CHAPTER
27
A new glacier inventory for
the Southern Patagonia Icefield and areal
changes 1986–2000
Gino Casassa, Jose´ Luis Rodrı´guez, and Thomas Loriaux
ABSTRACT
A revised glacier inventory comprising glacier
changes between 1986 and 2000 have been compiled
for the Southern Patagonia Icefield (SPI) based on
Landsat TM and Landsat ETMþ imagery acquired
on January 14, 1986 and October 27, 2000, respectively. Elevation data from the Shuttle Radar
Topography Mission (SRTM) and Advanced
Spaceborne Thermal Emission and Reflection
Radiometer (ASTER) Global Digital Elevation
Model (GDEM) were used to interpret ice divides.
The 1986 ice area of the 48 major SPI glaciers is
11,022 412 km 2 , which represents 85% of the
total SPI area of 13,003 282 km 2 . Our results
agree in general with Aniya et al. (1996), although
there are large differences in the basin limits for a
few glaciers. Area loss of 489 377 km 2 is obtained
for the period 1986–2000 for the whole SPI, of
which 68% corresponds to the 48 major glaciers
(333 106 km 2 ). Major (>5 km 2 ) area loss is
detected in 20 glaciers (268 87 km 2 ), which
accounts for 80% of the total area loss of the major
glaciers between 1986 and 2000. Smaller (<5 km 2 )
but significant area losses have occurred within 17
other glaciers, all of which have retreated more than
100 m. While our new results confirm the general
retreat of the SPI reported earlier (Aniya et al. 1997;
Rignot et al. 2003), we show that 9 glaciers within
the latitudes of 49 48 0 –50 25 0 S had relatively stable
frontal positions between 1986 and 2000, 8 of which
were previously retreating over the period 1944/
1986. Independent evaluation of ice thickness
changes within the SPI (Rignot et al. 2003) show
that significant thinning exists for only 2 of the 9
glaciers with stable fronts (excluding Moreno
Glacier which we regard as stable). The stable
frontal positions of the 13 glaciers might be due
to the recent increase of precipitation in the
central–south sector of the SPI. Although enhanced
precipitation has not yet been detected by observations, it is to be expected based on the intensification of the westerly circulation, as has already been
observed in the Southern Hemisphere since the mid1960s (Marshall, 2003).
27.1
INTRODUCTION
With an area of 13,000 km 2 (Aniya et al. 1996), the
Southern Patagonia Icefield (SPI) is the largest ice
body in the Southern Hemisphere outside Antarctica (Fig. 27.1). As such, identifying the spatiotemporal fluctuations of glaciers contained within
the SPI can greatly assist with paleoclimatic reconstructions in southern midlatitudes (Schwikowski et
al. 2006, Masiokas et al. 2009). Together with the
Northern Patagonia Icefield (NPI) and the Cordillera Darwin Icefield, the SPI was part of the
much larger Patagonian ice sheet that covered the
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A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Figure 27.1. Map of the Southern Patagonia Icefield
(SPI). NPI ¼ Northern Patagonia Icefield.
southwestern portion of South America during the
Last Glacial Maximum (LGM) from 40 to 55 S
(Hulton et al. 2002). The main objective of this
work is to provide a glacier inventory based on a
precise digital elevation model (DEM), which
before this work was not yet available for the SPI.
A detailed glacier inventory is available for the
NPI (Aniya 1988) based on 1:50,000-scale maps of
Chile’s Instituto Geográfico Militar (IGM) published in 1982, which were compiled from aerial
stereo photography acquired in 1974/1975. Aniya
(1988) identified 28 large outlet glaciers covering a
total ice area of 4,020 km 2 as of 1975. Due to lack of
elevation data in parts of the flat ice plateau, Aniya
interpreted ice divides based on trimetrogon and
other aerial photos. The inventory of Aniya was
updated and extended by Rivera et al. (2007) based
on Landsat ETMþ imagery of March 2001 and a
digital elevation model (DEM) generated from
ASTER satellite imagery of September 2001.
Rivera et al. (2007) mapped 48 additional glaciers
covering areas larger than 0.5 km 2 and determined
a total area for the NPI of 3,953 km 2 , which represents an ice loss of 140 km 2 (3.4%) with respect to
its area in 1979 as obtained from a Landsat MSS
image. Although large interannual variability has
been detected in the equilibrium line altitudes
(ELAs) of the NPI (Barcaza et al. 2009), practically
all of the glaciers experienced area loss in the period
1979–2001 (Rivera et al. 2007), although a few
advanced locally within shorter time intervals.
Coincident with this glacier retreat, large surface
thinning has been detected in the ablation areas
of the NPI (Rignot et al. 2003). Rivera et al.
(2007) also found smaller, but significant thinning
occurring in the accumulation areas of the NPI,
which indicates extensive downwasting of the NPI.
For the SPI the best glacier inventory currently
available is that of Aniya et al. (1996), based on
Landsat TM scenes of 1986 and the 1:250,000-scale
maps (Carta Preliminar) of the Chilean Military
Geographical Institute (Instituto Geográfico Militar, IGM). The Carta Preliminar was compiled
originally as preliminary charts at 1:250,000 scale
by the U.S. Air Force (USAF) from trimetrogon
aerial photos of 1944–1945 and published in the
mid-1950s, and is to date the only complete map
series of the SPI presently available. Assuming standard U.S. national map accuracies (1/30 inch for at
least 90% of the points in the horizontal, and one
half of the contour interval in the vertical—which is
250 m for the Carta Preliminar—for at least 90% of
the points), these maps have a horizontal accuracy
of 200 m and a vertical accuracy of 125 m. Large
data gaps exist in the Carta Preliminar within the
relatively featureless interior of the icefield due to
challenges in acquiring stereoscopic image pairs
there. Fortunately, the present availability of
high-quality DEMs, such as those derived from
the SRTM and ASTER GDEM, as well as increasingly more precise geometrically corrected
and orthorectified products now provides improved
databases for developing glacier inventories, particularly for resolving ice divides in low-relief areas.
In addition, there is a need for updating the inventory of 1986 and computing recent glacier variations, especially considering the large glacier
wasting currently observed at the SPI (e.g., Rignot
et al. 2003).
As our contribution to the Global Land Ice
Measurements from Space project (GLIMS), in this
chapter we present a revised glacier inventory for
the SPI. We use the same 1986 Landsat TM scenes
as used by Aniya et al. (1996), in combination with
SRTM and ASTER GDEM data, and a more
recent Landsat ETMþ image from 2000, providing
an updated inventory and analysis of glacier variations for the period 1986–2000.
Regional context 641
27.2
REGIONAL CONTEXT
27.2.1 Geographic setting
The SPI contains a north–south elongated area that
spans a latitudinal range from 48 14 0 S to 51 35 0 S
(Fig. 27.1). The icefield is 370 km long, and its
average center longitude is 73 30 0 W; the minimum width is 6 km (based on 2010 imagery from
Google Earth) and its mean width is 45 km (Casassa
et al. 2002). Ice elevations range from sea level on
the west to 3,607 m on the summit of Volcán
Lautaro, an active volcano (Motoki et al. 2006)
located on the ice divide between Pı́o XI Glacier
and O’Higgins Glacier. The geographic setting of
the SPI is very similar to the smaller NPI located 80
km to the north (Warren and Sugden 1993). The
SPI is composed of a central ice plateau lying within
an altitudinal range of 1,400–2,000 m, located along
the main north–south axis of the Patagonian
Andes. Outlet glaciers flow westward to the Pacific
fjords, while eastern glaciers usually terminate in
proglacial lakes located some 300 to 500 km westward of the Atlantic Ocean. Both the fjords and the
eastern proglacial lakes were formed during the
Pleistocene by erosional processes associated with
the extensive Patagonian Ice Sheet (Hulton et al.
2002). Large crustal uplift of up to as much as 39
mm yr1 has been measured at the SPI, the result
of a combination of recent deglaciation and low
mantle viscosity—the largest glacial isostatic rate
ever recorded globally (Dietrich et al. 2010).
stations around the SPI between 1933 and 1992
(Rosenblüth et al. 1995, 1997). Warming also prevails east of the SPI, with a weak trend observed at
Lago Argentino and a stronger warming rate of
1.4 C observed between 1938 and 1988 at Rı́o
Gallegos (Ibarzabal y Donangelo et al. 1996).
Warming is also reported by Rasmussen et al.
(2007), who detected a temperature increase of
0.5 C between 1960 and 1999 from NCEP/NCAR
reanalysis data at 850 hPa over the SPI.
There is a decreasing precipitation trend in northern Patagonia (41–47 S) which is well correlated
with Antarctic Oscillation (AAO) and Antarctic–
Pacific Oscillation (AAPO) indices (Aravena and
Luckman 2009). However, within the SPI region
there are no clear precipitation trends during the
period 1930–2000 (Aravena and Luckman 2009). A
nonsignificant decreasing precipitation trend was
detected in the period 1935–1990 at Lago Argentino
(50 18 0 S) in the east (Ibarzabal and Donangelo et
al. 1996), while an unusually strong positive trend
has been recorded since 1980 at Faro Evangelistas
(52 40 0 S) in the west (Aravena and Luckman 2009).
Although enhanced precipitation has not yet been
detected by observations in Patagonia, it is to be
expected eventually, based on (1) intensification of
westerly circulation, which has already been
observed in parts of the Southern Hemisphere since
the mid-1960s (Marshall, 2003), and (2) warming of
the sea surface, which globally will evaporate more
water, and warming of the troposphere, which will
carry and then precipitate more water.
27.2.2 Climate
The SPI is affected by westerlies and frontal systems
that occur all year round, with cloudy conditions
prevailing more than 70% of the time, particularly
on the western side (Carrasco et al. 2002).
Westerlies, which are weaker during the winter,
combined with the abrupt Andean mountain belt
produce a strong orographic effect with high precipitation falling to the west of the orogenic divide
that can reach 10 m yr1 and much drier conditions
to the east, which reach only a few hundred millimeters a year (DGA 1987, Ibarzabal y Donangelo
et al. 1996). Mean annual temperature at stations
around the SPI is about 7 C. The west of the SPI
enjoys a more maritime and more continental
climate than the east, where there are larger
seasonal oscillations (Carrasco et al., 2002).
There is a general warming trend observed south
of 45 S in Patagonia of 1.3 to 2.0 C century1 as
inferred from climatic records collected from
27.2.3 Glacier characteristics and changes
The glaciers of the SPI are typically temperate, with
large ablation rates in the lower areas and high
accumulation in the upper areas, and consequently
large ice turnover rates (Warren and Sugden 1993).
Maximum ice velocities have been detected at
calving fronts, with a record high of 50 m day1
measured within a 24 h period at the southern
terminus of Pı́o XI Glacier (Rivera et al. 1997b).
Snow accumulation rates can reach close to 20 m
yr1 in the upper plateau (Shiraiwa et al. 2002,
Kohshima et al. 2007), with ice ablation in the lower
reaches in excess of 10 m yr1 (Skvarca et al. 2010).
In the accumulation area snow redistribution by
wind can be significant, particularly on the mountain tops (Schwikowski et al. 2007). Temperate ice
conditions at depth have been detected, even in the
upper plateau, at elevations of 1,756 m on Tyndall
Glacier (Shiraiwa et al. 2002, Kohshima et al., 2007)
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A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
and at 2,600 m asl on the mountain plateau of Pı́o
XI Glacier (Schwikowski et al. 2007), as evidenced
by the presence of a water table at the firn/ice interface at a depth of 42.5 m for Tyndall (also detected
by means of an airborne ice radar survey, Casassa et
al. 2001) and 50.6 m for Pı́o XI observed during
drilling operations. Such water tables are typical of
temperate glaciers (e.g., Blindow and Thyssen
1986). At higher elevations, within the high summits
of the SPI (>3,000 m), cold-based ice is likely, as
already detected at a saddle of Mt. San Valentı́n at
3,750 m asl on the NPI (Vimeux et al. 2008).
The glacier inventory of the SPI compiled by
Aniya et al. (1996) based on Landsat TM scenes
of January 14, 1986 shows 48 major outlet glaciers,
of which 4 flow to the north, 25 to the west, and 19
to the east. The total ice area for the 48 glaciers as
determined by Aniya et al. (1996) is 11,259 km 2 . By
adding the 1,513 km 2 of ice included in small valley
and cirque glaciers this gives a total ice area of
12,772 km 2 . Further adding to this the 228 km 2
of rock exposures nested within the 48 glaciers, a
total area of 13,000 km 2 was obtained by Aniya et
al. (1996) for the SPI. Only 2 of the 48 outlet
glaciers were found to terminate on land, the rest
were calving in either freshwater lakes or in fjords.
Aniya et al. (1997) considered variation in all 48
outlet glaciers for the period 1944/1945–1985/1986
from aerial photos and satellite imagery, and found
that 39 had retreated, 7 had nearly stationary
fronts, and 2 had advanced (Pı́o XI and Moreno).
Glaciers on the northern half and on the east of the
SPI had retreated more than those on the southern
half and on the west (Aniya et al. 1997). Jorge
Montt and O’Higgins Glaciers have experienced
the largest recession in the SPI. Jorge Montt Glacier
retreated by 19.5 km between 1898 and 2011, opening a long fjord with water depths in excess of 390 m
(Rivera et al. 2012). O’Higgins Glacier retreated by
14.6 km between 1896 and 1995, due to frontal
detachment from a prominent island, which acted
as a pinning point, and subsequent retreat in an
overdeepened fjord lake (Casassa et al. 1997).
Glasser et al. (2011) and Davies and Glasser
(2012) reconstructed the volume loss and recession,
respectively, of the glaciers of the NPI and SPI
based on remote-sensing data and field observations of trimline and terminal moraine positions
since the Little Ice Age maximum in ad 1870, concluding that a marked increase in glacier loss
occurred during the 20th century.
Including bedrock outcrops (nunataks), the SPI
had an area of 13,500 km 2 in 1944/1945, as can be
seen from the 1:250,000-scale Carta Preliminar of
the IGM and trimetrogon aerial photos (Lliboutry
1956). Therefore, by 1986, when its area was 13,000
km 2 (Aniya et al. 1996), it had already lost 500 km 2
of ice if both datasets are to be believed. According
to De Angelis et al. (2007), in 2001 the SPI had an
area of 13,362 km 2 based on a Landsat TM image
of March 12, 2001, which indicates an area increase
of 362 km 2 in 1986–2001; this does not agree with
the measured retreat trend of most glaciers. More
recently, an area of 12,500 km 2 has been reported
for the SPI based on the analysis of high-resolution
ALOS satellite imagery of 2009 (Skvarca et al.
2010), revealing that the area reduction rate for
1986–2009 nearly doubled compared with that of
1944–1986. However, the accuracy of this conclusion about accelerating ice loss depends on the
uncertain accuracy of the differentials of the input
datasets. A more reliable softer conclusion is that
there has been a trend, perhaps accelerating, toward
overall ice area loss from the LIA to 1944/1945 and
from then until 2009.
Ice thickness changes have mostly been measured
on ablation areas based on ground studies (e.g.,
Naruse et al. 1987, Skvarca et al. 1995, Raymond
et al. 2005), photogrammetric analysis of aerial
photos and satellite imagery in combination with
ground studies (Rivera et al. 2005), airborne laser
scanning (Keller et al. 2007), and comparison of
DEMs derived from maps and SRTM data (Rignot
et al. 2003). The results show extensive thinning in
all ablation areas, with the greatest thinning
observed at the glacier termini (a maximum of 28
m yr1 at HPS12 Glacier), whereas accumulation
areas show no significant ice thickness change
(Rignot et al. 2003).
Rignot et al. (2003), based on analysis of SRTM
DEM and older cartography for a sample of Patagonian glaciers, calculated a mean ice volume loss
over the SPI equal to 13.5 0.9 km 3 /yr between
1975 and 2000. Combining also the amount lost
from the NPI, their total ice volume loss rate is
16.7 0.9 km 3 /yr. Converting their ice loss to water
loss, they calculated an equivalent sea level rise
(SLR) due to both icefields of 0.042 0.002 mm/
yr. This ice loss rate includes both an area average
thinning rate near 1.0 0.1 m/yr (a little greater rate
on the SPI and lower on the NPI) and frontal
retreat. Using a smaller sample size for the period
1995–2000, Rignot et al. (2003) also extrapolated
measurements to indicate a rapid increase in ice loss
rate of the SPI, and gave a SLR equivalency of
0.105 0.011 mm/yr for the 1995–2000 period.
Data and methods
There is evidence of mean thinning of 1.9 m yr1
in the upper area (1,320–1,450 m elevation) of
Chico Glacier (Rivera et al. 2005), which suggests
that at least some accumulation areas are also losing mass. Independent verification of large volume
losses in the Patagonian icefields came from analysis of GRACE satellite gravity data, which led to
contribution to sea level rise calculated as 0.078 mm
yr1 between 2002 and 2006 for the collective NPI
and SPI (Chen et al. 2007).
A recent calculation of glacier thickness changes
for the period 2000–2012 has been published for the
whole SPI, based on DEMs derived from 156
ASTER satellite images and a radar penetration
bias–corrected version of the SRTM DEM, showing extensive thinning on the SPI reaching higher
elevations in most cases (Willis et al. 2012). The
calculation of Willis et al. (2012) shows a mass loss
in excess of 20 Gt/yr for the period 2000–2012,
consistent with the results of Ivins et al. (2011),
Jacob et al. (2012), and Gardner et al. (2013), but
larger than the estimate of about 11 Gt/yr of
Floricioiu et al. (2012).
27.3
DATA AND METHODS
27.3.1 Satellite imagery
Obtaining cloud-free imagery of Patagonia is very
difficult due to the frequent overcast conditions.
Two sets of practically cloud-free satellite imagery
for the SPI were available for this study: Landsat
TM images of January 14, 1986 (five bands in
the visible-near infrared spectrum with a spatial
resolution of 30 m were used) and Landsat ETMþ
images of October 27, 2000 (the same five bands as
Landsat TM plus one panchromatic band with a
resolution of 15 m were used). Landsat images
corresponding to the following path/row designations, from north to south, include 231/094, 231/95,
231/96. They were obtained from the U.S. Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center. The images
provided by the USGS were orthorectified and processed to the L1T standard using digital elevation
models that included SRTM data. This resulted in
an estimated accuracy of less than 50 m (Tucker et
al. 2004). Two glaciers (HPS41 and Snowy) are
covered by clouds in the October 27, 2000 imagery.
A Landsat ETMþ image of October 14, 2001 was
instead employed to map those glaciers.
The image from 2000 was acquired in mid-spring,
when there was abundant snow cover in the upper
643
areas, so that glacier area change at high elevation
could not be mapped. Above the transient snowline
in the 2000 image, which was a few hundred meters
below the ELA, glacier margins and rock outcrops
were assumed to correspond to the 1986 image.
Since most change in glacier area occurs in ablation
areas, underestimates of glacier area in 2000 are
presumed to be small.
27.3.2 Glacier delineation
In order to discriminate between land cover classes,
different band arithmetic and index classification
schemes were tested. These included the normalized
difference snow index (NDSI), which is good at
distinguishing snow from soil, rock, and clouds
(Dozier 1989), and the TM4/TM5 band ratio,
which is useful for discriminating snow from ice
(Hall et al. 1987). Visual inspection of the imagery
showed that these classification schemes generally
performed relatively well, but in many places failed
to delineate the true extent of glacier ice and
bedrock outcrops within the icefield. Therefore,
the glacier margins were manually revised over
the entire SPI using 15 m pixel1 pansharpened,
false-color composite images (i.e., ETM bands 5,
4, 2 pansharpened with band 8 for October 27,
2000 images). We applied a 10–20% transparency
to the superimposed image (band 8) in order to
increase the spatial resolution to 15 m whilst preserving multiband information.
27.3.3 Ice divides
Ice divides were mapped for each of the 48 large
outlet glaciers identified by Aniya et al. (1996). For
this purpose the Spatial Analyst Basin Analysis tool
of ArcGIS 9.3 and SRTM version 2 elevation data
(3 arcsec 90 m resolution) were used. The program
was originally designed to analyze water drainage
boundaries (e.g., Fürst and Hörhan 2009). As a first
approximation, for the same surface morphology,
glacier flow direction resembles that of water flow.
This is particularly true for surface ice divides that
coincide with topographic highs (e.g., Waddington
and Marriott 1986). First, voids and sinks in the
DEM are filled automatically for consistent drainage. The program then calculates the direction of
the steepest surface slope and determines the potential direction of water flow. The program finally
delineates individual basins based on the cumulative and converging flow vectors.
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A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Careful visual inspection revealed a few problems
with automatic basin delineation: (i) basins did not
always correspond with the particular outlet glacier
of the 48 large glaciers identified by Aniya et al.
(1996) (e.g., the basin could be divided into smaller
subbasins); (ii) in a few basins close to the icefield
margins, the ice divides were incorrectly delineated
since they were affected by ice-free topography in
the lower reaches (this could be solved by masking
the glacier portion of the DEM using the NDSI
index and the TM4/TM5 band ratio and then correcting visually); and (iii) in two specific cases—
Chico–Viedma–Pı́o XI–O’Higgins, known as Paso
de los Cuatro Glaciares (Lliboutry 1956), and
Bernardo–Témpano–Occidental) basin estimation
differed notably from previous estimations (e.g.,
Aniya et al. 1996, Rignot et al. 2003). The first
two problems were fixed by redrawing the divides
manually by following the SRTM DEM for each
basin. The third problem was more of a challenge in
that the very flat topography in some portions of
the accumulation area made it difficult to precisely
identify the divides. However, they were delineated
with the help of past ground reconnaissance performed by some of the authors at Paso de los
Cuatro Glaciares, aerial photogrammetric studies
at Chico Glacier (Rivera et al. 2005), and those flow
features that could be identified on the Landsat
imagery. Aniya et al. (1996) did not have a precise
DEM available at that time, and had to use
1:250,000-scale preliminary maps. Rignot et al.
(2003) only applied the SRTM DEM and did not
employ field reconnaissance criteria or flow features
from satellite imagery. Delineation of all these
divides will hopefully improve when more precise
DEMs become available in the future.
SRTM data provide elevation values with a
global accuracy better than 9 m (Farr et al. 2007),
but in mountain areas it is possible that local errors
might be larger. The ASTER GDEM (30 m horizontal resolution) was used for independent determination of the ice divides on the SPI, again by
means of the ArcGIS 9.3 Spatial Analyst Basin
Analysis tool. Except for Paso de los Cuatro
Glaciares, no significant discrepancies with SRTM
divides could be found over the SPI. At Bernardo–
Témpano–Occidental the ASTER GDEM indicated a divide slightly higher in elevation for
Témpano Glacier, which was adopted since it was
a closer match to the original divide published earlier (Aniya et al. 1996, Rignot et al. 2003). At Paso
de los Cuatro Glaciares the ice divides obtained
with the ASTER GDEM were notably similar to
earlier published divides (Aniya et al. 1996, Rignot
et al. 2003, Rivera et al. 2005), so they were adopted
in this study. Although the ASTER GDEM has an
estimated vertical accuracy of 20 m (95% confidence, ASTER GDEM Validation Team, 2009),
in this particular basin it apparently performs much
better than the SRTM DEM. The main differences
between both elevation datasets lie in the central
plateau of Paso de los Cuatro Glaciares, between
Chico Glacier and Viedma Glacier. The plateau is
very flat with surface gradients of 0.01–0.02. Had
there been a þ5 m global correction to the ASTER
GDEM (ASTER GDEM Validation Team, 2009)
the result would be an even larger discrepancy
between the SRTM and ASTER GDEM divides
at Bernardo–Témpano–Occidental since it would
move the divide even higher.
27.3.4 Equilibrium line altitudes (ELAs)
ELAs were determined manually based on the mean
elevation of the snow/ice boundary as detected
visually on the 1986 Landsat TM imagery. The
images were acquired on January 14 (mid-summer),
which means that the real ELAs for that year could
be somewhat higher. More accurate ELAs for
Patagonia can be derived if imagery is acquired in
March (Barcaza et al. 2009). Based on ELAs
derived for 1986 the accumulation and ablation
areas were computed, as were the accumulation
area ratios (AARs). In the case of HPS10 Glacier
the ELA zone was obscured by clouds in the 1986
image, so the ELA was assumed to correspond to
the average value of the two neighboring glaciers,
Pio XI to the north and HPS12 to the south. The
Landsat ETMþ imagery of 2000 was acquired in
mid-spring (October 27), which meant that ELAs
could not be derived. ELAs obtained for 1986 were
used to estimate the accumulation and ablation
areas for 2000.
27.3.5 Glacier area errors
Area calculation errors (Ae) for each epoch are
based on pixel sizes of 1 to 2 pixel size for
1986 (30 to 60 m) and 1 to 4 pixel size for
2000 (15 to 60 m) for glacier basin delineation
(Williams et al. 1997, Rivera et al. 2007), which,
multiplied by ice area perimeter length ( p), results
in maximum area errors well below the calculated
areas for each of the 48 major glacier basins:
Ae ¼ n p
ð27:1Þ
Results
where n is delineation (precision) error expressed as
known or estimated precision of point measurements (e.g., 1–4 pixels in our case equates to a range
of precision between 30 and 120 m). Dividing this
calculated maximum area error, Ae, by calculated
glacier area and multiplying by 100 provides the
percentage error for each measured glacier entity.
This error is clearly not a consistent systematic
error as it can vary spatially depending on the type
of surface (i.e., the spectral contrast between adjacent land cover classes). For example, in an interface between clean ice and rock the margin
delineation error is expected to be small (e.g., 1 pixel
or smaller). But the error may be larger for a debriscovered glacier margin (not frequent in the SPI).
Year 2000 errors are larger since, as already stated,
the Landsat ETMþ image was acquired in midspring and the upper areas were snow covered.
Thus, for the whole SPI, which had perimeters of
4,693 and 4,162 km and ice areas of 13,003 and
12,514 km 2 in 1986 and 2000, respectively, the maximum area errors are 282 km 2 for 1986 (2.2%) and
250 km 2 for 2000 (2.0%). The smallest glacier in the
SPI is Snowy (19.1 and 15.5 km 2 of ice and perimeters of 35.0 and 15.5 km in 1986 and 2000,
respectively, Tables 27.1 and 27.2), such that maximum associated area errors are 2.1 km 2 in 1986
(11.0%) and 1.7 km 2 in 2000 (10.6%). Likewise, the
largest glacier is Pı́o XI (ice areas of 1,219.1 and
1,220.6 km 2 , and perimeters of 430.2 and 492.3 km
in 1986 and 2000, respectively), such that maximum
area errors are 25.8 km 2 (2.1%) and 29.5 km 2
(2.4%), respectively.
Note there is a more elaborate discussion of
errors in this type of measurement in Chapter 22
of this book on Mongolian glaciers by Krumveide
et al., who differentiate between accuracy, such as
defined above, and precision (as needed for determination of changes from one year to another, for
instance). For reasons they describe, precision is
generally much better than accuracy in cases where
clean ice boundaries exist, but where debris cover is
extensive, precision is only slightly better than
accuracy.
27.4
RESULTS
27.4.1 Glacier inventory
The glacier inventory included each of the 48 major
SPI glaciers identified by Aniya et al. (1996). All
glaciers within the inner perimeter of the SPI were
645
accounted for in 1986 (Fig. 27.2) and 2000 (Fig.
27.3), as were all rock areas. The boundaries of
the smaller cirque and valley glaciers, not included
in the 48 major glaciers, were determined using the
ArcGIS 9.3 Spatial Analyst Basin Analysis tool as
described earlier, but without applying any detailed
inspections or corrections, so no attempt to provide
a precise inventory of these smaller glaciers is made
here. Small cirque and valley glaciers that were
not contiguous to the SPI were excluded from our
glacier inventory, as was the case with Aniya et al.
(1996).
Total ice area for the SPI in 1986 was
13,003 282 km 2 (perimeter of 4,693 km), of which
11,022 412 km 2 (i.e., 85%) corresponded to the 48
major glaciers (with an associated perimeter of
6,863 km, Table 27.1 and Fig. 27.2). Total rock
exposures accounted for 553 km 2 , 465 km 2 of which
occur within the 48 glaciers. Our total SPI ice area
for 1986 is 234 km 2 larger than the ice area of
12,769 km 2 estimated by Aniya et al. (1996). It
should be noted, however, that basin classification
and corresponding areas for smaller glaciers (other
than the 48 glaciers) in our study is only preliminary
since precise manual boundary corrections were not
performed. Frequency distribution diagrams are
presented for 1986 (Fig. 27.4).
Comparison of the ice areas obtained by Aniya et
al. (1996) with those obtained in this study (Table
27.1) shows large differences for 8 glaciers. Of these,
Jorge Montt, Occidental, and HPS13 appeared with
smaller areas in Aniya et al. (1996), while Ofhidro,
Bernardo, Témpano, and HPS15 were calculated as
having larger areas by Aniya et al. (1996). In the
case of Bernardo, Témpano, and Occidental, the
area differences can be explained by a discrepancy
in estimated ice divides. For example, based on
both the SRTM and ASTER GDEM models, as
mentioned earlier, the divide for Occidental Glacier
was derived in this study to be 5 km to the northwest compared with the estimation of Aniya et al.
(1996).
According to Table 27.1 (1986 areas), the largest
glacier in the SPI, and in the whole of South America, is Pı́o XI Glacier (1,219 km 2 ), and the second
largest is Viedma Glacier which covers 1,054 km 2 ;
this basically agrees with the results of Aniya et al.
(1996). The third largest is O’Higgins Glacier (798
km 2 ), the fourth is Upsala Glacier, the fifth and
sixth in our study are Bernardo (562 km 2 ) and Jorge
Montt (530 km 2 ). Whereas for Aniya et al. (1996)
the third, fourth, fifth, and sixth largest are Upsala,
O’Higgins, Bernardo, and Penguin, respectively. In
646
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Figure 27.2. Landsat TM image mosaic of January 14, 1986 showing associated ice margins. Ice divides are
mainly determined from SRTM elevation and ASTER GDEM data, as explained in the text. Figure can also be viewed
as Online Supplement 27.1.
terms of glacier lengths the longest in 1986 was
Jorge Montt Glacier (73.3 km), followed by Viedma
Glacier (72.9 km), Pı́o XI Glacier (63.0 km), Upsala
Glacier (61.0 km), Bernardo Glacier (56.6 km) and
Témpano Glacier (49.5 km) (Table 27.1), although
by 2000 Viedma had become the longest because of
the major retreat of Jorge Montt (Table 27.2). The
terminus of Occidental Glacier, with an AAR of
0.29 in 2000, extends farther to the west than all
other glaciers in the SPI (74 10 0 W). Lopez et al.
(2010) determined a longer length of 53 km for
Occidental Glacier in 2005, based on an ASTER
image of January 3, 2005, using to all intents and
purposes the same ice divide as determined earlier
by Aniya et al. (1996). In our study the 2000 length
for Occidental Glacier was 46.2 km, which is
shorter than that of Lopez et al. (2010) since, as
stated earlier, the SRTM DEM and manual inter-
Results
647
Figure 27.3. Landsat ETMþ image mosaic of October 27, 2000 with corresponding ice margins. Ice divides are
mainly determined from SRTM elevation and ASTER GDEM data, as in Fig. 27.2. Figure can also be viewed as
Online Supplement 27.2.
pretation using flow features from Landsat imagery
indicates an ice divide that is 5 km to the northwest of the divide interpreted by Aniya et al. (1996).
This shows the importance of deriving accurate ice
divides when determining glacier parameters.
The mean AAR of all 48 glaciers in 1986 was
0.69, which was smaller but similar to the mean
value of 0.75 reported by Aniya et al. (1996). For
13 glaciers the AARs obtained in our study differ
notably (AAR 50.1) from the values estimated by
Aniya et al. (1996). The extreme AAR values found
were 0.28 for Occidental and 0.93 for both Penguin
and Europa (Table 27.1).
HPS41 and Snowy are cloud covered in the
imagery of October 27, 2000, so a Landsat ETMþ
image of October 14, 2001 was used for mapping
the glacier extent instead. An ice area of
12,514 250 km 2 was obtained for the SPI in
648
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Figure 27.4. Frequency diagrams showing area distributions in 1986 for (a) 48 major SPI glaciers which cover
11,022 412 km 2 ; (b), smaller valley and cirque glaciers within the SPI according to a preliminary classification of
335 basins, which cover 1,980 73 km 2 ; and (c) all glaciers of the SPI, covering a total ice area of 13,003 282
km 2 .
2000 (perimeter of 4,162 km), of which 10,689 424
km 2 (85%) was provided by the 48 glaciers (perimeter of 7,069 km, Table 27.2 and Figure 27.3). This
represents an ice area reduction of 4% with respect
to 1986. Rock exposures in 2000 accounted for 371
km 2 , representing a 33% reduction from 1986. This
reduction can be explained by the major retreat of
many glacier fronts and margins which previously
(1986) included rock nunataks, no longer found in
SPI glaciers as of 2000. Another possibility is that
there are more snow-covered outcrops and nunataks in the 2000 image, but they are snow covered at
that time. However, as mentioned earlier, the glacier margins above the transient snowline of the
2000 image were taken from the 1986 image, which
means that underestimation of rock areas due to
snow cover is likely to be minor.
27.4.2 Glacier variations 1986–2000
The total area reduction of the SPI for the period
1986–2000 is 489 377 km 2 . Major (>5 km 2 )
wasting is detected in 20 glaciers (268 87 km 2 ),
which accounts for 80% of total area loss of the
48 glaciers between 1986 and 2000. Small area losses
(<5 km 2 ) occurred within 27 glaciers. As indicated
above, with a glacier area delineation error of 2
pixels for 1986 (60 m) and 4 pixels for 2000 (60 m),
area reduction is significant for only 6 glaciers
(Jorge Montt, Bernardo, Greve, HPS12, Snowy,
and Upsala, see Table 27.2). However, because
significant (>100 m, see below) frontal retreat has
occurred in 37 glaciers, it can be considered that
glacier area reductions are only meaningful in
glaciers that have retreated more than 100 m (Table
27.2). In this regard our area errors are most probably overestimated since they reflect maximum
errors. Moreover, when comparing 1986 and 2000
glacier areas, most of the changes (mainly retreat)
occurred in the lowermost sections of the glaciers,
in the absence of snow cover and where glacier
delineation should encompass smaller errors. The
combined area loss of the 37 glaciers accounts for
303 km 2 , representing 62% of total area loss in the
SPI (489 km 2 ) and 91% of the loss of the major 48
glaciers (333 106 km 2 ).
Only one glacier (Pı́o XI) showed area gain (1.5
km 2 ), which is nonsignificant compared with maximum estimated error (39.2 km 2 ). In much the same
way as the above discussion, area error is probably
overestimated, considering the significant glacier
advance of Pı́o XI as shown later. Pı́o XI Glacier,
Discussion
together with Perito Moreno Glacier (with a minor
area loss of 2.9 km 2 between 1986 and 2000), have
traditionally been regarded as the only 2 large SPI
glaciers that have either been stable (in terms of
stationary frontal position and near-zero ice thickness change) (Moreno) or advancing (Pı́o XI)
during the last few decades (e.g., Warren and Sugden 1993, Aniya et al. 1997).
As already reported by Aniya et al. (1996), only 2
glaciers terminated on land in 1986 (Frı́as and
Bravo, Table 27.1), whereas the other 46 were
calving either into freshwater (30 glaciers) or into
tidewater fjords (16 glaciers) to the west (and to the
north in the case of Jorge Montt). By 2000, some of
the glacier ice close to the front of Frı́as Glacier was
already calving in freshwater, while much of Bravo
Glacier terminated in a proglacial lake.
Maximum error in estimating glacier length
change in the period 1986–2000 can be estimated
as 85 m, based on a 60 m (2 image pixels) maximum
error in 1986 and a 60 m (4 image pixels) in 2000, as
indicated earlier. The present study shows that 37
glaciers underwent significant frontal retreat (i.e.,
>100 m) between 1986 and 2000. Of these, 11
experienced a retreat larger than 1 km, with a maximum of 8.3 km for Jorge Montt, followed by 4.2
km for HPS12, 4.0 km for Upsala, 3.8 m for HPS8,
3.4 km for HPS38, 2.3 km for Bernardo, 2.1 km for
Onelli, 2.1 km for Greve, 1.9 km for Lucı́a, 1.2 km
for Snowy, and 1.1 km for Asia (Table 27.2).
Only 2 glaciers underwent significant advances
(i.e., >100 m) during this period: Pı́o XI (674 m
for the northern front and 210 for the southern
front, 442 m on average) and HPS19 (125 m). Nine
glaciers had fronts that advanced/retreated by less
than 100 m within the period; 6 of them flow to the
west (HPS13, HPS15, Penguin, Europa, HPS28,
and Calvo), while 3 flow east (Moreno, Spegazzini,
and Agassiz); they are all located in the central–
southern portion of the SPI (Figs. 27.2 and 27.3).
Farther north, 3 contiguous east-flowing glaciers,
Bravo, Mellizo Sur, and Oriental (Figs. 27.2 and
27.3), showed relatively small although significant
frontal retreat (i.e., >100 m) and/or area changes
compared with neighboring glaciers.
27.5
DISCUSSION
Our new results on glacier change between 1986 and
2000 confirm reports of general ice recession occurring in the SPI (e.g., Warren and Sugden 1993,
Aniya et al. 1997, Casassa et al. 2000, 2002). Pı́o
649
XI Glacier has been advancing since 1945, with
associated thickening in its ablation area (Rivera
and Casassa 1999), although its terminus has been
relatively stable since 1995 (Rivera et al. 1997a).
Perito Moreno Glacier is also observed to have a
relatively stable terminus position, near-zero ice
thickness change, and damming of Brazo Rico its
frontal lake, which has produced frequent ice ruptures and outburst floods (Stuefer et al. 2007).
In addition to Pı́o XI and Moreno, our study
indicates that 12 of the 48 large outlet glaciers
within the SPI also showed relatively stable frontal
positions and small area changes between 1986 and
2000. These 12 glaciers are HPS13, HPS15, HPS19,
Penguin, Europa, HPS28, Calvo, Spegazzini,
Agassiz, Bravo, Mellizo Sur, and Oriental (Table
27.2). Four of these had stable fronts between 1944
and 1986 (HPS13, HPS15, Calvo, Spegazzini) while
the other 8 had been retreating prior to 1986 (Aniya
et al. 1997) despite now being stable. Therefore, a
total of 14 glaciers had relatively stable termini
between 1986 and 2000, 8 of which flow west while
6 flow east. The latitudinal distribution of these
glaciers covers both the central–southern part
(49 10 0 S–50 40 0 S) and the northeastern part
(48 28 0 –48 39 0 ) of the SPI.
Nine of these 14 glaciers are located in the south–
central part of the SPI. According to Rignot et al.
(2003), one was thinning in the period 1995–2000
(HPS13), 2 were thinning in the period 1975–2000
(Calvo and Moreno), 3 showed no significant
thickness changes in 1995-2000 (HPS15, Penguin,
Europa), and there was no available thinning/thickening data for 3 glaciers (HPS28, Spegazzini, and
Agassiz) (Table 27.2). Moreno Glacier shows recent
(1990–1999) evidence of stability (no thinning or
thickening and a stable frontal position) despite
some thickening in 1999–2002 (Skvarca et al.
2004). So at present it can be considered a stable
glacier. Therefore, it can also be inferred that at
least 3 glaciers were stable (negligible ice thickness
change) over the 1986–2000 period (HPS15, Penguin, Europa), while 3 other glaciers for which we
lack information may also have been stable.
Furthermore, we can consider Moreno a stable
glacier according to recent ice thickness change
observations (Skvarca et al. 2004), although Rignot
et al. (2003) report a small degree of long-term
thinning at the end of the last century. In short,
there are 4 stable glaciers and 3 potentially stable
glaciers within the latitudinal band 49 48 0 –50 25 0 S
which flow both to the west (4 glaciers) and to the
east (3 glaciers). Pı́o XI (49 10’S), which has been
Glacier
Jorge Montt
Ofhidro
Bernardo
Témpano
Occidental
Greve
HPS8
HPS9
Pı́o XI
HPS10
HPS12
HPS13
HPS15
HPS19
Penguin
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Longitude West
( )
73 28 0
73 47 0
73 54 0
73 58 0
74 10 0
73 54 0
73 40 0
73 45 0
73 59 0
73 47 0
73 39 0
73 39 0
73 41 0
73 52 0
73 52 0
Latitude South
( )
48 19 0
48 25 0
48 36 0
48 43 0
48 50 0
48 56 0
49 00 0
49 03 0
49 10 0
49 31 0
49 41 0
49 43 0
49 48 0
50 00 0
50 02 0
Length
37.7
29.1
15.4
24.8
27.1
16.1
63.0
19.3
16.0
50.9
45.9
46.5
50.5
19.8
41.8
(km)
Aspect
NW
W
NW
N-W
S-W
W
W
W
SW
NW-W
W
W
W
NW
N
Calving
T
T
T
T
T
F
T
F
F
F
F
T
F
F
T
F/T/N
ELA
1,083
1,258
982
1,157
1,209
1,128
1,047
1,271
1,100
1,054
968
885
1,083
1,013
877
(m)
Slope
20.1
22.6
21.3
18.1
23.6
32.9
8.5
24.1
21.1
3.8
3.1
1.4
8.7
12.3
15.6
( )
3,172
2,593
2,226
2,728
2,636
2,640
3,554
3,446
1,577
3,554
1,536
2,381
2,332
1,610
2,426
hmax
(m)
43
51
63
63
63
27
17
154
163
154
26
49
78
78
9
hmin
(m)
Total ice area
465.5
167.3
102.8
216.6
181.6
66.2
1,219.1
56.2
46.0
519.2
182.2
292.6
561.7
82.4
529.7
(km 2 )
1986 parameters
Rock outcrops
2.00
1.00
0.60
0.10
6.50
5.00
17.70
0.40
4.70
14.20
0.51
5.66
7.10
0.10
9.10
(km 2 )
Accumulation area
433.3
136.2
88.2
197.4
113.6
40.5
959.8
29.6
23.6
253.5
51.6
184.9
442.5
58.0
359.8
(km 2 )
Ablation area
32.2
31.1
14.6
19.3
68.0
25.7
259.3
26.6
22.4
265.7
130.6
107.7
119.2
24.4
169.9
0.93
0.81
0.86
0.91
0.63
0.61
0.79
0.53
0.51
0.49
0.28
0.63
0.79
0.70
0.68
(km 2 ) (km 2 )
Aniya et al.
(1996)
527
176
174
141
204
61
1,265
55
38
438
244
332
536
116
464
(km 2 )
Ice area
Terminus 2000
AAR
Table 27.1. Glacier inventory and parameters obtained in this study (based on the Landsat TM image of January 14, 1986). Slopes were calculated using
ArcGIS 9.3’s Spatial Analyst Basin Analysis tool. The ice areas and AARs obtained by Aniya et al. (1996) are shown for comparison.
0.96
0.89
0.94
—
0.80
—
0.80
0.52
0.66
0.67
0.25
0.73
0.83
0.79
0.75
(km 2 )
AAR
650
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Europa
Guilardi
HPS28
HPS29
HPS31
Calvo
HPS34
Asia
Amalia
HPS38
HPS41
Snowy
Balmaceda
Tyndall
Pingo
Grey
Dickson
Frı́as
Moreno
Ameghino
Mayo
Spegazzini
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
73 51 0
73 57 0
73 34 0
73 34 0
73 33 0
73 21 0
73 31 0
73 43 0
73 41 0
73 41 0
73 34 0
73 33 0
73 18 0
73 17 0
73 20 0
73 14 0
73 09 0
73 07 0
73 02 0
73 11 0
73 20 0
73 20 0
50 17 0
50 22 0
50 25 0
50 28 0
50 36 0
50 41 0
50 43 0
50 49 0
50 56 0
51 02 0
51 19 0
51 21 0
51 23 0
51 16 0
51 00 0
51 00 0
50 47 0
50 45 0
50 28 0
50 26 0
50 22 0
50 14 0
18.5
16.5
21.2
33.7
12.8
19.0
34.4
12.5
39.7
14.8
9.2
17.7
19.6
24.2
18.6
17.5
13.6
23.6
19.0
14.0
29.8
35.7
E
N-NE
N-E
NE
E
SE
SE
SE
E
E
S
SW
W
W
W
NW
W
S-W
W
W
W
W
F
F
F
F
N
F
F
F
F
F
F
F
F
T
T
T
T
T
T
T
F
T
1,008
1,086
843
1,145
873
832
900
1,068
900
600
1,019
1,014
889
910
1,083
1,029
1,018
1,123
985
1,065
841
938
25.3
39.3
9.9
20.9
18.5
18.8
7.4
18.7
8.9
14.3
25.7
31.4
25.7
21.8
18.5
23.8
33.1
16.2
27.9
29.9
13.0
9.1
2,238
2,469
2,479
2,831
2,765
2,491
2,385
2,002
1,998
1,932
1,642
1,966
2,021
2,152
2,144
2,353
2,388
2,827
2,780
2,347
2,017
2,684
197
183
209
179
254
211
97
170
55
115
47
47
27
14
31
41
55
31
37
37
87
25
139.3
41.4
71.6
258.2
48.5
72.7
262.7
58.1
328.1
68.4
19.1
70.5
51.1
171.8
120.0
156.6
94.5
156.6
86.4
67.0
151.0
391.1
29.90
0.80
1.40
8.10
5.00
7.60
19.30
4.20
11.00
5.30
0.10
7.40
7.80
12.90
2.40
0.70
4.20
1.10
0.90
5.00
5.30
4.20
120.6
24.0
46.2
183.6
27.1
48.4
179.9
38.8
196.5
48.5
6.9
39.5
27.8
132.0
63.5
126.8
81.8
124.4
73.8
53.0
114.8
365.5
18.7
17.4
25.4
74.6
21.4
24.3
82.8
19.3
131.6
19.9
12.2
31.0
23.3
39.8
56.5
29.8
12.7
32.2
12.6
14.0
36.2
25.6
0.87
0.58
0.65
0.71
0.56
0.67
0.68
0.67
0.60
0.71
0.36
0.56
0.54
0.77
0.53
0.81
0.87
0.79
0.85
0.79
0.76
0.93
137
45
76
258
48
71
270
71
331
63
23
71
62
158
133
137
117
161
82
63
148
403
(continued)
0.85
0.62
0.42
0.73
0.62
0.59
0.62
0.79
0.64
0.67
0.48
0.55
0.44
0.80
0.65
0.89
0.97
0.88
0.85
0.75
0.85
0.94
Discussion
651
Onelli
Agassiz
Upsala
Viedma
Chico
O’Higgins
Bravo
Mellizo Sur
Oriental
Pascua
Lucı́a
38
39
40
41
42
43
44
45
46
47
48
Longitude West
( )
73 25 0
73 21 0
73 17 0
73 00 0
73 02 0
73 08 0
73 11 0
73 07 0
73 02 0
73 07 0
73 19 0
Latitude South
( )
50 07 0
50 06 0
49 57 0
49 31 0
49 00 0
48 55 0
48 39 0
49 37 0
48 28 0
48 21 0
48 21 0
Terminus 2000
Length
23.2
23.1
14.1
15.5
23.4
42.5
26.3
64.7
57.6
17.7
12.4
(km)
Aspect
N
N
E
SE
E
E
E
E-S
SE
E
E
Calving
ELA
13,002.8
176.3
93.4
44.1
31.6
118.2
797.9
202.6
1,053.7
796.5
51.6
82.7
(km 2 )
Total SPI
31
190
292
321
292
252
258
252
183
187
187
hmin
(m)
1,980.4
3,009
3,015
3,028
3,011
3,000
3,495
2,677
3,462
3,172
3,002
3,176
hmax
(m)
Total small SPI glaciers
24.0
18.0
22.5
14.6
27.8
6.2
7.0
2.7
9.3
10.0
33.0
( )
Slope
11,022.4
1,121
893
1,139
1,453
1,494
1,126
1,260
1,235
1,046
1,377
1,068
(m)
Total ice area
Total 48 glaciers
F
F
F
F
N
F
F
F
F
F
F
F/T/N
1986 parameters
Rock outcrops
552.8
88.1
464.7
18.50
8.80
7.30
4.10
11.60
41.30
21.90
36.90
66.50
7.30
21.20
(km 2 )
Accumulation area
8,043.4
108.3
62.0
24.9
26.7
76.4
719.1
130.0
703.9
602.6
39.0
54.6
(km 2 )
Ablation area
2,979.1
68.0
31.4
19.2
4.9
41.8
78.8
72.6
349.8
193.9
12.6
28.1
0.69
0.61
0.66
0.56
0.85
0.65
0.90
0.64
0.67
0.76
0.76
0.66
(km 2 ) (km 2 )
AAR
Aniya et al.
(1996)
11,262
200
88
74
37
129
820
275
904
902
50
84
(km 2 )
0.73
0.72
0.66
0.75
0.86
0.76
0.87
0.82
0.58
0.68
0.74
0.62
(km 2 )
F ¼ freshwater calving; T ¼ tidewater calving; N ¼ no calving (front terminating on land); hmax and hmin ¼ maximum and minimum elevation of each basin based on SRTM data.
Glacier
No.
Ice area
Table 27.1 (cont.)
AAR
652
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Discussion
gaining mass in recent decades, is considered a
special and isolated case. Its behavior is probably
due to local factors (e.g., increase in basal sliding
due to enhanced geothermal heat, surge-type
behavior, ice divide migration).
In light of the current atmospheric warming trend
observed in Patagonia (e.g., Ibarzabal y Donangelo
et al. 1996, Rosenblüth et al. 1997, Rasmussen et al.
2007) it is impossible to explain why these 4 glaciers
(and 3 potential others) are relatively stable while
the remaining large outlet glaciers of the SPI are
undergoing fast recession. It is known that glaciers
that have detached from stable pinning points and
started to calve into deep fjords, such as Upsala and
O’Higgins (Skvarca et al. 1995, Casassa et al. 1997),
typically experience rapid retreat until a new stable
position is reached, usually in shallower waters.
In this regard, it is possible that the 4 (and the 3
potential other) stable glaciers could be calving into
shallower waters and thus their frontal position
could be controlled by submarine or sublacustrine
geomorphic factors. However, stable termini would
not explain the slight (but possibly nonsignificant)
thinning observed, at least for 4 of these glaciers for
which data on ice thickness change do exist (HPS15,
Penguin, Europa, Moreno). The stable frontal
positions of the 4 (and 3 potential other) glaciers
mentioned above may be due to a recent increase
in snow precipitation in the SPI within the
latitudinal band 49 48 0 –50 25 0 S. All these glaciers
have relatively large accumulation areas, as
reflected by their AARs (mean of 0.83, Table
27.2), and large surface gradients at their ELAs
(Table 27.2). Large AARs indicate that processes
at work in accumulation areas may largely control
the climatic mass balance of the glacier, which tends
to be positive (Cogley et al. 2011). Large surface
gradients at the ELAs of these glaciers result in only
a small response to rising ELAs due to climate
warming since only a small area of the glacier would
be affected (Furbish and Andrews 1984, Naruse et
al. 1995). Therefore, the role played by solid precipitation in the way these glaciers behave may be
particularly relevant.
Strengthening of the circumpolar vortex coupled
with intensification of westerlies has been reported
in the Southern Hemisphere since the mid-1960s
(e.g., Marshall 2003), quantified as a positive trend
of the Southern Annular Mode (SAM). Since the
precipitation regime in Patagonia is highly influenced by the orographic obstacle of the Andes
(Schneider et al. 2003), any increase in westerlies
should result in an increase in precipitation, par-
653
ticularly on the western side of the Andes. Such a
signal has yet to be measured in Patagonia, but this
could at least partly be due to the lack of surface
monitoring stations, especially on the western margin of the SPI. The highest precipitation rate on the
outskirts of the SPI (7,220 mm yr1 ) has been measured at the Guarello station (50 21 0 , 75 21 0 W),
located 100 km west of the SPI, within the latitudinal band of the relatively stable glaciers described
above, where data are only available between 1949
and 1963 (Aravena and Luckman, 2009). South of
the SPI, at Faro Evangelistas (52 40 0 S) on the
coast, a strong positive trend in precipitation has
been observed, but there is as yet no consensus on
whether this is a real trend (Quintana 2004). A more
detailed study of climate trends in the region is
needed to elucidate this. Longer timescale proxy
records obtained from ice cores could also provide
valuable information in the future (Schwikowski et
al., 2006).
In major sectors of the SPI there is evidence that
ice wasting is on the increase (e.g., Rignot et al.
2003, Lopez et al. 2010), driven at least initially
by regional warming (Rasmussen et al. 2007,
Raymond et al. 2005). The very rapid retreat of
glaciers (e.g., Casassa et al. 1997) and associated
ice thinning reaching values of 28 m yr1 that
have been reported for the SPI (Rignot et al.,
2003) would seem to be the consequence. There
is initial evidence of increased glacial lake
outburst flood (GLOF) activity in Patagonia (e.g.,
Dussaillant et al. 2009) due to enhanced ice melt
and associated increase in the number of glacial
lakes. The recent GLOF event of May 2007
(Casassa et al. 2010) at Témpano Glacier (AAR
of 0.19, Table 27.1) is further evidence that glacier
retreat and enhanced ice melt are the main
contributors to the development of outburst
floods. This can be particularly relevant in glaciers
with small AARs, such as Témpano, where mass
balance is probably dominated by processes
occurring within the ablation area. If ice wasting
continues unabated, the consequences are likely to
impact the human population and infrastructure.
Enhanced glacier melt could also change the
hydrological regime of rivers originating in the
SPI (Casassa et al. 2009), thereby affecting water
resources. There could be problems for human
water consumption, particularly on the drier eastern side of the SPI, and for hydroelectric power
development (Skvarca et al. 2010), which is already
under way at both the eastern and northwestern
margins of the SPI.
Jorge Montt
Ofhidro
Bernardo
Témpano
Occidental
Greve
HPS8
HPS9
Pı́o XI
HPS10
HPS12
HPS13
HPS15
HPS19
Penguin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
43
51
63
63
108
27
12
158
163
154
26
49
78
78
26
hmin
(m)
37.6
29.3
15.8
24.8
22.9
15.9
63.1
18.8
12.2
46.9
44.9
45.9
49.8
18.3
457.7
165.3
102.6
214.9
168.5
64.1
1220.6
53.0
40.5
496.4
176.0
285.5
541.3
77.5
8.5
1.7
0.6
0.9
8.2
3.2
17.7
0.2
1.9
2.1
0.4
7.2
15.8
0.2
9.9
(km)
492.2
(km 2 )
Length
32.9
(km 2 )
Total ice area
2000
Rock outcrops
No. Glacier
Accumulation area
426.2
135.4
88.0
196.4
111.9
39.5
949.5
29.4
23.6
253.5
51.6
184.9
442.5
57.9
368.1
(km 2 )
Ablation area
31.5
29.9
14.7
18.5
56.6
24.7
271.0
23.6
16.9
242.9
124.4
100.6
98.8
19.5
124.1
(km 2 )
0.9
0.8
0.9
0.9
0.7
0.6
0.8
0.6
0.5
0.5
0.3
0.6
0.7
0.7
AAR
1986
0.93
0.81
0.86
0.91
0.63
0.61
0.79
0.53
0.51
0.49
0.28
0.63
0.79
0.70
0.68
P
(m)
L
(m)
709
85
(72)
(125)
100
1.8 7.8
0.2 4.8
2.1 9.4
7.8 17.1
13.0 11.9 4,225
2.1 7.1
147
179
3.2 5.4
1.5 39.2
3,780
5.6 6.2
22.8 14.1 2,119
6.2 9.0
7.2 10.7
20.3 13.9
4.9 5.8
37.5 18.3 8,319 1,780
1986–2000
(km 2 )
Area
change
674
172
601
N
(m)
2,331
NE
(m)
E
(m)
SE
(m)
210
954
953
S
(m)
SW
(m)
Mean glacier length change 1986–2000
432
NW
(m)
Table 27.2. Glacier inventory and parameters (based on the Landsat ETMþ image of October 27, 2000) and glacier variations for 1986–2000.
AAR
595
W
(m)
654
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Europa
Guilardi
HPS28
HPS29
HPS31
Calvo
HPS34
Asia
Amalia
HPS38
HPS41
Snowy
Balmaceda
Tyndall
Pingo
Grey
Dickson
Frı́as
Moreno
Ameghino
Mayo
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
183
212
179
262
211
97
172
55
118
55
47
29
13
31
41
55
31
37
37
87
25
16.1
20.3
33.7
12.0
17.9
31.8
12.4
38.4
14.3
8.2
15.7
16.3
23.9
17.8
17.5
13.6
23.2
17.1
14.0
29.6
35.7
40.9
65.4
255.3
42.9
68.6
254.3
57.4
315.2
60.3
15.5
67.5
47.4
169.5
115.8
151.8
93.4
155.8
83.0
64.7
149.0
387.5
1.1
1.1
9.5
7.5
2.9
20.6
3.2
10.9
5.9
—
7.4
4.2
3.9
3.2
2.3
4.9
3.8
2.1
5.6
3.3
7.4
23.6
42.0
181.0
23.3
47.2
179.2
38.5
196.7
47.8
7.0
39.5
28.1
131.5
63.2
124.8
81.6
121.5
72.0
50.8
114.5
362.3
17.3
23.3
74.2
19.6
21.4
75.1
18.9
118.5
12.6
8.6
28.0
19.2
38.0
52.6
27.0
11.7
34.2
11.0
13.9
34.6
25.3
0.93
0.85
0.79
0.67
0.6
0.71
0.36
0.56
0.54
0.77
0.53
0.81
0.87
0.6
0.6
0.7
0.5
0.7
0.58
0.65
0.71
0.56
0.67
0.70 0.68
0.7
0.6
0.8
0.4
0.6
0.6
0.8
0.5
0.8
0.9
0.8 0.798
0.9
0.8
0.8 0.768
0.9
58
846
418
-38
225
1,052
49
3,407
829
1,181
543
2.3 6.4
3.5 6.4
0.8 9.9
1.1 7.4
4.8 7.5
4.2 7.6
2.3 12.9
3.8 6.2
2.9 8.0
3.5 2.7
8.1 8.2
73
927
2.9 15.8
6.2 8.9
0.5 5.1
378
5.6 7.3
4.1 6.6
8.4 17.9
0.6 6.4
322
174
2.0 11.1
12.9 16.6
16
3.5 15.4
1,234
40
201
705
763
299
1,237
476
2,027
(continued)
376
Discussion
655
O’Higgins
Bravo
Mellizo Sur
Oriental
Pascua
Lucı́a
43
44
45
46
47
48
31
208
313
321
292
252
258
21.2
22.3
13.9
15.2
22.9
41.9
25.3
63.9
170.8
87.7
43.5
31.1
115.9
790.5
197.9
1037.5
748.1
19.8
7.6
1.0
4.1
12.5
34.6
22.6
33.6
74.8
1.8
107.4
60.7
25.0
26.5
75.8
716.9
128.7
696.0
591.0
37.5
50.1
63.4
27.0
18.5
4.6
40.1
73.6
69.1
341.5
157.1
12.3
23.5
4.7
0.6
0.7
0.6
0.9
0.7
0.9
0.7
0.7
0.8
0.8
0.7
1
0.61
0.66
1.10
0.85
0.65
0.9
0.64
0.67
0.76
0.76
0.66
0.87
P
(m)
83
181
214
186
266
157
398
4.7 16.4
7.4 33.3
2.2 12.9
0.5 5.7
0.6 4.6
5.7 8.8
333 106
5.5 18.4 1,919
227
16.2 43.1
48.5 41.1 4,047
1.8 6.5
9.1 11.9 2,129
13.8 14.6 30
1986–2000
(km 2 )
Area
change
P ¼ principal front; L ¼ left front; subsequent letters refer to the geographical orientation of the glacier front.
68.3
12,514.0 370.7
Chico
42
252
52.8
49.8
13.4
120.8
Total SPI
Viedma
41
183
17.7
73.6
13.9
1,824.8
Upsala
40
187
10.3
125.5
(km 2 )
1986
Total small SPI glaciers
Agassiz
39
210
18.6
(km 2 )
Accumulation area
10,689.2 302.4 7,971.1 2,718.1 0.71 0.70
Onelli
38
190
(km 2 )
(km 2 )
Length
(km)
Ablation area
Total 48 glaciers
Spegazzini
37
hmin
(m)
Total ice area
2000
Rock outcrops
No. Glacier
AAR
Table 27.2 (cont.)
AAR
L
(m)
N
(m)
NE
(m)
1,840
E
(m)
SE
(m)
S
(m)
SW
(m)
Mean glacier length change 1986–2000
NW
(m)
W
(m)
656
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
Conclusions
27.6
CONCLUSIONS
A revised glacier inventory has been compiled for
the Southern Patagonia Icefield (SPI) based on
Landsat TM imagery of 1986. The availability of
new elevation datasets and more precise geometric
corrections that can now be applied to satellite
imagery prompted us to revise the earlier inventory
completed by Aniya et al. (1996). Moreover, the
availability of more recent Landsat ETMþ imagery
from 2000 enabled us to assess glacier variation for
the period 1986–2000. The basins of the 48 major
outlet glaciers within the SPI have been delineated
in this study based on SRTM data and the ASTER
GDEM dataset, in addition to flow feature information from Landsat imagery. Ice divides as
interpreted by these two DEMs are generally in
agreement, except for the flat plateau area of Paso
de los Cuatro Glaciares, where there is a large
discrepancy between the divides for Viedma Glacier
and Chico Glacier. The divides inferred from
ASTER GDEM data were finally selected since
they are more consistent with earlier results. This
shows that care must be taken in the interpretation
of ice divides in large icefields and flat plateaus.
The 1986 ice area of the 48 major SPI glaciers
covers 11,022 412 km 2 , which represents 85% of
the total ice area of 13,003 282 km 2 determined
for the SPI. A preliminary basin classification was
also carried out for smaller valley and cirque
glaciers contiguous with the SPI, representing an
area of 1,980 73 km 2 distributed amongst 335
basins. Our results generally agree with those presented earlier by Aniya et al. (1996), although there
are differences in the delineations of a few of the
large outlet glacier basins. Aniya et al. (1996) compiled the first detailed glacier inventory for the SPI
from the same Landsat TM imagery of 1986, but
did not have precise DEMs available for the area
since only preliminary maps at 1:250,000 (Carta
Preliminar) were available at that time.
Glacier variation between 1986 and 2000, based
on Landsat ETMþ imagery, shows significant area
losses for 37 glaciers. Area change for the period
was 303 93 km 2 , which represents 62% of the
total loss for the SPI (489 km 2 ). Twenty of these 37
glaciers showed major (>5 km 2 ) wasting between
1986 and 2000, accounting for 80% of total area
loss of the 48 glaciers. Pı́o XI is the only glacier to
show areal gain in the SPI with its net advance in
the latter part of the 20th century likely the result of
local conditions (e.g., geothermal gradient, surging
behavior).
657
As far as frontal changes are concerned, 37
glaciers showed major retreat (i.e., >100 m), 9
had relatively stable fronts (retreat/advance <100
m), while only 2 advanced during the period: Pı́o XI
(442 m) and HPS19 (125 m). Moreover, whereas 2
of the 48 glaciers in 1986 had ice fronts terminating
on land, by 2000 one of these (Bravo) was entirely
calving into a lake and the other (Frı́as) was partially calving into a lake.
While these new results confirm the general ice
recession reported for the SPI in previous studies,
the new evidence on glacier change reported here
shows that 9 of the 48 large outlet glaciers maintained relatively stable frontal positions and small
area changes between 1986 and 2000. Four of these
9 glaciers had also been retreating between 1944
and 1986. One of the relatively stable glaciers is
Moreno, which is well known for its stable frontal
position and frequent ice damming. We also note
that independent evaluation of ice thickness change
within the SPI (Rignot et al. 2003) indicates that
only 2 of the 9 glaciers that had stable fronts experienced significant thinning (excluding Moreno which
we regard as stable), while 3 showed nonsignificant
thinning in the period 1986–2000, and another 3,
which lack ice thinning/thickening data, might be
stable as well. Of these 9 quasistable glaciers, 6 flow
west and 3 flow east; they are all located within the
latitude range 49 48 0 –50 25 0 S. These 9 glaciers also
have large AARs and steep surface gradients at
their ELAs, so they should be less sensitive to
climate warming observed in the region. Although
it is possible that at least the stable frontal positions
of these 9 glaciers could be controlled by calving
into shallow waters, this by itself would not explain
nonsignificant thinning in at least 3 of these glaciers.
The stable frontal positions of these 9 glaciers might
be due to a recent increase in snow precipitation in
the central–south sector of the SPI. Although
enhanced precipitation has not yet been documented in the region due to a large extent to lack
of established weather sensors, it would be an
expected consequence of westerly intensification,
which has been observed across the Southern
Hemisphere since the mid-1960s (Marshall 2003).
Major and accelerated wasting seen in many
sectors of the SPI is the cause of very rapid glacier
retreat (as described in this work) and associated ice
thinning (as reported by Rignot et al. 2003). The
inevitable consequence is increased GLOF activity
in Patagonia, which is likely to be occurring at the
moment, posing a major risk to local populations
and infrastructure. Enhanced glacier melt might
658
A new glacier inventory for the Southern Patagonia Icefield and areal changes 1986–2000
also result in changes in the hydrological regime of
rivers whose sources are in the SPI, which would
affect water resources for human consumption,
particularly on the drier eastern sectors, and for
hydroelectric power generation at the eastern and
northwestern margins of the SPI.
27.7
ACKNOWLEDGMENTS
This work was partly supported by the Centro de
Estudios Cientı́ficos (CECs). CECs is funded by the
Chilean Government through the Millennium
Science Initiative and the Centers of Excellence
Base Financing Program of CONICYT (Comisión
Nacional de Investigación Cientı́fica y Tecnológica
de Chile). Partial support was provided through
FONDECYT project 1090752. The manuscript
was completed while G.C. was staying at the Institute for Planetary Geodesy, Technological University of Dresden, thanks to an award from the
Humboldt Foundation, Germany. Kerstin Binder
helped with Fig. 27.1. We acknowledge the ice2sea
project, funded by the European Commission’s 7th
Framework Programme through grant number
226375, ice2sea manuscript number 138. ASTER
data courtesy of NASA/GSFC/METI/Japan Space
Systems, the U.S./Japan ASTER Science Team,
and the GLIMS project.
27.8
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