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The aim of this paper is to provide the community with a comprehensive overview of the studies of glaciers in the tropical Andes conducted in recent decades leading to the current status of the glaciers in the context of climate change. In terms of changes in surface area and length, we show that the glacier retreat in the tropical Andes over the last three decades is unprecedented since the maximum extension of the Little Ice Age (LIA, mid-17th-early 18th century). In terms of changes in mass balance, although there have been some sporadic gains on several glaciers, we show that the trend has been quite negative over the past 50 yr, with a mean mass balance deficit for glaciers in the tropical Andes that is slightly more negative than the one computed on a global scale. A break point in the trend appeared in the late 1970s with mean annual mass balance per year decreasing from -0.2 m w.e. in the period 1964-1975 to -0.76 m w.e. in the period 1976-2010. In addition, even if glaciers are currently retreating everywhere in the tropical Andes, it should be noted that this is much more pronounced on small glaciers at low altitudes that do not have a permanent accumulation zone, and which could disappear in the coming years/decades. Monthly mass balance measurements performed in Bolivia, Ecuador and Colombia show that variability of the surface temperature of the Pacific Ocean is the main factor governing variability of the mass balance at the decadal timescale. Precipitation did not display a significant trend in the tropical Andes in the 20th century, and consequently cannot explain the glacier recession. On the other hand, temperature increased at a significant rate of 0.10 °C decade-1 in the last 70 yr. The higher frequency of El Niño events and changes in its spatial and temporal occurrence since the late 1970s together with a warming troposphere over the tropical Andes may thus explain much of the recent dramatic shrinkage of glaciers in this part of the world.
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The Cryosphere, 7, 81–102, 2013
www.the-cryosphere.net/7/81/2013/
doi:10.5194/tc-7-81-2013
© Author(s) 2013. CC Attribution 3.0 License.
The Cryosphere
Current state of glaciers in the tropical Andes: a multi-century
perspective on glacier evolution and climate change
A. Rabatel
1
, B. Francou
2
, A. Soruco
3
, J. Gomez
4
, B. C
´
aceres
5
, J. L. Ceballos
6
, R. Basantes
2,7
, M. Vuille
8
, J.-E. Sicart
2
,
C. Huggel
9
, M. Scheel
9
, Y. Lejeune
10
, Y. Arnaud
2
, M. Collet
2,7
, T. Condom
2
, G. Consoli
2
, V. Favier
1
, V. Jomelli
11
,
R. Galarraga
7
, P. Ginot
1,12
, L. Maisincho
5
, J. Mendoza
13
, M. M
´
en
´
egoz
1
, E. Ramirez
13
, P. Ribstein
14
, W. Suarez
15
,
M. Villacis
7
, and P. Wagnon
2
1
UJF-Grenoble 1/CNRS, Laboratoire de Glaciologie et G
´
eophysique de l’Environnement (LGGE) UMR5183, Grenoble,
38041, France
2
IRD/UJF-Grenoble 1/CNRS/Grenoble-INP, Laboratoire d’
´
etude des Transferts en Hydrologie et Environnement (LTHE)
UMR5564, Grenoble, 38041, France
3
UMSA, IGEMA, Calle 27, Cota Cota, La Paz, Bolivia
4
ANA, UGRH, Huaraz, Peru
5
INAMHI, I
˜
naquito N36-14 y Corea, Quito, Ecuador
6
IDEAM, Carrera 10 N20-30, Bogot
´
a DC, Colombia
7
EPN, DICA, Ladr
´
on de Guevara E11-253, Quito, Ecuador
8
Department of Atmospheric and Environmental Sciences, University at Albany, Albany, NY, USA
9
Department of Geography, University of Zurich, 8057 Z
¨
urich, Switzerland
10
CEN, CNRM-GAME, M
´
et
´
eo-France/CNRS, Saint Martin d’H
`
eres, France
11
UPS-Paris 1/CNRS/UVM-Paris 12, Laboratoire de G
´
eographie Physique (LGP) UMR8591, Meudon, 92195, France
12
IRD/CNRS/IFSTTAR/M
´
et
´
eo-France/UJF-Grenoble 1/Universit
´
e de Savoie/Grenoble-INP, Observatoire des Sciences de
l’Univers Grenoble (OSUG) UMS222, St Martin d’H
`
eres, 38400, France
13
UMSA, IHH, Calle 30, Cota Cota, La Paz, Bolivia
14
UMPC/CNRS/EPHE, Sisyphe UMR7619, Paris, 75252, France
15
SENAMHI, av. Las Palmas s/n, Lima, Peru
Correspondence to: A. Rabatel (rabatel@lgge.obs.ujf-grenoble.fr)
Received: 13 June 2012 – Published in The Cryosphere Discuss.: 16 July 2012
Revised: 28 November 2012 – Accepted: 15 December 2012 – Published: 22 January 2013
Abstract. The aim of this paper is to provide the community
with a comprehensive overview of the studies of glaciers in
the tropical Andes conducted in recent decades leading to
the current status of the glaciers in the context of climate
change. In terms of changes in surface area and length, we
show that the glacier retreat in the tropical Andes over the
last three decades is unprecedented since the maximum ex-
tension of the Little Ice Age (LIA, mid-17th–early 18th cen-
tury). In terms of changes in mass balance, although there
have been some sporadic gains on several glaciers, we show
that the trend has been quite negative over the past 50yr,
with a mean mass balance deficit for glaciers in the tropi-
cal Andes that is slightly more negative than the one com-
puted on a global scale. A break point in the trend ap-
peared in the late 1970s with mean annual mass balance
per year decreasing from 0.2 mw.e. in the period 1964–
1975 to 0.76mw.e. in the period 1976–2010. In addition,
even if glaciers are currently retreating everywhere in the
tropical Andes, it should be noted that this is much more
pronounced on small glaciers at low altitudes that do not
have a permanent accumulation zone, and which could dis-
appear in the coming years/decades. Monthly mass balance
measurements performed in Bolivia, Ecuador and Colombia
show that variability of the surface temperature of the Pacific
Ocean is the main factor governing variability of the mass
balance at the decadal timescale. Precipitation did not dis-
play a significant trend in the tropical Andes in the 20th cen-
tury, and consequently cannot explain the glacier recession.
Published by Copernicus Publications on behalf of the European Geosciences Union.
82 A. Rabatel et al.: Current state of glaciers in the tropical Andes
On the other hand, temperature increased at a significant rate
of 0.10
Cdecade
1
in the last 70 yr. The higher frequency
of El Ni
˜
no events and changes in its spatial and temporal
occurrence since the late 1970s together with a warming tro-
posphere over the tropical Andes may thus explain much of
the recent dramatic shrinkage of glaciers in this part of the
world.
1 Introduction
The tropical Andes are host to more than 99% of all tropical
glaciers (Kaser, 1999) between Peru (71 %), Bolivia (20 %),
Ecuador (4%) and Colombia–Venezuela (4 %). Glacier in-
ventories have been conducted in almost all tropical moun-
tain ranges (Jordan, 1991; Poveda and Pineda, 2009; UGRH,
2010) from 1975 in Bolivia to 2006 in Peru. Based on these
inventories and current rates of retreat documented for a sam-
ple of glaciers in different cordilleras, Francou and Vincent
(2007) estimated the total glacier surface area in the tropical
Andes in the early 2000s to be around 1920km
2
.
The Intergovernmental Panel on Climate Change (IPCC)
pointed to the role of mountain glaciers as key indica-
tors of recent climate change (Lemke et al., 2007). Tropi-
cal glaciers are known to be especially sensitive to climate
change (e.g. Hastenrath, 1994; Kaser and Osmaston, 2002).
Due to the specific climate conditions in the tropical zone,
ablation occurs all year round on the lowest part of the
glaciers, resulting in a short-time response of the position of
the glacier terminus to changes in mass balance, and con-
sequently to changes in climate (e.g. Francou et al., 1995,
2003, 2004; Wagnon et al., 1999). An increase of more than
+4
C at elevations above 4000 m a.s.l. is projected for the
21st century using IPCC scenario A2 (Bradley et al., 2006;
Urrutia and Vuille, 2009). With no change in precipitation,
such a temperature change could lead to a major reduction
in glacial coverage and even to the complete disappearance
of small glaciers, whose upper reaches are located close to
the current equilibrium-line altitude (ELA). This is a serious
concern because a large proportion of the population lives
in arid regions to the west of the Andes (especially in Peru
and Bolivia, where the percentage of glaciers is the highest).
As a consequence, the supply of water from high altitude
glacierized mountain chains is important for agricultural and
domestic consumption as well as for hydropower (Vergara
et al., 2007). This is all the more true since these regions
also exhibit a combination of warm and dry conditions as
a part of the seasonal cycle, with limited seasonal temper-
ature variability and a dry season lasting from May/June to
August/September (Kaser et al., 2010). As a consequence,
mountain glaciers in the tropical Andes act as buffers against
highly seasonal precipitation at times when rainfall is low or
even absent (Vuille et al., 2008a).
To better understand glaciological processes, to link cli-
mate parameters and their variability to glacier mass bal-
ance, and to document current glacier changes, permanent
glacier monitoring networks have been set up in each coun-
try between Colombia and Bolivia. The oldest data series are
available in Peru, where partial surveillance of glaciers be-
gan in the early 1970s. Since the early 1990s, an important
effort has been made by IRD (the French Institute of Re-
search and Development), in association with Andean part-
ners in Bolivia, Ecuador and Peru, as well as other interna-
tional scientific teams such as the University of Innsbruck
(Austria), the Ohio State University (USA) and the Univer-
sity of Zurich (Switzerland). The observation system mainly
consists of measuring the glacier mass balance and surface
energy balance. In parallel, remote-sensing studies have been
performed using aerial photographs and satellite images to
reconstruct changes in the volume, surface area and length
of a large number of glaciers in the area since the middle of
the 20th century. In addition to this permanent monitoring,
considerable effort has been made to reconstruct glacier fluc-
tuations since the Little Ice Age (LIA) maximum across the
tropical Andes (e.g. Rabatel et al., 2005a, 2008a; Jomelli et
al., 2009).
The objective of this review is to provide the scientific
community with a comprehensive overview of studies per-
formed on glaciers in the tropical Andes in recent decades,
which will allow the current status of the glaciers to be deter-
mined. These are important issues to estimate the future be-
havior of glaciers and their impacts on the hydrological func-
tioning of high-altitude glacierized watersheds in coming
decades. The main topics being reviewed are (1) the magni-
tude of glacier changes since the LIA; (2) the glacier changes
since the mid-20th century; (3) the mass balance observa-
tions over the last two decades; and (4) the links of glacier
changes to local/regional climate at different timescales. Re-
search questions addressed also include whether the glacial
retreat of tropical glaciers in recent decades is unprecedent
since the LIA, and whether the glacial recession in the tropi-
cal Andes is related to the observed increase in atmospheric
temperature. Finally, this review brings a new perspective on
the nature of recent decadal glacier retreat, particularly on
the link between mass balance and maximum elevation and
size of the glaciers.
2 General settings and methodologies
2.1 Climate settings
From a climatological point of view, the tropical zone can be
divided into two zones with different characteristics. Troll
(1941) distinguished the inner tropical climate with more
or less continuous precipitation throughout the year and
the outer tropical climate which, when subtropical condi-
tions prevail, is characterized by a dry season from May
The Cryosphere, 7, 81–102, 2013 www.the-cryosphere.net/7/81/2013/
A. Rabatel et al.: Current state of glaciers in the tropical Andes 83
to September, and when tropical conditions prevail, by a
wet season from October to March. Here we consider that
Colombia and Ecuador belong to the inner tropics and Peru
and Bolivia to the outer tropics.
For both inner and outer tropics, the climate is charac-
terized by homogeneous temperature conditions throughout
the year with a slight seasonality of air temperature in the
outer tropics (1
to 2
C higher temperatures during the aus-
tral wet summer in October to March than during the austral
dry winter in May to September). In the tropical zone, inci-
dent solar radiation is also more or less constant throughout
the year, as the seasonality of the extraterrestrial irradiance in
the outer tropics is attenuated by pronounced cloud season-
ality (maximum cloud cover during austral summer). In the
inner tropics, humidity remains almost unchanged through-
out the year, whereas the outer tropics are characterized by
pronounced seasonality of specific humidity, cloud cover and
precipitation. Thus, notable accumulation occurs in the outer
tropics only during the wet season (Kaser, 2001). Precipita-
tion mainly results from an easterly flow of moisture from
the Amazon Basin (e.g. Garreaud et al., 2003). At interan-
nual timescales, the variability of precipitation has been de-
scribed in many studies and there is general agreement that
a significant fraction of this variability is related to the El
Ni
˜
no–Southern Oscillation (ENSO) phenomenon (e.g. Fran-
cou and Pizarro, 1985; Aceituno, 1988; Vuille et al., 2000;
Garreaud and Aceituno, 2001). These studies concluded that
El Ni
˜
no years (warm phase of ENSO) tend to be warm and
dry, while La Ni
˜
na years (ENSO cold phase) are associated
with cold and wet conditions on the Altiplano. However, the
climate characteristics of La Ni
˜
na/El Ni
˜
no are not uniform
across the tropical Andes region. Even at the scale of a coun-
try, the consequences of an El Ni
˜
no event may vary consider-
ably, for instance between the northern coast of Peru and the
southern Peruvian Altiplano region.
With the aim of linking changes in glacier mass balance
with climate variability and atmospheric circulation at a re-
gional to global scale, many recent studies have focused on
variables that are relevant for the glacier energy balance,
such as temperature, precipitation, humidity and convective
cloud cover (Wagnon et al., 1999; Francou et al., 2003, 2004;
Favier et al., 2004a; Sicart et al., 2005; Vuille et al., 2008b;
Salzmann et al., 2012). A common theme in all these studies
is the significant role of the tropical Pacific sea surface tem-
perature (SST) and the ENSO phenomenon in modulating
glacier mass balance at interannual timescales. Other stud-
ies have focused on temperature evolution in the last decades
from NCEP/NCAR reanalysis (Kalnay et al., 1996). Bradley
et al. (2009) showed that this data set is feasible to represent
near-surface temperature trends in the Andes. Nonetheless
it should be kept in mind that reanalysis data consider free-
tropospheric temperature based on a 2.5
resolution. Hence,
actual temperature measurements on or near the glaciers, for
example on Zongo or Antisana glaciers, may show absolute
temperatures that are somewhat higher than reanalysis data.
However, reanalysis temperature data and surface tempera-
tures are significantly correlated, as changes in temperature
are similar at the surface and in the adjacent free air (Hardy
et al., 2003; Bradley et al., 2009).
2.2 Reconstruction of LIA glacier changes
In the early 1980s, Hastenrath (1981) and Clapperton (1983)
had already mentioned that glaciers in the tropical Andes
were much larger during the LIA than today, but the date
of their maximum extent and the stages of their subse-
quent retreat remained highly conjectural. Historical sources
and mining settlements established in the colonial period
(Broggi, 1945) indicate that glaciers advanced considerably
during the 16th–19th centuries, then began to retreat after
1860AD in Peru (Ames and Francou, 1995) and Ecuador
(Hastenrath, 1981). Some authors tried to date the LIA in
the tropical Andes using glacier evidence with
14
C dating
(Gouze et al., 1986; Seltzer, 1992). In Bolivia, Gouze et
al. (1986) suggested 670–280calyrBP as the interval dis-
playing maximum ice extension. In Peru, on the basis of evi-
dence found in the ice core retrieved on the Quelccaya ice
cap, Thompson et al. (1986) assumed that the LIA lasted
from 1500AD to 1900AD. Lichenometry has also been
used to date very well preserved moraines on glacier fore-
lands (see the maps of the Bolivian eastern cordillera by
Jordan (1991), where the main moraine stages are repre-
sented). M
¨
uller (1985) applied this technique for relative
dating in Bolivia, and Rodbell (1992) dated Peruvian LIA
moraines to the period 750–1900AD, but without providing
a detailed chronology of glacier fluctuations during the pe-
riod. New detailed chronologies of glacier fluctuations dur-
ing the LIA concerning the tropical Andes have been pro-
posed in the past decade with systematic measurements of
Rhizocarpon Geographicum sp. made on each moraine in
several proglacial margins in Bolivia (Rabatel et al., 2005a,
2008a), Peru (Solomina et al., 2007; Jomelli et al., 2008),
and Ecuador(Jomelli et al., 2009). A new statistical approach
was developed to process data based on the extreme values
theory, as the largest lichens measured for moraine dating
are extreme values (Cooley et al., 2006; Naveau et al., 2007;
Jomelli et al., 2010).
Glacier length, surface area and ELA for the LIA maxi-
mum and the following moraine stages were reconstructed
using digital elevation models (DEM) on the basis of the
moraines (Rabatel et al., 2006, 2008a; Jomelli et al., 2009).
For five glaciers in Cerro Charquini Massif in Bolivia, Ra-
batel et al. (2006) computed changes in volume between the
most importantmoraine stagesby reconstructingglacier hyp-
sometry.
www.the-cryosphere.net/7/81/2013/ The Cryosphere, 7, 81–102, 2013
84 A. Rabatel et al.: Current state of glaciers in the tropical Andes
2.3 20th centuryobservations:from fieldmeasurements
to remote-sensing studies
2.3.1 Pioneering studies
Unlike mid-latitude glaciers where continuous mass bal-
ance series have been available for five to six decades, field
measurements of mass balance in the tropical Andes were
very scarce before 1990. Data on glacier terminus fluctua-
tions have been available for four glaciers in the Peruvian
Cordillera Blanca since the late 1940s, and since the late
1970s, a few years of mass balance measurements for three
of them (Kaser et al., 1990; Ames and Francou, 1995; Has-
tenrath and Ames, 1995a, b; Ames and Hastenrath, 1996).
2.3.2 Monitoring mass balance in the field
In 1991, a project by the French IRD and Bolivian partners
enabled instrumentation of two glaciers for full permanent
monitoring of their mass balance, hydrological balance, and
surface energy balance (Francou and Ribstein, 1995; Fran-
cou et al., 1995). The same monitoring system was set up
in Ecuador in 1994 (Francou et al., 2000), in Peru in 2003,
and in Colombia starting in 2006 (Table 1 and Fig. 1). This
collaborative effort is now part of a permanent monitor-
ing network called GLACIOCLIM (www-lgge.ujf-grenoble.
fr/ServiceObs/index.htm), and a joint international project
called GREAT ICE, involving academic and research insti-
tutions in France, Bolivia, Ecuador, Peru and Colombia. In
addition, two of the glaciers that belong to this monitoring
network, Zongo in Bolivia and Antisana 15 in Ecuador, are
among the benchmark glaciers in the tropics referenced by
the World Glacier Monitoring Service (WGMS, 2011).
Glacier mass balance is computed using the glaciologi-
cal method (Paterson, 1994). In the lower part of the glacier,
monthly measurements (in Bolivia, Ecuador and Colombia)
of stake emergence are made using a network of 10 to 25
stakes (depending on the glacier). Snow height and density
measurements are required as well as stake emergence mea-
surements because snowfall can occur at the glacier surface
at any time during the year. In the upper part of the glacier,
net accumulation (snow height and density) is measured at
the end of the hydrological year at two to four locations. To
compute the annual mass balance ofthe glaciers, glacier hyp-
sometry is calculated using a DEM computed by aerial pho-
togrammetry (Bolivia and Ecuador) or using maps from the
National Geographical Institute (Peru, Colombia).
2.3.3 Surface energy balance: measurements and
modeling
Climate controls glacier mass balance through energy and
mass fluxes at the ice or snow surface. The energy avail-
able for melt can be calculated as the residual of the en-
ergy balance equation, whose main terms on temperate trop-
ical glaciers are short-wave and long-wave radiation fluxes
and the turbulent fluxes of sensible and latent heat. Radiation
fluxes on glaciers can be accurately measured with radiome-
ters, whereas turbulent fluxes are generally derived from
aerodynamic profile methods with one or two levels of wind,
temperature and humidity measurements. These methods are
not very accurate and require parameters such as roughness
lengths or eddy diffusivity coefficients. Measurements of en-
ergy fluxes on tropical glaciers began in the 1960s but are
still relatively rare (e.g. Platt, 1966; Hastenrath, 1978; Hardy
et al., 1998). In 1995, automated weather stations began to
be used to monitor surface energy fluxes in the ablation area
of Zongo Glacier, in Bolivia, and Antisana 15 Glacier, in
Ecuador (Wagnon et al., 1999; Favier et al., 2004b).
The interpretation of point-scale energy flux measure-
ments can lead to erroneous generalizations of melt charac-
teristics when they are extrapolated to the whole glacier. For
example, albedo is highly variable near the snow line, so that
the contribution of solar radiation to melt energy depends on
the location of the weather station. A distributed energy bal-
ance model is thus required to investigate the link between
atmospheric forcing and the total glacier mass balance and
to quantify the contribution of glacier melt to water resources
downstream. With the objective of investigating seasonal cli-
mate forcing on the mass balance and meltwater discharge of
tropical glaciers, Sicart et al. (2011) applied the spatially dis-
tributed energy balance model of Hock and Holmgren (2005)
to the Bolivian Zongo Glacier at an hourly time step for an
entire hydrological year. The model calculates the surface
energy fluxes for each glacier grid cell from measurements
collected at a weather station located in the ablation area. It
is based on equations of mass and energy conservation, and
the parameters theoretically have a physical interpretation
(Beven, 1989), so they can be linked to measurable physi-
cal quantities. The model had to be adjusted to tropical high
mountains mainly for the calculations of albedo, due to the
frequent alternation of melt and snowfall periods during the
wet season, and of long-wave incoming radiation, due to the
pronounced seasonality of sky emission.
2.3.4 Contribution of remote sensing
To complete glaciological data time series in terms of
changes in surface area and volume before the beginning of
field measurements, and to calculate these changes at a re-
gional scale, remote-sensing techniques have proved to be
very efficient. Brecher and Thompson (1993) used terres-
trial photogrammetry to quantify the retreat of Qori Kalis
Glacier (Quelccaya ice cap, Peru). Aerial photographs (avail-
able since the 1950s) and satellite images (available since
the late 1970s) have been widely used for glacier invento-
ries (Jordan, 1991; Georges, 2004; Silverio and Jaquet, 2005;
Jordan et al., 2005; Morris et al., 2006; Raup et al., 2007;
Racoviteanu et al., 2007; Poveda and Pineda, 2009; UGRH,
2010) and to quantify variations in glacier surface area at a
decadal to interannual timescale since the mid-20th century
The Cryosphere, 7, 81–102, 2013 www.the-cryosphere.net/7/81/2013/
A. Rabatel et al.: Current state of glaciers in the tropical Andes 85
Table 1. Characteristics of the glaciers monitored and date of the beginning of the observations.
Zongo Chacaltaya Charquini Artesonraju Yanamarey Antisana Los La
Sur 15 Crespos Conejeras
a
Location 16
15
0
S 16
21
0
S 16
17
0
S 8
57
0
S 9
39
0
S 0
29
0
S 0
29
0
S 4
48
0
N
68
10
0
W 68
07
0
W 68
09
0
W 77
27
0
W 77
16
0
W 78
9
0
W 78
9
0
W 75
22
0
W
Surface area (km
2
) 1.94 0.32 5.39
b
0.60
b
0.63 1.71 0.22
Max. elevation (m a.s.l.) 6000 5396 5300 5979 5200 5760 5760 4960
Min. elevation (m a.s.l.) 4900 4985 4685 4725 4780 4680 4720
Aspect SE S S WSW SW NW SW NW
First year of mass-balance survey 1991
c
1991–2009
c
2002
c
2003
c
1971
c
1995
c
2005
c
2006
d
a
La Conejeras Glacier is located on Nevado Santa Isabel (Fig. 1).
b
Surface area in 2006.
c
Great Ice.
d
IDEAM.
52
Figure 1. a): Glaciers monitored in the tropical Andes. Glaciers with long-term mass balance series 1225
(small red hexagons) are labeled (see Table 1 for details). For these glaciers length (yellow cube) 1226
and area (blue circle) records also exist. The large red hexagon depicts a sample of 21 glaciers in 1227
the Cordillera Real, Bolivia, for which mass balance reconstructions are available from 1963 to 1228
2006 (Soruco et al., 2009a). Other glaciers whose changes in length and/or in area are monitored are 1229
numbered. ´MB initiated´ indicates glaciers with mass balance measurements starting in 2008 1230
(Incachiriasca), 2009 (Coropuna) and 2010 (Quisoquipinia). b): General atmospheric circulation 1231
over South America. The dotted frame shows the extent of the precipitation maps in figures 1c and 1232
d. c): and d): Mean hourly precipitation intensity (mm/h) in January (c) and July (d), respectively, 1233
from 1998 to 2010 based on TRMM (Tropical Rainfall Measurement Mission) Product 3B43 V6. 1234
1235
Fig. 1. (a) Glaciers monitored in the tropical Andes. Glaciers with long-term mass balance series (small red hexagons) are labeled (see
Table 1 for details). For these glaciers, length (yellow cube) and area (blue circle) records also exist. The large red hexagon depicts a sample
of 21 glaciers in the Cordillera Real, Bolivia, for which mass balance reconstructions are available from 1963 to 2006 (Soruco et al., 2009a).
Other glaciers whose changes in length and/or in area are monitored are numbered. “MB initiated” indicates glaciers with mass balance
measurements starting in 2008 (Incachiriasca), 2009 (Coropuna) and 2010 (Quisoquipinia). (b) General atmospheric circulation over South
America. The dottedframe shows the extent of the precipitation maps in (c) and (d). (c) and(d) Mean hourly precipitation intensity (mmh
1
)
in January (c) and July (d), respectively, from 1998 to 2010 based on TRMM (Tropical Rainfall Measurement Mission) Product 3B43 V6.
www.the-cryosphere.net/7/81/2013/ The Cryosphere, 7, 81–102, 2013
86 A. Rabatel et al.: Current state of glaciers in the tropical Andes
(Rabatel et al., 2006, 2011; Basantes, 2010; Caceres, 2010;
Collet, 2010).
Variations in glacier volume at a decadal timescale since
the mid-1950s were reconstructed for 26 glaciers in Bo-
livia (Rabatel et al., 2006; Soruco et al., 2009a, b) and two
glaciers in Ecuador (Caceres, 2010) on the basis of aerial
photograph pairs processed using photogrammetric restitu-
tion techniques. For three glaciers in the Cordillera Blanca,
Peru, Mark and Seltzer (2005) assessed changes in vol-
ume between 1962 and 1999 using similar photogrammet-
ric techniques, while Salzmann et al. (2012) applied a com-
bined remote-sensing data and modeling approach to esti-
mate changes in volume in the Cordillera Vilcanota, south-
ern Peru, for a similar period. This type of geodetic method
to compute volume variation over the whole glacier surface
is very useful to validate and adjust mass balance data cal-
culated using both glaciological and hydrological methods.
Such an adjustment was performed for Zongo Glacier in Bo-
livia by Soruco et al. (2009b).
Finally, Rabatel et al. (2012) showed that the method to
reconstruct annual mass balance based on snow line altitude
(SLA) measured on satellite images and used as a proxy
of the ELA can be used for glaciers in the outer tropi-
cal zone. This method was first developed for mid-latitude
glaciers (Rabatel et al., 2005b, 2008b), and was then success-
fully tested, validated, and applied on 11 Bolivian glaciers
(Bermejo, 2010; Consoli, 2011).
3 How did tropical glaciers change over time?
From centennial to annual scale
3.1 Glacier changes since the LIA maximum
Recent studies focused on glacier variations in the tropical
Andes from Venezuela to Bolivia during the LIA (e.g. Raba-
tel et al., 2005a, 2006, 2008a; Polissar et al., 2006; Solom-
ina et al., 2007; Jomelli et al., 2008, 2009). Figure 2 (up-
per panel) summarizes information on glacier advances in
the tropical Andes documented from moraine stages and lake
sediments. An early glacial advance at the beginning of the
last millenniumwas documented in Venezuela from lake sed-
iments (Polissar et al., 2006) and for some glaciers in Peru
and Bolivia from moraine stages (Jomelli et al., 2009). How-
ever, this 14th century glacial stage is absent in most valley
glaciers, suggesting that younger glacial advances extended
further than those that occurred in the 14th century.
The LIA period of maximum extent (PME) in the outer
tropics is dated to the 17th century, with dates varying
slightly from one mountain range to another. The licheno-
metric dates are around 1630± 27AD in Peru (Solomina et
al., 2007; Jomelli et al., 2008), and between 1657± 24AD
and 1686± 26AD in Bolivia (Rabatel et al., 2005a, 2008a).
These dates are concomitant with another glacial advance
documented from lake sediments in Venezuela (Polissar et
Fig. 2. Upper panel: comparison of moraine stages (triangles) dated
by lichenometry (Bolivia, Peru, Ecuador) and periods of glacier
advances (horizontal black bars) evidenced from lake sediments
(Venezuela). For each country, the triangles qualitatively represent
the position of the moraines along a schematic proglacial margin,
from the lowest one representing the period of maximum extent of
the LIA (the black triangle), to the uppermost moraine stage, clos-
est to the current glacier snout. For Ecuador, two schemes are rep-
resented, one for proglacial margins of glaciers with a maximum
elevation higher than 5700ma.s.l., and one for glaciers with a max-
imum elevation below 5700m a.s.l. Lower panel: proxies of climate
variations (wet/dry periods) in recent centuries. The P /A ratio in
the Sajama ice core (Liu et al., 2005) represents the relative change
in abundance of two pollen species (Poaceae and Asteraceae) and
is used as a proxy of moisture on the Altiplano. Data from ice cores
come from Thompson et al. (1985, 2006).
al., 2006). In the Ecuadorian Andes (inner tropics), the LIA
PME occurred in two distinct periods (Jomelli et al., 2009):
For glaciers with a maximum altitude above 5700m a.s.l.,
it was dated to the early 18th century (1730 ± 14 AD); for
those whose maximum altitude is lower, the PME was dated
to the early 19th century (1830± 11AD). The moraine stage
representative of the PME for glaciers with a maximum alti-
tude below 5700ma.s.l. is also found along proglacial mar-
gins of glaciers with a maximum altitude above 5700ma.s.l.,
but in the latter case it testifies to a smaller glacial advance
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 87
than the one that occurred during the maximum extent. The
advance dated from the early 19th century was also docu-
mented from reliable historical sources in Ecuador (Fran-
cou, 2004). Finally, the early 19th century advance was also
concomitant with an advance phase documented from lake
sediments in Venezuela (Polissar et al., 2006). Jomelli et
al. (2009) proposed that this difference in the timing of the
PME in Ecuador between glaciers with a maximum altitude
above/below 5700ma.s.l. could be the result of a cold and
dry period that would have followed a humid period. How-
ever, this difference is not yet clearly understood.
Following the PME, the evolution of glaciers in the in-
ner and outer tropics was remarkably homogeneous (Jomelli
et al., 2009). A slow withdrawal occurred during the late
18th and then during the first half of the 19th century. In
the outer tropics, among the moraine stages observed along
the proglacial margins, two are clearly the consequence of
an advancing glacier because they partly removed previ-
ous deposits (Rabatel et al., 2008a); they are dated to about
1730AD and about 1800 AD. After 1840 AD, withdrawal
was more pronounced and accelerated in the late 19th cen-
tury (from about 1870 to the early 20th century) in both inner
and outer tropics (Jomelli et al., 2009).
The withdrawal following the LIA maximum extent and
the absence of a major readvance in the 19th century (equiva-
lent to the magnitude of LIA maximum) in the entire tropical
belt are the main differences from the evolution of glaciers in
temperate latitudes of the Northern Hemisphere. In Bolivia,
Rabatel et al. (2006) observed that glaciers retreatedby about
1000m in length from the mid-17th to the late 19th century.
Figure 3 shows changes in surface area of Bolivian
glaciers since the LIA maximum. Only the glaciers with the
most complete time series for both the LIA period (Rabatel
et al., 2008a) and the recent decades are plotted. This figure
shows the current glacier withdrawal compared to the retreat
that occurred four centuries ago. Two main features are:
A general retreat has been underway since the PME of
the LIA (approximately 2nd half of the 17th century to
early 18th century), with two periods of accelerated re-
treat: one in the late 19th century and one in the last
three decades,the latter being the most pronounced. The
changes in surface area that occurred during the 18th
and 19th centuries are homogenous even though the
glaciers differ in size (ranging from 0.5 to 3.3km
2
), as-
pect (all are represented) and maximum altitude (rang-
ing from 5300m to 6000ma.s.l.).
Since the middle of the 20th century, the rate by which
Zongo Glacier retreated has differed from that of other
glaciers. Within the sample of glaciers plotted in Fig. 3,
Zongo Glacier is the only one with a maximum al-
titude above 5500ma.s.l. (reaching 6000ma.s.l.) and
hence still has a large accumulation zone. Glaciers with
a lower maximum altitude (i.e. <5400ma.s.l.) have al-
most completely disappeared.
Fig. 3. Changes in the surface area of eight glaciers in the Cordillera
Real, Bolivia, since the LIA maximum, reconstructed from moraine
stages (LIA maximum and before 1940) and aerial photographs
(1940 and after). 1963 was chosen as the common reference. Data
are from Rabatel et al. (2006, 2008a) and Soruco et al. (2009a).
3.2 Changes in glacier surface area in recent decades
Figure 4 presents a compilation of area loss rate quantified
for glaciers located between Venezuela and Bolivia, includ-
ing Colombia, Ecuador and Peru. In the following subsec-
tions we present a detailed description for each one of these
countries.
3.2.1 The Peruvian Andes
The Peruvian Andes are probably the best documented
glacial area in the tropics. In the Cordillera Blanca, Kinzl
(1969) reported that glacier retreat accelerated during the
late 19th century before slowing down during the first half
of the 20th century, with a small but marked readvance in
the 1920s. This event was followed by another significant re-
treat in the 1930s–1940s (Broggi, 1945; Kaser and Georges,
1997; Georges, 2004). In the period from 1950 to 1970,
glaciers retreated very slowly (Hastenrath and Ames, 1995a).
This period was followed by a general acceleration of re-
treat (Ames and Francou, 1995; Kaser and Georges, 1997).
Mark and Seltzer (2005) showed that glacier surface area de-
creased by about 35% in the Queshque Massif (southern part
of the Cordillera Blanca) between 1962 and 1999. Raup et
al. (2007) documented a 20% to 30 % retreat between 1962
and 2003 (depending on the source considered for 1962) for
glaciers inthe Huandoy–Artesonraju Massif(northern partof
the Cordillera Blanca). At the scale of the whole Cordillera
Blanca, several inventories were performed using digitized
maps and satellite images (e.g. Georges, 2004; Silverio and
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88 A. Rabatel et al.: Current state of glaciers in the tropical Andes
Fig. 4. Compilation of mean annual area loss rates for different time
periods for glaciated areas between Venezuela and Bolivia. Surface
areas have been computed from maps, aerial photographs, satellite
images and direct topographical measurements. Sources are given
in the text. Note that the average (smoothed using a 5-yr running
mean) is computed from a varying number of values depending on
the period concerned because fewer data were available for the first
decades of the study period. The grey box around the average rep-
resents the uncertainty corresponding to ±1standard deviation.
Jaquet, 2005; Racoviteanu et al., 2008; UGRH, 2010). Re-
sults differed slightly because of the methods used to de-
lineate glacier contours, for example whether or not peren-
nial snow fields were included. However, the main conclu-
sion was the same, i.e. a marked glacier retreat in the last
two decades. UGRH (2010) reported a 27% loss between
the 1960s and the 2000s, from 723km
2
to 527km
2
. Inter-
mediate estimations show that the mean annual loss in sur-
face area has increased since the late 1990s (Fig. 4). For
the second largest glacierized mountain range in Peru, the
Cordillera Vilcanota in southern Peru, Salzmann et al. (2012)
reported a 32 % loss in area between 1962 and 2006, in-
cluding an almost unvarying glacier area between 1962 and
1985. For Qori Kalis, an outlet glacier of Quelccaya ice cap
in the Cordillera Vilcanota, Brecher and Thompson (1993)
and Thompson et al. (2006) noted a 10 times greater loss in
area between 1991 and 2005 than between 1963 and 1978,
with an accelerated retreat in the 1990s. Finally, Racoviteanu
et al. (2007) reported that the glaciated area on Coropuna
(Cordillera Ampato, southern Peru) shrank by about 26%,
from 82.6km
2
in 1962 to 60.8km
2
in 2000.
Focusing on the Cordillera Blanca, Fig. 5 shows changes
in the length of ve glaciers in this mountain range. Direct
Fig. 5. Changes in surface area of ve glaciers in Ecuador and
Bolivia, and in length for five Peruvian glaciers. Observations of
changes in length start in 1949 except for Pastoruri Glacier (1980).
Observations in changes in surface area start in 1940 in Bolivia and
1956 in Ecuador.
annual measurements began between 1968 and 1980, but ad-
ditional information for 1949 was added based on aerial pho-
tographs. For glaciers for which data are available for the late
1970s, a change in the trend appeared in 1976–1977. Before
this date, changes in glacier length were limited (between
100 and 300m in about 30yr); Broggi Glacier even advanced
in the 1970s. Since the end of the 1970s, glacial withdrawal
has increased and the glaciers have retreated between 500
and 700m in length (i.e. more than twice the rate of the
former period). El Ni
˜
no years (e.g. 1982–1983, 1997–1998,
2004–2005) resulted in a more pronounced retreat, whereas
persistent La Ni
˜
na conditions at the turn of the 21st century
resulted in a slight slowdown in the retreating trend.
3.2.2 The Bolivian Andes
In Bolivia, Jordan (1991) published a complete inventory
of glaciers in both Cordilleras (Oriental and Occidental), on
the basis of aerial photographs taken in 1975 and field cam-
paigns from 1984. The total glacierized area was estimated
to be about 560 km
2
. Although there has been no more re-
cent glacier inventory of these cordilleras, glacier changes
in the Cordillera Real (part of the Cordillera Oriental) have
been documented (Ramirez et al., 2001; Rabatel et al., 2006;
Soruco et al., 2009a, b). Figure 5 illustrates changes in sur-
face area of the three glaciers with the most complete time
series of the Cordillera Real: Chacaltaya Glacier (Francou et
al., 2000; Ramirez et al., 2001), Charquini Sur Glacier (Ra-
batel et al., 2006) and Zongo Glacier (Soruco et al., 2009b).
Before the beginning of direct measurements in the early
1990s, estimates of surface areas were retrieved from aerial
photographs after 1940 at a decadal timescale. Zongo and
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 89
Charquini Sur glaciers showed a similar pattern as the one
previously described for the Peruvian glaciers, i.e. an almost
balanced situation during the 1950s and 1960s, which, at the
end of the 1970s, changed to a retreat that varied according
to the size and maximum altitude of the glacier concerned.
Chacaltaya Glacier retreated throughout the period, but the
retreat started to accelerate in the late 1970s.
3.2.3 The Ecuadorian Andes
In Ecuador, results obtained by Jordan et al. (2005) us-
ing photogrammetry on the Cotopaxi Volcano (5897ma.s.l.)
showed that Cotopaxi glaciers remained almost stagnant be-
tween 1956 and 1976 and then lost approximately 30% of
their surface area between 1976 and 1997. The calculated
loss of total mass (thickness) of selected outlet glaciers
on Cotopaxi between 1976 and 1997 was 78m, or 3–
4mw.e. yr
1
. Recent updates are mostly based on satellite
images (LANDSAT, ASTER and ALOS) which enabled re-
construction of changes in glacier surface area on Cotopaxi
Volcano, and additionally, documented glacier shrinkage on
both Antisana (5753ma.s.l.) and Chimborazo (6268ma.s.l.)
since the mid-20th century (Basantes, 2010; Caceres, 2010;
Collet, 2010). These studies showed that over the 1962–1997
period, the surface area of the glaciers on Chimborazo de-
creased from 27.7 to 11.8 km
2
(Caceres, 2010), which rep-
resents a loss of 57% or 1.6%yr
1
on average (Fig. 4).
For Cotopaxi and Antisana volcanoes, the loss in surface
area was respectively 37% and 33% for the period 1979–
2007. Intermediate data indicate that the retreat increased
during the second part of the period (Fig. 4). Changes in sur-
face area computed from satellite images at an almost an-
nual resolution on glaciers Antisana 12 and 15 also docu-
ment the increased loss of surface area since the early 1990s
(Fig. 5). However, small advances (few meters) by both
glaciers occurred in 2000 and 2008. These advances match
positive mass balance years and indicate the rapid response
of Ecuadorian glaciers to changes in mass balance.
3.2.4 The Andes of Colombia and Venezuela
In Colombia, a compilation of glacier surface-area mapping
at the scale of the most glacierized mountain range, Sierra
Nevada del Cocuy (Florez, 1991; Ceballos et al., 2006; Her-
rera and Ruiz, 2009), shows that (1) glaciers hardly changed
in the 1960s and 1970s, (2) there was a retreat of about 2 %
yr
1
from the late 1970s to the early 2000s, and (3) a ma-
jor increase in glacier retreat occurred during the 2000s. For
all the Colombian mountain ranges, Morris et al. (2006) and
Poveda and Pineda (2009) found that glacier area decreased
from 89.3 km
2
in the 1950s to 79 km
2
in the late 1990s and
to 43.8km
2
in the mid-2000s. This represents a total shrink-
age of about 51%, four times greater during the second pe-
riod. Glaciers in the Cordillera Central of Colombia are fre-
quently located on active volcanoes, and the strong glacier
loss was accelerated by several volcanic eruptions in recent
years (Huggel et al., 2007), most notably on Nevado del Ruiz
in 1985, and on Nevado del Huila where eruptions in 2007
and 2008 resulted in a 30% loss of glacier surface area in
two years.
In Venezuela, Morris et al. (2006) reported that glacier
surface area decreased from 2.03km
2
in 1952 to 0.3 km
2
in
2003, representing a total loss of 87%.
3.2.5 Summary at the scale of the tropical Andes
In terms of changes in surface area and length since the mid-
20th century (Figs. 4 and 5), the evolution of glaciers in the
tropical Andes can be summarized as follows:
Between the early 1940s and the early 1960s, infor-
mation was scarce, but evidence in Peru (Broggi, Uru-
ashraju, Yanamarey), Bolivia (Charquini, Chacaltaya)
and Colombia (Sierra Nevada del Cocuy) indicates a
moderate retreat ( 0.5%yr
1
).
From the mid-1960s to the second half of the 1970s,
glacier snout positions remained almost the same.
A clear change in glacier evolution can be seen in the
late 1970s, when the retreat accelerated but stepwise:
the first acceleration in the retreat occurred in the late
1970s, the second in the mid-1990s, and the third in the
early 2000s. These phases of accelerated retreat were
interrupted by 2 to 3yr with reduced retreat or even
short readvances such as in Ecuador in 1999–2000 and
in 2008–2009.
Glacier shrinkage in the three last decades appears to
be unprecedented since the PME of the LIA (mid-17th–
early 18th century).
3.3 Changes in glacier mass balance in recent decades
The longest mass balance series available are for Yanamarey
Glacier (Cordillera Blanca Peru, since 1971) and Zongo
Glacier (Cordillera Real – Bolivia, since 1973, reconstructed
from hydrological data, see Soruco et al., 2009b). It should
be noted that measurements on Yanamarey Glacier were in-
terrupted several times and to complete the missing data, a
linear trend was assumed. Among the glaciers where mass-
balance time series are available in the tropical Andes, two
subsets can be distinguished: glaciers with a maximum ele-
vation higher or lower than 5400ma.s.l. This elevation ap-
proximately matches the uppermost altitude reached by the
equilibrium-line on the studied glaciers during very negative
mass balance years. As a consequence, during such years the
glaciers with a maximum elevation higher than 5400ma.s.l
can preserve an accumulation zone (more or less important
depending on the maximum elevation of the glacier), and
conversely, glaciers with a maximum elevation lower than
5400ma.s.l. are completely exposed to ablation.
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90 A. Rabatel et al.: Current state of glaciers in the tropical Andes
Figure 6 shows the cumulative annual mass balance of
the eight glaciers between Colombia and Bolivia for which
field measurements have been conducted (Table 1). Over
the last 40 yr, two distinct patterns of loss can be dis-
tinguished: (1) Glaciers with a maximum elevation lower
than 5400m a.s.l. (Yanamarey, Chacaltaya, Charquini Sur
and La Conejeras glaciers) showed an average trend of
1.2mw.e. yr
1
, and (2) glaciers with a maximum eleva-
tion higher than 5400 m a.s.l. (Zongo, Artesonraju, Antisana
15 and Los Crespos glaciers) showed an average trend of
0.6mw.e. yr
1
. However, one can note that the changes
in mass balance at regional scale were homogeneous over
the whole period, especially when taking into account (1)
the link between the average mass loss trend and the max-
imum altitude of the glaciers; (2) the distance between the
glaciers monitored: 21
in latitude between Zongo and La
Conejeras; and (3) distinct hydrological year timing. This co-
herent glacier response suggests common large-scale forcing
influencing climatic variability at a regional scale (e.g. Fran-
cou et al., 2007).
A strong interannual variability was superimposed on
these long-term trends. Glaciers with a maximum elevation
higher than 5400 m a.s.l. experienced major fluctuations be-
tween a balanced or even a slightly positive mass balance
and deficits reaching more than 2m w.e. yr
1
. On the other
hand, glaciers such Chacaltaya, Charquini Sur, and Yana-
marey experienced a permanently negative mass balance in
recent years. Thus, it can be claimed that glaciers with a
maximum elevation lower than 5400ma.s.l. are very unbal-
anced and that, with a deficit of around 1.2mw.e.yr
1
,
many of them will probably completely disappear in one or
two decades (note that this is already the case for Chacaltaya
Glacier in Bolivia, which disappeared in 2010).
Figure 7 is a summary of mean annual mass balance
per period combining all available measurements made in
Colombia, Ecuador, Peru and Bolivia using different meth-
ods (geodetic, hydrological, glaciological and mass balance
reconstructions from variations in snow line altitude). The
quantity of available data has increased since the mid-90s
when mass balance data derived from the remote-sensing
method that uses ELA became available. Four important
points in this graph are worth emphasizing:
Although some glaciers sporadically had a positive
mass balance, the average signal over the past 50yr has
been permanently negative.
The late 1970s break point, already discussed with re-
spect to changes in surface area, is equally apparent
from mean annual mass balance per year, decreasing
from 0.2mw.e. yr
1
over the 1964–1975 period to
0.76mw.e. yr
1
over the 1976–2010 period.
A slight increase in the rate of glacier mass loss oc-
curred during the past two decades.
Fig. 6. Cumulative annual mass balance series computed for eight
glaciers in the tropical Andes. 2006 was chosen as the common ref-
erence.
Glaciers in the tropical Andes appear to have had more
negative mass balances than glaciers monitored world-
wide. In addition, tropical glaciers began to shrink at
an accelerated rate after 1976, while those located at
mid/high latitudes generally underwent an accelerated
retreat about 15yr later in the 1990s.
3.4 Synchronicity of the ablation rates throughout the
tropical Andes
Figure 8 shows the cumulative monthly mass balance of five
glaciers on which stake emergence was measured monthly,
taking into account the snow/ice density. The elevation
ranges include the whole glacier surfaces of Chacaltaya,
Charquini Sur and La Conejeras glaciers (mostly ablation
zones), the lower zone of Antisana 15 Glacier (4800 to
5000ma.s.l.) and the upper ablation zone of Zongo Glacier
(5000 to 5200ma.s.l.). At a monthly scale, the mass balance
in the ablation zone reflects changes in the energy balance
(melt energy) and snow accumulation at the glacier surface.
In the first decade, i.e. 1991–2001, the patterns remained al-
most the same: ablation peaked in 1995 and 1997–1998 on
the three glaciers monitored atthis time,whereas 1993–1994,
1996 and 1999–2000 were more balanced. In the 2001–2006
period a difference emerged between the inner and outer
tropics: the continuous high ablation rates of Antisana 15
Glacier did not appear in Bolivia until 2004. The 2006–2011
period was characterized by an almost balanced situation in
both the outer and inner tropics, even including a shortperiod
of mass gain on La Conejeras Glacier (2007–2008) followed
by a marked loss in 2009.
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 91
Fig. 7. Compilation of all available measured rates of change in
mass balance in the tropical Andes. Glaciological and hydrological
measurements are made annually; geodetic measurements are pluri-
annual and result from sporadic photogrammetric surveys. “SLA
represents the reconstruction of mass balance using variations in
snow line altitude. Each budget is drawn as a thick horizontal line
contained in a ±1 standard deviation box. The tropical Andes aver-
age was computed from available data and has been smoothed using
a 5-yr running mean, the light grey box around the average rep-
resents the ±1standard deviation. The global average comes from
Cogley (2012).
4 Which atmospheric factors control mass
balance processes on tropical glaciers?
4.1 Factors controlling seasonal changes in
mass balance
Long-term surface energy balance (SEB) field campaigns in
Bolivia (Wagnon et al., 1999, 2001; Sicart et al., 2005), Peru
(Juen et al., 2007) and Ecuador (Favier et al., 2004a, b) re-
vealed that in the tropics, the variability of SEB in the ab-
lation areas is mostly controlled by net short-wave radiation
(S), which is partly compensated by the negative net long-
wave radiation budget (L). On the one hand, S is closely
linked with cloud cover and surface albedo. As a conse-
quence, the surface albedo appears to be a primary variable
controlling the amount of melt energy at the surface of trop-
ical glaciers because of its strong variability and its feed-
back effect on the melt rate. Cloud cover on the other hand,
causing strong seasonal changes in L and solid precipitation,
controls the seasonal changes in energy fluxes and mass bal-
ance on tropical glaciers (Wagnon et al., 2001; Francou et al.,
2003, 2004; Favier et al., 2004a; Sicart et al., 2011).
Fig. 8. Cumulative monthly mass balance for ve glaciers in the
tropical Andes wherethe measurements were madeat this timescale
(the reference time for the cumulative time series is April 2005, ex-
cept for Conejeras Glacier where measurements began in March
2006). Zongo, Chacaltaya and Charquini Sur glaciers are located
in Bolivia, in the outer tropics, while Antisana 15 and Conejeras
glaciers are located in Ecuador and Colombia, in the inner tropics,
respectively. Note that for Zongo and Antisana 15 glaciers, only the
results of the ablation zone are plotted. Monthly mass balance mea-
surements on Chacaltaya Glacier were stopped in September 2005
because the glacier was too small. The light grey box highlights the
2001–2006 period when mass balance of Antisana 15 Glacier was
negative, diverging from the outer tropics.
Tropical glaciers are characterized by large vertical mass
balance gradients of about 2m w.e. (100m)
1
in the ablation
area (e.g. Kaser et al., 1996; Soruco et al., 2009b), implying
a significant contribution of the lowest areas to total ablation.
Kuhn (1984) noted the influence of the length of the ablation
period. Areal simulation of the energy fluxes at the scale of
the Zongo Glacier showed that the frequent changes in snow
cover throughout the ablation season were the main explana-
tion for the marked vertical mass balance gradients of tropi-
cal glaciers (Sicart et al., 2011). However, the seasonality of
precipitation is not the same in the inner and outer tropics.
Consequently, the seasonality of melting at the glacier sur-
face also differs.
4.1.1 Specificities of the inner tropics
On Antisana 15 Glacier (Ecuador), Favier et al. (2004a,
b) found that on seasonal timescales, mean ablation rates
remained almost constant throughout the year. Francou et
al. (2004) specified that the interannualvariability of ablation
was mainly controlled by year-to-year variations in air tem-
perature, which determine the snow line altitude. Glaciers
in the inner tropics are thus very sensitive to temperature
changes.
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92 A. Rabatel et al.: Current state of glaciers in the tropical Andes
In addition, albedo appears to be a major determinant in
melting. At a daily time step, a close relation was shown
between albedo and net radiation (Favier et al., 2004b).
Changes in albedo go hand in hand with changes in the short-
wave radiation balance. Consequently, the frequency and in-
tensity of snowfall, which can occur all year long, play a ma-
jor role in attenuating the melting processes.
As a consequence, both precipitation and temperature are
crucial for the annual mass balance, both during the main
precipitation period (between February and May) and the
secondary precipitation phase (September–October). Signif-
icant snowfall in February–May clearly reduces the ensuing
melting due to the albedo effect. Conversely, long periods
without snowfall lead to a significant increase in melt rate,
particularly in the periods close to the equinox (March–April
and September) when potential incoming short-wave radia-
tion is at a maximum.
Finally, the sensitivity of Ecuadorian and Colombian
glaciers to climate (in terms of dependence of mass balance
on climate parameters) is closely linked to the absence of
temperature seasonality. The 0
C isotherm constantly oscil-
lates through the ablation zone of the glaciers, and a minor
variation in air temperature can influence the melt processes
by determining the phase of precipitation and consequently
affect the surface albedo in the ablation zone (a temperature
increase of 1
C can move the snow–rain limit about 150 m
up the glacier).
4.1.2 Specificities of the outer tropics
In the outer tropics, where liquid precipitation is rare on
glaciers, the mass balance is closely related to the to-
tal amount and the seasonal distribution of precipitation
(Wagnon et al., 2001; Francou et al., 2003; Favier et al.,
2004a; Sicart et al., 2005).
Concerning the evolution of melt at the glacier surface
throughout the year, three seasons can be distinguished for
outer tropical glaciers (Sicart et al., 2011; Rabatel et al.,
2012): (1) In the dry season from May to August, melt is
low mainly due to a deficit in long-wave radiation of the sur-
face energy balance; this deficit is due to the low emissivity
of the thin cloudless atmosphere at very high altitudes. (2)
During the transition season from September to December,
when precipitation is not yet abundant, the meltwater dis-
chargeprogressively increases to reach itshighest annualval-
ues in November–December (Ribstein et al., 1995; Sicart et
al., 2011) due to high solar irradiance, with the sun close to
zenith, and low glacier albedo. (3) From January to April,
the frequent snowfall in the wet season reduces the melt rate,
which is nevertheless maintained by high long-wave radia-
tion emitted from convective clouds. Finally, the annual mass
balance depends largely on the beginning of the wet season,
which interrupts the period of high melt caused by solar ra-
diation (Sicart et al., 2011). Any delay in the beginning of
the wet season causes a very negative mass balance due to
reduced snow accumulation and very large ablation, an oc-
currence which is frequent during El Ni
˜
no events (Wagnon
et al., 2001). Indeed, Wagnon et al. (2001) showed that the
high melt rates measured at the Zongo Glacier weather sta-
tion during the 1997–1998 El Ni
˜
no year were mainly due to
reduced solid precipitation and associated low albedo.
4.1.3 Relation between air temperature and ablation on
tropical glaciers
Numerous studies, primarily from mid- to high-latitude
glaciers, have revealed a high correlation between glacier or
snow melt and air temperature (e.g. Zuzel and Cox, 1975;
Braithwaite, 1981). These correlations provide the basis for
degree-day models, which relate the melt rate to the sum of
positive temperatures, generally at a daily timescale, through
a constant degree-day factor. The degree-day factor depends
on the relative importance of each energy flux and gener-
ally is specific to the site and to the period considered. Few
studies have investigated the physical causes of the correla-
tion between air temperature and ice melt. Paradoxically, net
radiation generally is the greater incoming energy flux but
is poorly correlated to air temperature (Sicart et al., 2008).
At low latitudes, empirical models, similar to degree-day ap-
proaches, have been used to simulate the mass balance with-
out detailed examination of the hypotheses supporting the
model (e.g. Hostetler and Clark, 2000; Kull and Grosjean,
2000; Pouyaud et al., 2005), the main one being that the vari-
ability of melt rate is well correlated to the temperature (im-
plying constant degree-day factor). These hypotheses, must
be known and tested when the model is used outside the cal-
ibration experiment, such as in different climatic areas or for
mass balance forecasting or hindcasting.
Sicart et al. (2008) investigated the physical basis of
temperature-index models for Zongo Glacier in the outer
tropics and Antisana Glacier in the inner tropics. They
showed that during the melt season net short-wave radiation
controls the variability of the energy balance and is poorly
correlated to air temperature. The turbulent flux of sensible
heat is generally a gain in energy for the glacier surface,
whereas the latent heat flux is a sink. Both turbulent fluxes
tend to cancel each other out. Air temperature is a poor index
of melt, mainly because of (1) low and only slightly vary-
ing temperatures during the melt period, and (2) the low heat
content of the air at very high elevations. Albedo changes
due to frequent snowfalls that temporarily cover the melt-
ing ice surface contribute to, but are not the main cause of,
the poor correlations between temperature and melt energy.
As a consequence, the degree-day model is not appropriate
for simulating the melting of tropical glaciers at short time
steps. However, at the yearly timescale, air temperature is a
better index of the glacier mass balance because it integrates
ablation and accumulation processes over a long time pe-
riod. Indeed, temperature is a variable not only related to the
sensible heat flux, but also closely linked with the long- and
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 93
short-wave radiation balance through the phase of precipita-
tion which controls the albedo.
4.2 Regional forcing of the mass balance interannual
variability: the Pacific Ocean
Figure 9 shows time series of monthly mass balance anoma-
lies and Pacific sea surface temperature anomalies (SSTa).
The top graph focuses on the inner tropics and shows the
average monthly mass balance of Antisana 15 (1995–2011)
and La Conejeras (2006–2011) glaciers with the SSTa of the
Ni
˜
no 3.4 region. The bottom graph focuses on the outer trop-
ics, with the average monthly mass balance of Zongo (1991–
2011), Chacaltaya (1991–2005) and Charquini Sur glaciers
(2002–2011) and the SSTa of the Ni
˜
no 1+2 region.
In Ecuador (inner tropics), as shown by Francou et
al. (2004) and Vuille et al. (2008a), the two opposite phases
of ENSO explain the highly contrasted situations on the An-
tisana 15 Glacier. The SSTa peak in the central Pacific dur-
ing the austral summer (November–February) and the at-
mospheric response to ENSO over the Ecuadorian Andes is
delayed by three months, so that the year-to-year variabil-
ity of the mass balance is most important during the pe-
riod from February to May (Francou et al., 2004). During
warm ENSO phases, increasing temperatures favor precipi-
tation at the melting point up to 5100–5200ma.s.l., which,
together with the slight deficit in precipitation and cloudi-
ness, explains the consistently low values of the albedo and
the high melt rates (Favier et al., 2004a, b). In contrast, the
cold ENSO phase brings cooler temperatures, higher snow-
fall amounts and increased cloudiness, which, for long peri-
ods, prevent albedo from dropping below the typical values
of the fresh snow (0.8) and decreases available energy for
melt. To a lesser extent, stronger winds during austral winter
boost sublimation and reduce melting.
In Colombia, the impacts of ENSO on glaciers are similar
to those in Ecuador. It has been observed that monthly mass
balance was up to three anda half timesmore negative during
El Ni
˜
no events than in an average month. On the other hand,
the 2007/2008 La Ni
˜
na event resulted in a positive mass bal-
ance on La Conejeras Glacier.
In Bolivia, variations in the interannual glacier mass bal-
ance are also to a large extent controlled by SSTa in the
tropical Pacific (Francou and Ribstein, 1995; Francou et al.,
2003; Vuille et al., 2008a). During the ENSO warm phase (El
Ni
˜
no), precipitation decreases by 10–30% and dry periods
occur more frequently during austral summer (Vuille et al.,
2000). This situation increases incoming solar radiation, re-
duces snow accumulation and decreases albedo on the glacier
surface (Wagnon et al., 2001). On average, the near-surface
summer temperature is 0.7
1.3
C higher during El Ni
˜
no
than during La Ni
˜
na (Vuille et al., 2000), enhancing sensi-
ble heat flux to the glacier surface. During the relatively wet
and cold La Ni
˜
na periods, opposite conditions prevail, which
can lead to near equilibrium mass balance. However, as can
Fig. 9. Upper graph: time series of monthly mass balance anomalies
(mw.e.) for the inner tropics (mean of Antisana15 andLa Conejeras
glaciers) and Ni
˜
no3.4 SSTa (
C) between June 1995 and August
2011. Both time series were smoothed with a 12-month averaging
filter. Mass balance anomalies lag SSTa by 3months. Vertical bar
plot (inset) shows the correlation between mass balance anomalies
and the Ni
˜
no3.4 index, with the Ni
˜
no3.4 index leading mass bal-
ance anomalies by between 0 and 6months. Lower graph: the same
but for the outer tropics (mean of Zongo, Chacaltaya and Charquini
Sur glaciers) and Ni
˜
no 1 +2 SSTa. The best correlation between
both series was with a 4-month lag (see the vertical bar plot inset).
be seen on the lower graph in Fig. 9, the response of mass
balance to the SSTa forcing is not systematic, for example
over the 1992–1995 and 2001–2005 periods. Although the
positive mass balance anomaly in the 1992–1995 period has
been attributed to the cooling effect of the Pinatubo eruption
in June 1991 (Francou et al., 2003), the situation that oc-
curred between 2001 and 2005 is still being analyzed and
remains unresolved. Nevertheless, new characteristics ob-
served in ENSO variability (central Pacific/eastern Pacific or
“Modoki” ENSO) could explain the slight differences in the
response of glaciers in this region to the ENSO phenomenon,
particularly for the outer tropics. This last point will be the
focus ofa forthcomingpaper. Finally, ENSO influence on Sa-
jama Volcano glaciers has also been highlighted by Arnaud
et al. (2001), showing that the snow line elevation is related
primarily to precipitation and to a lesser degree to tempera-
ture.
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94 A. Rabatel et al.: Current state of glaciers in the tropical Andes
Concerning the Cordillera Blanca in Peru, the mechanisms
linking ENSO and glacier mass balance are similar to those
in Bolivia, with the SSTa exerting the prevailing large-scale
control on interannual mass balance variations. Typically, El
Ni
˜
no events result in negative mass balance anomalies, and
La Ni
˜
na in above average signals. However, these telecon-
nections are spatially unstable and ENSO events with re-
versed effects on glacier mass balance have been observed
(Vuille et al., 2008b).
During periods when ENSO is near neutral conditions,
other atmospheric forcing factors might also have an impact
on interannual mass balance variability, but their relative role
is poorly documented. Such factors might, for example, in-
clude variations in intensity and duration of the South Amer-
ican monsoon, or the so-called surazos, which cause precip-
itation during the dry period due to Southern Hemisphere
mid-latitude disturbances tracking abnormally north of their
usual path (Ronchail, 1995).
5 Climatic causes of tropical glacier changes
5.1 Causes of glacier retreat during the LIA (from the
PME to the late 19th–early 20th century)
The formation of moraines at a distance of about 800 to
1000m from the present glacier snout during the PME of
the LIA means that the specific mass balance was very posi-
tive, generating a significant transfer of ice downstream from
the glacier to offset increasing ablation at low altitude. From
sensitivity studies, Rabatel et al. (2006) suggested that con-
ditions may have been wetter during the LIA, thus increasing
accumulation rates, and, in conjunction with lower tempera-
tures, leading to a decrease in the freezing level. This hypoth-
esis isconsistent withother proxies,one basedon icecore ev-
idence (Fig. 2lower panel). For example, in several ice cores,
Thompson et al. (2006) and Vimeux et al. (2009) noted a
marked centennial-scale decrease in the δ
18
O of the snow/ice
between the late 16th and early 19th century. The minimum
δ
18
O content between 1620AD and 1730AD can be
considered to be related to increasing convective activity dur-
ing the PME (Vimeux et al., 2009). New δ
18
O records from
Andean speleothems and lake records also confirm that, in
this region, the LIA period must have been wet (Bird et al.,
2011). Pollen analyses from the Sajama ice core (Liu et al.,
2005) arealso in agreementwith wetter conditions during the
PME of the LIA.
Quantitatively, the application of simple climate/glacier
models (Polissar et al., 2006; Rabatel et al., 2008a; Jomelli
et al., 2009) highlights several points:
In Venezuela, for the period 1250–1820 AD, average
air temperature may have been 3.2± 1.4
C cooler, and
precipitation about 22% higher than at present.
In Ecuador, air temperature may have been 0.8
C to
1.1
C below today’s values, and a 25 % to 35% in-
crease in accumulation appears to have occurred in the
18th century.
In Bolivia, the PME of the LIA could be the result of a
decrease in temperature of 1.1
C to 1.2
C, and a 20 %
to 30% increase in accumulation.
In Colombia, the air temperature during the PME of the
LIA was estimated to be 1.2
C to 1.5
C lower than at
the turn of the 21st century (Baumann, 2006).
A major difference between tropical and mid-latitude
glaciers is that the tropical glaciers began to retreat just after
1740–1750AD, a long trend of recession which may have
been associated with drier conditions. Indeed, drier condi-
tions are indicated by the analysis of paleo-lake levels on the
Peruvian–Bolivian Altiplano (Chepstow-Lusty et al., 2003).
The shift to drier conditions between the late 18th and early
19th centuries is also apparent in pollen analyses of the Sa-
jama icecore (Liuet al.,2005) andnet accumulationfrom the
Quelccaya icecore (Thompson et al., 1985). However, the re-
cession was probably not continuous since distinct moraines
were deposited between the PME and the late 19th–early
20th century, indicative of small glacial advances, although
those never reached a magnitude as great as those in the
PME. Such small glacial advances occurred during the first
half of the 19th century in Bolivia and Peru as well (Rabatel
et al., 2006, 2008a; Jomelli et al., 2009), with moraine stages
dated from 1800 AD and 1860AD; they could be related
to relatively wetter conditions.
The last decades of the 19th century were characterized by
a substantial glacier retreat ata regional scale, whichcould be
due to dry conditions as documented in climate proxies and
the first instrumental measurements (Kraus, 1955; Torrence
and Webster, 1999).
5.2 Causes of the accelerated retreat in the last 30yr
5.2.1 Climate changes in recent decades
Recently, Vuille et al. (2008a) presented a review of climate
changes in the 20th century along the tropical Andes. These
authors reported that:
Precipitation changes are difficult to document be-
cause of the lack of high-quality long-term precipita-
tion records. Moreover, the variability at the decadal
timescale is higher than the multi-decadal trend, partly
due to ENSO effects. However, studies showed an in-
creasing trend in precipitation after the mid-20th cen-
tury (both at an annual scale and during the wet sea-
son) north of 11
S, i.e. in Ecuador and northern/central
Peru. Inversely, in southern Peru and the Bolivian Al-
tiplano, most weather stations indicated a decreasing
trend (Vuille et al., 2003; Haylock et al., 2006).
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 95
Changes in humidity are very hard to quantify as, in
the Andes, no long-term continuous records exist. How-
ever, based on CRU05 data, Vuille et al. (2003) found a
significant increase in relative humidity for the 1950–
1995 period ranging from 0.5%decade
1
(in Bolivia)
to more than 2.5%decade
1
(in Ecuador). Similarly,
based on NCEP reanalysis data, Salzmann et al. (2012)
found a significant increasing trend in specific humidity
in the southern Peruvian Altiplano over the past 50yr.
During the 1974–2005 period, outgoing long-wave ra-
diation (OLR) decreased in the inner tropics, suggest-
ing an increase in convective activity and cloud cover,
whereas in the outer tropics, the opposite trend is doc-
umented (Vuille et al., 2003). This pattern is consistent
with precipitation trends in the same period.
Temperature is by far the best documented climate pa-
rameter. Based on 279 weather stations located be-
tween 1
N and 23
S, Vuille et al. (2008a) showed that
near-surface air temperature increased significantly (by
0.10
Cdecade
1
) in the last 70yr, which represents
an overall temperature increase of 0.68
C since 1939.
These findings confirm results obtainedby other authors
in Peru (Mark and Seltzer, 2005), Bolivia and northern
Chile (Vuille et al., 2000), Ecuador (Quintana-Gomez,
2000; Villacis, 2008) and along the entire tropical An-
des from Ecuador to northern Chile (Vuille and Bradley,
2000), all of whom reported a significant warming trend
and a reduced daily temperature range (difference be-
tween daily minimum and maximum temperatures).
Consistent with this increase in temperature, Gilbert et
al. (2010) showed from englacial temperature measure-
ments in a 138 m deep borehole drilled near the sum-
mit of Illimani (6340 m a.s.l., Bolivia) that a warming
trend canalso be identified along the temperature profile
at very high altitudes. These authors quantified a mean
rise in atmospheric temperature of 1.1± 0.2
C over
the 20th century. It should be noted that this increase
in temperature is the only long-term evidence recorded
over the full 20th century in the Andes at the elevation
of glaciers, as most weather stations are located below
4000ma.s.l., or have only short-term records.
Figure 10 shows changes in freezing level height in the
Andean Cordillera documented based on NCEP/NCAR re-
analysis data. Freezing level height was computed using
monthly temperatures and geopotential height and plotted
as a 12-month running mean for the 1955–2011 period at
three sites (Antisana in Ecuador, Cordillera Blanca in Peru,
and Cordillera Real in Bolivia) using an elevation range be-
tween the glacier snout and the mean glacier altitude at each
site as a backdrop. For each site, the grid cell including the
site was selected. The freezing level height plotted as a 12-
month running mean provides an annual mean freezing line
elevation, with seasonality removed (albeit the seasonality is
Fig. 10. Changes in freezing level height in the Andean Cordillera
computed from NCEP/NCAR reanalysis data (1955–2011) for
three sites (Antisana in Ecuador, Cordillera Blanca in Peru, and
Cordillera Real in Bolivia) in parallel with a range of elevations
from glacier snouts to the mean elevation of glaciers at each site
(blue shaded area). These elevations are averages for each one of
the sites corresponding to values from the 2000 decade.
small in the tropics). In the inner tropics the freezing line is
closely associated with the ELA, while in the outer tropics
the ELA tends to be above the freezing line (due to moisture
limitations). From Fig. 10, one can note that, in the inner
tropics (Antisana in Ecuador), during the 1955–2011 period,
the ablation zone extended down to the freezing level, thus
explaining the year-round strong ablation rates. In the outer
tropics of Peru ( 9
S) and Bolivia ( 16
S), except during
strong El Ni
˜
no events, the ablation zone tended to be located
above the annual mean freezing line during the first half of
the study period. However, the marked increase in freezing
levels since the late 1970s–early 1980s led to a situation in
which the ablation zones of the Cordillera Blanca and today
even of the Cordillera Real are mostly located within the al-
titudinal range of the annual mean freezing level.
Quantitatively, the freezing level height has increased by
about 60m and 160 m over the last five and a half decades
in the inner and outer tropics, respectively. This increase can
be partially traced back to the increase in the tropical Pacific
SST (Diaz et al., 2003; Bradley et al., 2009).
Figure 11 shows cumulative temperature (monthly mean
values of NCEP/NCAR reanalysis data) at the current eleva-
tion of the glacier snout for the same three locations as in
Fig. 10. For each zone, Fig. 11 also shows the cumulative
glacier change, computed from the average of available sur-
face/length data. At Antisana and in the Cordillera Blanca,
the temperature was very close to 0
at the glacier snout
up to the late 1970s, when temperatures started to rise and
cumulative temperature became positive. Temperatures have
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96 A. Rabatel et al.: Current state of glaciers in the tropical Andes
Fig. 11. Cumulative degree months (NCEP/NCAR temperature re-
analysis data) at glacier snouts for the 1955–2011 period (red line)
at three locations: Antisana in Ecuador, Cordillera Blanca in Peru,
and Cordillera Real in Bolivia. For each zone, the symbols repre-
sent the cumulative glacier change in terms of surface (average of
the available data in Ecuador and Bolivia) or length (average of the
available data in Peru). The regression line associated with the sym-
bols match a 3rd order polynomial regression.
continued to increase ever since, meaning that, except for
short intervals associated with the cold phase of ENSO, they
have remained positive. For example, in 1997/1998 El Ni
˜
no
led to markedwarming and in1999/2000 LaNi
˜
na led to cool-
ing (Antisana) or at least stabilization of the cumulative tem-
perature (Cordillera Blanca), but this event was short-lived.
The situation in the Cordillera Real (Bolivia) appears to be
a little different, because in the outer tropics glaciers are lo-
cated in an area with a dryer climate and therefore at higher
elevations relative to the freezing line. Hence, from 1955 to
the mid-1990s, temperatures at the Zongo Glacier snout were
mainly below freezing and the cumulative temperature was
consequently negative. However, since the early 2000s, the
temperature hasreached the freezing point, and as a resultthe
cumulative temperature curve flattened out and even started
to rise since 2010.
5.2.2 Linking current climate change and glacier
evolution in the tropical Andes
The higher SST of the tropical Pacific Ocean off the coast
of South America observed after the 1976 Pacific climate
shift most likely helped to accelerate glacier retreat through-
out the tropical Andes. This strong signal is superimposed
on higher frequency, large-scale atmospheric events. The
Pinatubo eruption, an event of this type, occurred in June
1991 and for several months affected the glacier mass bal-
ance through a cooling effect of the volcanic sulfate aerosols
in the stratosphere, hence interrupting the long El Ni
˜
no pe-
riod (1990–1995), and causing theonly slightlypositive mass
balance in the entire decade on Chacaltaya Glacier (Francou
et al., 2003). Thus, we can assume that the higher frequency
and the change in the spatio-temporal occurrence of El Ni
˜
no
since the late 1970s, together with a warming troposphere
over the tropical Andes, explain much of the recent dramatic
shrinkage of glaciers in this part of the world.
Finally, it is interesting to note that the beginning of the
accelerated retreat of tropical glaciers occurred at the same
time as a major increase in the global temperature curve after
1976 (Trenberth et al., 2007), which incorporates the warm-
ing of the tropical Pacific and other tropical regions. Glaciers
all around the Pacific and Indian oceans underwent accel-
erated retreat since 1976 (with the exception of glaciers in
New Zealand until the early 2000s, which are also influenced
by ENSO, but in the opposite way). In the Northern Hemi-
sphere, from Alaska to northern Russia, and throughout Eu-
rope, the main forcing is the North Atlantic Oscillation, re-
sulting in (1) a slight time lag in the beginning of acceler-
ated retreat period (late 1980s/early 1990s) and (2) distinct
mechanisms of variability at a decadal scale (e.g. Francou
and Vincent, 2007).
5.3 Possible future changes in tropical glaciers in the
Andes: results of modeling
Using results from eight different general circulation mod-
els used in the 4th assessment of the IPCC, and CO
2
lev-
els from scenario A2, Bradley et al. (2006) showed that pro-
jected changes in mean annual free-air temperatures between
1990–1999 and 2090–2099 along the tropical Andes at an
elevation higher than 4000m a.s.l. will increase by +4
C
to +5
C. The maximum temperature increase is projected
to occur in the high mountains of Ecuador, Peru and Bo-
livia (Bradley et al., 2006). Urrutia and Vuille (2009), us-
ing a high-resolution regional climate model, came to similar
conclusions with respect to changes in near-surface temper-
ature. To test the response of glaciers to changes in air tem-
perature, Lejeune (2009) performed a sensitivity analysis of
glacier mass balance and equilibrium-line altitude by apply-
ing the CROCUS snow model to the Zongo Glacier in two
contrasted wet seasons (2004–2005 and 2005–2006). CRO-
CUS is a one-dimensional multi-layer physical model of the
snow cover, which can be adapted to account for glaciers.
The model explicitly evaluates at hourly time steps the sur-
face mass and energy budgets (for more details, refer to Brun
et al., 1989). The results of the sensitivity analysis on Zongo
Glacier showed that for a 1
C increase in air temperature,
the increase in ELA would be 150± 30m. With such a result
and assuming that changes in ELA are linearly proportional
to changes in temperature, the above mentioned projected
changes in air temperature (+4
C to +5
C) simulated at the
elevation of the glaciers for the end of the 21st century would
result in an ELA increase on the order of 480 to 900m. With
ELA
0
currently located at 5150ma.s.l. on Zongo Glacier
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A. Rabatel et al.: Current state of glaciers in the tropical Andes 97
(Rabatel et al., 2012), such an increase would locate the ELA
between 5630 and 6050ma.s.l. at the end of the 21st century,
i.e. in the upper reaches of the Zongo Glacier, with a sub-
sequent drastic reduction in its surface area. If extrapolated
to other glaciers in the Cordillera Real, such an increase in
the ELA would cause the disappearance of most glaciers in
this massif. However, these results are preliminary, and they
need to be supplemented and expanded by the analysis of the
sensitivity of mass balance and ELA to other meteorologi-
cal parameters (precipitation, humidity, radiation) at longer
timescales.
6 Summary, remaining challenges and concluding
remarks
This review of glacier changes over the last 50yr and two
decades of constant field observations of some representa-
tive glaciers in the tropical Andes enabled us to highlight the
following conclusions:
Consistent with most mountain glaciers worldwide,
glaciers in the tropical Andes have been retreating at an
increasing rate since the late 1970s. The rate of current
retreat appears to be unprecedented since the LIA max-
imum, i.e. since the second half of the 17th century and
the early 18th century.
The magnitude of glacier mass loss is directly related
to the size and elevation of the glacier. Glaciers with
a maximum altitude above 5400m a.s.l. (i.e. that still
have a permanent accumulation zone) have typically
lost 0.6m w.e. yr
1
over the last three and a half
decades, whereas glaciers with a maximum altitude
lower than 5400 m a.s.l. have shrunk at an average rate
of 1.2mw.e. yr
1
, i.e. at twice the rate of the former.
Although sporadic positive annual mass balances have
been observed on some glaciers, the average mass bal-
ance has been permanently negative over the past 50yr.
Interannual variability of mass balance is high, with
negative mass balance occurring much more frequently
than periods with near-equilibrium or positive mass bal-
ances, which occurred in only a few years. The variabil-
ity of the tropical Pacific SST is the main factor control-
ling thevariabilityof the mass balance at the interannual
to decadal timescale.
In the very high-altitude mountains of the tropical An-
des, radiationfluxes, andmore specifically the net short-
wave radiation budget, control the energy balance at the
glacier surface during the melt season. Furthermore, ab-
lation and accumulation processes are closely linked.
Indeed, through its effect on albedo, solid precipitation
mainly controls seasonal changes in energy fluxes and
hence in the mass balance of these glaciers.
Because precipitation has not displayed a significant
and spatially coherent trend in the tropical Andes
since the middle of the 20th century (unlike temper-
ature, which has increased at the significant rate of
0.10
Cdecade
1
in the last 70yr), we assume that at-
mospheric warming is the main factor explaining the
current glacierrecession. However, alarge proportion of
atmospheric warming is transmitted to a glacier through
precipitation via a change in phase.
Given the current climatic context, and the future
changes in atmospheric temperature projected by both
global andregional climate models, many glaciers in the
tropical Andes could disappear during the 21th century,
and those located below 5400ma.s.l. are the most vul-
nerable.
The ongoing recession of Andean glaciers will become in-
creasingly problematic for regions depending on water re-
sources supplied by glacierized mountain catchments, par-
ticularly in Peru and Bolivia. This issue was not specifically
discussed here as it is beyond the scope of the present re-
view, but it has been highlighted in several recent studies
(e.g. Bradley et al., 2006; Villacis, 2008; Kaser et al., 2010).
Hence, further efforts need to be undertaken on glacio-
hydrological modeling and analysis, together with locally
based water resource management studies(Bury etal., 2011),
in order to provide policy- and decision-makers with ade-
quate and useful information on how to manage water re-
sources in regions with rapidly shrinking glaciated areas.
Other outstanding issues should be answered by the com-
munity. For example, the following topics have to be ad-
dressed in the coming years: (1) Through what mechanisms
and dynamics are large-scale forcings (such as Pacific SST
and other not yet identified forcings) transmitted and scaled
down to the glacier surface? (2) How might future changes
in ENSO characteristics affect glaciers with different cli-
matic sensitivities in the inner and outer tropical Andes? (3)
In this review, mass balance and area/length changes have
been the main foci, but we did not consider glacier dynamics.
This reflects the fact that dynamics linking mass changes to
area/length changes remains poorly documented for tropical
glaciers. (4) Finally, the effects of glacier retreat and warm-
ing on natural hazards need to be better understood in the
Andes to be able to effectively reduce associated risks. For-
mation and growth of glacier lakes are of concern due to po-
tentially devastating lake outburst floods (Carey, 2005; Carey
et al., 2012).
www.the-cryosphere.net/7/81/2013/ The Cryosphere, 7, 81–102, 2013
98 A. Rabatel et al.: Current state of glaciers in the tropical Andes
Acknowledgements. This study was conducted within the frame-
work of the International Joint Laboratory GREAT-ICE, an
initiative of the French Institute of Research for Development
(IRD) and universities and institutions in Bolivia, Peru, Ecuador
and Colombia. IRD researchers: B. Pouyaud, R. Gallaire,
E. Cadier, P. Garreta and A. Coudrain are acknowledged for
their participation in the development of the glacier monitoring
program in the tropical Andes. We are grateful to all those
who took part in field campaigns for mass balance measure-
ments. We also appreciate collaboration with Nadine Salzmann,
Mario Rohrer, Walter Silverio, Cesar Portocarrero and Bryan
Mark. This study was funded by the French IRD (Institut de
Recherche pour le D
´
eveloppement) through the Andean part of
the French glaciers observatory service: GLACIOCLIM (http:
//www-lgge.ujf-grenoble.fr/ServiceObs/SiteWebAndes/index.htm)
and through the JEAI-IMAGE, and by the Grant SENESCYT-
EPN-PIC-08-506. We also acknowledge funding from, and
collaboration with, the Comunidad Andina de Naciones (CAN) and
the World Bank, the Inter-American Institute, the Swiss Agency
for Development and Cooperation, the Swiss Federal Office for the
Environment, the Servicio Nacional de Meteorolog
´
ıa e Hidrolog
´
ıa
del Per
´
u, Helvetas Swiss Intercooperation, US-NSF, the US State
Department and CARE. Special thanks to the USGS-EDC for
allowing us free access to Landsat images. SPOT images were
provided through the CNES/SPOT-Image ISIS program, contract
#503. Visualizations used in Fig. 1 were produced with the
Giovanni online data system, developed and maintained by the
NASA GES DISC. We also acknowledge the TRMM mission
scientists and associated NASA personnal for the production of the
data used.
Edited by: A. Klein
The publication of this article is financed by CNRS-INSU.
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... However, glaciers are decreasing in extent and mass during the dry season at an accelerated rate [1][2][3], due to the increase in global temperature caused by anthropogenic and natural causes [4][5][6]. As a result, glacier retreat is an essential indicator of climate change [2] and climate variability [7,8]. The tropical Andean glaciers (TAGs) are undergoing a rapid coverage reduction, which can economically impact populations in the tropical Andes, with effects on agriculture, drinking water, electricity generation, ecosystem integrity [9][10][11][12], cultural assets, and tourism activities [13][14][15], due to For the delimitation of the TAGs study area, we used the RGI (Version 6.0) [39], records to which we applied a catchment area or buffer of 1.5 km around each glacier to include the surrounding areas. ...
... Finally, we ought to mention as well that glacier surges are not approached in this work. Contrary to other Andean glaciers [44], where Remote Sens. 2022, 14,1974 4 of 21 these events are more commonly observed, glacier gains in the TAGs, usually resulting from surges, are sporadic [8] and occur to a minor extent. ...
... Finally, we ought to mention as well that glacier surges are not approached in this work. Contrary to other Andean glaciers [44], where these events are more commonly observed, glacier gains in the TAGs, usually resulting from surges, are sporadic [8] and occur to a minor extent. ...
Article
Full-text available
The fast retreat of the tropical Andean glaciers (TAGs) is considered an important indicator of climate change impact on the tropics, since the TAGs provide resources to highly vulnerable mountain populations. This study aims to reconstruct the glacier coverage of the TAGs, using Landsat time-series images from 1985 to 2020, by digitally processing and classifying satellite images in the Google Earth Engine platform. We used annual reductions of the Normalized Difference Snow Index (NDSI) and spectral bands to capture the pixels with minimum snow cover. We also implemented temporal and spatial filters to have comparable maps at a multitemporal level and reduce noise and temporal inconsistencies. The results of the multitemporal analysis of this study confirm the recent and dramatic recession of the TAGs in the last three decades, in base to physical and statistical significance. The TAGs reduced from 2429.38 km2 to 1409.11 km2 between 1990 and 2020, representing a loss of 42% of the total glacier area. In addition, the time-series analysis showed more significant losses at altitudes below 5000 masl, and differentiated changes by slope, latitude, and longitude. We found a more significant percentage loss of glacier areas in countries with less coverage. The multiannual validation showed accuracy values of 92.81%, 96.32%, 90.32%, 97.56%, and 88.54% for the metrics F1 score, accuracy, kappa, precision, and recall, respectively. The results are an essential contribution to understanding the TAGs and guiding policies to mitigate climate change and the potential negative impact of freshwater shortage on the inhabitants and food production in the Andean region.
... but over different surface states, including paramo, moraine, and glacier surfaces (Villacís, 2008;Maisincho, 2015) around the Antisana Glacier 15 (AG15) (Francou et al., 2000;2004). Initial results have shown the occurrence of an accelerated glacier retreat since the late 1970s (Cáceres et al., 2006;Rabatel et al., 2013). In addition, various Surface Energy Balance (SEB) modelings have been performed both at one point (Favier et al., 2004a) and at the scale of the entire glacier (Maisincho, 2015). ...
... y −1 for AG12 for 2005-2012 records) rough to the −0.6 m w.e. y −1 estimated by Rabatel et al. (2013) to the tropical glaciers. Therefore, results in AG12 can be used as a first proxy to determine the meltwater production and mass balance of Cayambe and Cotopaxi glaciers located in the Eastern Cordillera, which share similar climatic conditions. ...
Article
Full-text available
Tropical glaciers are excellent indicators of climate variability due to their fast response to temperature and precipitation variations. At same time, they supply freshwater to downstream populations. In this study, a hydro-glaciological model was adapted to analyze the influence of meteorological forcing on melting and discharge variations at Glacier 12 of Antisana volcano (4,735-5,720 m above sea level (a.s.l.), 1.68 km 2 , 0°29′S; 78°9′W). Energy fluxes and melting were calculated using a distributed surface energy balance model using 20 altitude bands from glacier snout to the summit at 30-min resolution for 684 days between 2011 and 2013. The discharge was computed using linear reservoirs for snow, firn, ice, and moraine zones. Meteorological variables were recorded at 4,750 m.a.s.l. in the ablation area and distributed through the altitudinal range using geometrical corrections, and measured lapse rate. The annual specific mass balance (−0.61 m of water equivalent-m w.e. y −1-) and the ablation gradient (22.76 kg m −2 m −1) agree with the values estimated from direct measurements. Sequential validations allowed the simulated discharge to reproduce hourly and daily discharge variability at the outlet of the catchment. The latter confirmed discharge simulated (0.187 m 3 s −1) overestimates the streamflow measured. Hence it did not reflect the net meltwater production due to possible losses through the complex geology of the site. The lack of seasonality in cloud cover and incident shortwave radiation force the reflected shortwave radiation via albedo to drive melting energy from January to June and October to December. Whereas the wind speed was the most influencing variable during the July-September season. Results provide new insights on the behaviour of glaciers in the inner tropics since cloudiness and precipitation occur throughout the year yielding a constant shortwave attenuation and continuous variation of snow layer thickness.
... Previous estimates of glacier mass changes throughout South America were based on either the glaciological method using in situ observations or gravimetric measurements from satellites with respective large uncertainties (Jacob et al., 2012;Gardner et al., 2013;Rabatel et al., 2013;Mernild et al., 2015). Both approaches have limitations in South America: the classic glaciological method based on stake measurements is restricted to a limited number of accessible glaciers and requires substantial extrapolation for regional assessments. ...
Thesis
Glaciers on Earth along other components of the cryosphere are important for the climate system. However, it is widely known that the vast majority of glaciers are retreating and thinning since the early part of the 20th century. Additionally, future projections have highlighted that at the end of the 21st century, glaciers are going to lose a considerable part of their remaining mass. These glacier changes have several implications for physical, biological and human systems, affecting the water availability for downstream communities and contribute to sea level rise. Unlike other regions, where glaciers are less relevant for the overall hydrology, glaciers in South America constitute a critical resource since minimum flow levels in headwaters of the Andean mountains are usually sustained by ice melt, especially during late summer and droughts, when the contribution from the seasonal snow cover is depleted. In the last decades, the number of studies has increased considerable, however, in the Southern Andes and the surrounding sub-Antarctic islands glaciers still are less studied in comparison with their counterparts in the Northern Hemisphere. The few studies on glacier mass balance in this region suggest a risk of water scarcity for many Andean cities which freshwater supply depends on glacial meltwater. Additionally, glaciers on sub-Antarctic islands have not been completely assessed and their contribution to the sea level rise has been roughly estimated. Hence, the monitoring of glaciers is critical to provide baseline information for regional climate change adaptation policies and facilitate potential hazard assessments. Close and long-range remote sensing techniques offer the potential for repeated measurements of glacier variables (e.g. glacier mass balance, area changes). In the last decades, the number of sensors and methods has increased considerably, allowing time series analysis as well as new and more precise measurements of glacier changes. The main goal of this thesis is to investigate and provide a detailed quantification of glacier elevation and mass changes of the Southern Andes with strong focus on the Central Andes of Chile and South Georgia. Six comprehensive studies were performed to provide a better understanding of the development and current status of glaciers in this region. Overall, the glacier changes were estimated by means of various remote sensing techniques. For the Andes as a whole, the first continent-wide glacier elevation and mass balance was conducted for 85% of the total glacierized area of South America. A detailed estimation of mass changes using the bi-static synthetic aperture radar interferometry (Shuttle Radar Topography Mission -SRTM- and TerraSAR-X add-on for Digital Elevation Measurements -TanDEM-X- DEMs) over the years 2000 to 2011/2015 was computed. A total mass loss rate of 19.43 ± 0.60 Gt a-1 (0.054 ± 0.002 mm a-1 sea level rise contribution) from elevation changes above ground, sea or lake level was calculated, with an extra 3.06 ± 1.24 Gt a-1 derived from subaqueous ice mass loss. The results indicated that about 83% of the total mass loss observed in this study was contributed by the Patagonian icefields (Northern and Southern), which can largely be explained by the dynamic adjustments of large glaciers. For the Central Andes of Chile, four studies were conducted where detailed times series of glacier area, mass and runoff changes were performed on individual glaciers and at a region level (Maipo River basin). Glaciers in the central Andes of Chile are a fundamental natural resources since they provide freshwater for ecosystems and for the densely populated Metropolitan Region of Chile. The first study was conducted in the Maipo River basin to obtain time series of basin-wide glacier mass balance estimates. The estimations were obtained using historical topographic maps, SRTM, TanDEM-X, and airborne Light Detection and Ranging (LiDAR) digital elevation models. The results showed spatially heterogeneous glacier elevation and mass changes between 1955 and 2000, with more negative values between 2000 and 2013. A mean basin-wide glacier mass balance of −0.12 ± 0.06 m w.e. a-1 , with a total mass loss of 2.43 ± 0.26 Gt between 1955–2013 was calculated. For this region, a 20% reduction in glacier ice volume since 1955 was observed with associated consequences for the meltwater contribution to the local river system. Individual glacier studies were performed for the Echaurren Norte and El Morado glaciers. Echaurren Norte Glacier is a reference glacier for the World Glacier Monitoring Service. An ensemble of different data sets was used to derive a complete time series of elevation, mass and area changes. For El Morado Glacier, a continuous thinning and retreat since the 20th century was found. Overall, highly negative elevation and mass changes rates were observed from 2010 onwards. This coincides with the severe drought in Chile in this period. Moreover, the evolution of a proglacial lake was traced. If drained, the water volume poses an important risk to down-valley infrastructure. The glacier mass balance for the Central Andes of Chile has been observed to be highly correlated with precipitation (ENSO). All these changes have provoked a glacier volume reduction of one-fifth between 1955 and 2016 and decrease in the glacier runoff contribution in the Maipo basin. The thesis closes with the first island-wide glacier elevation and mass change study for South Georgia glaciers, one of the largest sub-Antarctic islands. There, glaciers changes were inferred by bi-static synthetic aperture radar interferometry between 2000 and 2013. Frontal area changes were mapped between 2003 and 2016 to roughly estimate the subaqueous mass loss. Special focus was given to Szielasko Glacier where repeated GNSS measurements were available from 2012 and 2017. The results showed an average glacier mass balance of −1.04 ± 0.09 m w.e. a-1 and a mass loss rate of 2.28 ± 0.19 Gt a-1 (equivalent to 0.006 ± 0.001 mm a-1 sea level rise) in the period 2000-2013. An extra 0.77 ± 0.04 Gt a-1 was estimated for subaqueous mass loss. The concurrent area change rate of the marine and lake-terminating glaciers amounts to −6.58 ± 0.33 km2 a-1 (2003–2016). Overall, the highest thinning and retreat rates were observed for the large outlet glaciers located at the north-east coast. Neumayer Glacier showed the highest thinning rates with the disintegration of some tributaries. Our comparison between InSAR data and GNSS measurements showed good agreement, demonstrating consistency in the glacier elevation change rates from two different methods. Our glacier elevation and mass changes assessment provides a baseline for further comparison and calibration of model projection in a sparsely investigated region. Future field measurements, long-term climate reanalysis, and glacier system modelling including ice-dynamic changes are required to understand and identify the key forcing factors of the glacier retreat and thinning.
... Modern warming has created unsustainable conditions for high mountain glaciers, causing by enhanced warming, such as the transition from snow/firn to ice triggering albedo decrease, will further instigate the loss of Quelccaya's accumulation zone (Rabatel et al. 2013;Yarleque et al. 2018;Potocki, Mayewski, et al. 2022). Recently recovered ice cores from Quelccaya have exhibited loss of seasonal variations in stable isotopes due to melting and percolation through the firn, however, particle-based records still demonstrate preserved seasonality (Thompson et al. 2017). ...
Article
The goal of this dissertation is to assess past and present changes in the chemical climate of three high mountain regions: the Himalayas, Peruvian Andes, European Alps. In the first analysis, we report a comprehensive case study of chemical composition from streams, snow samples, and an ice core collected from around Mt. Everest Basecamp in the Khumbu region of Nepal during the late pre-monsoon period. Our findings document the 2019 pre-monsoon season, detailing chemistry from Cyclone Fani, spatial variability in snow and stream chemistry, and addressing potential pollution hazards, including the presence of heightened Pb in local streams and possible chemical signature of human waste. This is the first study to characterize the chemical composition of Khumbu Glacier ice/meltwater and create a detailed framework for pre-monsoon snow/water chemistry for the region. The second study details the correspondence in chemical climate resulting from two regional Central Andean firn cores (Quelccaya ice cap, Peru (5470 m asl) and Nevado Illimani glacier, Bolivia (6350 m asl)) and investigates the environmental proxies associated with Central Andean climate. The results show that the Quelccaya core has well-preserved environmental signals despite meltwater percolation, the two regional ice core records contain comparable signals and similar regional scale climatology, and past records of anthropogenic emissions, dust sources, volcanic emissions, forest fire signatures, evaporite salts, and marine-sourced air masses are evident within the cores. Moreover, annual layer thickness established from ultra-high-resolution chemical measurements on a near-basal ice core from the Quelccaya glacier indicates that Quelccaya ice cores drilled to bedrock may be ~1000 years older or more than previously suggested by depth-age models. Lastly, the third study focuses on a 2100-year record of heavy metals (Pb, As, Cd, Bi, Cu) extracted from a European Alps ice core, which is the first known case in which these elements have been observed in unison and at this resolution for this timespan in the European Alps. Chemical signatures resulting from decomposition analyses indicate a strong relationship between Pb, As, and Cd, a connection between Cu and marine chemistry (Na, K), and Bi retaining a unique signal that exhibits a distinct increase during the pre-industrial revolution between 1710-1850CE. We hypothesize that heavy metal concentrations likely reflect the timing of emerging technological advances related to the industrial revolution and periods of important past societal eras, including expanding mining activities, pandemics, and wars. In the Appendix, we address the importance of data analysis and visualizations in the field of Earth and Climate Sciences.
... Tropical glaciers are generally characterized by their small size, their position below the equilibrium line altitude (ELA), and by very intense long-and short-wave radiation as a consequence of the low latitudes and high atmospheric humidity conditions (Rabatel et al., 2013;Vuille et al., 2018). In Peru, Bolivia, and Ecuador there remain large glaciers that are expected to last for decades despite the extreme negative mass balance they have been subject to. ...
Article
This study extends knowledge of the evolution of glacier shrinkage in the Cocuy‐Güican Mountains since the maximum glacier extent of the Little Ice Age (LIA). Mass balance data for the Ritacuba Glacier since 2009 was acquired and compared with available data for the Conejeras Glacier (Los Nevados National Park). This study also investigated the hydrological significance of Colombian glaciers, which is still largely unknown because the available information is very limited. Glaciers in the Cocuy‐Güican Mountains covered 13.2 km2 in 2019 compared with 127.8 km2 during the maximum LIA, representing a shrinkage of 89.7%. Analysis of glacier cover observations made in 1955, 1994, 2010, and 2019 revealed that the rate of ice loss was greatest from 1994 to 2010 (0.59 km2 yr‐1), and was then almost halved from 2010 to 2019 (0.34 km2 yr‐1). This slowing of glacier retreat is consistent with the moderate negative mass balance measured for 2009‐2019, and an accumulated loss of 1766 mm w.e. (mm water equivalent). The progressive confinement of glaciers to higher elevations, ice accumulation in topographic locations providing shelter from solar radiation, and an absence of recent marked climatic anomalies could explain why the Cocuy‐Güican glaciers have temporally reached a near equilibrium state conditions. This contrasts with the Conejeras Glacier, where 47,000 mm w.e. has been lost in the same period. The available data on runoff and isotope tracers of streamflow and precipitation suggest that precipitation rather than glacier melt water exerts primary control over the hydrological variability at high elevation sites. This article is protected by copyright. All rights reserved.