Glaciers and Glacier Change in the Sierra Nevada, California
HASSAN J. BASAGIC; ANDREW G. FOUNTAIN
Department of Geography, Portland State University, Portland, Oregon, 97207
Model results
The Sierra Nevada extends over 640 km in eastern California and provides a vital
source of California’s water supply from alpine snow. The range also contains
numerous small high-elevation (~3500 m) alpine glaciers, which act to delay spring
runoff and are sensitive indicators of climate change. While knowledge of glacier
shrinkage is common from repeat photography, there is little quantitative information
on the subject. We present results here addressing three important questions: 1) How
many glaciers exist in the Sierra Nevada and where do glaciers occur; 2) How have
they changed over the past 100 years; and 3) Why have these glaciers changed?
Lyell Glacier
Seven glaciers were selected to quantify the change in glacier extent. Selection of
glaciers was based on the availability of past data and location. These glaciers include
Conness, East Lyell, West Lyell, Darwin, Goddard, Lilliput, and Picket glaciers (lower
left map). We reconstructed the glacier extents based on historic ground-based and
aerial photographs and field measurements. Aerial photographs were scanned and
imported into a GIS. Only late summer photographs, largely snow free, were used in
interpretation of the ice boundary. Aerial photographs were georeferenced to USGS
1944 aerial photograph
orthorectified imagery. The historic glacier extents were interpreted from aerial photographs by tracing the ice
boundary. Early 1900 extents are based on ground-based images and evidence from moraines. To obtain recent
glacier areas, the extent of each glacier was recorded using GPS in 2004. The GPS data was post processed (23m accuracy), and imported into the GIS database. Glacier area were calculated with in the GIS database.
To account for different glacier geometries, the net balance of each lobe (below left) was
multiplied by the glacier area altitude distribution (below right) to provide a mass balance
of -0.51 for the west lobe and -0.60 for the east lobe, indicating that both glaciers are out
of balance. The glaciers would continue shrinking under current climate conditions.
3900
3900
3800
3800
Elevation (m)
Glacier Change
Elevation (m)
Introduction
3700
West Lobe
East Lobe
3600
3700
West lobe
3600
East Lobe
3500
3500
Magnitude and Rate of change
G.K. Gilbert
50%
Conness
Lyell West
East Lyell
Darwin
25%
Goddard
Picket
H. Basagic
Lilliput
0%
1900
Repeat Photography
We gathered historical photos from the United States Geological Survey (USGS) Earth
Science Photographic Archive (http://libraryphoto.er.usgs.gov/) and re-photographed the scenes in
the field from the same vantage point (Harrison 1960, Klett et al 1984). During the
summers of 2003 and 2004, over 50 repeat images were collected from ten glaciers
located throughout the Sierra Nevada. The data serve as a visual comparison of change
through time. A loss in both glacier area and volume can be observed in the results from
Lyell Glacier, Yosemite National Park shown above.
1920
1940
1960
1980
Surface energy balance model
To better understand the role of local topographic controls on these glaciers, we apply a simple surface energy
balance model to the East and West Lobes of Lyell Glacier. The spreadsheet model solves for melt on a monthly
basis at seven individual locations (lower left) along the glacier’s surface based on PRISM climate data (lower
right) and Paterson’s (1994) surface energy balance equation:
Sierra Nevada Glacier
Inventory
250
16
Lyell Glacier
3512m elevation
The equation is made up of radiation and turbulent fluxes terms.
Precipitation (mm)
10
α = albedo values for fresh snow 0.84, Melting snow 0.74, ice 0.40
5
!
(
LW ↓↑ = Longwave = εaσTa4 - εsσTs4
where εa and εs are emissivity of the air, and the surface respectively. εa
accounts both cloudy and clear sky conditions. σ is the Stefan-Boltzmann
constant, and Ta and Ts are air and surface temperatures.
Conness
68
!
(
!
( 7 East
and West Lyell
4
!
(
Darwin
3
!
(
West Lobe
4
2
100
0
East Lobe
Lilliput
Glacier
15
30
2
1
!
(!
(
Picket
J
F
M
A
M
J
J
A
S
O
N
For more info on the glacier inventory visit: http://glaciers.us
Local redistribution of snow by avalanching doubles
precipitation in avalanche runout areas for points 1,4, and 7.
3600
Climate norm
scenario 1
scenario 2
-3
-2
-1
0
1
2
1
0
-170%
-1
-183%
-196%
-202%
Scenario 1
T = + 1.5 °C
P = + 21%
(Leung et al., 2004)
(50 yr trend April 1st SWE at Tioga Pass)
Scenario 2
T = + 1.5 °C
(Leung et al., 2004)
P = no change
-2
West Lobe
East Lobe
The results suggest that the East Lobe is more
sensitive to temperature changes as compared to
the west lobe, likely because of differences in
elevation (above). Glacier area altitude distributions
reconstructed for 2050 using scenario 1 (right). The
East Lobe has practically disappeared, while only
the upper region of the West Lobe remains.
3900
3800
3700
West lobe
East Lobe
3600
3500
0
0.05
0.1
2
Area (km )
Conclusion
-8
All seven glaciers study glaciers decreased in area over the past century ranging between
31% and 78%, an average of 51%. This range is assumed to be similar for the glacier
population as a whole. Our simplified point energy balance model supports this idea. The
modeled net balance for both lobes of Lyell Glacier are negative for the 1971-2000 climatic
norm period, suggesting the glaciers are out of balance with the current climate. Future
climate scenarios suggest the Lyell East Lobe glacier will practically disappear by
2050, while the Lyell West Lobe will continue to retreat. Encouraged by model results, we
plan to apply our energy balance model to a spatial model to more accurately model
topographic effects in other locations. Difficulties in local precipitation distribution need to be
resolved.
The 1971-2000 climate norm modeled PRISM data was used
as climate input for the model. A moist adiabatic lapse rate
was applied to each sample points (0.0065 °C m-1).
Melt estimates from the energy balance model were
compared to the 1967 summer balance data from
nearby Maclure Glacier. After winter snow
accumulation was adjusted to measured data
calculated summer melt matched measured.
data
model
2
1
0
-1
Glacier regions of the Western US
Climate norm
Scenario 1
Scenario 2
3700
-6
D
60
Kilometers
West Lobe
3800
-4
3
Study Site
3900
-2
balance (m WEQ)
!
(
6
0
$
0
8
150
50
QH = sensible heat = ρcpAu(Ta - Ti)
QL = latent heat = 0.622 Au(ea- ei)ρ0 /P0
where ρ is air density, cp is the specific heat capacity of air, A is a bulk
transfer coefficient (0.0015), and u is wind speed taken from nearby Tioga
Pass weather station. (Ta-Ti) and (ea- ei) is difference between air and
surface temperatures and water vapor pressure respectively (vapor pressure
values from Marks et al, 1992). ρ0/P0 is the atmospheric density at standard
pressure.
Goddard
12
200
SW = shortwave (global) calculated using Solar Analyst (Fu and Rich, 1999)
We performed an inventory of all
Sierra Nevada glaciers, perennial
ice, and snowfields to understand
the distribution and quantity of these
features. The GIS database is
based on USGS 7.5 minute
topographic quadrangle maps
(1:24,000 scale). The USGS
created these topographic maps
from aerial photos taken between
1975 and 1984. Our inventory of
glaciers and perennial ice features
throughout the range yields over
800 features with an area greater
than 0.01 km2, yielding a total area
of 34.8 km2. Additionally, we
identified over 800 ice features with
areas smaller than 0.01 km2.
14
0.1
2
Net balance (WEQ m)
2
Qm = SW ↓ (1 − α ) + LW ↓ − LW ↑ + QH + QL
0.05
Area (km )
We ran scenarios for possible climate conditions
in 2050: one with increased temperature (T) and
precipitation (P), and another with only increased
T. The net balance results on both lobes reveal a
large increase of melt (>100%) at lower elevations
only slight increase (+ 5%) in the accumulation
area of scenario 1 (upper right).
2000
Temperature (C)
September 5, 2004
0
2.0
Future scenarios
Balance (m W EQ)
Change in area
75%
1.0
Elevation (m)
August 7, 1903
0.0
Net balance (WEQ m)
Fractional area change (left) is plotted based on each glacier’s 1900 area.
The results indicate a large variation in overall magnitude of change. The
greatest loss in area occurred on the East Lobe of Lyell Glacier with a
loss of 0.17 km2 (-78%). The least change occurred in Lilliput Glacier with
a loss of 0.02 (-30%). Conness, Darwin, Goddard, and Picket glaciers all
lost approximately half of their surface area. Local variation in change is
most evident between the East and West Lobes of Lyell which have lost
78% and 39% respectively. These results suggest that local topography
strongly effects glacier response to changing climate conditions. Steep
headwall cliffs may enhance winter accumulation through avalanching
and reduce summer ablation by shading solar radiation.
100%
-1.0
Elevation (m)
Fractional Area Change
-2.0
3
4
5
6
7
month
8
9
10
References
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