Tree-Ring Based Estimates of Glacier Mass Balance in the Northern
Rocky Mountains for the Past 300 Years
1,2Gregory
T. Pederson, 3Emma Watson, 4Brian Luckman, 1Daniel B. Fagre, 5Stephen T. Gray, 2Lisa J. Graumlich
gpederson@montana.edu, Emma_Watson@ec.gc.ca, luckman@uwo.ca, dan_fagre@usgs.gov, stgray@usgs.gov, lisa@montana.edu
(1) U.S. Geological Survey - NRMSC - Glacier Field Station, West Glacier, MT 59936, USA, (2) Big Sky Institute - Montana State University, Bozeman, MT 59717, USA, (3) Climate Research Branch, Meteorological Service of Canada,
Environment Canada, Downsview, Ontario, Canada M3H 5T4, (4) Department of Geography, University of Western Ontario, London Ontario, Canada N6A 5C2, (5) Geological Survey - Desert Laboratory, Tucson, AZ, USA 85745
Athabasca Glacier
INTRODUCTION
Boulder Glacier
Summer 1988
Alpine glaciers in the U.S. and Canadian Rocky Mountains reached their maximum Holocene extent
during the Little Ice Age (LIA: Luckman 2000, Carrara 1989). Subsequently, glaciers throughout the
region have undergone dynamic and sometimes rapid phases of frontal recession (Carrara 1989,
Luckman 2000, Key et al. 2002). Recent glacier research has focused on developing detailed histories
of glacier fluctuations throughout the LIA. These data, though sparse, indicate multiple periods of glacier
advance during the LIA, and that the timing of maximum advance within the Canadian Rockies may not
have been synchronous (Figure 1a; Luckman 2000). Moraine dates at several of the northernmost
glaciers studied, e.g. in Jasper National Park, indicate maximum glacier extent between 1700 and 1750
(Luckman 2000) whereas further south, e.g. in Kananaskis and Glacier National Park (GNP), the LIA
maximum glacial extent occurred between ca. 1800-1850 (Carrara 1989, Smith et al. 1995, Luckman
2000, Key et al. 2002).
Mass balance records for this region are sparse consisting of two records from the Canadian Rockies
(Peyto 1965- present and Ram River Glacier 1965-75). In order to develop a better understanding of
glacier fluctuations in this region, Watson and Luckman (2004) and Pederson et al. (2004) independently
investigated the paleoclimatic drivers of glacier fluctuations using data derived from tree-ring
chronologies for two sites located along the Continental Divide (Figure 1b). We present a comparison of
these two proxy-based attempts of reconstructing glacier mass balance for Peyto Glacier in Alberta, and
the glaciers in GNP, Montana. We also use instrumental and proxy climate data to investigate whether
differences between these proxy mass balance series reflect regional differences in mass balance over
time, or whether they result from differences in the approach and data used to develop the
reconstructions. In doing so, we begin to explore differences in timing of the LIA maximum glacial
advance.
(b)
Photo by J. DeSanto
Summer 1932
mean SWE (n=25)
mean SWE Northern PC (n=10)
mean SWE Southern PC (n=7)
Photo by G. Grant
(Left) The Snout of the Athabasca Glacier – Jasper National Park, Alberta - viewed from the same
point in 1917 and 1986. Over this 70-year period the Athabasca Glacier retreated 1.5 km.
(Right) The Boulder Glacier located in Glacier National Park, Montana. During the mid-19th century the
boulder glacier extended across boulder pass leaving a prominent terminal moraine. Today all that
remains is a stagnant ice apron.
Number of Moraines
1300
1500
Figure 3. Plots of April 1st standardized SWE
records over the 20th century. (a) standardized
SWE values at 25 stations (see Figure 1). (b)
mean annual SWE values for the 25 stations in
(a) and smoothed with a 10-year spline.
SUMMARY AND CONCLUSIONS
Figure 4. Plots of standardized seasonal maximum
(Tmax) and minimum (Tmin) temperature records over the
20th century. (a) mean summer Tmax anomalies for 14
meteorological stations (see Figure 1). (b) mean
summer Tmin anomalies for the 14 stations in (a). All
records in (a) and (b) have been smoothed using a 10yr
spline. (c) mean annual and smoothed (10yr spline)
values for spring (MAM) and summer (JJA) Tmin.
Though careful interpretation and record selection is required, tree-ring based proxy records of
climate allow the reconstruction of continuous records of glacier changes. In the absence of longterm instrumental records, they permit exploration of the relative contribution of changes in
temperature and precipitation to net mass balance. Here, both proxy records indicate decadal- and
longer-term trends in winter snowpack and summer temperature have a significant influence on the
net mass balance of regional alpine glaciers. Instrumental records suggest the minor north to south
differences in the relative extent of 18th and 19th century glaciers may have been due to slight
differences in SWE (e.g. Figure 3) or reduced temperatures along the gradient. These two
preliminary attempts at modeling past glacier fluctuations helped to identify the various scales of the
forcing factors and therefore possibly identify their causes. Results also indicate the potential to
develop a more comprehensive picture of how glaciers have fluctuated in the past, thereby providing
insight on how future modeled or actual climate changes may influence them into the 21st century.
(b)
(a)
(a)
1700 1900
Figure 1. (a) Dated LIA moraines in the Canadian Rockies. Moraines ages are based on
dendrochronology at 48 glaciers and lichenometry at another 18 glaciers. The ages are grouped into
25-year increments. Graph reproduced from Luckman (2000). (b) Location of Peyto Glacier, Alberta
and Glacier National Park, Montana. Selected meteorological stations and tree-ring chronology sites
used to develop the Peyto Glacier mass balance reconstructions are shown. The larger scale map
shows stations located along the Continental Divide from which snow water equivalent (SWE) and
temperature records were obtained.
COMPARISON OF MASS BALANCE PROXY RECORDS
A fundamental difference between these two mass balance reconstructions is that approximately 30
years of mass balance data were available for Peyto Glacier that allow direct calibration of mass balance
estimates, whereas no mass balance data were available for the GNP study . Therefore the latter case
relied on a detailed history of glacier length and estimated retreat rates for the Agassiz and Jackson
Glaciers (Carrara 1989, Key et al. 2002) to evaluate correspondence between the glaciers and inferred
mass balance histories.
Though the methods and data used to infer and reconstruct glacier mass balance are quite different
between studies, interesting similarities and differences arise between the balance estimates. The
winter and summer balance records in both regions exhibit strong decadal and multi-decadal variability,
with the winter balance proxies often sharing common periods of above and below average accumulation
events (Figure 2). For example, there is general correspondence between records from the 1770s1790s, throughout the 19th century, and a common period of high accumulation beginning in the mid1940s to the late-1970s. Differences in estimated winter balance arise with above average accumulation
predicted for the early-1700s and early-20th century at Peyto Glacier, while the D’Arrigo et al. (2001)
PDO reconstruction (used to estimate GNP winter balance) indicates average to below average
accumulation for the GNP region. Fewer similarities exist between the regional records of summer
balance; however, there are two prominent periods of correspondence between records with (cool)
summer conditions predicted for the early-1700s and early- to mid-1800s. Pronounced disparities
between summer balance estimates arise with average to high summer ablation rates predicted between
ca. 1720s-1770s and 1850s-1880s for GNP, when low ablation conditions are reconstructed for Peyto
Glacier. The most striking difference however, is the long-term linear trend towards higher summer
ablation rates (ca. 1880) at Peyto Glacier, which consequently drives the 20th century negative trend in
net mass balance.
Figure 2. Comparison of seasonal and net mass balance proxies for Peyto Glacier and Glacier National
Park. (Top) Standardized seasonal and net mass balance estimates for Peyto Glacier including the
regression beta values for tree-ring data (locations given in Figure 1). (Bottom) Reconstructions used to
infer potential seasonal mass balance for the Glacier National Park region. All annual values have been
smoothed using a 10 yr spline (thick colored line).
EVALUATING THE DIFFERENCES – Instrumental and Proxy Records
Throughout the region SWE records display coherent decadal-scale variability that corresponds with sea
surface temperature patterns in the North Pacific (i.e. PDO: Figure 3). PCA analysis reveals northern and
southern regions differing primarily in magnitude of major periods of above and below average SWE.
Records of summer Tmin and Tmax exhibit strong regional coherence (Figure 4a,b). Estimated summer
balance at Peyto Glacier and for the GNP region are strongly correlated with instrumental summer Tmax
(with 11/15 stations [r=-0.22 to -0.528] for Peyto, and 13/15 stations [r=-0.36 to -0.65] for GNP). Between
1920 and 1990 no linear trend is present in summer Tmax – interdecadal variability dominates. However,
records of spring and summer Tmin show a strong trend over the 20th century (Figure 4c), which is consistent
for all seasons. Such changes in Tmin may extend the period of melting, shift the rain:snow input ratio, and
maintain higher summer ablation rates. Similarities between instrumental temperature and SWE records
suggest mass balance patterns should be regionally similar. Therefore, differences between reconstructions
may be more readily explained by differences in proxies used to estimate net mass balance values.
Each winter mass balance proxy relied on ocean-atmosphere teleconnections related to temperature and
pressure patterns in the North Pacific (i.e. PDO) since no winter precip. sensitive local chronologies were
found. However, this assumes that the PDO operated in a similar manner (with similar variability) prior to the
period of instrumental record. Disagreement, between published PDO reconstructions prior to ca. 1840
suggests this assumption may be violated (Figure 5; Gedalof et al. 2002), thus it is important that inferred
trends and variability in winter SWE conditions are confirmed by multiple lines of independently derived
evidence. Another interesting difference between regional mass balance proxies is the long-term trend
towards negative summer and net mass balance at Peyto Glacier. The linear trend towards higher ablation
rates at Peyto Glacier is caused by the Athabasca summer Tmax reconstruction, which exhibits increases in
Tmax beginning ca. 1825. This trend is not evident in the GNP summer and net balance proxies, in part,
because the tree-ring chronologies contain a mixed temperature and precipitation signal that may not
consistently track summer melt. Also, the mixed climate signal contained in drought reconstructions does
not retain the centennial-scale variability seen in temperature reconstructions. Neither reconstruction
includes, or necessarily captures the aforementioned trends in Tmin, making the assessment of this
potentially important variable difficult.
Figure 5. Tree-ring derived records that are related to sea surface temperatures in the Pacific Ocean and
winter mass balance in the study area. The Gedalof and Smith, 2001 and Biondi et al., 2001 series are
reconstructions of PDO. Miners Well is a mountain hemlock tree ring-width chronology from the Gulf of
Alaska developed by G. Wiles, P.E. Calkin and D. Frank. All series have been smoothed with a 10-yr
spline.
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