The importance of the altitude of the principal accumulation areas of a glacier, when considering its response to temperature changes, has been repeatedly stressed by Ahlmann,Reference Ahlmann 1 CooperReference Cooper 2 and others, but the reason for the decisive influence of this altitude is not always very obvious to workers in related fields. It may, however, be clearly demonstrated by means of a simple diagram such as the one accompanying this note.
1. Basis of the Values Used
The ablation and accumulation values, shown in the diagram as water equivalent, are those obtained by Professor AhlmannReference Ahlmann 1 on the southern side of Vatnajökull, extending up to about 400 m. above the firn line. The increase of accumulation with altitude was found to average 0.28 per cent (nearly 3 mm. per metre of altitude) for the three years 1936 to 1938. The slope of this line refers to the particular conditions on the south side of Vatnajökull but may be rather typical for the sides of an ice cap facing relatively warm ocean water. The same applies to the variation of ablation in altitude, which was found to average minus 0.48 per cent (nearly 5 mm. per metre) over all except the lowest ice, but in this case the slope probably has a wider significance for it is conditioned by the climate rather than by the local physical features.
At altitudes more than about 400 m. above the firn line the decrease of accumulation has been estimated from various data, and little weight need be given to this part of the curve. A slope of minus 0.15 per cent has been chosen on the assumption that the accumulation would have fallen to about 30 cm, on a plateau 2 km. above the firn line in this climate. (Owing to the extreme cold at this level the air can carry little moisture beyond the margin of the plateau.)
2. The Net Accumulation
The importance of the annual addition of firn remaining at the end of the ablation season needs no emphasis, and its distribution in altitude is also a very significant factor for the glacier’s response to variations of climate. The curve in the diagram (p. 575) shows that the increase of this net accumulation immediately above the firn line is extremely rapid. For Vatnajökull the slope is about 0.75 per cent in the first 200 m. above the line. At greater heights this rate of increase rapidly diminishes until it must change into a decrease for ice fields which lie far above the firn line. In the Vatnajökull climate the decrease might be expected to begin some 500 to 700 m. above.
In this connection the height at which the ablation falls to insignificant values is an important factor. It is estimated to lie between 500 and 600 m. above the line in the climate of the south side of Vatnajökull. Presumably it varies directly with the accumulation and inversely with the length of the ablation season at the line, but the tendency near the limit of ablation is for the slight melting, on a few days in summer, to be refrozen in the firn, and this may considerably reduce the height of effective ablation in very cold dry climates.
3. Examples of firn Fields at Various Heights
The three vertical strips, marked a, b and c in the diagram, within the curve of net accumulation, refer to the altitudes of the principal accumulation areas of three hypothetical firn fields. The broken line curve indicates the net accumulation if the firn line rose 100 m. above its former position, on account of an increase of about 0.7° C. in the temperature of the ablation season, without change of precipitation. It is at once evident that the effect of such a change in firn line altitude is very much greater on ice field (a) than on the field at (b) or (c), in fact there is every reason to suppose that the effect at (c) will be opposite, although probably much smaller.
In general, the effects of such an increase in the altitude of the firn line may be summarized as follows for ice fields at various higher levels which lie in a region of considerable precipitation:
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a. Firn field up to about 400 m. above the original line. Strongly negative. Drainage glaciers will be extremely sensitive to slight temperature changes Iasting only a decade or two. Examples: Juneau Ice Field (LawrenceReference Lawrence 3 ) with up to 15 terminal moraines formed in the last two centuries. Jostedalsbreen is another well-known example of this type.
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b. 500 to 700 m. Indifferent. Amount of ice passing the fink Line little affected. Glacier tongues move back to adjust their ablation areas to the new firn line. Vatnajökull during the nineteenth century is a possible example, as much of the firn area lay at 500 to 600 m. above the line, and the outlets showed little net movement (ThorarinssonReference Thorarinsson 4 ).
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c. Above about 700 m. Slightly positive with more ice passing the new firn line. Drainage glaciers will at first retreat to adjust their ablation areas, but after a long interval under the new conditions they should advance again. Although Greenland covers too great a range of climate for generalizations of this kind, one could anticipate a recent increase of accumulation on most of the plateau. The, effects of intensified ablation are very striking at low levels. Zone (c) covers almost all of the Antarctic continent down to sea-level, but reactions to temperature changes would be exceedingly slow.
It is not the purpose of this note to consider the very complex climatic response of steep glaciers on high mountains, beyond noting that it should be governed by their relative amounts of
firn area in the different zones. In high latitudes some of these glaciers should have a substantial proportion of their accumulation area in zone (c), and might be expected to show a positive tendency in response to a rise in temperature, the more so if the latter is accompanied by greater storminess and precipitation as seems to be the case in these latitudes. Wind drift is another factor of great importance to the climatic response of valley glaciers.
Before closing a word should be said about the mysterious case of the Taku Glacier in the Juneau Ice Field, Alaska, because the recent behaviour of this great glacier serves as a useful warning that simple generalizations seldom have a universal application to complex natural phenomena.
The Taku comes from a fire field lying entirely in the lowest zone (a), but has advanced steadily by no less then 5 km. during the past half-century. Meanwhile, about ten other large tongues from this field have receded rapidly in accordance with the vast majority of glaciers in North America. The Juneau Ice Field is at present under active investigation in the capable hands of Mr. W. O. FieldReference Field and Miller 5 of the American Geographical Society and Mr. M. Miller, and perhaps we may anticipate an early solution of the Taku mystery.
