Disintegrating Ice Shelves
The Wilkins Ice Shelf in West Antarctica has been in the news again lately. As ice shelves go, it is rather small, but it has claimed attention because it is falling to pieces. This is not the breaking off and drifting away of one large iceberg, but a disintegration into lots of slivers, each a few tens of metres thick.
Why not one big chunk? Evidently forces are at work that cause cracks to appear and to propagate through a previously homogeneous block of floating ice, but they are more distributed in the Wilkins collapse than in the really big calving events we sometimes hear about. The slab of ice, weakly supported by the water underneath but pinned more firmly at its sides and at the grounding line, is not strong enough to resist what the engineers call “bending stresses”, originating from variations in the vigour of ice flow or perhaps from the ocean tides.
Why now? Why has the Wilkins joined the growing list of spectacular recent ice-shelf failures? The trouble with the bending explanation is that it does not account for change. Ice shelves are always at risk of bending until they crack, and I know of no evidence that they are suffering more of this now than in the past. In the ordinary way, the loss by calving of icebergs (and perhaps melting on the underside) would balance the gain by snowfall and flow across the grounding line. The shelf would stay roughly the same size.
At this point, enter a now-familiar suspect: global warming. Cracks in ice shelves behave very differently when they have water in them. It is a thousand times denser than air, and magnifies greatly the forces encouraging the tip of the crack to propagate downwards. If the crack propagates all the way to the bottom of the shelf, and also sideways until it meets another crack or the shelf edge, then the job of making a new sliver is done. If there are lots of cracks, and they all fill with water and propagate right through the shelf, then the shelf falls to pieces. This seems to have happened to the Wilkins in recent months. On the other side of the Antarctic Peninsula, continuing study of the collapse of the Larsen B Ice Shelf, an even more dramatic event in summer 2002, is showing that it too disintegrated when, and probably because, its cracks filled with meltwater.
The water can only have come from melting at the surface. All of the West Antarctic ice shelves that have collapsed recently, in whole or in part, are now on the warm side of the -5ºC isotherm of mean annual air temperature at the surface. This isotherm has been migrating rapidly southwards in West Antarctica, a part of the world which, according to weather-station records, has warmed faster than almost any other during the past 50 years.
It would be wrong to think that -5ºC is a kind of tipping point. All it means, given the rather equable climate of West Antarctica, is that an ice shelf which crosses from the cold to the warm side of the isotherm begins to suffer a “significant” summer melt season. At the other end of the world, the ice shelves which once fringed the north coast of Ellesmere Island in Canada have all but disappeared over the last century. The mean annual temperature is much colder than in West Antarctica, but the annual range is much greater, and in northern Canada too the culprit seems to be increased summertime melting.
Why worry? The shelf ice made its contribution to sea-level rise when it flowed across the grounding line, roughly balancing the sea-level fall due to snow accumulating on the grounded ice. The main reason for concern is back stress. At the grounding line, just as there is a force pushing the shelf ice further out to sea, where it will eventually calve and drift away, so there is an opposing force - the back stress - discouraging the grounded ice from accelerating seawards. Take away the ice shelf and you remove the back stress.
Again Larsen B is instructive. Its collapse has provoked the land-based glaciers which formerly fed it to calve icebergs from their grounding lines at greatly increased rates. So the loss of an ice shelf is indeed a cause for concern in the context of sea-level rise. And we think we know why these losses are happening more frequently now.
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