Dynamic thinning: just add warm water?
Dynamic thinning is even more exciting than basal lubrication, but recent work by Hamish Pritchard and colleagues shows up the difference. For basal lubrication of the flow of glacier ice, just water will do, but it is now clearer than ever that for dynamic thinning what you need is warm water. Or rather, and this is not the same thing, if you supply warm water what you get is dynamic thinning.
Dynamic thinning is thinning over and above what can be expected from an imbalance between the ability of the climate to generate meltwater and the ability of the glacier to replenish the stock by flow. There is a simple reason for getting excited when we observe dynamic thinning. It means that the ice must be moving faster, and shedding more mass at its terminus, than it did before the dynamic thinning began.
Most glaciers are thinning at present. Strictly speaking, we mean that their surfaces are getting lower, but we can correct for the possibility that part of that is due to change of the bed elevation. Measuring the change of surface elevation is within the grasp of several methods. In particular, the ICESat laser altimeter on which the Pritchard work relies can give very accurate estimates.
In Greenland the most thinning you can expect from the climate – snowfall minus melting – is a few m/yr. In Antarctica, less than that is usual, and there are plenty of outlet glaciers down there where even today there is no melting even at sea level, and all of the loss is by discharge across the grounding line. Dynamic thinning, and the implied faster flow and faster discharge, are signs of trouble.
What Pritchard and colleagues have shown is that dynamic thinning was widespread around the margins of Greenland and Antarctica during 2003–2007, and is on the increase. Both of these findings are scary. There ought not to be any dynamic thinning in a well-regulated world. Perhaps their most telling finding is that there is a definite difference between fast-flowing tidewater glaciers and slower parts of the ice-sheet margins.
Roughly, the slow-flowing margins are showing just run-of-the-mill accelerating loss with no evidence for dynamic thinning. We are learning rapidly about basal lubrication as one of the reasons for the run-of-the-mill acceleration. But if it were the main or only reason then there would be no difference between the land-terminating and the tidewater margins. In fact, all hell is busting loose on the tidewater glaciers. Pritchard and colleagues have now documented dynamic thinning in the north of Greenland as well as the south, and in the east of Antarctica as well as the west – but only on tidewater outlets, and only on fast-flowing tidewater outlets.
These results point squarely at the ocean as the culprit. The better the ocean is at melting the base of a fringing ice shelf, or at sapping a grounded calving front, the faster does the glacier go in its efforts to maintain the supply of ice, and the greater the resulting dynamic thinning inland. At this point, our explanations of what is happening begin to get rather hand-waving. Measuring the temperature of the ocean near to tidewater glaciers is an extreme challenge, and we know very little about whether it is changing. But there are indeed some signs that warmer water is getting at the ice.
For example, David Holland and colleagues have shown dramatic maps, based on measurements by fishing boats, of the arrival of warm water in Jakobshavn Fiord in western Greenland. The warm Irminger Current, deriving from the Gulf Stream, curls clockwise around southern Greenland. Between 1996 and 1997, it flooded Jakobshavn Fiord with water 2–4 °C warmer than what was there before. In 1997, the floating tongue of Jakobshavn Glacier, the largest outlet of the Greenland Ice Sheet, began to disintegrate, and simultaneously the dynamic thinning of the glacier began. By 2007 it was 30 m thinner 70 km inland. At 15 km inland, it was more than 200 m thinner. It is still thinning, and nobody can tell when it will stop thinning.
Not all of the dots connecting global warming to the dynamic thinning of Jakobshavn Glacier have yet been joined up, but I don’t know anybody who is betting on this being the wrong explanation, or, after the demonstration by Pritchard and colleagues, on Jakobshavn being an isolated example. This makes the world look even more complicated, and for the moment at least it makes predicting the future contribution of glaciers to sea-level rise even harder.
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