Meteorological Factors Affecting Variation of Glaciers Along the Alaskan Coast: Encyclopedia Arctica Volume 1: Geology and Allied Subjects

The glaciers of Alaska and the meteorological conditions that account for their existence are described in the article “Glaciers in the Arctic.” Also mentioned are their changes in volume and length which are usually determined by observations of the advance or recession of the termini. These variations in size may be due to changes in meteorological conditions, variations in solar radiation, or to the effects of volcanic activity and earthquakes. This article is concerned with the meteorological factors, which in the Alaskan coastal area appear to be dominant agent affecting most variations in the size of the glaciers.

The volume and length of glaciers in temperate regions vary in accord– ance with changes in the rate of snow accumulation in their upper portions and the rate of annual melting at lower altitudes. Both these factors are dependent on air temperature, the former primarily on temperatures conducive to accumulation of snow from precipitation, rime, and hoarfrost; the latter on temperatures sufficiently high to melt the ice after it has flowed from the n e ^ é^ v e ^ é^ area to lower elevations. Any change in the average temperature, no matter how slight, will, therefore, cause a change in the annual rate of accumulation and the rate of wasting of the glacier.

A rise in temperature would tend to modify the volume of precipitation and the proportion of snowfall to the total precipitation over a period of years, and would increase the rate of ablation. The zone of maximum accumu– lation of snow would tend to shift and there would be a corresponding change in the areas of the different glaciers which would receive the greatest supply of snow.

A lowering of the average temperature in the same locality would tend to produce the opposite results. Any change in the mean temperature of different months of the year at different elevations, or lengthening or shortening of the ablation season, will also influence the regimen of glaciers. The terminal melting of glaciers that reach tidewater is also affected by the temperature of the water, which itself reflects any change in the air temperature over the North Pacific Ocean.

The a ^ e^ ffects on the glaciers of both temperature and snow precipitation ^ ✓^ are further modified by any change in the average force and in the prevailing direction of the wind. The rate of melting at a given temperature is sub– stantially increased with any increase in wind velocity. Any change in the direction of prevailing winds is also important since this may cause a change in the temperature of the air coming in contact with the glacier and either increase or decrease the rate of ablation. Wind also plays a significant part in the areas where snow accumulates, for any change in its velocity or direction changes the amount of snow which accumulates in different areas from direct precipitation, rime, hoarfrost, and from drifting. This will determine which slopes become overburdened with snow to the point where avalanching further modifies the distribution of the snow.

Although any one factor such as temperature, precipitation, or wind may vary more than the others, they are all so closely interrelated that any change in the general circulation of air above that portion of the earth’s surface will be reflected in some modification of each.

Individual glaciers will respond to any such meteorological changes somewhat differently, depending on such factors as their orientation, elevation, relationship of area to elevation, geographical position, thickness, rapidity of motion, character of surrounding area, and the general nature of their areas of accumulation and ablation. Thus, the effects of the same change in meteorological conditions will not necessarily be similar on two glaciers but may be opposite. This accounts for the not unusual phenomenon of some glaciers increasing in volume and experiencing an advance of their termini, while others nearby are retreating and shrinking in size.

The response of glaciers, according to orientation and geographic position to changes in temperature, snow accumulation, and wind, are not as difficult to assess as the effect of such changes at different elevations. The position and elevation of the summer snow line on the glacier, known also as the firn line, is critical to the regimen of the glacier. This is the point at which snow accumulation since the peak of the previous ablation season exactly equals melting in the subsequent ablation season, and, therefore, marks the division during that period of about one year between the area of accumulation and the area of ablation. Its position on the glacier tends to vary somewhat each year according to where this balance is achieved. Above this point there is a net accumulation during the year. Below it there is a net loss through melting. The surface of the former is snow or n e ^ é^ v e ^ é^ not yet consolidated into glacier ice; the surface of the latter at the peak of the ablation season is ice.

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The dominant factor that determines the position of this snow line on the glacier is temperature. Any considerable departure from the mean may cause a substantial increase or decrease in the volume of the glacier by reducing or enlarging the area of the glacier subjected to melting during that par– ticular period and the extent of the area in which accumulation will occur. An important consideration, which illustrates the complexity of the character of surface ablation, is the fact that at a given temperature, ablation on ice is some 30% greater than on snow. Thus in the years that this snow line is high, ablation is proportionately much greater. Ablation also increases relatively more at higher than at lower temperature intervals. Thus a few abnormal seasons may have such a profound effect on a glacier as to alter its entire regimen either tending to further expansion or further contraction until a balance is again achieved between accumulation and ablation.

Meteorological conditions not only account for the existence of glaciers but in a local sense are themselves influenced by the extent of the glaciers themselves. Glaciers and ice fields cool the air passing over them and thus further modify total precipitation, the proportion of snowfall to the total precipitation, and the character of the local winds. It follows, therefore, that any reduction or increase in the size of such ice masses will also exert an important influence on local air temperatures and result in further modifica– tions of snow accumulation and changes in the circulation of the air. Thus, if the balance of a glacier’s regimen is strongly negative or positive, its influence on meteorological factors will tend to accentuate that condition until a balance determined by the proper combination of different meteoro– logical factors is again restored. Another important consideration is that

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the effect of a given set of meteorological conditions on a glacier of a given form and elevation will be quite different in another decade or century when that glacier may be either more or less expensive with a corresponding change in the elevation of its surface.

A further complicating factor is that, in general, the effects of any change in snow accumulation are long delayed while the results of any increase or decrease in melting are immediate and often measurable in that particular year. The effects of a change in the amount of snow accumulation in the n e ^ é^ v e ^ é^ area of a glacier, many miles in length, may not be observable in ^ ✓^ the terminal portions of the glacier for many years, while a change in the rate of melting below the snow line on the glacier will take effect immediately. In addition to this time lag, it has also been observed that glaciers will respond to the meteorological changes causing their expansion at a slower rate than to changes i ^ o^ f similar magnitude favoring contraction. The recession of glacier ^ ✓^ termini is, therefore, usually more rapid than glacier advance. In other words, a few years with summer temperatures well above the mean will do more damage to the glaciers than would be restored in a series of correspondingly cooler years.

The observed results along the Alaskan coast are that, in general, the last half-century has been a period of recession, varying from rapidly wasting glaciers in some areas to slow recession in others. However, there are also a few significant exceptions where glaciers appear in a semblance of balance or have advanced over terrain not covered by ice within at least 500 years. The general climatic changes that can be detected are a slight increase in the mean temperature during the last half-century and a slight lengthening of the ablation season. The net effect of this is to increase the rate of ablation and probably to raise the level of maximum snow accumulation. Most glaciers are affected by a proportionately higher rate of ablation than of accumulation and are, therefore, in a state of shrinkage or recession.

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A few glaciers, however, are so situated that the increase of ablation is more than compensated by an increase in accumulation at higher elevations, so the net result is expansion revealed by a slow advance of the terminus.

To date meteorological records are almost entirely confined to coastal points at or near sea level and a few localities in valleys on the inner side ^ ✓^ of the coastal ranges. These data are helpful but contribute l o ^ i^ ttle to our knowledge of the interplay of meteorological factors affecting the regimen of the glaciers themselves. This vital aspect of glaciologic studies has been almost entirely neglected in the northwestern part of this continent so that no detailed evaluation of the meteorological changes now affecting Alaskan coastal glaciers is available. Because of this, the general principles herein set forth are largely derived from the detailed glaciologic and meteorologic studies of observers of somewhat similar coastal glaciers in Scandinavia and the islands of the North Atlantic.

William O. Field, Jr.