RECESSION AND SHRINKAGE OF THE GLACIERS.
Every glacier about Mount Rainier that was examined by the writer furnished evidence of a recent recession of its terminus and of a lowering of its surface. In two instancesthe Carbon and Willis glaciersrough measurements of the amount of these changes during the past fifteen years were obtained.
A recent recession of the extremities of the glaciers is shown by barren areas below them on which vegetation is advancing. All of the primary glaciers extend below timber line, and in a climate like that of the region about Mount Rainier any area where there is sufficient soil soon becomes covered with young trees. The barren area about the ends of the glacier may in part be accounted for by the fact that the streams from the glaciers shift their positions from time to time and sweep away the earth in their courses or make deposits of bowlders and stones; but about the extremity of each glacier there are areas not thus affected which are similar in soil conditions to adjacent areas on which large trees are growing. The forests are advancing on the barren areas and gradually taking possession of them. This evidence, even if actual observations of the recession of the extremities of the glaciers were not available, is sufficient to show that the ice streams have for a number of years been growing shorter and shorter.
The fact that abandoned lateral moraines bordering the glaciers below their névé fields are in many instances not yet clothed with vegetation, while older moraines of a similar character are forest covered, is evidence that their surfaces have been recently lowered. The elevation of the crests of these recently abandoned lateral moraines above the surface of the glacier that they border is least, or perhaps not at all, noticeable just below the lower borders of the névés, and becomes progressively greater and greater downstream. The lowering of the surfaces of the glaciers is due largely to surface melting. This leads to a concentration of englacial débris at the surface. For this reason the lower portions of the glaciers are almost always heavily covered with stones and earth. As the superficial débris increases in thickness it affords greater and greater protection to the ice beneath, and surface melting is correspondingly retarded. The influence of climate on the melting of the glaciers is thus counteracted to a marked degree. While in the case of glaciers that are free of morainal material or but lightly loaded surface melting would be in excess of subsurface melting, it is apparent that in the case of heavily moraine-covered ice bodies the rate of melting is greatest below the surface portion. The subsurface melting, it is apparently due to the descent of surface streams into crevasses and moulins and to the heat furnished by streams from the adjacent land which flow through englacial or subglacial tunnels.
The marked recession and shrinkage in progress in the case of the glaciers on Mount Rainier is evidence of a climatic change which is accompanied either by a decrease in the snowfall or by an increase in the mean annual melting, or, what is more probable, of both. Whether the recession and surface lowering of the glaciers is a continuous process or is varied by minor periods of advance and rise of the surface is not known. Judging by what has been observed in the case of the glaciers of the Alps, however, it is to be expected that the glaciers about Mount Rainier will be found to pulsate, as it werethat is, their extremities, although in process of retreat when a period of ten or twenty years is considered, will be found to alternately advance and retreat when smaller periods of time are considered, the algebraic sum of the fluctuations being in the direction of recession. It is for the purpose of obtaining more detailed information in reference to the fluctuations of the glaciers that records of the positions occupied by their extremities, suggested in a preceding page, are desired.
The rapid recession and marked lowering of the surfaces of the glaciers about Mount Rainier is not peculiar to that system of ice streams, but is shared by all of the glaciers of the west coast of North America, with but one known exception. In no instance have the minor fluctuations of the glaciers referred to been observed, but the fact of a general retreat of the extremities of the glaciers, from those in the High Sierra of California to western Alaska, is well known.1 Not only are the glaciers of the Cordilleran region of North America undergoing a gradual decrease, but all others in the northern hemisphere, with the possible exception of those of Greenland and adjacent regions, are likewise shrinking back into the higher portions of the mountains from which they flow, although in some instances exhibiting local advances.
The recession of the glaciers referred to is evidence that a climatic change has been in progress for at least a score of years, and probably for over a century, which is unfavorable to the existence of perennial ice. Evidently the mean annual snowfall is becoming less, the mean summer temperature greater, the prevalence of clouds and fogs in the summer season decreasing, or a combination of these changes in conditions throughout the entire northern hemisphere is leading, on an average, to an excess of melting over snow accumulation. Observations on the fluctuations of the extremities of the glaciers about Mount Rainier thus have increased interest because these fluctuations form a part of a series of changes which is affecting probably the entire earth. The study of the climatic changes indicated by the fluctuations of existing glaciers has a still wider bearing, as it may be expected to throw light on the much-discussed problem of the cause of the cold of the Glacial period.
Last Updated: 28-Mar-2006