Mass Balance of West Gulkana Glacier

American Geographical Society

Mass Balance of West Gulkana GlacierAuthor(s): Fred B. Chambers, Melvin G. Marcus and L. Sam ThompsonSource: Geographical Review, Vol. 81, No. 1 (Jan., 1991), pp. 70-86Published by: American Geographical SocietyStable URL: http://www.jstor.org/stable/215177 .

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MASS BALANCE OF WEST GULKANA GLACIER*

FRED B. CHAMBERS, MELVIN G. MARCUS, and L. SAM THOMPSON

ABSTRACT. Annual and longer-term mass-balance calculations provide a means to relate glacier behavior to climate. Selected representative glaciers are monitored for this purpose. West Gulkana Glacier, in the eastern Alaska Range, was among the glaciers mapped during the 1957-58 International Geophysical Year. It was remapped in 1986, after which twenty-nine-year mass balance was calculated. Annual budgets were also calculated for 1985-86 and 1986-87 glacier years. The three-decade record shows steady retreat and mass loss in the glacier's transitional marine-continental climate zone. West Gulkana Glacier is regionally representative of small, low-elevation valley systems. Caution must be exercised in relating its climatic responses to glaciers situated above 1,800 m.

A,principal objective of the 1957-58 International Geophysical Year was to establish baseline data that could be used later in interpreting en- vironmental trends. An important contribution by American scientists

to the effort was the mapping of nine small alpine glaciers that were deemed to be representative of various geographical zones in Washington State and Alaska. The glaciers, which were mapped at the 1:10,000 scale and a 5-m contour interval, stretched from Blue Glacier on the Olympic Peninsula in Washington to Lemon Creek Glacier in southeastern Alaska to six others located poleward from the Gulf of Alaska to McCall Glacier in the Brooks Range. One of the glaciers, West Gulkana, was intended to provide an index to glacierized terrain lying in the eastern Alaska Range, astride the divide between the Delta-Tanana lowlands to the north and the Copper River basin to the south (Fig. 1).

Three factors were probably paramount in the choice of the representative glaciers: inclusion of different glacier-climate regions, the prevailing wisdom that small glaciers were the best medium for understanding relationships between glacier regimen and climate (Heusser and Marcus 1964), and ac- cessibility. Although the representativeness of the small glaciers and, to a lesser extent, the selection of geographical zones may be debated, the existing maps provide a base for subsequent studies. Indeed, at the time of the original

* This research was made possible in part by financial support from the American Geographical Society, Arizona State University, and the following U.S. agencies: Army Research Office, Army Cold Region Research and Engineering Laboratory, Northern Warfare Training Center and Cold Regions Test Center, Defense Mapping School, and the U.S. Military Academy. W. 0. Field, L. R. Mayo, W. Strauss, H. Brecher, and B. Trapido are thanked for their contributions to the research and its presentation.

* DR. CHAMBERS is a visiting lecturer in geography at the University of California, Davis, California 95616. DR. MARCUS is a professor of geography at Arizona State University, Tempe, Arizona 85287. DR. THOMPSON is a permanent associate professor at the U.S.

Military Academy, West Point, New York 10996.



WEST GULKANA GLACIER 71

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FIG. 1-Regional setting of West Gulkana Glacier, Alaska.

mapping, it was anticipated that "continuing surveys, if carried on over a sufficient period of time, would give te history of wastage and accumulation, and the pattern of variation would provide the basis for more satisfactory and more accurate interpretation of the response of these glaciers to meteorological and other factors" (AGS 1960, 3).

From 1986 to 1987, the first such efforts were made in Alaska, when a team of scientists remapped West Gulkana Glacier and determined its mass- balance changes. This article summarizes the findings of this survey and reports on the inferences that the data permit for three decades of regional climate.

West Gulkana Glacier is located in the eastern Alaska Range, only 8 km east of the Richardson Highway and Isabel Pass. It is one of three glaciers that drain the watershed of Phelan Creek and eventually feed into the Delta River; the other two are Gulkana and College glaciers. Some 4 km in length, West Gulkana Glacier flows southward from 2,100 m elevation to 1,325 m; it is well entrenched below 1,800 m (Fig. 2). It attained its maximum post- Pleistocene advance during the Little Ice Age, circa 1830 (Fig. 3). Neighboring



72 THE GEOGRAPHICAL REVIEW

WEATHER OBSERVATION

FIG. 2-West Gulkana Glacier 1986. This black-and-white map is a reduced version of the mul- ticolor 1:10,000 original. Copies of the original map may be obtained from the Department of Geography, Arizona State University, Tempe, Arizona 85287, for a period of two years after the publication date of this issue of the Geographical Review.

Gulkana Glacier had been the focus of intensive studies by University of Alaska glaciologists in the 1960s (Mayo and Pewe 1963; Rutter 1965), and its regimen has since been monitored by the United States Geological Survey. West Gulkana Glacier, however, has not been investigated, except by aerial photographs, since the 1957-58 mapping project.




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West Gulkana Glacier is an alpine site within a larger subarctic climate zone. The Alaska Range and, to a lesser extent, the Wrangell Mountains serve as both battleground and border zone for continental and maritime air masses. The West Gulkana area is transitional, under the variable influence of both air masses, which in turn are altered by local topography. Generally, West Gulkana winters are very cold, with temperatures below -20? C in January; the short three-month glacier summer has temperature readings between 50 C and 15? C.

The map from the 1957 expedition was based on eight control points occupied and triangulated by Wild P-30 theodolite (AGS 1960). Two other points, though not occupied, were triangulated from several locations and used as vertical control during the phase of aerial triangulation and plotting. To recover the original control points, the 1986 team used the 1957 survey data, which had been deposited at the Byrd Polar Institutes, Ohio State University; the original glass diapositives, taken by the United States Navy Heavy Photographic Squadron 61; and the map that resulted from the 1957 study. The original study had been limited to triangulation from a 200-m baseline; by contrast, the 1986 survey utilized an electronic distance-mea- suring device (DN 60) and a Wild T-2 theodolite. A baseline of nearly two kilometers was established. As evidence of the precision of the original survey, the electronically measured 1,783.5-m distance from control point Shirley to control point Velta varied by only 30 cm from the distance triangulated by the 1957 expedition.

A flight line and a mission plan were prepared independently of any previous work. The conclusions were almost identical: a flight line running NW to SE, four 60 percent overlapping photographs, and a photographic scale of approximately 1:27,000. Air Photo Tech Inc., now AeroMap U.S., an Alaskan company, flew the mission, photographing with a Zeiss Jena MRB camera and a Lamegon PI lens with a focal length of 151.964 mm. VEP Associates of Caldwell, New Jersey, working from the glass diapositives supplied by Air Photo Tech, formed the models and densified the control using a DSR-11 Analytical Stereorestitution Instrument.

The ground coordinates were computed using Kern's version of Shut's polynomial solution. The DSR-11 was used to digitize the contours and morainic and other glacier features, gathering a digit every 0.05 inch at the publication scale of 1:10,000. Inked contours were drawn on mylar by a Calcomp 1076 Belt Plotter, driven by computer-assisted drafting software on a Microvax II minicomputer at a Tektronix 4111 Graphic Workstation. The contour lines, outline of the glacier, prominent ice fields, and obvious morainic deposits, as in the original, were plotted at a scale of 1:10,000 with a 5-m contour interval. Ice-free land masses were plotted at a contour interval of 25 m. The inked manuscript was photographically superimposed on scribe and peel-coat materials by the Department of Graphic Arts, Defense Mapping School, which also printed the final map.




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Am 2 1986 Ice Margin AA1 + AA2 --- ----1957 Contours

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FIG. 4-Schematic illustration of contour-displacement method for calculations of mass balance.

MASS BALANCE 1957-86

Glacier mass balance is the annual net gain or loss of water for a glacier system. A glacier year consists sequentially of one winter season of accumulation and one summer season of ablation; thus the year is seldom exactly 365 days. Several direct methods are used to determine annual balances (Meier 1962; AASH 1969; 0strem and Stanley 1969; Mayo, Meier, and Tang- born 1972). The integrated budget B over the entire glacier surface S is generally given as

B = J'bn dV

where b4 is the net annual budget at incremental elevations or points and dV is change of ice volume. Glacier mass balance is also estimated by indirect methods, including equilibrium line altitude (ELA) and annual photography or imagery.

For periods of several years, the most effective measure of change is a comparison of two topographic maps of the same scale and contour interval (Finsterwalder 1960; Haumann 1960; Marcus 1964; Konecny 1966). Essen- tially, the method computes change in ice volume through calculations based on contour-line displacement (Fig. 4). For West Gulkana Glacier, contours of




TABLE I-CHANGES OF ICE VOLUME AND THICKNESS IN WEST GULKANA GLACIER 1957-87a

CONTOUR INTERVAL (m) dh (m) dV dV,

1325-1425 -33.00 -11.4809 -10.3328 1425-1525 -50.66 -19.4630 -17.5167 1525-1625 -33.99 -21.0125 -18.9113 1625-1725 -20.82 -16.2427 -14.6184 1725-1825 -7.57 -4.4258 -3.9832 1825-1925 -2.13 -2.4316 -2.1884 1925-2025 +3.80 +1.8540 +1.6686 2025-2125 +2.73 +0.5736 +0.5162 2125-2225 -7.17 -0.1563 -0.1407 2225-2325 -16.64 -0.5016 -0.4514 2325-2350 -5.88 -0.0441 -0.0397

Total -73.3309 -65.9776

a dh = change in height. dV change in ice volume; millions of m3. dV, = change in water- equivalent volume; millions of m3.

the new map were superimposed on the 1957 map. Lines and areas were then planimetered, allowing solutions for volume and ice-thickness changes between 1957 and 1986; these are given for every 100 m (Table I). Water- equivalent (mass) values are based on a mean ice density of 0.90.

The most pronounced loss of ice volume and extent occurred at the lowest elevations along the glacier snout. Ablation below the 1,700-m contour ac- counted for 93 percent of the total volumetric loss of the glacier from 1957 to 1986, while the terminus had retreated approximately 350 m. Interestingly, the glacier had sustained a net mass gain between the 1,900-m and 2,100-m contours. This represents an average increase of about 3.25 m in ice thickness and a volume increase of roughly 2.4 million m3. A similar thickening has been detected on the neighboring Gulkana Glacier and is believed by one observer to be a kinematic wave. Ice losses occur again above the 2,100-m contour, but at a much slower rate than at lower elevations of the glacier. Ice above this contour is in a couloir of Institute Peak and is probably not a significant contributor to the mass budget of the glacier.

The total volumetric ice loss during the twenty-nine-year period as determined by the comparison of topographic maps is 73.3 million m3 (66.0 million m3 water equivalent), representing an average annual loss of ice volume of 2.53 million m3 (2.28 million m3 water equivalent). Similarly, the areal extent of the glacier has diminished by approximately 13 percent during the interval.

Volume changes can be calculated from digital data as well as from traditional planimetric methods. Two approaches were attempted. The first employed GRIDZO, a common engineering-geology program from the Den- ver, Colorado, firm Rockware that is used primarily to determine cut-and- fill volumes at construction sites. Exactly the same 1957 and 1986 glacier boundaries were used in the execution of this program. The second method involved digitizing the glacier and all other snow and ice masses appearing on the maps, using the UNIRAS program (Thompson and Smith 1988).

Results from digital data essentially verified those obtained through pla-




TABLE II-VOLUMETRIC CHANGES IN WEST GULKANA GLACIER AS DETERMINED BY PLANIMETRIC AND DIGITAL METHODS 1957-86

LOSS IN WATER METHOD EQUIVALENT' AVERAGE ANNUAL LOSSa

Planimetric 65.98 2.28 GRIDZO 1 55.98 1.93 GRIDZO 2 58.05 2.00 UNIRAS 37.89 1.31 1985-86 - 1.81 1986-87 0.98

a Millions of m3.

nimetric techniques (Table II). Using several different algorithms to calculate the variations between the two digitized maps, GRIDZO placed the total volumetric ice loss of the glacier at 62.2-64.5 million m3 (56.0-58.1 million m3 water equivalent), or up to 88 percent of the value determined by the planimetric techniques. The discrepancy is explained on close examination of how the computer interprets the change in z-axis, vertical values. Both maps were digitized to include both ice and nonice areas; obviously only the z values of the ice showed a net change between the two maps. However, GRIDZO does not recognize the boundary between glaciated and unglaciated parts of the maps. Instead, it interpolates the edge of the ice mass gradually to phase into the ice-free region where there is no change in z values. This interpretation creates a boundary error: GRIDZO "sees" more ice than is actually present. Attempts to resolve the discrepancy with other software were unsuccessful, and the boundary problem remains a potential source of error.

The UNIRAS calculation showed a still smaller volumetric ice change in the twenty-nine-year period: 42.1 million m3 (37.9 million m3 water equivalent). This was no doubt due to the inclusion of all snow and ice surfaces in the topographic basin, leading to a greater area digitized above the 1,900-m elevation where thickening or no-loss conditions occurred.

SHORT-TERM MASS BALANCE 1985-86

The short-term mass balance of West Gulkana Glacier was determined by employing the stratigraphic system (IASH 1969). Measurements of snow depth and density were completed throughout the summer of 1986 and the spring and summer of 1987. Snow depth was measured approximately every 100 m longitudinally up the center axis of the glacier to an elevation of approximately 2,000 m. Five snowpits were excavated at different elevations to provide representative snow-density information. Additional snow-depth measurements were made every 100 m from each snowpit latitudinally to the sides of the glacier.

Holes were drilled into the ice and stakes inserted to assess ice loss after an area became free of snow during the ablation season. The distance from the top of a stake to the ice surface was measured and remeasured periodically.




TABLE III-ANNUAL MASS-BALANCE WEST GULKANA GLACIER 1985-86 AND 1986-87

VERTICAL CHANGE (m of

AREA water equivalent) VOLUME CHANGE (m3 of water)

LOCATION (m2) 1985-86 1986-87 1985-86 1986-87

Low pit 209,000 -1.22 -1.14 -254,980 -238,260 Middle pit 302,000 -1.12 -0.78 -338,240 -235,560 High pit 433,000 -0.66 +0.33 -285,780 +142,890 Cirque pit 1,355,000 -0.69 -0.48 -934,950 -650,400

Total -1,813,950 -981,330

Differences between the measurements provided the depth and the rates of ice ablation. The mass balance for each of the five zones was determined from the measurements described above and then summed to provide the annual balance for the glacier as a whole.

Snow and ice measurements on West Gulkana Glacier were recorded regularly from 26 June to 25 July 1986. Two additional site measurements were logged on 12 September 1986 to assess ice loss after 25 July. The research team returned to the glacier on 10 May 1987. Regular measurements of mass balance resumed and continued until 25 July 1987. It is probable that the initial readings in May represent the maximum accumulation of the winter season. Snowfalls continue in this region into April; any snow melt at the surface would most likely be refrozen at depth within the snowpack, re- sulting in little runoff and mass loss.

On 26 June 1986, when mass-balance measurements commenced, snow depth on the glacier varied from a minimum of 0.59 m near the glacier terminus to 2.68 m in the vicinity of the cirque snowpit. Glacial ice first appeared near the terminus on 2 July. The mean snowline was at the low snowpit elevation on 4 July, at the middle site on 8 July, at the high pit on 19 July, and at the cirque pit location on 22 July. This shift was a snowline- elevation rise of 230 m in twenty-two days, or 10.5 m per day. Most of the glacier was free of snow on 25 July, and September measurements further indicated a severely negative mass balance for 1985-86.

On average, the higher elevations suffered roughly one-half the vertical water loss of the lower ones (Table III). Nowhere on the main body of the glacier was an increase of mass observed. In total, West Gulkana Glacier lost more than 1.81 million m3 of water during the 1985-86 budget year, or 79 percent of the mean annual twenty-nine-year mass-balance rate of -2.28 million m3. Thus, although 1985-86 was a negative budget year, it was by no means atypical of the recent past for this glacier.

SHORT-TERM MASS BALANCE 1986-87

Snow began to fall on the glacier and remained on the surface during the second week of September 1986, effectively marking the end of the 1985- 86 budget year and the start of the 1986-87 period. After the return of the




FIG. 5-West Gulkana Glacier, showing residual avalanche lobe in late July 1987. (Photograph by F. B. Chambers)

research team, measurements recommenced on 10 May 1987, when the snowpack varied from a minimum depth of 1.55 m near the terminus to 4.3 m approximately 300 m west of the high snowpit site. Subsequent snow probes revealed a lobe of deep snowpack along a southwest-northeast line in the vicinity of the high snowpit. This additional snow probably came from avalanches originating on Institute Peak to the northeast and the ridges to the west of the glacier. Residual snow, derivative of the avalanche zones, is

apparent in a late-July photograph (Fig. 5). Avalanche-related accumulation is probably unusual for West Gulkana

Glacier. Ordinarily, as in 1985-86, the snowpack on surround'ing peaks and slopes develops gradually through the accumulation season; a large propor- tion of that snow remains stabilized on the valley walls into and through the summer season. However, in 1986-87 deep winter snowfall was relatively light. It was followed by very heavy snowfall in April immediately before the onset of ablation. The dense, unstable snowpack avalanched almost in its entirety. The evidence was the disappearance of the valley-wall ne've by the end of June, which had remained into August during the 1985-86 budget year. Also, residual avalanche debris was preserved throughout the summer in the glacier outwash zone and lower stream-cut canyons; no valley-bottom snow had been apparent after 1 July in the previous year.

The avalanche flow significantly moderated the negative 1986-87 balance. The snowline retreated slowly, and midglacier areas remained covered with




snow the entire summer, although high zones had exposed ice. A total of some 0.98 million m3 of water equivalent was lost during the 1986-87 balance year. Although this value is almost half that seen the previous year and only 43 percent of the twenty-nine-year mean annual negative budget rate, it still shows a glacier with a highly negative mass balance, especially if the avalanche contribution is excluded. Thus, recent mass-balance years sustain the three-decade trend.

IMPLICATIONS FOR CLIMATE

Glacier regime and climate change are inextricably interwoven, and glaciers are a sensitive index to both local and regional atmospheric behavior. Several glacier-climate interrelationships were investigated in coordination with the West Gulkana mapping and mass-balance efforts. Scalar connections between surface-energy balance fluxes, ablation, and upper-air soundings at 850, 700, and 500 mb were interpreted (Marcus, Chambers, and Brazel 1991), and daily glacier-surface radiation was analyzed (Miller 1988). Such studies provide insights into intraseasonal glacier-climate interactions but do not address the long-term climatic perturbations associated with the three-decade mass-balance record. The latter requires interpretation of the climate record and changing synoptic patterns.

West Gulkana Glacier has suffered serious mass loss since 1957. Some 93 percent of that loss occurred below 1,700 m elevation, which accounts for 65 percent of the total glacier area. Although the annual rate of water loss, which averaged 2.28 million m3 from 1957 to 1986, actually varied from year to year, a marked and continuing negative balance exists. This pattern implies that the amount of annual snowfall or the length of the accumulation season has decreased, or that the amount of heat available for melting during the ablation season has increased.

In reality, it is probable, to some degree, that both occurred. If accumulation had remained constant over twenty-nine years, it is doubtful that such extreme diminution of the glacier surface would be seen. Deep winter snowfall, however, is not the key factor. Studies have indicated that the preponderance or dearth of spring, summer, and fall accumulation events may be of greater consequence (Marcus 1964; Hoinkes 1968; Alt 1978; Mayo and Trabant 1984; Hay and Fitzharris 1988). A preponderance of such events, for example, will increase accumulation to the system but will also retard the onset or diminish the effectiveness of the ablation-season melting pattern. Therefore, synoptic patterns during that season may control whether a glacier experiences annual net gain or loss.

Various other scenarios of ablation and accumulation behavior can be hypothesized. For example, the rise or fall of the regional snowline elevation can lead to loss or gain of water mass. But a rise in snowline also may be associated with warmer, wetter air masses that, before and after the onset of the ablation season, can increase snowfall on the upper glacier reaches.




This is especially nourishing for glaciers with large proportions of their accumulation areas above the seasonal snowline.

West Gulkana Glacier experienced net gain between 1,900 m and 2,100 m. Similar thickening has been noted on the larger Gulkana Glacier. This bulge may be a kinematic wave, but it also may relate to snowline elevation. The thickening occurs at elevations much higher than most of the accumulation area of West Gulkana Glacier. Because much of this area of Gulkana Glacier lies above 1,900 m elevation, it is possible that greater snow and ice are a precursor to glacier advance. Thus, the two adjacent glaciers, because of their topographic settings, will react quite differently to the same climatic regime.

Another factor can locally enhance onset of the ablation season: warmer winter temperatures. If the accumulated winter snowpack is warmer than in the past, less energy is required in spring to bring the snowpack to the melting point, and the ablation season is thereby lengthened.

Interpretation of climate trends for interior and southeastern Alaska sheds light on the scenarios posed above. Temperature and precipitation records for stations at Fairbanks, Big Delta, Trims Camp, Paxson, and Gulkana were analyzed for the period 1950-86. The stations form a transect of almost 500 m. It should be noted that the record at Paxson is abbreviated to 1968-86 and at Trims Camp to 1954-78.

Five-year moving averages of temperature were used to interpret trends. Little if any change occurred during the period of record from April through October, with the exception of Trims Camp, which experienced some warming of approximately 30 C in April with an offsetting cooling in June. Tem- peratures for November through March reveal greater variation (Fig. 6). Fairbanks, Big Delta, and Gulkana show similar trends throughout the thirty- five-year period; starting from 1950, temperatures warmed 10 C-2? C during the 1958-64 interval but then cooled back 2T C-3? C from 1964 through 1968. Temperatures fluctuated during the late 1960s and early 1970s before a pronounced warming of 30 C-4? C from the late 1970s into the 1980s. This pattern resulted in the warmest winter temperatures recorded during the thirty-five- year period at all three stations. Temperature trends for Paxson and Trims Camp paralleled those at the other two stations. Temperature moderation in the months of December and January was especially notable, with the average for 1982-86 50 C-7? C warmer than in the 1950-54 period.

Not surprisingly, the five-year moving averages of precipitation are much less consistent from station to station (Fig. 7). There is no clear pattern of increasing or decreasing moisture at any location, with the exception of Big Delta, where the July trend was one of definite increase for the period of record. July precipitation at that station averaged 6 cm during 1950-66 and increased to a mean 9.5 cm for 1980-86.

An attempt to link winter precipitation with warming temperatures revealed no consistent relationship. Regression analyses of precipitation (de-




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1950-54 1955-59 1960-64 1 965-69 1970-74 1975-79 1980-84

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FIG. 6-Five-year moving averages of winter temperature (November-March) for Fairbanks, Big Delta, and Gulkana, Alaska 1950-86.

pendent variable) versus temperature (independent variable) for the three stations gave R2 values of .26 for Fairbanks, .24 for Big Delta, and .18 for Gulkana. Closer to the glacier, winter precipitation increased at Paxson during the 1978-82 period, which may relate to the thickening at the upper elevations of West Gulkana and Gulkana glaciers.




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1950-54 1955-59 1960-64 1965-69 1970-74 1975-79 1980-84

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FIG. 7-Five-year moving averages of winter precipitation (November- March) for Fairbanks, Big Delta, and Gulkana, Alaska 1950-86.

Although station weather records provide generally inconclusive evidence, it does appear that warmer winter temperatures may play a small role in the negative mass balances on West Gulkana Glacier. Conclusions regarding the relationship between mass balance and climate are more likely to emerge from analysis of annual variations of synoptic patterns and the




incorporation into the research design of data from other glacier regimes, spread over a latitudinal range. Work has been initiated in this direction by synoptic typing (Brazel, Chambers, and Kalkstein 1991) and by remapping of another representative ice mass, Lemon Creek Glacier in southeastern Alaska.

COMPARATIVE GLACIER REGIMES

If the overall twenty-nine-year mass-balance record for West Gulkana Glacier displays a strong negative trend, the question remains: does this represent a glacier-climate relationship in the region? Furthermore, can the West Gulkana experience be linked to glacier behavior in other regions of western North America?

West Gulkana Glacier appears, at first inspection, to have been a poor choice as a regional representative. Its negative thirty-year regime has been at odds with results reported for other glaciers in the area since at least 1976. For example, Gulkana Glacier has thickened in the accumulation zone since 1976. It has been asserted that the other glaciers in the Alaska Range have behaved in similar fashion and that this pattern suggests that the climate variations affecting Gulkana are widespread (Mayo and Trabant 1986). In the late 1950s and the 1960s, on the other hand, it was noted that, if periodic glacial surges were excluded, most glaciers in the range were retreating (Meier and others 1971). Thus West Gulkana Glacier serves well as an index only for glaciers ensconced in lower-elevation topography; glaciers with high accumulation areas simply respond differently to vertical migrations of the freezing layer and seasonal snowline position. It will be instructive to learn whether other glaciers selected for the 1957 project, which are also at relatively low elevations, exhibit the same temporal tendencies.

The linkage of West Gulkana Glacier to ones in other regions cannot be fully assessed without detailed knowledge of latitudinal shifts of synoptic patterns and their effect on local glacier regimen and snowline. Nevertheless, comparative mass-balance data can be the source of useful insights. To the north, McCall Glacier followed the West Gulkana pattern during 1958-71: mass-balance estimates were consistently negative (Dorrer and Wendler 1976), and similar conditions were noted for the Brooks Range. In southeastern Alaska, a surrogate-based analysis for forty-two glaciers also showed general negative balances through 1975 (Pelto 1987). Lemon Creek Glacier in the Juneau Icefield behaved transitionally, with a mixture of positive and negative years (Miller 1972). Further south, however, key representative glaciers experienced positive to near-equilibrium balances from the late 1950s into the 1970s. These included Peyto Glacier in the Canadian Rockies and Sentinel Glacier in the British Columbia Coast Range (Letreguilly 1988), along with glaciers in the North Cascades (Tangborn 1980).

The post-1976 period was substantially different. Although West Gulkana Glacier continued to experience net loss, glaciers to the south, including




Sentinel, Blue Glacier in the Olympic Mountains (Spicer 1989), and Wolverine Glacier on the Kenai Peninsula, exhibited near-equilibrium to positive reg- imens. More generalized surveys for forty-seven glaciers in the North Cas- cade range (Pelto 1988) and for a global-monitoring survey (Wood 1988) also indicate a preponderance of positive budgets; the latter study noted that advancing glaciers constituted more than 55 percent of observed glaciers.

An interesting exception to this general pattern is Place Glacier in British Columbia. As at West Gulkana Glacier, the trend has been consistent decline (Letreguilly 1988), and it is likewise located in a transitional zone between maritime and continental climatic regimes. It is probable that such transitional glaciers are sensitive indicators of climate change, especially for warming (Yarnal 1984a, 1984b). Research at Wolverine and Sentinel glaciers pro- vides some indication that warming may enhance glacier growth by increased snowfall in coastal regions. This relationship has been postulated for southern Alaska on the basis of thermal analysis of summer heat and winter precipitation (0strem, Haakensen, and Eriksson 1981).

The reality is that no single glacier can satisfy the requirements for representativeness, except as it relates to its own special locality and char- acteristics. Selected baseline glaciers must be viewed not only individually but in the context of linkages among them. Those linkages, in turn, are interactive with latitudinal shifts of synoptic activity. Also, it is impossible to examine mass-balance trends in northwestern North America without considering the implications for the current debate about global warming. The topic is significant for ongoing research, because glacier behavior often reveals climate change before the meteorological record does. The irony is that the mass balance of glaciers such as West Gulkana can be used to argue both for and against the global-warming thesis.

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Mass Balance of West Gulkana Glacier