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Thursday, July 8, 2010

More on the Pine Island Glacier (PIG) melting and acceleration mechanisms

Exploring ice shelves

by Graham Cogley,, July 5, 2010

In a remarkable paper about Pine Island Glacier just published in Nature Geoscience, Adrian Jenkins and co-authors describe the latest step in maturation of an emerging glaciological technology: autonomous underwater vehicles — AUVs — or in other words unmanned submarines, for exploring the undersides of ice shelves.

The United States Navy has been sending manned submarines beneath the ice pack of the Arctic Ocean for more than 60 years. But pack ice, a few metres thick, is small beer by comparison with the ice shelves that fringe much of the Antarctic Ice Sheet. These are typically a few hundred metres thick, and there is no question of surfacing by punching your way through the ice if you run into trouble. Indeed, the cavities beneath ice shelves, between the base of the shelf and the sea floor, must be among the most inaccessible of all the theoretically accessible places near the Earth’s surface. Being so hard to reach — until now — the sub-shelf cavity is also one of the least observed, but not necessarily least understood, parts of the climate system.
Climate, you say? Well, the sub-shelf cavity is where the ocean meets the ice sheet. Assuming (safely) that thermodynamic equilibrium prevails, the contact between shelf ice and seawater must be at the freezing point of the seawater, which depends on the pressure of the overlying shelf and the saltiness of the water. But the water and ice at some distance from the contact will not be at the freezing point, so there must be a heat source or sink, and therefore melting or freezing, at the base of the shelf.

If you add warm water to the sub-shelf cavity, you should expect additional melting. In 2002, Rignot and Jacobs inferred, from temperature profiles in the ocean offshore, astonishingly high rates of basal melting near several grounding lines: more than 50 m of ice per year. But this does not mean 50 m/yr of thinning of the shelf. A central part of their analysis was measurement of ice flow across the grounding line by radar interferometry. The inference of rapid melting was required to explain why the floating shelf was “pulling” ice so aggressively out of the grounded ice sheet.

So the contents of the sub-shelf cavity, and what goes on within it, are just as much part of the puzzle of glacier response to climate as are the grounding line itself, the shape of the floor of the cavity, the water at the base of the grounded ice, and of course things that happen in the atmosphere and the wider ocean.

Jenkins and his co-authors have contributed the first large set of in-situ observations from the sub-shelf cavity. What strikes me most forcibly about this dataset is how triumphantly it confirms earlier theoretical analysis of the way things ought to be down there. According to theory, warm water should be flowing inward at depth, melting the shelf base aggressively near the grounding line and — having thus become cooler, fresher and more buoyant — flowing upward and outward along the base. This is exactly what the autonomous underwater vehicle observed, confirming that we did know a thing or two even before measurements became possible on this scale in the sub-shelf cavity.

There are other noteworthy points about this study. For example the AUV found a ridge in the sea floor beneath the floating ice, in just the right place to explain why the grounding line of Pine Island Glacier has been retreating inland since the first observations in the early 1970s, and to confirm that this is something we ought to be worried about.

But perhaps the most noteworthy point of all, looking ahead, is the AUV. Buried in the Methods section of the paper is this, describing an incident part-way through the field campaign: “… the AUV lost track of the rugged ice-shelf basal topography, ascended into a crevasse, collided with the ice and executed avoidance manoeuvres that prompted it to abort its program and take a direct route to the recovery waypoint. After minor repairs, …”. Putting it another way, the world is now a little bit smaller, but not less dramatic, than it used to be.


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