7, 2009 3:00 PM | Permalink