The Zwally Effect: new evidence of its broader application to Greenland's ice sheet melting and glacier outflows

The Zwally effect: It won’t go away

by Graham Cogley, environmentalresearchweb.org, July 19, 2010

The Zwally effect is an acceleration of the flow of marginal ice in the ice sheets due to lubrication of the bed by meltwater percolating from the surface. Up to a point, this phenomenon is not surprising. It is well documented on smaller, thinner valley glaciers. The surprise, first documented by Zwally and co-authors in 2002, is seeing the same phenomenon in ice as thick as 1,200 m.

The Zwally paper has stimulated a growing literature with two main threads. One thread tries to explain how meltwater can find its way through more than a kilometre of ice. The other tends to show that the Zwally effect is not the reason for dramatic increases in the speed of tidewater outlet glaciers, where the evidence favours, quite strongly, warm ocean water as the culprit. But that doesn’t mean that seasonal acceleration is uninteresting.

Ian Bartholomew and co-authors report on more dramatic seasonal acceleration than has been measured hitherto. It still doesn’t rival the speed-ups observed on some tidewater outlets, but the observations highlight the potential of GPS from a different angle, and suggest fascinating insights into how the surface meltwater does its subglacial work.

This new report relies on time series of positions obtained with four Global Positioning System receivers deployed along 35 km of a land-terminating flowline at 67.1° N in southwest Greenland. The data include not just horizontal but also vertical velocities, as well as near-surface air temperature. Averaged over the summer, the speed-up from winter background values was rather modest. But the fascinating bits are the details.
The further up-glacier, the later the onset of speed-up, by more than a month. The natural explanation is a later onset of melting at higher elevations. The highest site was at 1,063 m and the lowest at only 390 m above sea level.

More interesting is that the horizontal velocity correlates very nicely with the vertical acceleration, or in other words with the rate of uplift of the surface. The ice goes faster when the surface is uplifting rapidly. Or rather, rapid uplift seems to provoke speed-up. This is a subtle observation in more ways than one. For one thing, the amounts of uplift are a few decimetres at most. That we can detect such subtle vertical motions is a payoff for all the trouble it took to loft a couple of dozen GPS satellites into orbit.

More interesting still is the authors’ subdivision of the summer into three phases. In phase 1, there is no particular surface uplift or speed-up: the meltwater, if any, has yet to reach the bed. In phase 2, the cumulative uplift increases towards a maximum, and so do the horizontal velocities, more or less. (You need the eye of faith to see these phases in the noisy data. But I buy them.) The concluding phase 3 sees repeated episodes of uplift and speed-up, but the course of the surface elevation is downward and so, more or less, is that of the horizontal velocity.

Phases 2 and 3 add up to another picture of an invisible world beneath the authors’ feet. The meltwater, once it reaches the bed, pressurizes the ice and forces it upwards, filling and enlarging cavities and promoting basal sliding. But the enlargement proceeds at least in part by melting of roofs and walls, implying the creation of connections and, in short, of a network. The network grows steadily better at discharging the arriving meltwater. Phase 2 becomes phase 3 when the network becomes more than able, on average, to cope with the spate of water. Phase 3 ends when the supply of meltwater gives out, and the ice starts winning again, resuming its regular wintertime job of squeezing the summertime channels shut.

If you want real glaciological drama, visual or acoustic, you should probably go to tidewater terminuses, at which most of the ice leaves the ice sheet. But there is still plenty of land-terminating ice, and the main things about the Zwally effect, granting that it is real, are that it must be real everywhere; and that if the surface of the ice sheet gets warmer, then the bed of the ice sheet is bound to get busier.

Link:  http://environmentalresearchweb.org/blog/2010/07/the-zwally-effect-it-wont-go-a.html

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.

Link:  http://environmentalresearchweb.org/blog/2010/07/ups-and-downs-of-the-glacier.html

F. Sigmundsson et al., Trans. Roy. Soc. A, Vol. 368, Climate effects on volcanism: Influence on magmatic systems of loading and unloading from ice mass variations, with examples from Iceland

Philosophical Transactions of the Royal Society A (May 28, 2010), Vol. 368, No. 1919, pp. 2519-2534; doi: 10.1098/rsta.2010.0042

Climate effects on volcanism: Influence on magmatic systems of loading and unloading from ice mass variations, with examples from Iceland

Freysteinn Sigmundsson*, Virginie Pinel, Björn Lund, Fabien Albino, Carolina Pagli, Halldór Geirsson, and Erik Sturkell


Abstract

Pressure influences both magma production and the failure of magma chambers. Changes in pressure interact with the local tectonic settings and can affect magmatic activity. Present-day reduction in ice load on subglacial volcanoes due to global warming is modifying pressure conditions in magmatic systems. The large pulse in volcanic production at the end of the last glaciation in Iceland suggests a link between unloading and volcanism, and models of that process can help to evaluate future scenarios. A viscoelastic model of glacio-isostatic adjustment that considers melt generation demonstrates how surface unloading may lead to a pulse in magmatic activity. Iceland’s ice caps have been thinning since 1890 and glacial rebound at rates exceeding 20 mm yr⁻¹ is ongoing. Modelling predicts a significant amount of ‘additional’ magma generation under Iceland due to ice retreat. The unloading also influences stress conditions in shallow magma chambers, modifying their failure conditions in a manner that depends critically on ice retreat, the shape and depth of magma chambers as well as the compressibility of the magma. An annual cycle of land elevation in Iceland, due to seasonal variation of ice mass, indicates an annual modulation of failure conditions in subglacial magma chambers.

*Correspondence e-mail: fs@hi.is

Link to abstract:  http://rsta.royalsocietypublishing.org/content/368/1919/2535.abstract

P. Huybers & C. Langmuir, Earth Planet. Sci. Lett., 286 (2009), Feedback between deglaciation, volcanism, and atmospheric CO2

Earth and Planetary Science Letters, 286 (2009) 479–491

Feedback between deglaciation, volcanism, and atmospheric CO2

Peter Huybers and Charles Langmuir

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, U.S.A.


Abstract

An evaluation of the historical record of volcanic eruptions shows that subaerial volcanism increases globally by two to six times above background levels between 12 ka and 7 ka, during the last deglaciation. Increased volcanism occurs in deglaciating regions. Causal mechanisms could include an increase in magma production owing to the mantle decompression caused by ablation of glaciers and ice caps or a more general pacing of when eruptions occur by the glacial variability. A corollary is that ocean ridge volcanic production should decrease with the rising sea level during deglaciation, with the greatest effect at slow spreading ridges.
 

CO2 output from the increased subaerial volcanism appears large enough to influence glacial/interglacial CO2 variations. We estimate subaerial emissions during deglaciation to be between 1000 and 5000Gt of CO2 above the long term average background flux, assuming that emissions are proportional to the frequency of eruptions. After accounting for equilibration with the ocean, this additional CO2 flux is consistent in timing and magnitude with ice core observations of a 40-ppm increase in atmospheric CO2 concentration during the second half of the last deglaciation. Estimated decreases in CO2 output from ocean ridge volcanoes compensate for only 20% of the increased subaerial flux.  If such a large volcanic output of CO2 occurs, then volcanism forges a positive feedback between glacial variability and atmospheric CO2 concentrations: deglaciation increases volcanic eruptions, raises atmospheric CO2, and causes more deglaciation. Such a positive feedback may contribute to the rapid passage from glacial to interglacial periods. Conversely, waning volcanic activity during an interglacial could lead to a reduction in CO2 and the onset of an ice age. Whereas glacial/interglacial variations in CO2 are generally attributed to oceanic mechanisms, it is suggested that the vast carbon reservoirs associated with the solid Earth may also play an important role.

Link to complete paper here:  http://www.people.fas.harvard.edu/~phuybers/Doc/volc_ice.pdf