Discussion of Mr. J. T. Hollin’s Paper

Dr. J. F. Nye: You make the point that you cannot reconcile an accumulation which has been constant for, say, 10,000 yr. with a positive mass balance now. In the course of this you quote a figure of 5,000 yr. from my work. I would not want that to be taken too seriously. First, it is, mathematically, not the total time taken for readjustment, it is more a sort of half-life. I would expect things to go on for times of 10,000 to 20,000 yr. And then, quite apart from that there is what I might call the “theoretical error” that might change the constant by a factor of two to four—the constant might be 10,000 yr. and the ice sheet might still be reacting, building up.

Mr. Hollin: Yes, but if so it is rather surprising that it is all building up in phase.

Dr. Nye: The small glaciers near the margin might already have adjusted themselves, so that if you look at the end of a glacier you do not see changes any more, but the ice sheet itself is such a big thing, it might not have reacted yet.

Mr. Hollin: Let us do a rough calculation; suppose the accumulation has doubled from, say, 7 g./cm.² yr. to 14 g./cm² yr., then, at a point where the thickness was previously 2,000 m., the new thickness (since the width of the ice sheet is essentially fixed by sea-level (Reference HollinHollin, 1962)) will be (Reference NyeNye, 1959, p. 498)

. This build-up could take at most 3,600 yr. If it took longer some of the material would have to escape sideways and the budget would cease to be out of balance. Also, in reality, some of the increased accumulation will be carried to the edge with no delay at a velocity which cannot be less than the old steady-state velocity. This is what Dr. Nye has estimated as dying away to 1/e of its value in 5,000 yr. In practice such perturbations must be concentrated in the marginal part of the ice sheet, where the ice circulation is most rapid, and the time constant for such areas is such that a large part of any accumulation increase must be removed horizontally in hundreds rather than thousands of years.

Dr. Nye: One has to distinguish between the edge where you have glaciers finishing on land and the edge where there is ice going off into the sea. I understand that the glaciers look pretty stationary, but of course we do not know whether the rate of calving into the sea has increased or decreased.

Dr. J. W. Glen: Surely it does not matter what the velocity with which ice is coming out by calving is provided it has been assessed correctly. If we have a positive mass balance in Antarctica, then either the middle is getting thicker and it has not yet affected the edges, or the edges are pushing out. This is just as true of the part which is calving as anywhere else, and there is no evidence over admittedly a rather short period of time that the calving glaciers have pushed out markedly.

Dr. H. Lister: There is one little bit of evidence that I feel is fairly reliable, and that is that the snow surface across Antarctica has the most minute grains from the surface to some metres down. There is very little change and the only way we pick out the annual layers is by trying to understand the depth hoar which is dominant in autumn. Now if the Antarctic is not receiving a reasonably adequate layer, the surface grains would be much bigger, they would get that savage temperature gradient through them each spring and each autumn which I think produces the big hoar crystals, and if we do not get large grains most of the way across the continent, then surely we must be having some centimetres of deposit added each year, and even this “thin” blanket cannot be got rid of by the rate of flow according to our existing knowledge, but, agreed, our existing knowledge is inadequate.

Mr. Hollin: Yes, this is one line of evidence, but it is based on limited information, particularly about ablation of course, but I think these other lines of investigation, sea-level, isostasy, glacial geology, and the accumulation records are in their ways just as good evidence.

Dr. G. De Q. Robin: I would like to reinforce what Mr. Hollin has said about the typicality of accumulation measurements. The same certainly applies with regard to the major ice streams. Dr. G. W. M. Swithinbank is making a major investigation of several major ice streams. We know nothing about the volume discharge of the Lambert Glacier for example and yet this is draining an enormous basin. Most of the figures of discharge are for relatively small glaciers. We do not know our major loss terms as yet.

Mr. J. Macdowall: I was interested in the suggestion that some stations may have nonrepresentative accumulation measurements. Obviously this is a possibility; people are likely to try more sheltered areas for a base. However the implications of this on your measurements depend very much on what happens to snow when the wind is blowing. We need to know whether this snow subsequently falls elsewhere in the vicinity.

Mr. Hollin: I simply wanted to draw attention to factors that might be influencing us to a heavy budget. I have a photograph from “Wilkes Station” at the end of the accumulation season. One half of the field of view is beautifully white, while the other half, the Vanderford and John Quincy Adams Glaciers area, is blue because there has been no real accumulation there.