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Impact of Climate Change on North Cascade Alpine Glaciers, and Alpine Runoff

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Analysis of key components of the alpine North Cascade hydrologic system indicate significant changes in glacier mass balance, terminus behavior, alpine snowpack, and alpine streamflow from 1950 to 2005. North Cascade glacier retreat is rapid and ubiquitous. All 47 monitored glaciers are currently undergoing a significant retreat and four of them have disappeared. Annual mass balance measured on ten glaciers, averaging 30–50 m in thickness, yields a mean cumulative annual balance for the 1984–2006 period of −12.4 m water equivalent (m we), a net loss of 14 m in glacier thickness and 20–40% loss of their total volume in two decades. The data indicate broad regional continuity in North Cascades glacial response to climate. The substantial negative annual balances have accompanied significant thinning in the accumulation zone of 75% of North Cascade glaciers monitored. This is indicative of glacier disequilibrium; a glacier in disequilibrium will not survive the current warmer climate trend. Alpine snowpack snow water equivalent (SWE) on April 1 has declined 25% since 1946 at five USDA Snow Course sites. This decline has occurred in spite of a slight increase in winter precipitation. The combination of a decline in winter snowpack and a 0.6° increase in ablation season temperature, during the 1946–2005 period in the North Cascades, has altered alpine streamflow in six North Cascade basins. Observed changes in streamflow are: increased winter streamflow, slightly declining spring streamflow and a 27% decline in summer streamflow. Only in the heavily glaciated Thunder Creek Basin (> 10% glaciated) has summer streamflow declined less than 10%; this is attributable to enhanced glacier melting.
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65Climate Change and Alpine Runoff
Mauri S. Pelto
1
, Nichols College, Dudley, Massachusetts 01571
Impact of Climate Change on North Cascade Alpine Glaciers, and
Alpine Runoff
Abstract
Analysis of key components of the alpine North Cascade hydrologic system indicate significant changes in glacier mass balance,
terminus behavior, alpine snowpack, and alpine streamflow from 1950 to 2005. North Cascade glacier retreat is rapid and ubiquitous.
All 47 monitored glaciers are currently undergoing a significant retreat and four of them have disappeared. Annual mass balance
measured on ten glaciers, averaging 30-50 m in thickness, yields a mean cumulative annual balance for the 1984-2006 period of
-12.4 m water equivalent (m we), a net loss of 14 m in glacier thickness and 20-40% loss of their total volume in two decades.
The data indicate broad regional continuity in North Cascades glacial response to climate. The substantial negative annual
balances have accompanied significant thinning in the accumulation zone of 75% of North Cascade glaciers monitored. This is
indicative of glacier disequilibrium; a glacier in disequilibrium will not survive the current warmer climate trend. Alpine snowpack
snow water equivalent (SWE) on April 1 has declined 25% since 1946 at five USDA Snow Course sites. This decline has occurred
in spite of a slight increase in winter precipitation. The combination of a decline in winter snowpack and a 0.6° increase in abla-
tion season temperature, during the 1946-2005 period in the North Cascades, has altered alpine streamflow in six North Cascade
basins. Observed changes in streamflow are: increased winter streamflow, slightly declining spring streamflow and a 27% decline
in summer streamflow. Only in the heavily glaciated Thunder Creek Basin (> 10% glaciated) has summer streamflow declined
less than 10%; this is attributable to enhanced glacier melting.
1
E-Mail: peltoms@nichols.edu
Introduction
Glaciers have been studied as sensitive indica-
tors of climate for more than a century. Glacier
behavior integrates water and energy balance fac-
tors exhibiting an enhanced climate change signal
(IPCC 1996). The North Cascades, Washington
extend from Snoqualmie Pass (47°
N) north to
the Canadian Border (4 N), and are host to
700 glaciers. These 700 glaciers cover 250 km
2
and yield 800 million m
3
of runoff each summer
(Post et al. 1971). The North Cascade Glacier
Climate Project (NCGCP) was founded in 1983
to monitor glaciers throughout the North Cascade
Range and identify the response of North Cascade
glaciers to regional climate change. Annual balance
measurements on 10 glaciers, periodic terminus
surveys on 47 glaciers, and longitudinal profile
mapping on twelve glaciers over a twenty-three
year period have been completed. This monitoring
effort has enabled the identification of consistent
trends from glacier to glacier.
The North Cascades region experienced a
substantial climate change beginning in 1976,
to generally warmer conditions (Ebbesmeyer
et al. 1991; Mote 2003). A climate warming of
0.6-0.9°C in mean annual temperature has been
observed in the past century in the North Cascades
(Kovanen 2003 and Mote 2003). A resultant de-
cline in winter snowpack, due primarily to rising
winter temperatures in the Pacific Northwest,
has also been observed since 1976; the impact
is most pronounced at lower elevations (Figure
1) (Mote 2003). Several climate models indicate
that the Pacific Northwest will likely experi-
ence a 1.7-2.8°C temperature increase and 1-10
cm increase in winter precipitation during the
early 21st century (Parson 2001). A portion of
this warming has already placed some glaciers
in jeopardy (Pelto and Hedlund 2001), reduced
April 1 winter snowpack, and decreased summer
alpine streamflow.
Glacier mass balance and terminus data from
NCGCP, USDA Snow course, and SNOTEL data
from eight stations, and USGS runoff records from
six basins are utilized in this study to address the
following questions: What is the current condition
of North Cascade glaciers, and can most of the
glaciers reach a new point of equilibrium with
the current and forecasted future climate? How
substantial has the change in alpine winter snow-
pack been? What has been the impact of changes
in glacier annual balance and winter snowpack on
alpine streamflow?
Northwest Science, Vol. 82, No. 1, 2008
© 2008 by the Northwest Scientific Association. All rights reserved.
66
Climate
The North Cascade region has a temperate mari-
time climate. Approximately 80% of the regions
precipitation occurs during the accumulation season
(October-April) when the North Cascades are on the
receiving end of the Pacific storm track. Moisture-
rich Pacific cyclonic systems travel northeasterly
into the region; these fronts are slowed by the Cas-
cades and produce low intensity but frequent and
lengthy orographic precipitation events (Schermer-
horn 1967). Occasionally the warm fronts elevate
temperatures, leading to rainfall and rapid snowmelt
resulting in potential winter season flooding. From
late spring to early fall, high pressure to the west
keeps the Pacific Northwest comparatively dry.
These seasonal variations in climate are related
to changes in large-scale atmospheric circulation
occurring over the Pacific Ocean, including the
Gulf of Alaska.
Climate west of the Cascade Crest is temper-
ate with mild year-round temperatures, abundant
winter precipitation, and dry summers. Average
annual precipitation in the North Cascades typically
exceeds 200 cm west of the divide. Climate east
of the Cascade Crest is more continental, creating
a sharp contrast with the maritime climate west
of the Cascade Crest (Mote 2003).
Two changes in atmospheric circulation have
been noted for the region. In particular, during
winter, southwesterly air flow
has been more prevalent since
1977 (Trenberth 1990; Vaccaro
2002). The increased southwest-
erly flow has produced more fre-
quent warm winter cyclonic air
masses and more frequent win-
ter rain events (Moore and McK-
endry 1995). During summer,
more persistent anti-cyclonic
systems are associated with a
stable high pressure ridge over
the Pacific Northwest and weak
airflow off the Pacific Ocean
(Vaccaro 2002). Both of these
changes reflect the general long
term trend of a positive Pacific
Decadal Oscillation (PDO).
The PDO Index is the leading
principal component of North
Pacific monthly sea surface tem-
perature variability (poleward of
20N for the 1900-93 period) (Mantua et al. 1997).
During the positive PDO phase warm weather is
favored in the Pacific along the northwest coast and
over the Pacific Northwest. During the negative
phase ocean water off the northwest coast is cooler
and cooler temperatures prevail across the Pacific
Northwest (Mantua et al. 1997). In the past century:
“cool” PDO regimes prevailed from 1890-1924
and again from 1947-1976, while “warm” PDO
regimes dominated from 1925-1946 and from
1977 through the present (Mantua et al. 1997). It
was postulated in the late 1990’s that a negative
PDO phase may have been starting, however this
has not transpired (Gedalof and Smith 2001). We
have continued to have a dominantly positive PDO
through 2006, which yields warmer conditions in
the North Cascades
Glacier Mass Balance
Mass balance is a more valuable indicator of cli-
mate than terminus change over short time periods
because mass balance is a direct measure of annual
climate conditions, while terminus behavior is
determined by the cumulative impact of climate
and other glaciologic factors over many years.
Surface mass balance is the difference between
accumulation of water in winter and loss of water
by ablation in summer. It is typically measured on
a water year basis, beginning October 1 and end-
Figure 1. Mean ablation season temperature (June-September) at Diablo Dam (As), and
April 1 snow water equivalent (SWE) at five USDA SNOTEL sites (Rainy
Pass, Stevens Pass, Harts Pass, Miners Ridge and Fish Lake).
Pelto
67Climate Change and Alpine Runoff
ing September 30. Annual balance
is defined as the change observed
annually on a glaciers surface on a
specific date (Mayo et al. 1972).
Since 1984, NCGCP has moni-
tored the annual balance of eight
glaciers, with two additional gla-
ciers added in 1990 (Pelto 1996,
1997; Pelto and Riedel 2001). The
glaciers represent a range of geo-
graphic characteristics and span the
North Cascade Range (Table 1 and
Figure 2). Key geographic variables
are orientation of the glacier, mean
elevation, accumulation sources and
distance to the mountain ranges
watershed and climate divide. NC-
GCP methods emphasize surface
mass balance measurements with
a comparatively high measurement
density on each glacier, consis-
tent measurement methods, and
fixed measurement locations. The
methods are reviewed in detail
by Pelto (1996, 1997) and Pelto
and Riedel (2001). The average
density of measurements NCGCP
utilizes in the accumulation zone of
each glacier ranges from 180-300
points/km-2 (Pelto 1996; Pelto and
Riedel 2001). Measurements are
completed at the same time each
year in late July and again in late
September near the end of the abla-
TABLE 1. The geographic characteristics of the nine glaciers where annual balance has been monitored annually (Figure 2).
Accumulation sources: wind drifting = WD, avalanche accumulation = AV, direct snowfall = DS.
Glacier Aspect Area (km2) Accumulation To Divide Elevelation (m)
Columbia SSE 0.9 DS, DW, AV 15 km west 1750-1450
Daniels E 0.4 DS, WD 1 km east 2230-1970
Easton SSE 2.9 DS 75 km west 2900-1700
Foss NE 0.4 DS At divide 2100-1840
Ice Worm SE 0.1 DS, AV 1 km east 2100-1900
Lower Curtis S 0.8 DS,WD 55 km west 1850-1460
Lynch N 0.7 DS,WD At divide 2200-1950
Rainbow ENE 1.6 DS,AV 70 km west 2040-1310
Sholes N 0.9 DS 70 km west 2070-1630
Yawning N 0.2 DS At divide 2100-1880
Figure 2. Map of North Cascade glaciers indicating the location of glaciers where
mass balance is measured, SNOTEL stations with long term SWE records
and weather stations utilized in this study.
68
tion season. Ablation occurring after the last visit
to the glacier is measured during the subsequent
hydrologic year.
Glaciers in the North Cascades exhibit con-
sistent responses to climate from year to year.
Figure 3 illustrates the closely correlated pattern
of annual mass balance fluctuations. In most
years, all glaciers respond in step with each other
to variations in winter precipitation and summer
temperature. There is a mean annual range of 0.8
m in annual balance from the highest to the lowest
annual balance in a given year, but the inter-annual
trend is the same for each glacier. This regional
response to climate is also indicated by the high
cross correlation values of mass balance between
individual glaciers ranging from r
2
= 0.75 to 0.99
(Table 2). The South Cascade Glacier monitored
by the USGS is included and likewise has a high
correlation coefficient (Krimmel 2001).
The mean annual balance has been -0.54 m/a
for the 1984-2006 period on the eight glaciers
monitored annually (Table 3). The mean cumulative
mass balance loss has been -12.4 m w.e, which is
a minimum of 14.0 m of glacier thickness lost.
North Cascade glacier average thickness ranges
from 30-60 m (Post et al. 1971; Pelto and Hed-
lund 2001). Thus, 20-40% of the volume of these
glaciers has been lost since 1984. Observations
by the USGS at South Cascade Glacier indicate
that since the mid-1950s, South Cascade Glacier
has a cumulative mass balance loss of 25m; mean
annual balance averaged -0.15 m/a from 1956-
1975, and averaged -1.00 m/a from 1976-2003
(Krimmel 2001). The cumulative mass balance is
trending more negatively, indicating that instead
of approaching equilibrium as the glaciers retreat
they are increasingly in disequilibrium with cur-
rent climate (Figure 4).
Terminus Behavior
Since the maximum glacier advance during the
Little Ice Age (LIA) there have been three climate
changes in the North Cascades sufficiently large
to substantially alter glacier terminus behavior.
Figure 3. Annual balance records on ten North Cascade glaciers. South Cascade Glacier mass balance data is from the USGS
(Bidlake, et al., 2007; Krimmel, 2001). In any given year the mean range from the most negative to the most positive
glacier mass balance is 0.8 m. Because the response of each glacier to annual weather conditions is similar the year to
year trend of glacier balance is similar causing the lines to overlap.
Pelto
69Climate Change and Alpine Runoff
TABLE 2. Cross-correlation of annual balance on North Cascade glaciers 1984-2005, except for Easton Glacier 1990-2005.
South Cascade Glacier observed by the USGS, all other glaciers data from NCGCP.
Columbia Daniels Easton Foss Ice Worm Lower Curtis Lynch Rainbow Sholes Yawning
Daniels 0.90
Easton 0.90 0.89
Foss 0.94 0.97 0.93
Ice Worm 0.89 0.95 0.93 0.94
Lower Curtis 0.96 0.95 0.94 0.97 0.94
Lynch 0.93 0.98 0.90 0.98 0.93 0.95
Rainbow 0.96 0.94 0.95 0.97 0.98 0.98 0.96
Sholes 0.94 0.96 0.94 0.98 0.97 0.98 0.97 0.97
Yawning 0.97 0.95 0.94 0.98 0.97 0.98 0.96 0.99 0.98
S.Cascade 0.85 0.84 0.89 0.82 0.87 0.81 0.79 0.81 0.85 0.92
TABLE 3. The annual and mean cumulative annual balance of the 10 North Cascade glaciers examined by the NCGCP and of
the South Cascade Glacier observed by USGS (Bidlake et.al., 2007; Krimmel, 2001).
Glacier Columbia Daniels Easton Foss I. Worm L. Curtis Lynch Rainbow Sholes Yawning SCascade
Year NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP NCGCP
1984 0.21 0.11 0.51 0.86 0.39 0.33 0.58 0.09 0.12
1985 -0.31 -0.51 -0.69 -0.75 -0.16 -0.22 0.04 -0.23 -1.20
1986 -0.20 -0.36 0.12 -0.45 -0.22 -0.07 0.20 -0.10 -0.61
1987 -0.63 -0.87 -0.38 -1.39 -0.56 -0.30 -0.26 -0.47 -2.06
1988 0.14 -0.15 0.23 -0.24 -0.06 0.17 0.43 -0.06 -1.34
1989 -0.09 -0.37 0.09 -0.67 -0.29 0.03 -0.24 -0.19 -0.91
1990 -0.06 -0.68 -0.58 -0.27 -0.92 -0.51 -0.12 -0.46 -0.32 -0.32 -0.11
1991 0.38 -0.07 0.41 0.30 0.63 0.04 0.36 0.44 0.48 0.23 0.07
1992 -1.85 -1.70 -1.67 -1.92 -2.23 -1.76 -1.38 -1.65 -1.88 -2.06 -2.01
1993 -0.90 -0.38 -1.01 -0.73 -1.02 -0.48 -0.62 -0.80 -0.96 -0.66 -1.23
1994 -0.96 -0.45 -0.92 -0.68 -1.23 -0.55 -0.40 -0.72 -0.88 -0.62 -1.60
1995 -0.45 0.24 -0.31 0.31 0.47 -0.21 0.18 -0.20 -0.25 -0.26 -0.69
1996 -0.62 0.45 0.22 0.34 0.57 -0.18 0.53 0.12 0.06 034 0.10
1997 0.35 0.88 0.53 0.50 0.76 0l27 0.62 0.51 0.42 0.50 0.63
1998 -1.46 -1.82 -1.87 -1.95 -1.64 -1.38 -1.97 -1.49 -1.56 -2.03 -1.60
1999 1.75 1.52 1.61 1.56 2.15 1.55 1.45 1.84 1.76 1.63 1.02
2000 0.40 -0.25 -0.10 -0.10 -0.33 -0.25 -0.24 0.15 -0.08 -018 0.38
2001 -1.52 -1.75 -1.93 -1.92 -2.15 -1.88 -1.82 -1.71 -1.83 -1.94 -1.57
2002 0.60 -0.18 0.18 0.10 0.05 0.13 -0.13 0.12 0.21 0.26 0.55
2003 -1.17 -1.52 -0.98 -1.35 -1.40 -1.25 -1.20 -0.98 -1.12 -1.85 -2.10
2004 -1.83 -2.13 -1.06 -1.94 -2.00 -1.51 -1.98 -1.67 -1.86 -1.78 -1.65
2005 -3.21 -2.90 -2.45 -3.12 -2.85 -2.75 -2.62 -2.65 -2.84 -3.02 -2.45
2006 -0.98 -1.25 -0.79 -1.02 -1.35 -1.06 -1.05 -0.61 -0.71 -0.93
70
During the LIA mean annual temperatures were
1.0-1.5°C cooler than at present (Burbank 1981:
Porter 1986). The lower temperatures in the North
Cascades led to a snowline lowering of 100 to 150
m during the LIA (Porter 1986; Burbank 1981).
North Cascade glaciers maintained advanced
terminal positions from 1650-1890, emplacing
one or several terminal moraines.
This first substantial climate change following
the LIA was a progressive temperature rise from
the 1880s to the 1940’s. The warming led to
ubiquitous rapid retreat of North Cascade Range
alpine glaciers from 1890 to 1944 (Rusk 1924;
Burbank 1981; Long 1955; Hubley 1956). The
average terminus retreat of glaciers on Mt. Baker
from their LIA moraines to their 1950 positions was
1440 m. For 38 North Cascade glaciers monitored
across the range the retreat over the same period
was 1215 m (Pelto and Hedlund 2001).
The second substantial change in climate in the
Northern Cascades began in 1944 when conditions
became cooler and precipitation increased (Hubley
1956; Tangborn 1980). Hubley (1956) and Long
(1956) noted that many North Cascade glaciers
began to advance in the early 1950s, following
30 years of rapid retreat. All 11 Mount Baker
glaciers advanced during this 1944-1975 period
(Pelto 1993; Harper 1993).
The third climate change was towards warmer
conditions that reduced April 1 snowpack, led to
glacier retreat and prompted negative mass bal-
ances (Harper 1993; Krimmel 1994; Pelto 1993
and 2001). By 1984, all Mount Baker glaciers,
which were advancing in 1975, were again retreat-
ing (Pelto 1993). The average retreat from 1984
to 2007 of Mount Baker glaciers is 340 m, and
ranges from 290-460 m. The NCGCP had been
monitoring the terminus position of 47 glaciers
from 1984-2007, all have retreated substantially.
By 2004, four had disappeared: David Glacier,
Lewis Glacier, Spider Glacier and Milk Lake
Glacier; in each case no glacier ice mass exceed-
ing 0.01 km
2
remains.
The 38 North Cascade glaciers where the
terminus history has been determined for the
1890-1998 period exhibit one of three distinct
behavior patterns (Pelto and Hedlund 2001): 1)
Retreat from 1890 to 1950 that was followed by
a period of advance from 1950-1976, followed
by another retreat after 1976, 2) Rapid retreat
from 1890 to approximately 1950, followed by
slow retreat or equilibrium from 1950-1976 and
moderate to rapid retreat since 1976, 3) Continu-
ous retreat from 1890 to 2006. Today, regardless
of glacier type rapid retreat is underway.
The time between the onset of a mass balance
change and the onset of a significant change in
terminus behavior is called the initial terminus
response time or reaction time (Ts) (Johannes-
son and others 1989). For the Northern Cascade
glaciers initial observed terminus response time
to climate change is invariably less than 16 years
(Pelto 1993; Hubley 1956; Harper 1993). Thus,
by 1993, the termini of all North Cascade gla-
ciers had begun their initial response to the 1976
climate change. The current climate change is of
sufficient magnitude that equilibrium is not being
approached, even after two decades of retreat.
The glacier margin retreat has been as sig-
nificant at the head of many glaciers as at the
terminus. This indicates thinning in the upper
reaches of the glacier. From 1984-2005 Ice Worm
Glacier has retreated 165 m at its head and 144
m at its terminus. Columbia Glacier has retreated
72 m at its head and 119 m at its terminus from
1984-2005. If a glacier is thinning not just at its
terminus, but also at its head then there is no point
to which the glacier can retreat to achieve equi-
librium (Pelto 2006). That thinning is occurring
in the accumulation zone of the glacier indicates
that it no longer has a substantial consistent ac-
cumulation zone. The accumulation zone is the
region of the glacier that even at the end of summer
melt season still retains snowpack from the winter
season. To be in equilibrium a glacier during the
average year needs approximately 65% of its area
to be in the accumulation zone at the end of the
Figure 4. Cumulative annual balance record of North Cascade
glaciers in meters of water equivalent, the trend is
continuing to become more negative.
Pelto
71Climate Change and Alpine Runoff
summer melt season. If the upper reaches of the
glacier, where the accumulation zone is located,
the glacier is thinning significantly, then it is no
longer consistently an accumulation zone. Of the
12 glaciers where the change in glacier surface
elevation through time has been mapped 9 are
experiencing substantial thinning, at least 10 m
since 1984, in the accumulation zone at the head
of the glacier. Thus, 75% of the North Cascade
glaciers observed are thinning appreciably in
the accumulation zone and are in disequilibrium
with current climate. Foss Glacier (Figure 5) is
an example of a glacier that is in disequilibrium
and rapidly melting away.
Snowpack Changes
The decline in snow pack since 1950 has been
noted throughout the Pacific Northwest (Mote
et al. 2005; Pelto 2006). This has been shown to
be the result of rising temperatures not declin-
ing precipitation (Mote 2003; Mote et al. 2005).
Further, Mote et al. (2005) noted that the snow
water equivalent (SWE) record was well corre-
lated with temperature. Winter season snowfall
is directly measured by the USDA at a series of
SNOTEL sites, and the April 1 SWE provides
an excellent measure of winter season snow ac-
cumulation on glaciers (Pelto 2006). The April 1
SWE record is summarized from five long-term
Snow Course and now SNOTEL stations; Rainy
Pass, Lyman Lake, Stevens Pass, Miners Ridge
and Fish Lake (Table 4). These are the only North
Cascade snowpack records that exist for the entire
interval. The snowpack maximum for most of the
SNOTEL sites is reached near April 1 at most
SNOTEL sites (Pelto 1993; Mote 2004). If March
1 or April 15 data are used the results are similar
for the sites. The April 1 date is also the date for
which longer term snow course measurements
were completed prior to the SNOTEL continu-
ous measurement program started in 1980. In
Figure 1, April 1 mean SWE has declined by 25%
at these stations since 1946, while winter season
precipitation (November-March) has increased
3% at Concrete and at Diablo Dam, the most reli-
able long term weather stations. Kovanen (2003)
noted the strong link in precipitation between
weather stations with a correlation coefficient
of 0.93, indicating the strong regional nature of
the precipitation events.
Mote (2003) in examining 40 stations in the
Washington and British Columbia noted that
Figure 5. Foss Glacier in 1988 and 2005 viewed from the
east. This glacier has lost 40% of its total area
since 1984. This glacier feeds the Skykomish
River. The decreased area results in less summer
glacier runoff released to the river. Note reference
points A and B.
TABLE 4. Location of USDA SNOTEL (Snowpack Telemetry) measurements sites utilized for April 1 SWE data.
Elevation Latitude Longitude Source Mean April 1 SWE Change 1946-2005
Lyman Lake 1805 48 12 120 55 USDA 1.62 m -.37 m
Rainy Pass 1460 48 33 120 43 USDA 1.01 -.29
Stevens Pass 1245 47 44 121 05 USDA 1.20 -.75
Fish Lake 1030 47 31 121 04 USDA 0.89 -.40
Miners Ridge 1890 48 10 120 59 USDA 1.32 -.29
A
B
A
B
72
substantial declines in SWE coincide with sig-
nificant increases in temperature, and occur in
spite of increases in precipitation. Mote (2003)
also identified that the lower elevation SNOTEL
sites generally experienced the most significant
decline in SWE, as would be expected with a
moderate rise in the mean snow line. A key ratio
that can be used to identify the relationship be-
tween the snowpack and precipitation is the ratio
between winter precipitation (November-March)
and April 1 SWE. An increasing ratio indicates a
greater percentage of precipitation is falling and
remaining as snow. A declining ratio indicates
that greater percentages of precipitation occur
as rain instead of snow and/or that melt of winter
snowpack is increasing. The declining ratio is
evident in Figure 6, demonstrating that reduced
April 1 SWE is not due to precipitation decline,
but to reduced accumulation of snowpack and
winter ablation of existing snowpack.
Alpine Streamflow
Glaciers are key sources of alpine summer stream-
flow and a critical water supply source in the
North Cascades (Bach 2002). Comparison of
1963-2003 USGS runoff records for six alpine
North Cascades basins with varying percentages
of glacier cover reveal the impact of the climate
change and changing glacier extent (Table 5). The
seasonal changes in flow, the slope of the linear
trend in discharge, and the percent of change an-
nually for the winter, spring and ablation period
are all noted in Table 5 for six alpine basins in
the North Cascades. The basins have a differing
percentage of glacier cover, causing a difference
in dependence on glacier runoff (Table 5). The
discharge response is consistent with all basins
having increased streamflow during the winter,
declining streamflow in the summer and nearly
unchanged streamflow in the spring. Comparison
of changing streamflow in two adjacent basins,
Thunder Creek and Newhalem Creek, the former
with 14% glacier cover the latter with 0% glacier
cover, allow determination of the impact of glaciers
on streamflow.
Alpine runoff throughout the mountain range
increases in the winter (November-March) as more
frequent rain on snow events enhance melting and
Figure 6. The annual ratio between winter precipitation mea-
sured at Diablo Dam and Concrete and April 1 SWE
at five USDA SNOTEL stations. The decline in the
ratio indicates that a greater percentage of the total
precipitation falling at the low elevations station’s,
is not accumulating in the snowpack at the higher
elevations SNOTEL sites. This can be due to more
rainfall or to greater snowpack melting during the
winter.
TABLE 5. Runoff data from six USGS monitored basins in the North Cascades. Change in flow is in percent per year for the
1963-2003 period. Bold numbers indicate reductions in flow. Winter flow increased in all six basins. Summer flow
decreased in all six basins.
USGS Mean % Winter Spring Summer
Station ID Elevation Glaciation % change/year % change/year % change/year
Thunder 12175500 1755 13.6 0. 50 0. 31 -0. 04
Newhalem 12178100 1320 0.2 0. 43 0. 09 -0. 73
Stehekin 12451000 1561 3.1 0. 27 -0. 06 -0. 37
Nooksack 12210500 1141 2.1 0. 01 -0. 20 -0. 66
NF Nooksack 12205000 1311 6.1 0. 55 0. 01 -0. 52
Skykomish 12134500 1127 0.3 0. 30 -0. 01 -1.11
Mean 0. 344 0. 0233 -0.48
Pelto
73Climate Change and Alpine Runoff
reduce snow storage (Mote 2003). The streamflow
increase rises substantially in Newhalem Creek
(17%), and Thunder Creek (20%), with a mean of
13.8% in all six basins examined. The increased
streamflow occurred in part due to a precipitation
increase of 3% at Diablo Dam, within 5 km of
both basins, but mostly is due to increased winter
snowmelt and increased winter rain (Table 5).
The earlier release of meltwater in the North
Cascades due to warmer spring conditions and
reduced winter snowpack has become more pro-
nounced since 1990 when first documented by
Pelto (1993). This trend is seen throughout the
western United States (Stewart et al. 2004). How-
ever, total spring runoff (April-June) has changed
little in each basin. Earlier snowpack melting
leading to greater contributions to runoff from
higher altitudes, and by reduced contribution
from lower altitudes due to the reduced snowpack
depth, duration and extent. The two factors largely
counteract each other and the net change in spring
streamflow is small. Stewart et al., (2004) noted
an earlier peak runoff ranges from 10-30 days in
the Pacific Northwest.
Summer runoff has decreased markedly, 28%,
in the non-glacier Newhalem Creek due to the
earlier melt and the reduced winter snowpack. In
Thunder Creek runoff has, in contrast, decreased
just 3%. The difference is accounted for in part
by enhanced glacier melting. Glaciers contribute
35-45% of Thunder Creek’s summer streamflow
(Tangborn 1980). The observed net loss of -0.54
m a
-1
in glacier mass spread over the melt season
is equivalent to 2.45 m
3
s
-1
of discharge in Thunder
Creek, which is 9% of the mean summer stream-
flow. The increased glacier melting resulting in
reduced glacier volume has enhanced summer
streamflow. The increased glacier runoff due to
glacier volume loss from 1984-2004 has offset
a significant portion of the summer streamflow
reduction observed in non-glacier basins. At some
point the reduction in glacier area will exceed
the increase in melting per unit area. This results
in glacier runoff declines even with higher melt
rates. For example, Ice Worm Glacier, which is
the headwaters of Hyas Creek, has lost 30% of
its area since 1984, while during the same period
August streamflow has declined 25%. Glaciers
act as natural frozen reservoirs in a watershed. In
the North Cascades these natural reservoirs are
shrinking rapidly, as is the summer runoff they
can provide. The observed seasonal changes in
streamflow are summarized in Figure 7.
There has been a significant change in the
timing of peak spring streamflow events from
alpine streams since 1950. During the period
of glacier equilibrium from 1950-1975 summer
melt events in Thunder Creek accounted for 17
of the 26 highest peak flows. From 1984-2004 the
peak streamflow events dominantly resulted from
winter rain on snow melt events accounting for
eight of the thirteen yearly peak flow events, the
other five occurring in the summer.
Conclusions
The climate drivers for the hydrologic changes
in North Cascade alpine regions are: a 0.6°C
summer temperature rise, a 0.9°C rise in winter
temperature and a minor increase in precipitation
(Mote 2003).
The increase in winter temperature has led to
a 25% decline in April 1 Snow water equivalent
at eight USDA snow course sites since 1946.
This decline has occurred despite an increase in
winter precipitation. A comparison of April SWE
and winter precipitation indicates a decline in this
ratio reflecting warmer conditions, yielding more
rainfall events, leading to more winter melt and
less snowpack accumulation.
The impact of the warmer temperatures and
reduced winter snowpack on North Cascade
glaciers has been an average annual balance of
-0.54 m a
-1
over the past 23 years. The net loss,
-12.4 m w.e., represents a significant portion of
the total glacier volume, 20-40%, that has resulted
in substantial retreat and thinning. The resultant
retreat is ubiquitous, rapid and increasing. There
is no evidence that North Cascade glaciers are
close to equilibrium. Their ongoing thinning
indicates that all of the glaciers will continue to
retreat in the foreseeable future. In cases where
the thinning is substantial along the entire length
of the glacier, then no point of equilibrium can be
achieved with present climate and the glacier is
unlikely to survive (Pelto 2006). Of the 12 North
Cascade glaciers where longitudinal thinning has
been examined, nine are in disequilibrium with
current climate.
North Cascades alpine streamflow has increased
18% in the winter from 1963-2003 due to the
increased snow melt and the extent and frequency
74
of rain events in the alpine zone. Mean spring
streamflow has increased by only 1% in the alpine
basins. Mean summer streamflow declined 20%.
In Thunder Creek Basin with 10% glacier cover,
streamflow declined 3%; it is the only alpine
basin where summer streamflow
did not decline more than 15%. The
smaller reduction in Thunder Creek
discharge is in large part attributable
to the enhanced glacier melting that
occurred. As North Cascade glaciers
continue to retreat and the area avail-
able for melting declines, overall
glacier runoff will decline, providing
less of a buffer during low summer
streamflow. Peak streamflow events
in glacier dominated Thunder Creek
have shifted from summer snowmelt
primarily during the 1950-1975 pe-
riod to winter rain-melt dominated
events 1984-2004. The observed
changes represent significant shifts
in the annual hydrologic cycle in the
alpine zone and alpine fed water-
sheds in the North Cascades.
Continued glacier retreat is in-
evitable, 75% of the North Cascade
glaciers we observe are in disequi-
librium and will melt away during
this century with the current cli-
mate. The loss of glacier area will lead to further
declines in summer runoff in glacier-fed rivers
as the glacier area available for melting in the
summer declines.
Figure 7. Seasonal changes in alpine streamflow in six North Cascade basins.
The consistent, increase in winter streamflow and decrease in summer
streamflow, regardless of basin is evident. The reduction in summer
streamflow is least pronounced in the most heavily glaciated basin,
Thunder Creek.
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Accepted for Publication 3 December 2007
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Annual net balance eight North Cascade glaciers during the 1984-94 period has been determined by measurement of total mass loss firn and ice melt and ice melt and, residual snow depth at the end of the Summer season. Overall spatial density of measurment points is 200 points km ⁻² . Mean annual balance of North Clascade glaciers from 1984 to 1994 has been −0.38 ma ⁻¹ . The resulting 4.2 m loss in water-equivalent thickness is significant, since North Cascade glaciers have an average thickness of 30–50 m. Cross-correlation of annual net balance Ior eight glaciers ranges from 0.83 to 0.97. This indicates the mass balances of the eight glaciers have been responding similarly to elimate conditions despite their range of topographic and geographic characteristics. Annual net balance of individual glaciers was correlated with climate records. The highest ablation-season correlation coefficient is mean May–August temperature, ranging from 0.63 to 0.84. The highest accumulation-season correlation coefficient is total accumulation-season precipitation, ranging from 0.35 to 0.59.
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Two models based on standard observations of precipitation, temperature, and run-off at low-altitude weather and gaging stations have been devised to calculate annual glacier balances in the North Cascades of Washington. The predicted glacier balances of the Thunder Creek basin glaciers, determined by a run-off–precipitation (RP) model during the 1920–74 period, are compared with balances predicted by a precipitation–temperature (PT) model for the same period. Annual balances determined by the PT model are also compared with balances measured by field techniques at South Cascade Glacier since 1958. In the PT model, winter snow accumulation (winter balance) is determined by winter (October–April) precipitation observed at the Snoqualmie Falls weather station. Summer (May–September) ablation (summer balance) on the glaciers is estimated by a technique which utilizes maximum and minimum air temperatures, also observed at Snoqualmie Falls. Ablation calculations incorporate summer cloud cover as a variable by using a relationship between cloud cover and the range in daily maximum and minimum air temperatures. Annual mass changes for the 1884–1974 period in both South Cascade Glacier and the Thunder Creek glaciers were reconstructed by utilizing the PT model. The fluctuations in glacier mass during this period generally agree with historical observations and show that a definite change in glacier activity from marked recession to stability or an advancing state occurred about 1945. During the 1900–45 period, South Cascade Glacier lost mass at a rate of 1.4 m per year and the Thunder Creek glaciers (which are at a higher altitude) at 1.1 m per year. These models suggest that the relationship of glacier mass balance to precipitation and temperature is a very sensitive one. It appears from these studies that a decrease in summer air temperature of just over 0.5 deg or an increase in winter accumulation of slightly more than 10% (350 mm) from the 1920–74 average would cause these glaciers to grow continuously.
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Mass-balance quantities at specific points on a glacier as defined in [IHD] (1970) relate either to annual maxima or minima in ice mass at that point (the stratigraphic system), or to values at the beginning and end of a hydrologic year (the annual or fixed-date system). Most quantities measured in the field relate to summer surfaces, which correspond to the annual minima at the measurement points. When stratigraphic system point values are integrated over a whole glacier, the result may be meaningless because annual maxima and minima and summer surfaces may form at different times at different places. The combined system utilizes several kinds of data to derive meaningful area-average results that can be directly related to other hydrologic and meteorologic information. Measurements to summer surfaces at certain specific times, including the beginning and end of a hydrologic year, are added together with proper recognition of the types of material involved: old firn and ice, snow and superimposed ice of the year under study, new firn formed during that year, and late snow deposited toward the end of the year. Other “balance increment” terms relate values at the beginning and end of a hydrologic year to corresponding area-average balance minima. As a result, two types of “net balance” and many other terms are given precise meaning for a glacier as a whole. The scheme is sufficiently versatile to be used on any glacier, although the terms relating to summer surfaces are not defined on a glacier in which ablation or accumulation is continuous throughout a year.
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Reply to the comments of Meier and others on “Annual net balance of North Cascade glaciers, 1984–94” by Mauri S. Pelto - Volume 43 Issue 143 - Mauri S. Pelto
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Lichenometric studies permit close dating for the timing of stabilization of the late Holocene moraines built by North Mowich, Carbon, Winthrop, Cowlitz, and Ohanapecosh glaciers on Mount Rainier. The moraine chronologies indicate synchronous responses among these glaciers during the past 200 yr. Periods of glacier recession began between 1768-1777, 1823-1830, 1857-1863, 1880-1885, 1902-1903, 1912-1915, and 1923-1924. Since the early 19th century, the mean equilibrium-line altitude has risen about 160 m on Mount Rainier. Minimum ages for earlier glacier variations are based on lichenometric, dendrochronologic, and tephrochronologic data. These data indicate that recessional phases commenced about 1328-1363, 1519-1528, 1552-1576, 1613-1623, 1640-1666, 1690-1695, 1720, and 1750. Whereas the pattern of glacier fluctuations at Mount Rainier agrees with the general chronologic framework of late Holocene variations from many other areas, comparisons of the detailed moraine chronologies from Mount Rainier for the past two centuries with those from Swedish Lapland indicate several differences in the timing of moraine stabilization. These differences imply some nonsynchrony in Northern Hemisphere glacier variations during the late Holocene.
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The terminus positions of six glaciers located on Mount Baker, Washington, were mapped by photogrammetric techniques at 2- to 7-yr intervals for the period 1940-1990. Although the timing varied slightly, each of the glaciers experienced a similar fluctuation sequence consisting of three phases: (1) rapid retreat, beginning prior to 1940 and lasting through the late 1940s to early 1950s; (2) approximately 30 yr of advance, ending in the late 1970s to early 1980s; (3) retreat though 1990. Terminus positions changed by up to 750 m during phases, with the advance phase increasing the lengths of glaciers by 13 to 24%. These fluctuations are well explained by variations in a smoothed time-series of accumulation-season precipitation and ablation-season mean temperature. The study glaciers appear to respond to interannual scale changes in climate within 20 yr or less. The glaciers on Mount Baker have a maritime location and a large percentage of area at high elevation, which may make their termini undergo greater fluctuations in response to climatic changes, especially precipitation variations, than most other glaciers in the North Cascades region.