Blog Archive

Wednesday, July 14, 2010

Graham Cogley: Ups and downs of glaciers

Ups and downs of glaciers

by Graham Cogley, environmentalresearchweb, July 12, 2010

Last year I had occasion to take a 500-km trip by taxi. The taxi had a GPS unit — a talking GPS. I didn’t pay much attention along the way, but I had to be impressed when the taxi pulled up on the main street at our destination and the GPS announced, smugly but correctly, “You have reached your destination.”

The Global Positioning System has become part of our lives in the last decade or two, but it is much more than talking taxis. Some recent work illustrates dramatically the ability of accurate positioning devices to tell us things about how the world works.

The toothpaste in the Earth’s mantle complicates attempts to measure glacier mass balance by the gravimetric method, and also by the geodetic method. (For the latter, you need two maps of surface elevation. Subtract the earlier map from the later, and divide by the time span. The result is nearly a map of the glacier’s mass balance, the only missing ingredient being an estimate of the density of the mass gained or lost.)

But what if the elevation of the glacier bed has changed, in conflict with our assumption that surface elevation change equals thickness change, or equivalently that all of the gravity signal is due to the glacier? It can and does happen, and the glacier itself is often to blame. Loading the underlying bedrock, it forces the soft mantle material, at depths below about 100-200 km, away from where the ice is building up. A glacier that is shedding mass constitutes a “negative load”, and the mantle material flows back. (The negative load ends up as a positive load spread more thinly over the ocean.)

The trouble is that the mantle deforms viscoelastically. The elastic deformation is instantaneous and reversible, just like that of an elastic band. Very crudely, it amounts to about a third of the equilibrium response to the load. The remaining viscous part of the deformation is what we think of as flow. To model it, though, we need an accurate model of the variation of viscosity (stiffness) throughout the 2,800 km thickness of the mantle. That is a formidable challenge.

The mantle flows so slowly that it is still responding today to the loss of ice at the end of the Ice Age, roughly 10,000 to 15,000 years ago. The toothpaste is pushing the bed of the glacier upward slowly, and before we can interpret a change in its surface elevation as a change in its mass we have to remove the bed-elevation component of the change.

But now Yan Jiang and co-authors offer an ingenious twist on the monitoring of elevation change with GPS. They have collected five or more years’ worth of GPS readings of surface elevation from several fixed sites around the North Atlantic. The sites are all on bedrock, not on glaciers. (They wouldn’t bear on this particular problem if they were on the ice.)

The surface’s vertical velocity varies from place to place. The ingenious twist is to focus on the vertical acceleration of the surface, which turns out to be systematically greater near to large ice masses (in Greenland, Iceland and Svalbard). The authors argue persuasively that, while the vertical velocity will reflect delayed viscous adjustment, the acceleration is a signal of the Earth’s elastic response to recent increases in the rate of glacier mass loss.

There are some rough edges: sites with large accelerations and not much glacier ice nearby, and one site not too far from the ice but with relatively low vertical acceleration. But I can’t think of a mechanism to explain these observations other than elastic response of the solid earth to recent removal of glacier ice. The viscous response to this unloading has barely begun, and the viscous response to deglaciation cannot possibly change by so much over a period as short as a few years. The authors even have a go with an elastic-response model at estimating the mass balance that would account for the acceleration in west Greenland, and get plausible answers.
The talking GPS on my taxi ride demonstrated the power of positioning accuracy at the few-metre level. For measurements of glacier mass balance we would like millimetre-level (vertical) accuracy, but there are technical and conceptual problems to be ironed out before that becomes reality. For now, Yan Jiang and co-authors have shown that decimetre-level accuracy will do nicely to be going on with.


No comments: