Glacier mass balance measurements help to provide an understanding of the
behavior of glaciers and their response to local and regional climate. In
2005 the United States Geological Survey established a surface mass balance
monitoring program on Sperry Glacier, Montana, USA. This project is the
first quantitative study of mass changes of a glacier in the US northern
Rocky Mountains and continues to the present. The following paper describes
the methods used during the first 11 years of measurements and reports
the associated results. From 2005 to 2015, Sperry Glacier had a cumulative mean
mass balance loss of 4.37 m w.e. (water equivalent). The mean winter,
summer, and annual glacier-wide mass balances were 2.92,
The worldwide retreat of glaciers in the past century is seen as both an
effect of and evidence for global climate change (Roe et al., 2016). In the
United States, one example of this global trend is the retreat of
glaciers in Glacier National Park (GNP), which is located in the Rocky
Mountains of northwest Montana (Fig. 1). In 1850, during a period known as
the Little Ice Age, approximately 150 glaciers existed in the area now
encompassed by GNP, making it one of the largest concentrations of glaciers
in the US Rocky Mountains (Key et al., 2002; Pedersen et al., 2004).
The amount of glacier-covered area has receded dramatically since that time,
a trend documented by decades of photographs and measurements of glacier area
(Alden, 1914, 1923; Dyson, 1948; Johnson, 1980; Carrara and McGimsey,
1981, 1988; Carrara, 1989; Key et al., 2002). By 1998, 37 named glaciers
remained; 11 of these had a total surface area of 8.25 km
An overview map of Sperry Glacier which displays the locations of the long-term measurement sites and the weather station, plus changes in glacier extent between 2005 and 2015. Glacier surface elevations are contoured from a DEM derived from imagery pairs taken on 2 September 2005. The aerial photo in this figure was taken on 26 August 2005 by the US National Agricultural Imagery Program.
This trend and its effects are expected to continue. One geospatial model scenario predicts a complete disappearance of five glaciers in GNP by 2030 with continued warming (Hall and Fagre, 2003). Another process-based model that specifically examines Sperry Glacier suggests the glacier may last until about 2080 given the current climate and glaciological conditions (Brown et al., 2010). The retreat of glaciers in GNP has had and will continue to have hydrologic and ecological effects in the region's mountain ecosystems and some degree of economic effect for its human communities (Clark et al., 2015).
It is widely accepted that the regional retreat of glaciers has been driven by climate change (Roe et al., 2016), at least some of which is anthropogenic (IPCC, 2013). However, no quantitative measurements of mass changes have ever been conducted for any glacier in GNP. Without such studies, it is difficult to determine whether or how the retreat of GNP's glaciers directly reflects regional climate trends. To address this gap, the US Geological Survey (USGS, 2016) established a long-term mass balance monitoring program in which glaciological, surface area, and hypsometric measurements provide a quantitative estimate of mass changes for one glacier in the park, Sperry Glacier. The changes and trends measured on this glacier are meant to serve as a reference for others in the region, an approach outlined by Fountain et al. (1997) and since adopted widely (Kaser et al., 2003). Sperry Glacier is now one of four benchmark glaciers studied by the USGS Glaciers and Climate Program, with two benchmark glaciers located in Alaska and one in Washington (USGS Glacier Studies).
This paper presents the data collection methods and results from the first 11 years of measurements, from 2005 to 2015. Results include measurements of glacier area, snow depth, snow density, and ablation of snow, firn, and ice. We calculate the associated point balance values (Cogley et al., 2011) for the snow depth and ablation measurement sites. We also compute and report conventional glacier-wide seasonal and annual mass balances (Cogley et al., 2011) for the same period. Results from this work will improve our understanding of glacier responses to climate in GNP as well as the role of glaciers as a water resource. The data sets included in this paper and the associated Supplement may also be used for general analysis of regional and global glacier mass balance trends.
Sperry Glacier (48.623
Oblique photo showing overview of the glacier, the bergschrund, and headwall. Photo taken by Adam Clark (USGS) on 25 September 2015.
Sperry was chosen as a site for surface mass balance measurements because of two factors that are rare in the region. First, it has a history of previous scientific studies that documents its progressive retreat over the past century (Johnson, 1980). Second, its physical characteristics best meet those recommended for detailed mass balance studies and regional benchmark glacier status (Fountain et al., 1997; Kaser et al., 2003). These characteristics include a well-defined drainage basin and topographic features that are representative of many of GNP's glaciers.
Sperry is a winter-accumulation type glacier composed of temperate ice. In
2015 the total surface area measured 0.78 km
The GNP climate is influenced by both maritime and continental air masses.
However, given Sperry Glacier's position on the western and predominantly
windward side of the continental divide, Pacific storm systems dominate the
weather that affects the glacier during the non-summer months. These storms
bring heavy precipitation and moderate temperatures as relatively warm,
moist Pacific fronts collide with and lift over the Rocky Mountains. July
and August are typically dry months with the warmest temperatures of the
year (Finklin, 1986). Temperature and precipitation patterns are marked by
strong elevational gradients. For valley sites at about 1000 m, mean
temperatures for July, which is generally the warmest month of the year, are
typically 15–17
Snowpacks in the GNP region are strongly influenced by multi-decadal climate patterns (McCabe and Dettinger, 2002; Selkowitz et al., 2002) but exhibit no significant trends since 1922 based on instrument records (Selkowitz et al., 2002). However, Pederson et al. (2011) showed recent snowpack declines, predominantly since the 1980s, using snowpack reconstructions from 66 tree-ring chronologies in key runoff generating areas of the Colorado, Columbia, and Missouri River drainages. The snowpack at 2000 m typically peaks about 1 May each year but can vary by up to four weeks earlier or later (US Natural Resources Conservation Service).
List of acquisition dates for aerial photographs and measured terminus positions used to derive each DEM and AAD.
Initially, the surface area of Sperry was mapped using aerial photographs taken in September 1998. To ensure that mass balance calculations included the most current estimates of the glacier surface, we mapped the terminus using multi-channel GPS receivers at the end of the 2003, 2005, 2007, 2009, and 2013 ablation seasons. We conducted the mapping by walking the terminus (Fig. 2) of the glacier, with the terminus defined as any ice contiguous with the main body even if debris-covered. We did not map the glacier margin below the near-vertical cirque walls that rise above the upper elevation margins of the glacier for safety reasons.
We differentially corrected the GPS data collected during the mapping, then smoothed the track by removing obvious errors such as loops caused by the mapper's movement. We overlaid each mapped terminus with a polygon of the 1998 glacier outline and calculated the area of each resulting polygon for each year. For these years, the resulting values should be considered the maximum possible glacier extent for a given year because they do not include any small changes at the glacier margins below the steep cirque walls or on the ridge above the headwall. In 2015 the complete glacier margin was mapped again from 0.5 m resolution aerial imagery and included area changes across the entire glacier including the margins along the flanks and head of the glacier.
During years 2005, 2009, 2010, and 2013 we walked the seasonal snow line with a GPS unit at the end of the ablation season. This end-of-summer snow line separates the ablation and accumulation areas on the glacier since the area at elevations above this line were still covered with snow that accumulated the previous winter. We divided these mapped accumulation areas by the total area of the glacier to obtain the accumulation area ratio (AAR).
Dates for mass balance years and winter and summer seasons for the study years, 2005 through 2015.
Area altitude distributions (AADs) were calculated for 8 years of the 11-year study period (Table 1) from a time series of digital elevation models (DEMs). These DEMs were derived from aerial photographs taken of the glacier in August or September so as to capture the glacier near its mass minimum (Fahey, 2014). Fahey used Next-Generation Automatic Terrain Extraction (NGATE; Zhang et al., 2006a, b) and SOCET SET® software to derive DEMs from the aerial imagery. Horizontal grid cell size was 5 m and the relative horizontal and vertical errors were less than 1 m. Since we used each DEM in isolation to generate Sperry Glacier altitude distributions for selected years, the DEMs were not co-registered to each other and we consider the relative error more pertinent to our analysis. The raw DEM coverage reached beyond the glacier's margin so each DEM was clipped by the most recently measured glacier margin so that all cells in the DEM represent snow-, firn-, and ice-covered area. Each cell from these glacier DEMs was then binned into 50 m bands and then summed to derive the total area for each elevation band.
For this study, we determined conventional (Cogley et al., 2011) seasonal and annual surface mass balances for Sperry Glacier using the glaciological method combined with the most recently mapped glacier margin and AAD. We followed established protocols for measuring snow depth, sampling snow density, and installing and reading ablation stakes (Ostrem and Brugman, 1991; Kaser et al., 2003). The snow depth measurements included all snow deposited on the glacier surface during the winter season, including seasonal snowfall, avalanche debris, and wind-transported snow. Because we were not able to directly measure melt that occurred during the winter season, we did not specifically account for it in the winter balance terms of mass balance calculations. Similarly, we did not include summer-season precipitation in our summer balance terms because it fell primarily as rain and was assumed to run off and not measurably contribute to the glacier's mass change. In addition, local climate data (Finklin, 1986) show that both winter-season melt and summer-season precipitation likely have minimal contributions to the net balances of each period. Because Sperry Glacier is a small, temperate glacier, we assumed that runoff transported nearly all melt off the glacier with negligible mass retained by refreezing and/or the formation of superimposed ice. This is a common assumption (Kaser et al., 2003) and is supported by our field observations and observations in the North Cascade Mountains, USA (Pelto, 1996).
We determined the mass balance year for field measurements and balance calculations using a time system that combines the stratigraphic and floating-date systems (Cogley et al., 2011). Ideally, measurements of accumulation are conducted when the glacier's mass is at its maximum for the year and ablation measurements during its minimum, with the balance year the period between two consecutive minima. For Sperry, such timing is impractical because difficulties with access preclude continuous monitoring. Also the time of the maxima and minima can vary by several weeks between years.
We therefore defined each mass balance year as the period between the latest
ablation stake readings in successive summer seasons (Table 2). These
readings were timed to occur as late as possible in September or early
October of each calendar year, so as to coincide as closely as possible with
the formation of that year's end of summer surface, which represents the
minimum annual mass for that year. We used the previous summer's surface as
the reference surface for snow depth probing at the end of the subsequent
accumulation season and defined winter balances
We adhered to the notation and signing conventions delineated in Cogley et
al. (2011) and used terms as defined in that glossary. The dimensions of the
depth and surface altitude measurements are length (m). Multiplying these by
the sampled snow density or, for firn and ice, an assumed density of 720 and
874 kg m
A winter mass balance point measurement (
A summer mass balance point measurement (
An annual mass balance point measurement (
The primary method for taking snow depth measurements was to probe vertically through the seasonal snowpack to the previous summer surface. A sectional solid aluminum probe was used, and depths were measured to the nearest 0.01 or 0.05 m depending on the year. Measurement locations were recorded using handheld GPS receivers. Depending on the GPS equipment available, some locations were differentially corrected. For safety reasons, we made no measurements on the steep headwall above the bergschrund (Fig. 2).
All measurements of snow density were taken in the spring. In 2005, density measurements were made in two snow pits (Supplement). The first pit was located roughly 275 m above the terminus; the second pit was 500 m farther up the glacier, roughly 125 m below the bergschrund. During 2006, 2007, and 2008 measurements were made at one pit dug at the lower location. From 2009 to 2013 no density measurements were made on the glacier and a bulk density value derived from the relationship between snow depth and density using the 2005–2006 data was employed to calculate balances. Density measurements resumed in 2014 and were taken at ablation stake sites A, C, and D that year, and at sites A and D in 2015.
Logarithmic curve fit to the three-pit mean data from June
2005 and 2006. Here
For the years 2005–2008, direct density measurements were made by weighing
samples of snow from the shaded face of a snow pit. The samples were
collected in 10 cm increments with a 1000 cm
In 2014 and 2015 density was measured by weighing snow samples collected at three or four specific depths in a 1.5 m deep snow pit. One sample was taken within 0.10 m of the snow surface, the second at about 1.0 m, and the third at about 1.5 m. At depths greater than 1.5 m from the snow surface, snow samples were obtained using a coring cylinder at 0.10 to 0.30 m intervals until the previous summer's surface was reached. Densities were then derived by weighing these samples.
During years 2005 and 2006 we adopted the methods of Jansson (1999) and
calculated the mean of the density measurements at 0.5 m intervals,
providing a bulk density for the first 0.5 m below the snow surface, then
1.0, and 1.5 m, continuing down to the deepest measurements for each
snow pit. A logarithmic curve was then fit to these values, resulting in a
function,
In 2007 and 2008 the data did not trend like the measurements from 2005 and 2006 and no model could adequately explain the relationship between depth and density. To calculate winter balances we instead used the bulk mean density derived from all values measured in the snow pit.
For balance years 2014 and 2015, we applied the measured densities taken
from their respective depths in the column to calculate water equivalents
layer by layer and then summed these values to get
We measured the surface change at the ablation stake locations and obtained
a water-equivalent length value for the total ablation by multiplying the
height loss of snow, firn, and ice by their respective densities. We used
the same density value for the snow component of
Glacier-wide mass balances represent the mean balance for the infinite
number of possible points across the glacier's surface. These values cannot
be directly measured and are typically estimated from point measurements or
interpolations of point measurements. We estimate glacier-wide mass balances
for the winter (
Sperry Glacier areal extent, 2005–2015.
Differences between winter point balances within bands varied between 0 and 5.05 m w.e. In most cases winter point balances in the same band differed by less than 1.5 m w.e. There were two instances in the 11-year record when there was bare ice and a high amount of accumulation in the same elevation band, resulting in differences greater than 4 m w.e. Summer point balances within bands varied much less, ranging between 0.01 to 0.82 m w.e. In 68 % of cases the difference was less than 0.5 m w.e. Annual point balances located in the same elevation band were slightly more variable than the summer balances but still less variable than winter balances. Differences ranged from 0.01 to 1.98 m w.e. and in 71 % of cases the difference was less than 1 m w.e.
During some years no measurement points were located at the very lowest
elevations of the glacier below 2300 m. In these cases, measurement points
within 10 m of 2300 m were used to assign balances to this elevation band.
In situations where this was not possible, then a single point balance,
taken from the lowest elevation measurement point, was used instead
(Supplement). For some balance years there were no measurements taken at the
higher elevations of the glacier between 2550 and 2650 m (Supplement). The
winter and summer balances for these bands were derived using a gradient
found between two point balances and their respective elevations at two
different measurement sites. The stakes used to calculate these gradients
are presented in the data tables included in our Supplement. Furthermore, no
measurements of any kind have been collected at the uppermost elevations of
the glacier on the steep southern headwall above 2650 m. With respect to the
winter balance, observations show that frequent avalanches prevent large
amounts of snow from accumulating on this steep slope. Thus it is likely
winter balances will be lower on areas above the bergschrund than those
immediately below it. For the area located on this headwall, we used the
mean winter point balance taken from all measurement points on the glacier
to represent winter balances. Ultimately the snow depths on the headwall are
unknown, yet it is necessary to assign balances to this region. Using the
mean
Annual balances for elevation bands with no point balances were derived by
summing the winter and summer values assigned to them via the extrapolation
methods discussed above. This sometimes resulted in negative balances on the
steep southern headwall. This is consistent with observations showing much
of this headwall will melt down to firn and ice during years with a strongly
negative
Summary statistics for 2005–20105 winter point balances.
Mapped outlines of Sperry Glacier for the period 1998–2015.
The cumulative mass balance is the total mass gained or lost over multiple
balance years. For this study, we calculated cumulative mass balances for
the 11 study years by summing the glacier-wide annual (
Sperry Glacier has decreased by 0.08 km
In years 2005, 2009, 2010, and 2013 the accumulation area ratios were
34, 46, 48, and 36 % respectively (Supplement). The average
value is 41 %. While we did not measure the AAR for any other years, we
noted that during the two years with the most positive
When combining all points from all years (
Summary statistics for 2005–2015 summer point balances.
Winter and summer mass balance measurements from all points/all
years which were grouped by elevation (NGVD29 and NAD 83) into 50 m bins. We
plotted the mean for each bin against the mean elevation for each bin. The
number of measurements in each bin varies within years and among years
depending on the year's measurements. The
A somewhat complicated relationship exists between winter balance and
elevation after averaging the
In 2005, 2006, and 2007 the number and locations of ablation stakes varied (Supplement). Beginning in 2008, when seven stakes were placed in the same locations each year, the winter, summer, and annual balances were measured consistently at each site every year with the exception of one stake in 2009. Stake Z was added in 2015 to better measure the uppermost accumulation zone and to compare balances found at nearby stake D.
Summary statistics for 2005–2015 annual point balances.
Ablation, as reflected in the change of the glacier surface elevation at the
ablation stake sites, followed a similar temporal pattern in all 11 years.
The surface change was greatest during June and July and sometimes early
August with surface lowering rates ranging between
When combining all stake measurements from all years (
Glacier-wide balances. Units are m w.e. for all values.
Bars represent the seasonal and annual mass balances on the glacier. The black line depicts the running cumulative balance for the period of record.
The relationship between summer balance and elevation varied between years
and sometimes in a complex manner. When all
When combining the annual point balances from all stakes from 2005 to 2015
(
The site index method resulted in
Winter mass balances for years 2005–2015 plotted against each elevation (NGVD29 and NAD83) band. Area altitude distribution is from 2005. Gray bars represent the percentage of glacier area within each elevation band. Colored lines plot the winter mass balances across the elevation range of the glacier.
Similar to the winter balances, summer mass balances did not always vary
consistently with elevation (Fig. 8). For years 2005, 2006, 2008, and 2010
Summer mass balances for years 2005–2015 plotted against each elevation (NGVD29 NAD83) band. Area altitude distribution is from 2005. Gray bars represent the percentage of glacier area within each elevation band. Colored lines plot the summer mass balances across the elevation range of the glacier.
The first three balance years, 2005–2007, were strongly negative with 2007
having the lowest
Again, similar to the winter and summer balances, the changes in annual
balances with elevation were variable (Fig. 9 and Supplement). For years
2005, 2006, 2010, and 2012
Annual mass balances for years 2005–2015 plotted against each elevation (NGVD29 and NAD83) band. Area altitude distribution is from 2005. Gray bars represent the percentage of glacier area within each elevation band. Colored lines plot the annual mass balances across the elevation range of the glacier.
Cumulatively the glacier lost 4.37 m w.e. between 2005 and 2015 by way of the site index method. About 70 % of that loss occurred during the first three years of the study. Both seasonal and annual balances exhibited more variability in the remaining eight years of the study.
Cumulative balances in the ablation zone, which averaged about 60 % of the
glacier area for the four years it was measured, ranged from
Errors associated with measurements of snow depth and density, ablation, stake and probe point locations, and the fact that measurements are never taken on the exact day of glacier maximum and minimum balances all may affect the individual point balances. There is uncertainty as to how well the point balances represent their respective elevation bands and what this effect may have on the glacier-wide values. Our accumulation area measurements have shown that some bands contain zones of both accumulation and ablation during some years. And finally certain areas of the glacier simply cannot be measured due to safety concerns. A developing understanding of the accumulation and ablation patterns over time has been used to develop the current stake network and now fewer measurements are made at strategic locations. Yet we acknowledge that the optimal number and distribution of measurement points on the glacier still needs to be resolved. We have not yet done a complete quantitative error analysis. However we summarize the major sources of uncertainty associated with the Sperry program and also offer error estimates based on other similar work.
Glacier-wide annual mass balances (
Because they are time- and labor-intensive to conduct, density measurements
are sparse compared to depth measurements. Due to the small amount of data,
it is uncertain how much density varies temporally and spatially. During the
2009–2013 balance years no density measurements were made, which introduces
additional uncertainty for those years. Nevertheless, our limited
measurements suggest spatial variability of snow density across the glacier
surface is minimal near the time of maximum balance. Measurements made on
the same day but at different locations result in mean values that vary by
less than 50 kg m
A more critical source of error lies in the problem of accurately measuring snow depths. It can be difficult to determine the previous summer's surface through several meters of snow, especially in the accumulation zone. Multiple probe measurements made at one point were used to address this problem. One approach to quantifying the effects of errors follows Jansson (1999), who identified systematic probing errors as the most likely to create relatively large uncertainties in the winter balance. He argued that small random errors will cancel out, larger random errors will be detected as islands of anomalous depth, but systematic errors in resolving the previous summer surface are restricted to the accumulation area and are often proportional to snow depth. Beginning in 2014 we cored through the entire winter's snowpack and obtained a definitive depth to ice or firn at two to three of the stake sites. These depths were consistent with those sounded by probing at the same locations.
The ablation stake measurements are subject to stake sinking, which will underestimate summer balance, and to recording blunders made in the field. Partitioning the snow, firn, and ice components of height loss also proved difficult in some instances. Firn density was not measured, forcing us to use an estimated value. The spatial variability of ablation across the glacier surface could be examined more thoroughly through the use of more stakes placed in different locations.
Jansson (1999) suggests a glacier-wide mass balance uncertainty value of
approximately 0.10 m w.e. for glaciers with sparse networks of measurement
points. Riedel and Larrabee (2011) with the US National Park Service have
derived error values for the winter, summer, and annual point balances on
four glaciers in the North Cascades, USA, years 1993–2009. Winter point
balance errors ranged from 0.19 to 0.26 m w.e., summer point balance errors from
0.26 to 0.33 m w.e., and annual point balance errors from 0.31 to 0.42 m w.e. A
complete description of this program's error analysis can be found in Riedel
et al. (2008). At this time it is unclear what error values are appropriate
for both the point and glacier-wide mass balances on Sperry Glacier. The
Sperry program methods are similar to those used by the National Park
Service in the North Cascades and by many other programs worldwide cited in
Jansson (1999). Uncertainties ranging between
There are no other glacier mass balance programs in the US northern Rockies that can be compared to Sperry Glacier. However some general comparisons can be made with the Peyto Glacier mass balance record from the Canadian Rockies and mass balances from four glaciers in the North Cascades, USA (WGMS, 2016; Riedel and Larrabee, 2016), where mass balances are calculated with methods similar to those used on Sperry Glacier.
At Peyto Glacier between 2005 and 2015, the cumulative mass balance was
A comparison of annual and cumulative balances with four glaciers in the
North Cascades – Noisy Creek, Silver, North Klawatti, and Sandalee glaciers
– showed similar general trends to those observed on Sperry (Fig. 10).
Cumulative balances for years 2005–2015 were all more negative than Sperry
Glacier and ranged from
Conventional glacier-wide seasonal and annual mass balances were estimated using glaciological methods from 2005 to 2015 at Sperry Glacier and reported along with all the accompanying glaciological measurements. Measurements of glacier extent and accumulation areas were also measured during certain years and are included in this paper as well.
Results from our site index methods reveal that, on average, Sperry Glacier
loses about 0.40 m w.e. each year (mean
The range between point balances was greater than the glacier-wide values.
Some winter point balances exceeded 5.00 m w.e., while at other locations on
the same day there were patches of bare ice with no winter accumulation.
During particularly hot and long summers, summer point balances approached
Between 2005 and 2015, it is estimated Sperry Glacier lost 4.37 m w.e.
averaged over its entire area. In the ablation zone, cumulative balances
reached as low as
Sperry's surface area has decreased by 0.08 km
The variable relationship between mass balance and altitude on Sperry is likely affected by concurrent multiple drivers that are not yet fully understood. Further research is needed to improve our knowledge of the processes that control accumulation and ablation patterns on this glacier. Specific to ablation, local factors that regulate the balance of heat energy input from solar radiation such as aspect, slope angle, and cirque-wall shading, combined with the type of material present on the glacier's surface, may have a greater influence on ablation rates than the gradient between altitude and air temperatures. The winter balance is likely bolstered by the snow added to the glacier from avalanches and wind, especially in the areas directly beneath the cirque walls, but this re-distribution of mass needs to be better quantified. Our findings are comparable with other work which shows small alpine cirque glaciers are more sensitive to these topographic factors that can influence the surface mass balance (Kuhn, 1995; Hoffman et al., 2007; Fountain, 2007; Fountain and Vecchia, 1999). Future work will focus on obtaining better data, especially in the accumulation zone with specific attention given to adding and/or adjusting the measurement site locations and to sampling the depth and density of snow and/or firn at the end of the ablation season. In addition we plan to complete a thorough examination of the errors and uncertainties associated with the methods we use on Sperry and to compare our glaciological results against geodetic mass balances.
All data have been submitted to the World Glacier Monitoring Service and are available at
This research was supported by the US Geological Survey's Climate and Land Use Change Research and Development Program. Help with fieldwork was provided by a large group of field technicians and volunteers over the years. The authors would like to thank Mauri Pelto, Jon Riedel, and one anonymous reviewer for their helpful comments and insight, which greatly improved this paper. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government. Edited by: R. Drews Reviewed by: M. Pelto and one anonymous referee