By Graham Cogley
Plenty of evidence has emerged recently to show that the beds of glaciers can be complicated places, especially when we consider the liquid water down there and the fact that much of that water must have come from the surface.
In a paper just published in Nature, Christian Schoof explores this complexity and explains at least some of it.
One of Schoof’s insights is that cavities and channels are two very different ways to store subglacial water. The size of a bed cavity, in the lee of a bump for example, is governed by the respective rates at which the basal ice continues to flow horizontally down-glacier, opening the cavity, and creeps downwards, lowering the cavity roof. The size of a channel is governed by the rate at which its wall expands by melting and the rate at which the wall shrinks by inward creep of the ice. Cavity or channel, the creep rate depends on the difference between the pressure of the ice on the void and the pressure of whatever is in the void, water or air, on the ice.
For a given thickness of ice overburden, this pressure difference depends on the fluid pressure. Air is useless at opposing the weight of the ice, so we are only interested in the water pressure, which requires that we acknowledge the importance of meltwater from the surface. Water melted at the bed, by geothermal heating and by friction between the ice and the bed, is insignificant, and the pressure of the void-filling water on the overlying ice is likely to depend entirely on how fast water is arriving at the bed from above.
So in fact three variables determine how the meltwater at the bed organizes itself: the melting rate at void walls, the opening rate due to down-glacier flow, and the closure rate due to the pressure difference at void walls, the latter depending in turn on the rate of delivery of surface meltwater. These variables are entangled with each other, but Schoof combines them ingeniously, and consistently, in a model that shows that this is a one-thing-or-the-other problem. A collection of linked cavities can be a stable way to organize the meltwater, and so can a tree-like network of channels, but any other arrangement of the voids will evolve into one of these two.
Linked cavities can be kept full, and can transfer meltwater not too inefficiently, if, or rather because, the water pressure is high. At high water pressure, the ice will flow faster because less of it is in contact with its solid bed, meaning that cavity opening will proceed faster. More of the ice will reach lower, warmer elevations sooner, increasing the production of surface meltwater.
But channels are different. The more meltwater in them, the faster their walls melt and the bigger they get, lowering the water pressure and so tending to drive pressurized cavity water towards and into them. In Schoof’s simulations a few big channels end up discharging the meltwater. But because more of the water is in big channels, less is spread over the bed. More of the ice is in contact with the bed and not with water, and the ice will slow down.
But now comes an intriguing twist in the plot. Surface meltwater tends to reach the bed in pulses, once a day. Closure of the channels by creep is a slower process, requiring days or longer. So the daily pulses raise the water pressure in the channel network, driving water out of the channels, weakening the contact of the ice with the solid bed, and thus speeding the ice up. This speedup is not fully integrated into Schoof’s analysis, but is clearly a way for the subglacial drainage network to have its cake and eat it. More meltwater implies channelization, reduced water pressure, and deceleration of the glacier. But more meltwater arriving in pulses means that a glacier can still slide rapidly over its bed even though the drainage network at its bed has become channelized.
If, over the next century or two, we lose a large fraction of the ice now in the Greenland Ice Sheet — or, perish the thought, the Antarctic Ice Sheet — then greenhouse gases will have a lot to answer for. But Christian Schoof’s analysis shows that so will the Sun. Or, to be more accurate, so will the Earth, because it turns to meet the Sun once a day.