Introduction

Valley and mountain glaciers and ice caps respond to changes in regional climate on the scale of decades; thus, they can serve as indicators of regional climate change. During the last century, most valley and mountain glaciers and ice caps have receded (Meier, 1984; Haeberli et al., 1989), although some advances have occurred during periods of cooling (Wood, 1988).

Images acquired by satellite sensors can provide a regional overview for studying changes in glacier area, terminus position, glacier facies and the position of the regional snow line (Williams et al., 1991; Hall et al., 1992), while in situ studies usually provide measurements only at a single point. Major changes in glaciers may be obvious from initial inspection of satellite images; however, detailed study of digital image data can reveal the presence of small changes that are useful indicators of regional trends in glacier mass balance. If observed over a long enough period of time in many different geographic settings, glacier-terminus position changes can be related to changes in regional and even global climate.

Background

In-situ measurements are required for precise measurement of a glacier's mass balance; however, this labor-intensive method can be performed on just a few small glaciers. Worldwide, only a few glaciers are monitored on a regular basis (Wood, 1988), and they may not necessarily be representative of other glaciers in the same region.

Landsat multispectral scanner (MSS) and thematic mapper (TM) data have been available since 1972 and 1982, respectively, and are valuable for studies of glacier movement. MSS 79-m pixel resolution and TM 28.5-m resolution may be used to locate the position of a glacier terminus by comparing sequential images. In addition, Landsat data have been found to be useful for measuring the position of the equilibrium line, and for estimating mass-balance change when calibrated with previous field measurements of glacier mass balance (Ostrem, 1975). ( The equilibrium line is the line that separates the area of a glacier that has a net gain of mass over the year (accumulation area) from the area having a net loss (ablation area)).

Indications of a trend toward a negative mass balance may be evident in a number of ways. A glacier may begin to recede and thin, with a concomitant increase in snowline elevation and expansion of its ablation area. Often these attributes may be measured with satellite techniques.

On land, the terminus of an advancing glacier can be seen clearly from space because the high reflectance of the cleaner ice, which often characterizes an advancing glacier, sharply contrasts with the lower reflectance of the surrounding moraine. Conversely, it can be difficult to determine precisely the terminus of a debris-covered, stagnant, or retreating glacier. The lack of contrast in these cases may hinder accurate measurement of the exact amount of glacier recession. This, however, is not true of retreating tidewater glaciers because the contrast between the glacier and the water is usually quite high.

Satellite Measurements

Vatnajokull is the largest ice cap in Iceland (8050 km2). An ice cap is a dome-shaped mass of glacier ice that flows radially from highland areas and usually has several lobate or valley outlet glaciers on its margins.

Overall, outlet glaciers in the southwestern part of Vatnajojull have receded several hundred meters or more between 1973 and 1987 according to both ground and satellite measurements (Hall et al., 1992; Williams et al., in press). For example, a recession of 1140 m was measured in the middle of Tungnaarjokull, an outlet glacier of Vatnajokull. In the middle of another outlet glacier in the southwestern part of Vatnajokull, Sidujokull, was also measured using Landsat data for the same period and found to have receded 485 m. Measurements of the margins of Vatnajokull, from 1973 to 1992, using Landsat and ground measurements, are reported in Williams et al. (in press).

The Pasterze Glacier is located in the eastern Alps of Austria. Since 1856, the Pasterze Glacier has experienced an almost continuous retreat according to ground and, more recently, satellite measurements. Since the winter of 1965/66, for example, the terminus receded almost 400 m and much of the lower part was covered with supraglacial debris (Hall et al., 1992; Bayr et al., 1994). Long-term meteorological data show a general increase in average summer temperature and a concurrent decrease in snowfall (Bayr et al., 1994). Thus, the recession of the Pasterze Glacier can be attributed, at least in part, to long-term meteorological conditions. Between 1984 and 1990, in-situ measurements show that the terminus of the Pasterze Glacier receded 102 m, while measurements using Landsat TM data show a 90-m recession, or an average recession of 15 m a-1. Thus the satellite-derived measurements are accurate within one TM pixel (28.5 m).

Glaciers of Glacier Bay, Alaska

Glacier Bay, Alaska, contains both tidewater and non-tidewater glaciers, many of which have been in a state of rapid retreat since the late 1700s (Field, 1947; Hall et al., 1995). About 200 years ago there was a massive ice field that extended into Icy Strait and filled the bay. By 1794 the ice had retreated and opened a small bay, and by 1879 the retreat had exceeded 60 km (Molnia, 1982). From 1794 to 1892, there was an 80-km recession in the western side of Glacier Bay and a 40-km recession in its eastern side. Recession continued another 15-20 km in the western bay until 1920-1930, when the Johns Hopkins and Grand Pacific Glaciers began to advance.

The Muir Glacier has been retreating steadily since the late 1700s although there was some sporadic advance activity during that time. Landsat-derived measurements show that between 1973 and 1992, the Muir Glacier retreated >7.3 km +/- 79 m, with the rate of retreat being greater between 1973 and 1980. The average retreat, as measured using Landsat imagery, was 0.73 km a-1 between 1973 and 1982, and only 0.04 km a-1 between 1982 and 1992. This is consistent with reports from ground observations indicating a slowing of the Muir's retreat by 1982 (Krimmel and Meier, 1989).

Other glaciers of Glacier Bay that retreated significantly (>1 km) from about 1966 to 1986, according to field and Landsat-derived measurements, are the Burroughs, Casement, Cushing, McBride, Plateau and Morse. Others have advanced slightly (<1 km), while only the Carroll advanced significantly during that approximately 20-year period (5.1 km) (Hall et al., 1995).

The Glacier Bay area is a fine example of rapid deglaciation, even though it is not clear how much recession is the result of climate amelioration and how much is the result of the tidewater glacier cycle which is independent of short-term, regional climate. Available average air temperature data from nearby meteorological stations show a general warming trend, especially over the last 20 years. Such a warming trend may help to explain recession of the non-tidewater glaciers in Glacier Bay.

Glaciers in College Fjord, Chugach Mountains, Alaska

College Fjord, a 40-km long fjord in the northwestern part of Prince William Sound, cuts into the heart of the Chugach Mountains and contains five tidewater glaciers, five large valley glaciers and dozens of smaller glaciers. The two largest tidewater glaciers in the fjord are the Harvard and Yale Glaciers. They emanate from the same snowfields. Observations show that the Harvard Glacier has advanced, while there has been a simultaneous retreat of the Yale Glacier (Sturm et al., 1991).

The Harvard Glacier has been advancing since 1905, and possibly earlier. It has advanced at an average rate of nearly 20 m a-1 since 1931, while the adjacent Yale Glacier has retreated at a rate of approximately 50 m a-1 during the same time period. The striking contrast between the terminus behavior of the Yale and Harvard Glaciers, which parallel each other in the same fjord, and are derived from the same snowfield, supports the hypothesis that their terminus behavior is largely the result of dynamic controls rather than changes in climate. If climate were controlling the terminus behavior, more synchronous behavior between the two glaciers would be expected (Sturm et al., 1991).

Conclusion

In general, the terminus positions of tidewater glaciers, such as those found in Glacier Bay and College Fjord, are thought to be the result of a complex interaction of fjord depth, ice thickness and calving rate, with climate and mass balance playing a secondary role (Meier and Post, 1987).

Although most of the Earth's small glaciers have been retreating, many glaciers are advancing. Some are advancing due to local climatic conditions and others are advancing due to factors not directly tied to climate such as the tidewater glacier cycle. Landsat, in use for almost a quarter of a century, is an excellent source of global information on decadal-scale glacier-terminus changes. Along with ancillary measurements, an assessment of changes in regional mass balance of glaciers can be made. Many of the glacierized regions on Earth have diminished in size during the last century or more. Interpretation of satellite data can reveal the magnitude of these changes and permit regional monitoring of glaciers over time.

References

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