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Quaternary Research 62 (2004) 9–18

Tree-ring-based mass-balance estimates for the past 300 years at

Peyto Glacier, Alberta, Canada

Emma Watson*,1 and Brian H. Luckman

Department of Geography, University of Western Ontario, London, Ontario, Canada N6A 5C2

Received 17 September 2003

Abstract

Tree rings were used to reconstruct mass balance for Peyto Glacier in the Canadian Rocky Mountains from A.D. 1673 to 1994. Summer

balance was reconstructed from tree-ring estimates of summer temperature and precipitation in the Canadian Rockies. Winter balance was

derived from tree-ring data from sites bordering the Gulf of Alaska and in western British Columbia. The models for winter and summer

balance each explain over 40% of the variance in the appropriate mass-balance series. Over the period 1966–1994 the correlation between

the reconstructed and measured net balances is 0.71. Strong positive mass balances are reconstructed for 1695–1720 and 1810–1825, when

higher winter precipitation coincided with reduced ablation. Periods of reconstructed positive mass balance precede construction of terminal

moraines throughout the Canadian Rockies ca. 1700–1725 and 1825–1850. Positive mass balances in the period 1845–1880 also

correspond to intervals of glacier readvance. Mass balances were generally negative between 1760 and 1805. From 1673 to 1883 the mean

annual net balance was +70 mm water equivalent per year (w.e./yr.), but it averaged �317 mm w.e./yr from 1884 to 1994. This reconstructed

mass balance history provides a continuous record of glacier change that appears regionally representative and consistent with moraine and

other proxy climate records.

D 2004 University of Washington. All rights reserved.

Keywords: Glacier fluctuations; Mass balance; Dendrochronology; Tree-ring reconstructions; Canadian Rockies; Peyto Glacier

Introduction

Glacier fluctuations have frequently been used as indi-

cators of past climate variations on many different time-

scales. However, the links between changes in glacier size

and climate are often complex and best evaluated from

detailed studies of glacier mass balance. Unfortunately

direct measurements of mass balance are labor-intensive,

expensive to maintain, and few in number. Most of these

records are short (rarely more than a few decades) and from

the late 20th century, a period characterized by strongly

negative glacier mass balances. The short mass-balance

record for Peyto Glacier is the longest and best documented

available for any glacier in the Canadian Rockies (Demuth

et al., in press). Longer mass-balance records through

periods of more variable climate could improve our inter-

0033-5894/$ - see front matter D 2004 University of Washington. All rights rese

doi:10.1016/j.yqres.2004.04.007

* Corresponding author. Fax: (416) 739-5700.

E-mail address: Emma.Watson@ec.gc.ca (E. Watson).1 Current address: Climate Research Branch, Meteorological Service of

Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4.

pretation of the climate signal inferred from records of

glacier fluctuations.

Research over the past two decades has provided detailed

histories of glacier fluctuations during the Little Ice Age

(LIA) at many sites in the Canadian Rockies. These studies

used dendrochronologic, lichenometric, and historical sour-

ces (Luckman, 2000). In addition, several dendrochronolog-

ical studies have provided reconstructions of precipitation

and temperature variables in the region (e.g., Luckman et al.,

1997; St. George and Luckman, 2001; Colenutt, 2000;

Watson and Luckman, 2001, 2004; Wilson and Luckman,

2003). The availability of these proxy precipitation and

temperature records offers, for the first time, the possibility

of reconstructing glacier mass-balance history in the Cana-

dian Rockies independent of the evidence of past glacier

fluctuations. This independence allows cross validation of

several different sources of paleoclimate information. These

records allow a detailed evaluation of the controls of glacier

fluctuations and mass balance in this region prior to the short

period of direct measurements.

Measured mass-balance records in Scandinavia and the

Alps have been extended using tree-ring data (e.g., Nicolussi

rved.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–1810

and Patzelt, 1996; Raper et al., 1996). However, these studies

only involved temperature-sensitive tree-ring series as prox-

ies for either the summer or annual balances, and did not

incorporate a separate estimate of winter balance. In this

paper, we present the first attempt to derive a mass-balance

record from independently estimated summer and winter

balances for a glacier. For the Canadian Rockies, we provide

the only continuous record of glacier changes through the

past three centuries and the latter part of the LIA for this

region.

Mass-balance studies at Peyto Glacier

Peyto Glacier is a ca. 12-km2 outlet glacier from theWapta

Icefield in Banff National Park, central Canadian Rockies

(Fig. 1). During the 1960s it was selected as representative of

glaciers in the central Canadian Rockies and included in the

Canadian mass-balance studies designed to contribute to the

International Hydrological Decade (1965–1974). Subse-

quently, Peyto Glacier has been the focus of many classical

glaciological studies (Demuth et al., in press) and has one of

the longest continuously monitored glacier mass-balance

records in Canada, spanning over 35 years.

Mass-balance studies at Peyto Glacier were initiated in

1965 and have been maintained to the present. As noted in

previous studies (e.g., Yarnal, 1984; Bitz and Battisti, 1989;

Walters and Meier, 1989), maximum correlations with winter

Fig. 1. Location of Peyto Glacier and the sites from which instrumental and

tree-ring data used in the study were obtained.

mass balance (Bw) are positive with winter precipitation

(e.g., correlations with October–April precipitation at Banff

and Jasper are 0.70 and 0.65, respectively). Summer mass

balance (Bs) correlates most strongly (negatively) with local

summer temperatures (e.g., the correlation between Bs and

June–August temperatures at Jasper is �0.78). The annual

net balance (Bn) is derived by summing Bw (accumulation)

and Bs (ablation). Between 1966 and 1995 only 6 yr (1966–

1968, 1973–1974, and 1976) show a positive balance and

there has been considerable frontal recession of the glacier.

Themeasured winter balance at Peyto Glacier may be divided

into two distinct periods. From 1976 to 1995 the mean winter

balance was only 67% of the 1965–1976 mean (Luckman,

1998; Demuth and Keller, in press). This abrupt decrease

coincides with the well-documented changes in atmospheric

circulation patterns in 1976 that have been linked with large-

scale interdecadal variations in the North Pacific Ocean (Bitz

and Battisti, 1999; McCabe et al., 2000; Walters and Meier,

1989). These variations are documented by indices of the

Pacific Decadal Oscillation (PDO; Mantua et al., 1997) and

Pacific North America pattern (PNA; Wallace and Gutzler,

1981) and have a widespread influence on streamflow,

precipitation, snow pack, glacial mass balance, temperature,

and other environmental variables along the west coast of the

Americas (Ebbesmeyer et al., 1991; Villalba et al., 2001;

Moore and McKendry, 1996; Brown and Braaten, 1998).

Alaskan climate is also strongly related to these patterns, and

tree-ring chronologies from the Gulf of Alaska have been

used to reconstructMarch–September temperatures (Wiles et

al., 1996, 1998), North Pacific SSTs, and the PDO itself

(D’Arrigo et al., 1999, 2001; Gedalof and Smith, 2001).

Positive PDO values reflect a warming of coastal waters

and a cold pool in the central and western North Pacific that

lead to an enhanced Aleutian Low and an amplified high-

pressure ridge over the southern Canadian cordillera and

Pacific Northwest (Mantua et al., 1997). These changes

result in wetter and warmer conditions in Alaska (enhanced

advection of warm moist air from the south) and warmer

and drier winters in the Canadian Rockies. Atmospheric

flow is more zonal in negative PDO winters, which

increases storm frequency in western Canada and decreases

it in Alaska (McCabe et al., 2000). Winter temperatures in

both regions are cooler during negative PDO years. Since

winter climates in both regions are strongly related to a

common forcing, tree-ring chronologies from Alaska may

serve as an important source of winter climatic information

for Peyto Glacier, where winter-sensitive tree-ring chronol-

ogies are not currently available.

Data and methods

Tree-ring chronologies

Tree-ring chronologies have been developed from a vari-

ety of treeline species (Larix lyallii, Picea engelmannii, and

Table 1

Location, length, and source of records used in this study

Lat. N Long. W Elev. (m) Prov./state Length

Monthly climate records

Banff

precipitationa51 11 115 34 1389 Alberta 1895–1995

Banff

temperaturea51 11 115 34 1389 Alberta 1895–2001

Jasper

precipitationa52 53 118 04 1061 Alberta 1936–1995

Jasper

temperatureb52 53 118 04 1061 Alberta 1916–1994

Mass balance

Peytoc 51 41 116 32 2140–3180 Alberta 1966–1997

Tree–ring data

Miners Well

(MW)d60 00 141 41 650 Alaska 1428–1995

Athabasca

(ATHA)e52 13 117 14 2000 Alberta 869–1994

Waterton (WA)f 49 28 113 34 1200 Alberta 1673–1996

Lytton (LY)f 50 14 121 35 258 B.C. 1468–1996

a Obtained from the Historical Canadian Climate Database (Mekis and

Hogg, 1999; Vincent, 1998; Vincent and Gullett, 1999).b From the dataset assembled by Luckman and Seed (1995).c Winter, summer, and net mass-balance records provided by the National

Glaciology Programme of the Geological Survey of Canada and the

National Water Research Institute. Data are not available for 1991 and

1992. The standard error for the mass-balance measurements is 150–200

mm w.e. (Demuth and Keller, in press).d Total ring-width chronology (Tsuga mertensiana) downloaded from the

International Tree-Ring Databank (ITRDB) submitted by G. Wiles, P.E.

Calkin, and D. Frank.e May–August maximum temperature reconstruction (adjusted R2 is 0.51)

developed by Luckman and Wilson (Unpublished ms).f Precipitation reconstruction (July–June) from Watson (2002) and Watson

and Luckman (2004). The Lytton reconstruction model is based on a

ponderosa pine chronology from Lytton, and the Waterton reconstruction is

derived from two Douglas-fir chronologies from within Waterton Lakes

National Park. The Waterton climate record is a combination of the records

from Claresholm and Carway. Location and approximate elevation for the

tree-ring data refer to the meteorological stations not the chronology sites.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–18 11

Pinus albicaulis) in the Canadian Rockies, which have been

used to reconstruct summer temperatures for the region (e.g.,

St. George and Luckman, 2001; Colenutt, 2000; Wilson and

Luckman, 2003). The longest of these summer temperature

reconstructions (April–August mean temperatures, 1073–

1983) was developed by Luckman et al. (1997) using ring

width (RW) and maximum tree-ring density (MXD) data

from sites near Athabasca Glacier. Additional RWand MXD

series from living and sub-fossil trees at the Athabasca, Peyto,

and Robson Glaciers have been used to provide an extended,

more regionally representative reconstruction for May–Au-

gust maximum temperatures from ca. A.D. 950 to 1994

(unpublished data). Previous work showed strong relation-

ships between the reconstructed May–August temperatures

at the Athabasca Glacier and both local and regional glacier

fluctuations (Luckman et al., 1997; Luckman, 2000). The

new May–August reconstruction can be used to calculate Bs

at Peyto Glacier.

Recent work in the Rockies and southern British Colum-

bia has developed a network of annual (July–June) pre-

cipitation reconstructions from Pseudotsuga menziesii

(Douglas-fir) and Pinus ponderosa (ponderosa pine) chro-

nologies, three of which are in the Alberta Rockies (Banff,

Jasper, and Waterton; Watson and Luckman, 2001, 2004).

Although these are reconstructions of annual precipitation

they are strongly influenced by summer precipitation and

may not be well suited to estimate winter precipitation or Bw.

Therefore, several chronologies of Tsuga mertensiana

(mountain hemlock) from theGulf of Alaska region (obtained

from the International Tree-Ring Data Bank, http://www.

ngdc.noaa.gov/paleo/treering.html) and Vancouver Island

(D. Smith, personal communication, 2003) were used in the

reconstruction trials. Tsuga ring-width series on Vancouver

Island are sensitive to winter snow pack and summer temper-

atures and have been used to estimate mass balance for

glaciers on Vancouver Island (Smith, 2002; Lewis, 2001).

Reconstruction strategy

Stepwise multiple regression analysis was used to gener-

ate models that can be used to develop reconstructions of Bw

and Bs at Peyto Glacier. Potential predictors for the winter

model include mountain hemlock chronologies from Alaska

and British Columbia and precipitation reconstructions from

the southern cordillera. The pool of potential predictors for

the summer model includes the precipitation reconstructions

for the southern cordillera and the updated Athabasca sum-

mer temperature reconstruction. Given the limited length of

the mass-balance records (28 yr), verification of the models

was accomplished by comparing the instrumental values with

an independent dataset generated using the ‘‘leave-out-one’’

method (Gordon, 1982). The validity of the models is also

assessed by comparing the measured net mass balance record

with reconstructed net mass-balance (i.e., the difference

between the reconstructed winter accumulation and summer

ablation). Finally, the annual and cumulative Bn are com-

pared with the LIA moraine record from the Canadian

Rockies.

Results

Statistical models were developed that explain more than

40% of the variance in the observed measurements (Table 1)

of both Bw and Bs balances (Table 2; Fig. 2). The Miners

Well chronology (Table 1) enters the winter model negatively,

indicating that wider rings, which correspond to positive

PDO years and therefore warm, wet winters in Alaska and

warm, dry winters in the Rockies, are associated with lower

winter balance values. The most westerly precipitation re-

construction (Lytton;Watson and Luckman, 2004) also enters

the winter balance model positively. The tree-ring data used

to develop the Lytton reconstruction correlate significantly

with monthly winter precipitation totals (Watson, 2002),

Table 2

Calibration and verification results for the tree-ring-based mass-balance reconstructions developed for Peyto Glacier

Calibration Verification

Years Predictorsa SEb R R2adj D-W d c Standard reconstructions First differenced data

Period r REd Sign test r RE Sign test

Winter balance (1468–1995)

1966–1995 MW (�0.61);

LY (0.31)

269.55 0.67 0.41 2.04 1966–1995 0.58* 0.38 22/6* 0.08

Summer balance (1673–1994)

1966–1994 ATHA (�0.59);

WA (�0.51)

303.01 0.71 0.46 2.05 1966–1994 0.62* 0.41 21/6* 0.59* 0.40

ns indicates that a result is not statistically significant ( p > 0.05).a Tree-ring data used to develop the model (for abbreviations see Table 1). Beta weights are given in parentheses.b Standard error of the estimate in mm w.e./a.c The Durbin–Watson d statistic was evaluated to test for first-order serial correlation in the residuals. These values are not significant at the 0.05 level.d Reduction of error statistic (Fritts, 1991). Positive values indicate that the reconstruction has some skill and is a better predictor than simply using the mean of

the calibration period.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–1812

explaining the positive weighting of this variable in the

winter model. The revised summer temperature reconstruc-

tion from Athabasca (Luckman and Wilson, unpublished

data) and the precipitation reconstruction from Waterton

Lakes (Watson and Luckman, 2004) both enter the summer

model negatively. This indicates that warmer summers are

associated with more negative summer balances (i.e., greater

ablation). Although the Waterton reconstruction is of annual

precipitation, the chronologies used to develop the recon-

struction correlate more highly with monthly summer tem-

peratures than with precipitation totals in any individual

month (Watson and Luckman, 2002). Both the summer and

winter reconstructions pass all standard verification tests,

demonstrating that the models do have predictive skill (Table

2). The summer model passes two of three verification tests

repeated after the instrumental and predicted series are trans-

formed to first differences indicating that it performs well at

higher frequencies (Table 2). The winter reconstruction fails

two of the three verification tests performed using first

differenced data (it marginally passes RE, which is the most

rigorous verification test of the three), indicating that its skill

is found at lower frequencies. A greater proportion of the

variance in the winter mass-balance record is related to trend

and autoregression than in the summer balance time series, so

this is not considered to be a significant limitation (Demuth

and Keller, in press).

Estimates of net mass balance were calculated as the

difference between the reconstructed winter and summer

series rather than a separate net mass-balance reconstruction

being developed. The correlation between this series and the

measured net mass-balance record is 0.71 (Fig. 2) providing

additional verification of the reconstructions. The tree-ring

based net mass-balance reconstruction also correlates sig-

nificantly (r = 0.51, n = 82, p < 0.05) with an independent

extension of the net mass-balance measurements derived

from instrumental precipitation and temperature records

from Banff and Jasper (not shown).

Peyto Glacier mass balance, 1673–1994

The tree-ring-based reconstructions of Bw, Bs, and Bn

are shown in Fig. 3. The 1966–1994 mean for all three

parameters is below the long-term reconstructed mean (i.e.,

there is less winter accumulation, more summer ablation,

and a more negative balance during the period of measure-

ment). The Bs and Bn show the greatest differences, which

are related to increasing temperatures during the 20th

century in this region (Luckman, 1998). The reconstructed

mass balance is positive for most of the 19th century

corresponding with the period of maximum glacier extent

during the Holocene (Luckman, 2000).

The mean reconstructed Bn for 1673–1994 is negative

(�63.5 mm w.e./yr). Prior to 1883 the balance is variable

but the mean is positive (+70 mm w.e./yr). After 1883 the

mean net loss has been 317 mm w.e./yr (Figs. 3 and 5).

These balance estimates are calculated per unit area of the

glacier surface from late 20th century values; detailed data

on changes in glacier area and hypsometry are needed to

convert these point estimates into absolute changes in

glacier volume. Peyto Glacier had an estimated area of

17.15 km2 in 1897 (Wallace, 1995), decreasing to 13.35

km2 in 1966 and 11.81 km2 in 1993 (Demuth and Keller, in

press). The glacier surface has also downwasted several

hundred meters (Wallace, 1995). Based on these data, a net

winter balance of 1.5 m w.e. on the glacier surface in the

1890s would be at least a 30–40% greater volume gain than

the equivalent balance on the 1990 glacier surface. Esti-

mates of the total volume of accumulation on the glacier

based on these reconstructed winter balances would be

significantly underestimated because they are not corrected

for changes in glacier size. Therefore, the net balances in the

18th and 19th centuries may have been more strongly

positive than the data in Figs. 3 and 5 suggest.

Three pronounced intervals of positive Bn are 1695–

1720, 1810–1825, and ca. 1845–1880. In each of these

Fig. 2. Actual and predicted winter, summer, and net mass balance for Peyto Glacier. The R, adjusted R2, and Durbin–Watson d (DW) statistics are listed for

the winter and summer reconstruction models. The tree-ring data used to develop each model are listed and their beta weights are given in parentheses (see

Table 2 for further details). The correlation between the actual and reconstructed net mass balance series is given in the top right corner of the bottom plot. Note

that the predicted net mass-balance series is calculated as the difference between the winter and summer series and is not a separate reconstruction.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–18 13

intervals, winter accumulation is above the long-term mean

and summer ablation is less than the long-term mean. It

should be noted that the winter balance time series, and

therefore the net series, shows reduced interdecadal swings

after ca. 1850. This has been found in many published

reconstructions of the PDO (Gedalof and Smith, 2001;

D’Arrigo et al., 2001; Villalba et al., 2001). However, as

all of these reconstructions include some temperature-sen-

sitive Gulf of Alaska chronologies, this is not an indepen-

dent verification of this finding. Four major intervals of

predominantly negative mass balance are seen in the record

between ca. 1680–1690s, 1760–1805, part of the 1830s

and 1840s, and most of the 20th century. The interval

1976–1994 is the longest strongly negative period in the

entire record (Fig. 3), averaging �633 mm w.e./yr. and

including the first (1994), third (1979), and fourth (1990)

most negative years in the 322-yr reconstructed record. The

measured Bn for 1998 was –2210 mm w.e., considerably

higher than any reconstructed year (WGMS, 2001).

Relationships between the seasonal mass balance records

Correlations between the reconstructed and measured net

and seasonal balance series over the instrumental period

(1966–1995) are presented in Table 3. Similar calculations

Fig. 3. Winter, summer, and net mass-balance reconstructions (1673–1994) for Peyto Glacier based on tree-ring data. The thick line fitted to each series is a 10-

yr smoothing spline. Note that the tree-ring data used in the winter model and the Athabasca series used in the summer model are considered reliable (i.e., SSS >

0.85) well beyond 1673. However, the short Waterton precipitation reconstruction used in the summer balance model is only considered reliable back to 1709

(Watson, 2002). The means for the full reconstructions over the interval 1673–1994 are delineated by a thick grey horizontal line. The means for the measured

mass-balance records over their full length (1966–1995) are shown using a black horizontal line.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–1814

were made over the entire record using a running 30-yr

period to investigate the changing relationships between the

three balance series (Fig. 4). Over both the instrumental

period (Table 3) and the full reconstruction (Fig. 4), Bn has

a higher correlation with Bs (summer ablation) than Bw

(winter accumulation). This result was expected given that

most other studies indicate that summer temperatures are a

more important control of mass balance at Peyto and other

continental glaciers (e.g., Walters and Meier, 1989; Bitz and

Battisti, 1999). However, Fig. 4 indicates considerable

variation in the relationships between net and seasonal mass

balance over time (e.g., Bs is clearly more important during

the 19th and late 20th centuries, whereas Bw is more highly

correlated with values of Bn for most of the 18th century.

There are other periods (see below) when the series are of

almost equal importance (e.g., around 1700 and the early

1800s).

Over the full reconstruction, Bw and Bs are significantly

correlated (r = 0.23, n = 322, p < 0.05), but more detailed

analysis using a 30-yr window indicates a weaker, nonsig-

nificant relationship for most of the record. The two periods

of significant correlation in the early 18th and 19th centuries

correspond with the major intervals of positive mass balance

(Fig. 3) and subsequent moraine formation (Fig. 5). These

positive correlations reflect periods when wetter winters

(positive Bw) corresponded with cooler summers (i.e., less

Table 3

Correlations between the three measured and three reconstructed mass

balance series (Bn, Bw, and Bs) over the calibration interval (1966–1995)

Measured series Reconstructed series

Bn Bn

Bw 0.67 0.81

Bs 0.78 0.89

Measured series Reconstructed series

Bw-Bs 0.06 0.46

Notes. (1) Bold correlations are statistically significant at or beyond the

0.05 level. (2) Over the period of observed measurement (1965–1995)

correlations between the measured summer and winter balance series are

not statistically significant ( p > 0.05) but correlations between the

reconstructed series are (r = 0.46; p < 0.05). The Miners Well chronology

from Alaska correlates significantly with the Waterton precipitation and

Athabasca temperature reconstructions used in the summer models (r =

0.22 and 0.13 respectively, n = 322), indicating that the predictors used in

the winter and summer models exhibit a weak relationship with each other.

This is perhaps not surprising, as changing low-frequency atmospheric

circulation patterns affect both temperature and precipitation patterns at the

annual scale.

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–18 15

negative ablation). Significant positive correlations also

occur near the end of the 20th century, but in this case drier

winters are associated with warmer summers, resulting in

pronounced recession of the glacier terminus. Correlation

between the two reconstructed seasonal balances over the

measurement period is much stronger (r = 0.46, 1966–

1995; Table 3) than the mean correlation over the entire

reconstruction. The only interval where there is a prolonged

negative relationship (albeit nonsignificant) between Bw

and Bs is during the late 18th century when cooler summers

are associated with average winter precipitation resulting in

Fig. 4. Correlations between the seasonal and net mass-balance series. The corre

year). The dashed horizontal lines denote statistical significance ( p < 0.05). Correla

legend entry.

a series of years where Bn was close to equilibrium (Figs. 3

and 4). These results highlight the importance of both winter

accumulation and summer ablation in determining changes

in the net mass balance of Peyto Glacier. The observed

mass-balance record is too short to demonstrate the full

range of these relationships.

Comparison with the record of historical glacier

fluctuations in the Rockies

Fig. 5 displays the summary record of dated moraines

from 66 glaciers in the Canadian Rockies (Luckman, 2000).

It also shows the reconstructed annual and cumulative mass-

balance changes at Peyto Glacier between 1673 and 1994.

The cumulative data better represent low-frequency changes

in mass balance. The most striking feature of the cumulative

Bn record is the dramatic decrease since the 1880s. This

corresponds quite well with Wallace’s (1995) estimate that

Peyto Glacier has lost 70% of its volume over the past 100

years.

The regional glacier history (Fig. 5) contains two major

periods of glacier advance, one in the early 1700s and the

other in the second quarter of the 19th century (Luckman,

2000). The LIA maximum in this region is thought to have

occurred in the mid-19th century. Eighteenth-century mor-

aines occur immediately downvalley of 19th-century mor-

aines in some forefields but were overridden by the more

extensive 19th-century advance in most cases. In many

glacier forefields several smaller readvance moraines were

formed short distances upvalley of the LIA maximum in the

late 19th and early 20th centuries. During the 20th century,

lations are calculated using a moving 30-year window (plotted at the 15th

tions over the full period (1673–1994) are given in parentheses beside each

Fig. 5. Cumulative reconstructed net mass balance (black line) for Peyto Glacier and the Little Ice Age (LIA) moraine record (25-yr increments) for the

Canadian Rockies. In most cases, the date of the oldest moraine is also the date of the maximum extent of each glacier during the LIA. Moraines of less

extensive readvances are also shown. The diagram summarizes results from 66 glaciers with moraine data based on dendrochronology (48 glaciers) and

lichenometry (18 glaciers; see Luckman, 2000). The bottom diagram shows the net mass balance reconstruction for comparison (for caption see Fig. 3).

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–1816

glacier fronts at most sites have been receding except for a

minor readvance at some sites in the 1970s and 1980s

(Luckman et al., 1987).

The dates for regional moraine-building episodes corre-

spond with major intervals of positive balance in the Peyto

reconstruction (Fig. 5). The two longest and most extreme

periods of positive mass balance immediately precede the

two major moraine-building intervals. Major moraine-build-

ing intervals (those marking the downvalley limits of

glaciers) are associated with, or follow, the change from

positive to negative mass balance. Shorter intervals of

positive balance are associated with mid-18th- and late-

19th-century moraines. Although no evidence has been

reported for an advance at Peyto in the 1970s, the measured

record does confirm several years of positive mass balance,

and other nearby glaciers have built small moraines during

this period (Luckman et al., 1987).

The LIA maximum at Peyto Glacier appears to have been

between ca. 1836 and 1841 based on trees tilted or killed by

the glacier along the north lateral moraine (Luckman, 1996,

in press). In situ overridden stumps indicate that the glacier

advanced to within 200 m of this maximum position by 1768

but there is no evidence for an early 18th-century advance at

this site. Several small moraines upvalley of the terminal

position appear to have been formed between 1880 and 1908,

based on the oldest trees growing on their surfaces and an

assumed 12-year lag between moraine formation and colo-

nization (Heusser, 1956). Other data from the site suggest this

ecesis estimate is too short and these moraines are probably

10–20 years older (Luckman, 1996) and therefore corre-

spond with the several shorter periods of positive mass

balance in the late 19th century (Fig. 5).

Conclusions

The mass balance for Peyto Glacier has been recon-

structed over the period from 1673 to 1994. Summer mass

E. Watson, B.H. Luckman / Quaternary Research 62 (2004) 9–18 17

balance was reconstructed from tree-ring estimates of sum-

mer temperature and precipitation in the Canadian Rockies

but winter-sensitive tree-ring chronologies are not available

from this region. A review of the mesoscale controls of

winter mass balance in the western Americas suggested that

winter balance could be modeled using the documented

teleconnection between winter climate along the Gulf of

Alaska and the Canadian Rockies. Therefore, a tree-ring

chronology from Alaska sensitive to winter and spring

conditions (Villalba et al., 2001) and a precipitation recon-

struction from British Columbia were used to model winter

mass balance for the glacier. The models for winter and

summer balance explain over 40% of the variance in each

measured mass balance series and pass conventional verifi-

cation tests. The resulting net balance is significantly

correlated with the measured net balance figures (r = 0.71,

1966–1994).

Inferences about paleoclimate based on glacier histories

are limited by the biased and truncated nature of glacial

record. Later glacial advances obliterate the evidence of

earlier, less extensive, advances and almost no evidence is

preserved of the periods between successive glacier advan-

ces. For example, in the Canadian Rockies there is no

evidence for glacier positions between the 18th- and 19th-

century advances. The reconstruction presented here sug-

gests that there was not substantial recession and mass loss

over this interval, although the late 1700s was a period of

generally negative mass balances. Our reconstruction dem-

onstrates that the 20th-century recession is exceptional in

the context of the mass-balance history of Peyto Glacier

over the last 300 yr. As Peyto was initially selected as a

regionally representative example of glacier behavior, and

similar justifications may be made for the tree-ring predic-

tors used, we suggest that this mass-balance history is also

regionally representative. The cumulative and net mass-

balance series agree with the independently derived LIA

regional moraine record and show that moraine building

episodes followed periods of reconstructed positive mass

balance. Mass balances are positive over much of the 19th

century, consistent with the dating of the LIA maximum for

many glaciers in the region. Cumulative mass balance is

consistently negative after 1883 and reflects the trend of

increasing summer and winter temperatures in the region.

This reconstructed mass-balance record allows the de-

velopment of a more complete picture of glacier changes

placing the observed mass-balance record in perspective.

During the past 30 yr, the net balance at Peyto Glacier has

been dominated by changes in the winter balance (Demuth

and Keller, in press), apart from isolated extreme years of

high summer ablation (e.g., 1970, Fig. 2; and more recently,

1998, WGMS, 2001). However, the correspondence in

dating between the moraine record and reconstructed sum-

mer temperatures over the past 300 yr suggests that glacier

fluctuations were primarily responding to summer condi-

tions (Luckman, 2000; Wilson and Luckman, 2003). This

apparent contradiction between the relative significance of

precipitation and temperature during the period for which

we have instrumental mass-balance data and the past 300 yr

of temperature reconstructions and moraine records is re-

solved by the longer reconstructed record. This clearly

shows that the relative contribution of summer and winter

balances to net balance varies over time and therefore

glacier fluctuations cannot be interpreted in terms of a

simple, single climatic control. Indeed, the most pronounced

and extended episodes of positive mass balance at Peyto

Glacier reflect periods when higher winter accumulation

coincides with cool summers that reduce ablation.

Acknowledgments

This work was supported by a grant from the Canadian

Foundation for Climate and Atmospheric Sciences. The

dendrochronological work was funded by the Natural

Sciences and Engineering Research Council of Canada

and the Meteorological Service of Canada. We thank Mike

Demuth, National Glaciology Programme of the Geological

Survey of Canada and the National Water Research Institute

for providing the mass balance data; Dan Smith and the

contributors to the ITRDB for providing tree-ring data and

Rob Wilson for the revised Athabasca reconstruction. Fig. 1

was prepared by Patricia Connor of the Cartographic

Section, Geography Department, UWO.

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