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The Zwally effect: it won’t go away

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.

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