Recent glacier and climate variations in the Pacific Northwest
Stephen C. Porter
Quaternary Research Center, University of Washington
The rugged peaks of Washington's Cascade Range support more than 800 mountain glaciers, the longest and most voluminous of which descend the flanks of lofty dormant volcanoes. Unlike the glaciers of northwestern Europe, which lie close to human settlements and have been observed for hundreds of years, those in the Pacific Northwest were not visited until the second half of the last century, and most had not been seen or studied before the early decades of the 20th century.
Glaciers fluctuate in size in response to changes in their climatic environment, and their yearly changes in mass (i.e., their annual mass balance) is determined primarily by the amount of winter snowfall and the amount of summer melting. Small mountain glaciers in temperate latitudes tend to respond rapidly to climate variations. The glacier margin, or terminus, will advance or retreat depending on the glacier's recent mass balance history, which in turn reflects the recent pattern of climate variation.
Studies of glacier history throughout the world have shown that, following glacier recession at the end of the last glacial age, mountain glaciers expanded in size during the last three millennia, many reaching their greatest postglacial size during the last 700 years. This recent interval of glacier growth, widely referred to as the Little Ice Age, culminated in major glacier expansions during the early to middle 17th century and the early to middle 19th century. Since about 1850, glaciers worldwide have experienced fluctuating retreat, so that today we commonly see belts of sparsely vegetated deglaciated terrain beyond receding ice margins. Whereas the largest glaciers have retreated a mile or more during the past century, some small glaciers have disappeared entirely in recent decades in response to the general 20th century warming of the climate.
Glacier-clad Mount Rainier, seen from the west. The longest glaciers originate at the summit of the volcano and descend the mountain flanks to terminate below treeline.
The glaciers of Mount Rainier, in the southern Washington Cascades, are among the best studied of our western mountain glaciers. The largest reach lengths of 5-6 miles and end below treeline in coniferous forest. The most accessible is Nisqually Glacier, which originates at the summit of Mount Rainier (14,400 feet altitude) and descends to an altitude of about 5000 feet. The position of the glacier front was first recorded in 1857, and sporadically thereafter until the end of the century. More-or-less continuous recordings of its frontal position were made after the mountain became a National Park, which brought increasing numbers of visitors, as well as scientists, to observe its glaciers. Together with geologic dating of moraines that record past glacier-margin positions, this observational record provides a relatively detailed picture of the variations of Nisqually Glacier and of the climatic trends which controlled its fluctuations.
Extent of Nisqually Glacier in 1976 and early in the 19th century during its greatest advance of the Little Ice Age. Glacier terminus positions recorded sporadically between these times are identified by year of observation. Since 1976, the glacier terminus has continued to retreat and now lies well upvalley from its 1976 position. (after Porter, 1981)
Remnants of the terminal moraine of Nisqually Glacier lie close to the modern highway bridge across the Nisqually River and record the greatest expansion of the glacier since the end of the last ice age about 10,000 years ago. Botanical dating of the moraine shows that it was built early in the last century, probably in the mid-1820's. By the time the terminus was visited in 1857, the ice margin had retreated about 300 meters upvalley. As shown in the accompanying map, slow general retreat continued until the end of the century. By 1916, when the national park facilities were developed, the margin lay about 800 m behind the outermost moraine. Continued retreat, broken by several halts and readvances, has brought the glacier front to its present position, nearly 2500 m (more than a mile) upvalley from its outermost moraine. Today one can barely see the glacier front from the Nisqually River bridge, but clearly visible are the lateral moraines and vegetation trimlines on the valley walls that record its former much greater extent.
The present terminus of Nisqually Glacier lies near upper left in this view, more than a mile upvalley from the Nisqually River bridge, visible in the lower part of the photograph. The maximum downvalley limit of the glacier during the Little Ice Age lay at the bottom edge of the photo where the river passes a small forested moraine remnant.
The reconstructed mass balance history of Nisqually Glacier is consistent with the over-all recessional trend of its terminus, for it shows a progressive long-term negative trend of mass balance, indicating progressive loss of mass by the glacier. Retreat rates were highest at times when summer temperatures were high and winter snowfall was low, a pattern also seen in the European Alps where glaciers experienced a similar retreat history during this time interval.
Long-term mass balance trend for Nisqually Glacier based on a reconstructed climate record since 1850 (after Burbank, 1982). The long-term negative trend since the culmination of the Little Ice Age in the early to middle 19th century was accompanied by a decrease in glacier-covered area and glacier volume.
Like Nisqually Glacier, other glaciers on Mount Rainier have undergone substantial retreat during the past 100 years, as have other glaciers in the Pacific Northwest. Lyman Glacier, in the North Cascades, is a good example.
The terminus of Lyman Glacier has retreated far upvalley from the arcuate moraine that it built during the first decade of the 20th century. In the process the glacier has decreased in area and volume by more than 60 percent.
The overall pattern of 20th century glacier retreat is consistent with the warming trend seen in the later part of the global temperature trend for the last 350 years, which displays an abrupt warming in the early part of the century to values well above those of the Little Ice Age.
Average temperature trends for the last 350 years, showing the rise in tempeature at the end of the Little Ice Age.
Projection of warming trends into the next few centuries implies continued increase in global temperatures, with consequent further reduction in mountain glacier cover. Ultimately, as the world enters the next ice age, average temperatures will decline and a full-scale glaciation, much like the last one, will culminate ca. 23,000 years from now:
Past and future temperature trends. The lower part of the curve (prior to 10,000 years ago, is based on paleoclimatic evidence from deep-sea cores and terrestrial glacial evidence. The Holocene part of the curve (the last 10,000 years, is constructed on the basis of evidence of mountain glacier variations and other types of climate-proxy data, including historical information. Projections for the next glaciation are based on Milankovitch (Earth-orbital) variations. Dashed curves (red) show possible scenarios for future warming using various assumptions about greenhouse gas addtions to the atmosphere.
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