TCThe CryosphereTCThe Cryosphere1994-0424Copernicus PublicationsGöttingen, Germany10.5194/tc-10-681-2016Thinning of the Monte Perdido Glacier in the Spanish Pyrenees since 1981López-MorenoJuan Ignacionlopez@ipe.csic.esRevueltoJesúshttps://orcid.org/0000-0001-5483-0147RicoIbaiChueca-CíaJavierJuliánAsunciónSerretaAlfredoSerranoEnriqueVicente-SerranoSergio MartínAzorin-MolinaCesarAlonso-GonzálezEstebanGarcía-RuizJosé MaríaDept. of Geoenvironmental Processes and Global Change, Pyrenean Institute of Ecology, CSIC, Campus de Aula Dei, P.O. Box 13.034, 50.080 Zaragoza, SpainDept. of Geography, University of the Basque Country, Prehistory and Archeology, Vitoria, SpainDept. of Geography, University of Zaragoza, Zaragoza, SpainDept. of Graphic Design and Engineering, University of Zaragoza, Huesca, SpainDept. of Geography, University of Valladolid, Valladolid, SpainJuan Ignacio López-Moreno (nlopez@ipe.csic.es)17March201610268169410August201529September20159March201610March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://tc.copernicus.org/articles/10/681/2016/tc-10-681-2016.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/10/681/2016/tc-10-681-2016.pdf
This paper analyzes the evolution of the Monte Perdido Glacier,
the third largest glacier in the Pyrenees, from 1981 to the present. We
assessed the evolution of the glacier's surface area by analysis of aerial
photographs from 1981, 1999, and 2006, and changes in ice volume by geodetic
methods with digital elevation models (DEMs) generated from topographic maps
(1981 and 1999), airborne lidar (2010) and terrestrial laser scanning (TLS,
2011, 2012, 2013, and 2014) data. We interpreted the changes in the glacier
based on climate data from nearby meteorological stations. The results
indicate that the degradation of this glacier accelerated after 1999. The
rate of ice surface loss was almost three times greater during 1999–2006
than during earlier periods. Moreover, the rate of glacier thinning was 1.85
times faster during 1999–2010 (rate of surface elevation change =-8.98±1.80 m,
glacier-wide mass balance =-0.73±0.14 m w.e. yr-1)
than during 1981–1999 (rate of surface elevation change =-8.35±2.12 m, glacier-wide mass balance =-0.42±0.10 m w.e. yr-1).
From 2011 to 2014, ice thinning continued at a slower rate (rate
of surface elevation change =-1.93±0.4 m yr-1, glacier-wide
mass balance =-0.58±0.36 m w.e. yr-1). This deceleration in
ice thinning compared to the previous 17 years can be attributed, at least
in part, to two consecutive anomalously wet winters and cool summers
(2012–2013 and 2013–2014), counteracted to some degree by the intense thinning that
occurred during the dry and warm 2011–2012 period. However, local climatic
changes observed during the study period do not seem sufficient to explain
the acceleration of ice thinning of this glacier, because precipitation and
air temperature did not exhibit statistically significant trends during the
study period. Rather, the accelerated degradation of this glacier in recent
years can be explained by a strong disequilibrium between the glacier and
the current climate, and likely by other factors affecting the energy
balance (e.g., increased albedo in spring) and feedback mechanisms (e.g., heat
emitted from recently exposed bedrock and debris covered areas).
Introduction
Most glaciers worldwide have undergone intense retreat since the culmination
of the Little Ice Age (LIA), believed to have been in the mid-19th century,
as indicated by measurements of ice surface area and volume (Vincent et al.,
2013; Marshall, 2014; Marzeion et al., 2014 and 2015; Zemp et al., 2014).
This trend has apparently accelerated in the last three decades (Serrano et
al., 2011; Mernild et al., 2013; Carturan et al., 2013a; Gardent et al.,
2014; López-Moreno et al., 2014). Marshall (2014) and Zemp et al. (2015)
noted that loss of global glacier mass during the early 21st century
exceeded that of any other decade studied. Several studies have examined
this phenomenon in Europe. In the French Alps, glacier shrinkage has
accelerated since the 1960s, mainly in the 2000s (Gardent et al., 2014). In
the Ötztal Alps (Austria), the loss of glacier area was calculated to be
-0.4 % per year from 1969 to 1997 and -0.9 % per year from 1997 to 2006
(Abermann et al., 2009). In the Central Italian Alps, the average annual
decrease in glacier area was found to be approximately 10-fold greater
during 1990–2007 than during 1860–1990 (Scotti et al., 2014), while the rate
of ice mass loss in the long term monitored at Careser Glacier (Italian Alps)
during 1981–2006 (-1.3 meter of water equivalent per year; hereafter m w.e. yr-1)
was about twice that for the period 1933–1959 (-0.7 m w.e. yr-1)
(Carturan et al., 2013b). Over a similar period (1980–2010), the
rate of ice mass loss calculated for the Swiss Alps was -0.65 m w.e. yr-1
(Fischer et al., 2015), which clearly exceeds the values presented
by Huss et al. (2010) for the same region over the 20th century (close to
-0.25 m w.e. yr-1). In the Sierra Nevada of southern Spain, the Veleta
Glacier, which formed during the LIA, evolved into a rock glacier during the
mid-20th century and has suffered marked degradation during the last two
decades (Gómez-Ortiz et al., 2014).
The glaciers in the Pyrenees, which are among the southernmost in Europe
(Grunewald and Scheithauer, 2010), have also undergone significant retreat.
In 1850, these glaciers had an estimated area of 2060 ha, but this had
decreased to 321 ha by 2008 (René, 2013). Since 1850, the
different massifs have had variable reductions in area covered by ice, with
a 59 % reduction in the Vignemale Massif and an 84 % reduction in the
Posets–Llardana Massif (Gellatly et al., 1995; René, 2013). A total of
111 glaciers disappeared in the Pyrenees from 1850 to 2005, and only 31
actual glaciers (with ice motion) remain. There has been a rapid glacial
recession since the 1990s, and many of these glaciers face imminent
extinction. Chueca et al. (2005 and 2008) reported that the rates of glacial
shrinkage during the last two decades of the 20th century and the
beginning of the 21st century were similar to those observed from 1860
to 1900, immediately after the end of the LIA. A similar conclusion has been
reached by Marti et al. (2015) for the Ossoue Glacier (French Pyrenees).
Most studies agree that global warming is responsible for the observed
glacier shrinkage and the recent acceleration of this shrinkage and glacier
thinning. The air temperature increase has been particularly strong since
the 1970s in most mountain ranges of the world (Haeberli and Beniston, 1998;
Beniston et al., 2003; Nogués-Bravo et al., 2008; Gardent et al., 2014).
Global warming has increased the equilibrium line altitudes (ELAs) and
reduced the accumulation area ratios (AARs) of glaciers, such that most
glaciers are not in equilibrium with current climate (Mernild et al., 2013)
and many cannot persist for much longer (Pelto, 2010). In the case of the
Pyrenees, the annual air temperature has increased by a minimum of
0.9 ∘C since the culmination of the LIA (Dessens and
Bücher, 1995; Feulliet and Mercier, 2012). More recently, Deaux et al. (2014)
reported an increase of 0.2 ∘C decade-1 for the
1951–2010 period. This air temperature increase explains the ∼255 m
increase in the elevation of the ELA of the glaciers of the Maladeta
Massif since the culmination of the LIA, which is currently close to 2950 m a.s.l.
(Chueca et al., 2005). The decreased accumulation of snow and the
increase in air temperature during the ablation season are thought to be the
principal causes of recent glacier decline on the southern (Spanish) side of
the Pyrenees (Chueca et al., 2005).
Glaciers are very good indicators of climate change due to their high
sensitivity to anomalies in precipitation and air temperature (Carrivick and
Brewer, 2004; Fischer et al., 2015). However, it is not always easy to
establish a direct relation between annual fluctuations of climate and the
changes in the area and mass of a particular glacier. This is difficult
because only glaciers of small size respond rapidly to changes in annual
snowfall and snow/ice melt, whereas mid-sized and large glaciers respond
much more slowly (Marshall, 2014). Moreover, very small glaciers may develop
and evolve for reasons unrelated to the regional long-term, monthly or
seasonal climatic evolution, for example, from avalanches, wind drift and
new rock exposure. In the case of shrinking glaciers, the latter can be a
key driver of glacier thinning (Chueca and Julián, 2004; Serrano et al.,
2011; Carturan et al., 2013c). Local topography also has a considerable
effect on the development of ice bodies, and can cause notable variations in
the ELAs of different glaciers in the same region (Reinwarth and
Escher-Vetter, 1999; Carrivick and Brewer, 2004; López-Moreno et al.,
2006). Moreover, many studies of recent changes in glaciers examined the
evolution of glacier surface areas or lengths. These parameters respond to
climate fluctuations, but their relationship to climate is also affected by
geometric factors (Haeberli, 1995; Carturan et al., 2013a). Thus, direct
mass-balance estimations or geodetic methods that determine changes in ice
volume provide better information on the relationship between changes in
glacier characteristics and changes in climate (Chueca et al., 2007; Cogley,
2009; Fischer et al., 2015). In the Pyrenees, only a few estimates of ice
volume loss have been published (Del Río et al., 2014; Sanjosé et
al., 2014; Marti et al., 2015), whereas there is abundant research on recent
changes in glaciated surface areas (Chueca et al., 2005; López-Moreno et
al., 2006; González-Trueba et al., 2008). Annual estimates of glacier
mass fluctuations based on the glaciological method have only been performed
on the Maladeta Glacier (Spanish Pyrenees) and on the Ossoue Glacier (French
Pyrenees), with the findings indicating a mean glacier thinning of -14 m
during the last 20 years for the Maladeta Glacier and -22 m for the Ossoue
Glacier (Arenillas et al., 2008; René, 2013; Marti et al., 2015). Other
studies in the Spanish Pyrenees compared digital elevation models (DEMs)
derived from topographic maps from 1981 and 1999 in the Maladeta Massif
(Chueca et al., 2008) and the Monte Perdido Glacier (Julián and Chueca,
2007), reporting rates of loss (glacier-wide mass balance) of -0.36
and -0.39 m w.e. yr-1, respectively.
Monte Perdido study area and extent of ice cover at the
culmination of the Little Ice Age (according to the map of Schrader, 1874)
and in 2008. Red squares mark the scanning positions, numbered points
indicate the positions of the fixed targets used for georeferencing and
merging the different clouds of points. Red circles in the right panel
inform of the location of the analyzed meteorological stations.
This paper focuses on the recent evolution of the Monte Perdido Glacier, the
third largest glacier in the Pyrenees. We document changes in the glacier
surface area from 1981 to 2006 and provide updated information on surface
elevation changes by comparing DEMs derived from topographic maps from 1981
and 1999 (Julian and Chueca, 2007), a new DEM obtained in 2010 from airborne
lidar, and four successive terrestrial laser scanning (TLS) surveys that
were performed during the autumns of 2011, 2012, 2013, and 2014. We examined
these data in connection with data on precipitation, snow depth, and air
temperature since 1983 from the closest meteorological station, and three
longer air temperature and precipitation records (1955–2013) from
neighboring stations. Identifying changes during recent years in this region
is particularly important because in the 21st century snowfall accumulation
has been higher and the air temperatures slightly cooler than in the last
decades of the 20th century. This shift is associated with a
persistently positive North Atlantic Oscillation (NAO) index during the beginning
of the 21st century (Vicente-Serrano et al., 2011; Buisan et al., 2015). The
recent response of remnant ice bodies to this climatic anomaly is as yet
unknown. Moreover, the availability of annual TLS data in recent years
permits detailed examination of the relationship between changes in climate
and glaciers.
Study area and review of previous research on the Monte Perdido
Glacier
The Monte Perdido Glacier (42∘40′50′′ N 0∘02′15′′ E) is
located in the Ordesa and Monte Perdido National Park (OMPNP) in the Central
Spanish Pyrenees (Fig. 1). The ice masses are north-facing, lie on
structural flats beneath the main summit of the Monte Perdido peak (3355 m),
and are surrounded by vertical cliffs of 500–800 m in height
(García-Ruiz and Martí-Bono, 2002). At the base of the cliffs, the
Cinca River flows directly from the glacier and the surrounding slopes, and
has created a longitudinal west–east basin called the Marboré Cirque
(5.8 km2).
Scientists have studied glaciers in the Marboré Cirque since the
mid-19th century (Schrader, 1874), and many subsequent studies examined the
status and extent of the ice masses and moraine features deposited during
the LIA (Gómez de Llarena, 1936; Hernández-Pacheco and Vidal Box,
1946; Boyé, 1952). More recent studies have established the locations of
moraines to deduce the dynamics and extent of LIA glaciers (Nicolás,
1981 and 1986; Martínez de Pisón and Arenillas, 1988; García
Ruiz and Martí Bono, 2002; Martín Moreno, 2004) and have analyzed
environmental changes during the Holocene through the study of sediments in
Marboré Lake (Oliva-Urcía et al., 2013) and by dating of Holocene
morainic deposits (García-Ruiz et al., 2014).
The map of Schrader (1874), numerous old photographs, and the location of
the LIA moraines (García Ruiz and Martí Bono, 2002) indicate a
unique glacier at the foot of the large north-facing wall of the Monte
Perdido Massif (Monte Perdido, Cilindro and Marboré peaks) (Fig. 1).
The map of Schrader (1874) distinguishes the Cilindro–Marboré Glacier,
with three small ice tongues that joined at the headwall, from the Monte
Perdido Glacier, which was divided into three stepped ice masses connected
by serac falls until the mid-20th century. The glacier that existed at the
lowest elevation was fed by snow and ice avalanches from the intermediate
glacier, but disappeared during the 1970s (Nicolás, 1986;
García-Ruiz et al., 2014). The two remaining glacier bodies, which are
currently unconnected, are referred to in this paper as the upper and lower
Monte Perdido Glaciers. The glacier beneath the Cilindro and Marboré
peaks has transformed into three small and isolated ice patches
(García-Ruiz et al., 2014). It is noteworthy that Hernández-Pacheco
and Vidal Box (1946) previously estimated a maximum ice thickness of 52 m
for the upper glacier and 73 m for the lower glacier. In 2008, 82 % of the
ice cover present at the end of the LIA had already disappeared. The upper
and lower ice bodies have mean elevations of 3110 and 2885 m (Julián
and Chueca, 2007). Despite the high elevation of the upper glacier, snow
accumulation is limited due to the minimal avalanche activity above the
glacier and its marked steepness (≈40∘).
No direct observation has been made of the current location of the ELA in
the upper Cinca valley, but studies at the end of the 20th and beginning of
the 21st century placed it at about 2800 m in the Gállego Valley, west
of the OMPNP (López-Moreno, 2000), and at about 2950 m in the Maladeta
Massif, east of the OMPNP (Chueca et al., 2005). The mean annual air
temperature at the closest meteorological station (Góriz at 2250 m a.s.l.,
2.7 km from the glacier) is 5.03 ∘C, although this
station is on the south-facing slope of the Monte Perdido Massif. Assuming a
lapse rate of 0.55 to 0.65 ∘C every 100 m,
the annual 0 ∘C isotherm should be roughly at 2950 to
3150 m a.s.l. The climate in this region can be defined as high-mountain
Mediterranean. Precipitation as snow can fall on the glacier at any time of
year, but most snow accumulation is from November to May, and most ablation
is from June to September.
Data and methodsComparison of DEMs
DEMs from different dates can be used to
calculate surface elevation changes. This technique is well established for
the study of glaciers in mountainous areas (Favey et al., 2002), and we have
previously applied it in several studies of the Pyrenees (Chueca and Julián, 2004;
Chueca et al., 2007; Julián and Chueca, 2007). Thus, we used three DEMs to
estimate the surface elevation changes in the Monte Perdido Glacier. Two
DEMs (1981 and 1999) were derived from topographic maps and one (2010) was
from airborne lidar measurements. All three DEMs had a cell size of 2×2 m,
and they were used in the context of a geographic information system (GIS),
working under the European Datum ED50 (UTM projection, zone 30).
The 1981 DEM was obtained from the cartography published by the Spanish
Instituto Geográfico Nacional (IGN) (Sheet 146-IV, Monte Perdido; Topographic National Map Series, scale
1 : 25 000). This map was published in 1997 and its cartographic restitution
was based on a photogrammetric flight in September 1981. The 1999 DEM was
also derived from cartography published by the IGN (Sheet 146-IV, Monte
Perdido; Topographic National Map Series MTN25, scale 1 : 25 000). It was
published in 2006 and its cartographic restitution was based on a
photogrammetric flight in September 1999. The 2010 DEM was obtained from an
airborne lidar flight (MDT05-LIDAR) made by the IGN in September 2010 in the
context of the National Plan for Aerial Orthophotography (NPAO).
The root mean squared error (RMSE) in elevation calculated by the IGN for
their digital cartographic products at 1 : 25 000 scale is ±1.5 and
±0.2 m for their lidar-derived DEMs. To verify these accuracies, we
compared 2010–1999, 2010–1981 and 1999–1981 pairs of DEMs in areas of
ice-free terrain situated near the studied glaciers. The results showed good
agreement with the accuracy indicated by the IGN in almost all areas,
although larger vertical errors were identified in several sectors with very
steep terrain (slopes > 65∘ in most cases)
located in the Monte Perdido glacial cirque (sharp-edged crests and abrupt
cliffs linked to the geological and structural disposition of the area). In
those sectors, differences between the DEMs reached 10–15 m. As both the
upper and lower Monte Perdido glaciers are placed well outside those areas
and have smoother topographical surfaces, it was assumed that the altimetric
data provided by the IGN has an appropriate consistency over the glaciated
terrain.
The combined vertical RMSE for DEM differences was < 2.5 m for 1999
minus 1981, and < 2.0 m for 2010 minus 1999. In the latter case it
must be noted that different geodetic methods (photogrammetric and airborne
lidar) were used in the comparison and that this may affect the accuracy of
the surface elevation changes (Rolstad et al., 2009). In any case, both
these errors were considered precise enough for the purposes of the present
work, as the surface elevation changes obtained in our analysis were
generally much greater than these errors. The estimation of surface
elevation changes was performed in ArcGIS comparing, by cut and fill
procedures, pairs of glacier surface DEMs (1981–1999 and 1999–2010). The
glacial perimeters associated with each DEM date were retrieved from aerial
photographs (1981: Pirineos Sur flight, September 1981, scale of 1 : 30 000, black and
white; 1999: Gobierno de Aragón flight, September 1999, scale of 1 : 20 000, color). There were
no high quality flights for 2010, so 2006 aerial photographs were used
(PNOA2006 Flight, August 2006, scale of 1 : 5000, color). The 1999 and 2006
photographs were already orthorectified, but we had to correct the geometry
and georeference the aerial survey of 1981 by use of the georeferencing
module of ArcGIS. The reference for the control points was from the
orthophotos and DEM data from 1999. The horizontal RMSE accuracy of the set
of control points ranged from 2.1 to 4.7 m, and was considered sufficiently
precise for our study. The maximum horizontal error was used to calculate
the uncertainty in the glaciated areas and their temporal changes. This
uncertainty was calculated using the buffer tool in ArcGIS. This tool allowed
quantification of the area of the polygon generated with the maximum
horizontal error around the perimeter of the glacier. A resampling procedure
using cubic convolution was used to generate the final rectified images.
The most recent estimates of the evolution of the glacier were from annual
TLS surveys. Lidar technology has developed rapidly in recent years, and
terrestrial and airborne lidar have been used in diverse geomorphology
studies, including monitoring changes in the volume of glaciers (Schwalbe et
al., 2008; Carturan et al., 2013b). The device employed in the present study
is a long-range TLS (RIEGL LPM-321) that uses time-of-flight technology to
measure the time between the emission and detection of a light pulse to
produce a three-dimensional point cloud from real topography. The TLS used
in this study employed light pulses at 905 nm (near-infrared), which is
ideal for acquiring data from snow and ice cover (Prokop, 2008;
Grünewald et al., 2010; Egli et al., 2011), a minimum angular step of
0.0188∘, a laser beam divergence of 0.0468∘, and a maximum working distance of 6000 m.
When TLS is used for long distances, various sources of error must be
considered, in particular, the instability of the device and errors from
georeferencing the clouds of points (Reshetyuk, 2006). We used an almost
frontal view of the glacier (similar to the view used for the photos shown
in Fig. 4) with minimal shadow zones in the glacier and a scanning
distance of 1500 to 2500 m. We also used indirect registration, also called
target-based registration (Revuelto et al., 2014), so that scans from
different dates (September of 2011 to 2014) could be compared. Indirect
registration uses fixed reference points (targets) that are located in the
study area. Eleven reflective targets of known shape and dimensions
(cylinders of 10×10 cm for targets located closer than 200 m, and 50×50 cm
squares for longer distances) were placed at reference points on rocks
situated 200 to 500 m from the scan station. Using standard topographic
methods, we obtained accurate global coordinates for the targets by use of a
differential global positioning system (DGPS) with post-processing. The
global coordinates were acquired in the UTM 30 coordinate system in the
ETRS89 datum. The final precision for the set of target coordinates was
±0.05 m in planimetry and ±0.10 m in altimetry. A total of 65
reference points around the ice bodies (identifiable sections of rocks and
cliffs) were used to assess measurement accuracy. Ninety percent of the
reference points had an error of less than 0.40 m. Thus 0.4 m was taken as
the uncertainty (error bars) when calculating the ice thinning and mass loss
rates. The conversion of mean surface elevation change to annual mass budget
rate was performed by applying a mean density of 900 kg m-3 (Chueca et
al., 2007; Marti et al., 2015). Use of this value assumes the absence of
firn, which has a lower density. This assumption was mostly valid at the end
of the study period, but likely some firn was present during the early 1980s
(Fig. 3a suggests the presence of firn). Unfortunately, a lack of
additional information forced us to adopt this generalization, which may
have led to a slight overestimation of the mass loss rate for 1981–1999.
Climatic data
The Spanish Meteorological Agency (AEMET) provided climatic data from the
Góriz manual weather station, located at 2250 m a.s.l. on the southern
slope of the Monte Perdido Massif. The absence of changes in instrumentation
and observation practices in the meteorological station since 1983, and the
proximity of the meteorological station to the glacier (2.7 km), suggest
that the station accurately recorded the climate variability over the
glacier. The climatic record consists of daily data of air temperature,
precipitation, and snow depth. From these data, we derived annual series of
maximum and minimum air temperatures for the main periods of snow
accumulation (November–May) and ablation (June–September), precipitation
during the accumulation season, and maximum snow depth in April (generally
the time of maximum snowpack at this meteorological station). The lack of
detailed meteorological or mass balance data over the glacier made it
necessary to define the accumulation and the ablation seasons in a
subjective manner based on our experience. We are aware that May and October
are transitional months between accumulation and ablation conditions
depending on specific annual conditions. However, we set these periods
because June and November are the months when ablation and accumulation,
respectively, become generally evident over the surface of the glacier. The
statistical significance of the linear climate trends was assessed using the
non-parametric correlation coefficient of Mann–Kendall's τ-b (Kendall and
Gibbons, 1990). Results obtained for Góriz were contrasted with those
from three other observatories (see Fig. 1) with precipitation (Pineta,
Aragnouet and Canfranc), and air temperature (Mediano, Aragnouet and
Canfranc) data for 1983–2013, and also for 1955–2013. The non-parametric
Mann–Whitney U test (Fay and Proschan, 2010) was used to detect
statistically significant differences in the medians of precipitation and
air temperature when the 1983–1999 and 2000–2010 periods were compared.
ResultsClimatic evolution and variability from 1983 to 2014
Figure 2 illustrates the high interannual variability of climate at
Góriz station since 1983. The average maximum air temperatures at
Góriz during the snow accumulation and ablation seasons showed no
significant trends, with τ-b values close to 0 (Fig. 2a and b). The
range between the highest and lowest average seasonal anomalies during the
study period exceeded 3 and 4 ∘C during
the accumulation and ablation periods, respectively, for maximum and minimum
air temperatures. The average minimum air temperatures exhibited very weak
increases in both seasons, but these were not statistically significant
(p < 0.05). The interannual air temperature range was larger for the
accumulation period (∼ 5∘C) than for the
ablation period (∼ 2.5∘C). Table 1 shows that
the evolution of air temperature at Góriz is in line with that observed
at the three other meteorological stations (Mediano, Aragnouet and
Canfranc). No statistically significant trends were observed for maximum or
minimum air temperature during the 1983–2013 period. When maximum and
minimum air temperature data were considered on a monthly basis, the four
analyzed observatories exhibited statistically significant increases only in
May and June, and statistically significant decreases only in November and
December. The Mann–Whitney test did not reveal statistically significant
differences in the medians of the series for the accumulation and ablation
seasons at any observatory when the periods 1983–1999 and 2000–2010 were
compared.
Interannual fluctuations and overall trends (straight lines) of
minimum and maximum air temperatures during the accumulation and ablation
periods, precipitation during the accumulation period, and maximum snow
depth during April, based on data from the Góriz meteorological station
(1983 to 2014). Box plots at the right of each panel show the interannual
variability during the last three years of the study period (2011/12, 2012/13,
and 2013/14), when terrestrial laser scanning measurements were available.
Box: 25th and 75th percentiles, bars: 10th and 90th percentiles, dots: 5th
and 95th percentiles, black line: median, red line: average.
τ-b values of the trends for the period 1983–2013 for air
temperature and precipitation in the analyzed stations. Asterisks indicate
statistically significant trends (p<0.05). Bold numbers are
statistically significant differences in the medians of the period 1983–1999
and 2000–2010 according to the Mann–Whitney test.
Interannual fluctuations of minimum and maximum air temperatures
during the accumulation and ablation periods and precipitation during the
accumulation period at the Aragnouet, Canfranc, Mediano (only air
temperature) and Pineta (only precipitation) stations for 1955–2013. Numbers
give the τ-b values f the trends. Asterisks indicate statistically
significant trends (p<0.05).
Photographs of the Monte Perdido Glacier during late summer in
1981 and 2011.
Surface area (ha), change of surface area (ha), and annual rate of
surface area loss (ha yr-1) of the Monte Perdido Glacier.
Precipitation at Góriz during the accumulation period also exhibited
strong interannual variability, with a range of approximately 600 to 1500 mm (Fig. 2e).
The trend line showed a slight increase, but this was not
statistically significant. Similarly, maximum snow accumulation during April
varied from less than 50 to 250 cm, and there was no evident trend during
the study period (Fig. 2f). Monthly trend analysis (Table 1) found a
significant increase in precipitation at Góriz only during May, and near
zero τ-b coefficients for most other months. Very similar results were
obtained for the other three analyzed stations (Pineta, Aragnouet and
Canfranc), with no statistically significant trends for the accumulation and
ablation periods. Significant increases in precipitation were observed at
Aragnouet only in May, and at Pineta only in March. No statistically
significant differences in the median precipitation during the accumulation
and ablation seasons of the 1983–1999 and 2000–2010 periods were found at
any of the analyzed meteorological stations.
Figure 3 shows the interannual evolution of air temperature and
precipitation series for a longer time period (1955–2013). The data
illustrate that the climate observed during the main study period
(1983–2013) is not necessarily representative of the longer climate series.
Thus, the 1955–2013 period exhibited statistically significant (p < 0.05)
warming during the ablation period, and the accumulation exhibited
positive τ-b values but did not reach statistical significance.
Precipitation during the accumulation period did not exhibit statistically
significant trends during the period 1955–2013 in any of the three
observatories analyzed.
Figure 2 also shows that 2011–2014, the period for which TLS measurements of
annual glacier evolution were available, showed extremely variable
conditions. Mid-September 2011 to mid-September 2012 was one of the warmest
recorded years (especially during the ablation period, which was in the 96th
and 74th percentiles for maximum and minimum air temperature, respectively)
and with a rather dry accumulation period (27th percentile). The period of
2012 to 2013 had an accumulation period that was more humid than average
(59th percentile) and the coolest recorded summer (1st and 18th
percentiles for maximum and minimum air temperatures, respectively), while the
accumulation period of 2013 to 2014 was very wet (78th percentile) and
slightly cooler than average, with air temperatures around or below the
average (22th and 48th percentiles for maximum and minimum temperature,
respectively) during the ablation period.
Glacier evolution from 1981 to 2010
Figure 4 shows photographs of the Monte Perdido Glacier taken in late summer
in 1981 and 2011. A simple visual assessment shows the degree of degradation
of the glacier during this 30-year period. In 1981, the upper and lower
glaciers were no longer united (they became disconnected between 1973 and
1978), and they exhibited a convex surface and significant ice depth with
noticeable seracs hanging from the edges of the cliffs. Both ice bodies were
heavily crevassed, with evidence of ice motion over the whole glacier. The
photograph from 2011 shows that the two ice bodies are further separated, as
well as showing a dramatic reduction in ice thickness, evident in the
concave surface, the disappearance of almost all seracs, and the retreat of
ice from the edges of the cliffs. Crevasses are only observed in the eastern
part of the lower glacier, indicating that the motion of the glacier has
slowed or stopped in most of the two ice bodies. Moreover, there are rocky
outcrops in the middle of the lower glacier and areas that are partially
covered by debris deposits, presumably originating from crevasses and rock
falls in the upper glacier.
Table 2 shows the surface area of the ice in 1981, 1999, and 2006. From 1981
to 1999 the glacier losses were -4.50±1.27 ha (a change of
-1.50±0.52 ha in the upper glacier and -3.00±1.21 ha in the
lower glacier), corresponding to an overall rate of -0.25±0.07 ha yr-1.
From 1999 to 2006, the glacier losses were -5.40±1.20 ha
(a change of -2.00±0.46 ha in the upper glacier and -3.40±1.16 ha
in the lower glacier), corresponding to an overall rate of -0.77±0.17 ha yr-1, more than three times the rate of the previous 18 years.
Comparison of the elevation of the glacier's surfaces derived from the DEMs
(1981 to 1999 vs. 1999 to 2010) also indicates an acceleration of glacier
thinning over time (Fig. 5). During the 1981–1999 period, the ice thinning
was -6.20±2.12 m in the upper glacier and -8.79±2.12 m in the
lower glacier (-8.35±2.12 m overall); thus, the mean rates of glacier
thinning were -0.34±0.11 m yr-1 and -0.48±0.11 m yr-1
(-0.46±0.11 m yr-1 overall or -0.42±0.10 m w.e. yr-1
as a glacier-wide mass balance), respectively. Moreover, the
changes in glacier elevation surface were not spatially homogeneous. No
sector of either glacier showed increased thickness, but some small areas of
the lower glacier showed only minor thinning, with declines in thickness of
less than 5 m. The largest losses of glacier thickness were in the lower
elevations and western regions of the upper and lower glaciers, with
decreases that exceeded 25 and 35 m, respectively. During the 1999–2010
period, the thinning was -7.95±1.8 m in the upper glacier and
-9.13±1.8 m in the lower glacier (-8.98±1.80 m overall),
corresponding to rates of -0.72±0.16 m yr-1 and -0.83±0.16 m yr-1 (-0.82±0.16 m yr-1 overall or -0.73±0.14 m w.e. yr-1), respectively. The spatial pattern of thinning
resembled the pattern from 1981 to 1999, but areas of noticeable glacier losses
were also observed further eastward. The smallest decreases were observed in
the higher elevation parts of the lower glacier and upper glacier, probably
due to more effective shading of these areas, and the greatest decreases
were observed in the lower reaches and central-eastern parts of both ice
bodies.
Surface elevation change in the upper and lower Monte Perdido
Glacier from 1981 to 1999 and from 1999 to 2010 based on comparison of DEMs.
Surface elevation change of the upper and lower Monte
Perdido Glacier based on terrestrial laser scanning from September of 2011
to 2012 (Fig. 5a), 2012 to 2013 (Fig. 5b), 2013 to 2014 (Fig. 5c), and 2011
to 2014 (Fig. 5d).
Surface elevation changes over the whole glacier, lower glacier,
and upper glacier for the same four time periods examined in Fig. 5. Box:
25th and 75th percentiles, black line: median, red line: average, bars: 10th
and 90th percentiles, dots: 5th and 95th percentiles.
Evolution of the Monte Perdido Glacier from 2011 to 2014 from TLS
measurements
Figure 6 shows the differences in glacier surface elevation between
consecutive annual scans (12 September 2011, 13 September 2012, and
14 September 2013) and the total change from 2011 to 2014. Figure 7 shows
the frequency distribution of change in surface elevation measured over the
glacier for these periods.
The period of mid-September 2011 to mid-September 2012 was very dry during
the accumulation period and very warm during the ablation period. These
conditions led to dramatic glacier thinning, with an average decrease of
-2.10 ± 0.40 m (-2.08±0.40 m in the upper glacier and
-2.12±0.40 m in the lower glacier). Ice thinning affected almost the
entire glacier (AAR = 3.5 %), and was particularly intense in the
western sectors of the upper and lower glaciers, where thinning exceeded
4 m. The few scattered points indicating depth increases in the
middle of the lower glacier likely derive from a rising of the ice surface due
to motion of existing crevasses.
Conditions were very different from 2012 to 2013, with a rather wet
accumulation period and very cool ablation period. These conditions led to
changes that contrasted sharply with those of the previous year, in that
large areas of the glacier showed increased surface elevation. Most of these
increases did not exceed 1.5 m, and most were in the highest elevation
areas of both ice bodies. Nonetheless, during this year, large areas
remained stable (AAR = 54 %) and some areas even exhibited noticeable
ice thinning (more than -1.5 m in the upper and lower glaciers). Despite the
excellent conditions for glacier development from 2012 to 2013, the average
increase in glacier surface elevation was only +0.24 ± 0.40 m
(+0.22 ± 0.40 m in the upper glacier and +0.28 ± 0.40 m in the
lower glacier). Very similar conditions occurred in 2013–2014, with very wet
accumulation months and below average air temperature during the ablation
period. Again, large areas exhibited moderate increases in surface elevation
(AAR was 41 %, sometimes exceeding 3 m), although there were
still areas that showed significant ice loss, with an average thinning of
-0.07±0.40 m (-0.08±0.40 m in the upper glacier and
-0.07±0.40 m in the lower glacier).
The overall result of a very negative year (2011–2012) for glacier
development followed by two years (2012–2013 and 2013–2014) of positive
conditions led to a net average thinning of -1.93±0.40 m
(-0.58±0.36 m w.e. yr-1 as glacier-wide mass balance), with some
regions experiencing thinning that exceeded 6 m. Only the areas of
the eastern part of the lower glacier that were at high elevations (around
the bergschrund) exhibited some surface elevation gain during this period
(AAR for the three years was 16 %), and this was typically less than
+1.5 m. Interestingly, the areas with the highest and lowest thinning
during 1981–2010 were similar to those with the highest and lowest thinning
during 2011–2014, indicating a consistent spatial pattern of glacier
shrinkage over time.
Discussion and conclusions
The results of this study indicate that the recent evolution of the Monte
Perdido Glacier was similar to that of many other glaciers worldwide
(Marshall, 2014; Vincent et al., 2013), especially those in Europe (Gardent
et al., 2014; Abermann et al., 2009; Scotti et al., 2014; Marti et al.,
2015), where glacier shrinkage has been occurring since the culmination of the LIA
and has clearly accelerated since 2000. More specifically, the annual loss of
area of the Monte Perdido Glacier was nearly three times greater during
1999–2006 than 1981–1999, and the glacier thinning from 1999 to 2010 was
almost double that observed from 1981 to 1999. Acceleration in glacier
thinning has also been reported for the Ossoue Glacier (French Pyrenees),
where the mass balance during the period 2001–2013
(-1.45 m w.e. yr-1) was almost 50 % greater compared to that
during 1983–2014 (-1 m w.e. yr-1) (Marti et al., 2015). Climatic
analyses suggest that the recent acceleration in the thinning of the Monte
Perdido Glacier cannot be explained solely by an intensification of climate
warming or by a decline in snow accumulation. Climate data (1983–2014) of a
nearby meteorological station, and three other Pyrenean meteorological
stations, suggest that during most of the year air temperature has not
exhibited statistically significant trends. The Mann–Whitney test did not
reveal statistically significant differences in air temperature when the
period 1983–1999 was compared to 2000–2010. Precipitation in the four
analyzed stations during the accumulation period and maximum annual snow
depth at Góriz were also stationary or slightly increased. Previous
studies of the Pyrenees and surrounding areas showed that air temperature
increased significantly throughout the 20th century, especially after the
relatively cold period from the 1960s to the mid-1970s (López-Moreno et
al., 2008; El Kenawy et al., 2012; Deaux et al., 2014). Similar trends were
detected in the three air temperature series analyzed for this study covering
the 1955–2013 period. At the same time, there was a regional significant
decline in snow accumulation from mid-March to late April/early May from 1950
to 2000 in the Pyrenees (López-Moreno, 2005). The trends during this
period of decreasing precipitation and milder air temperatures during winter
and early spring can be related to changes in the NAO index (López-Moreno et al., 2008). More recent studies that used
updated databases (including data of the 21st century) confirmed that a
shift towards more negative NAO has affected the recent evolution of air
temperature and precipitation over the Pyrenees. Thus, for the period 1983 to
2013, which does not include the effects of the cold and wet period of the
1960s to 1970s, no temporal trends of either variable are found near Monte
Perdido. Vicente-Serrano et al. (2011) found that the increased occurrence of
very wet winters during the 2000s was associated with frequent strong
negative NAO winters. In agreement, Buisan et al. (2015) reported that for
the period 1980–2013 the overall number of snow days in the Pyrenees
remained stationary and even slightly increased in some locations. In a more
recent study, Buisan et al. (2016) observed stationary behavior or slight
increases in snow water equivalent for the period 1985–2015 in the central
Spanish Pyrenees. The findings of Macias et al. (2014) support the view that
southern Europe and some other regions of the world have undergone clear
moderations of the warming trends that were recorded at the end of the 20th
century. Nonetheless, it is necessary to bear in mind that the longest
climatic records or dendroclimatological reconstructions for the Pyrenees
still point to the period considered in this study (1980–2014) as a very
strong positive anomaly of air temperature and a dry period compared to the
period since the end of the LIA (Büngten et al., 2008; Deaux et al.,
2014; Marti et al., 2015). More research is needed to fully assess the
implications of the air temperature increase detected in May and June at the
four analyzed meteorological stations. This warming could lead to less snow
accumulation at the end of the accumulation season and a longer ablation
period, and an early rise of albedo that may affect the mass and energy
balance of the glacier (Qu et al., 2014). Another factor that should be
considered in future research is the effect of increases in the slope of the
glacier due to greater thinning in the lower reaches. Increasing slope is
expected to affect snow accumulation on the glacier and might constitute
another feedback mechanism underlying the recent evolution of the glacier.
The glacier-wide mass balance rates presented in this study for 1980–1999
and 1999–2010 (-0.42 ± 0.1 and -0.73 ± 0.14 m w.e. yr-1,
respectively) are similar to those reported by Chueca et al. (2007) and
Pastor (2013) for the Maladeta Massif (-0.36 m w.e. yr-1 for 1981–1999
and -0.7 m w.e. yr-1 for 1991–2013). The most recent mass balance
values obtained for the Monte Perdido Glacier are more similar to those
reported for glaciers in the Swiss Alps (Fischer et al., 2015) or for the
best preserved glaciers in some areas of the Italian Alps (Carturan et al.,
2013a), but are lower than those of the fastest retreating glaciers in the
Alps (Carturan et al., 2013b) or that reported for the Ossoue Glacier
(French Pyrenees, -1.45 m w.e. yr-1 for 1983–2014). The smaller rates
of mass loss on the Spanish side of the Pyrenees than on the French side may
be explained by the location of the remnant ice bodies on the southern side
of the range, confined to the most elevated and the least exposed locations
in their respective cirques (López-Moreno et al., 2006). In contrast,
the Ossoue Glacier has maintained a considerable glacier tongue with an
eastward slope. In this context, the only explanation for the rapid
degradation of the Monte Perdido Glacier after 1999 is that the progressive
warming observed since the end of the LIA was responsible for a dramatic
reduction in the AAR, and most of this glacier is below the current ELA (at
3050 m a.s.l. during the three-year period 2011–2014, Fig. 6). Such a
reduction in AAR would lead to an imbalance that would likely be exacerbated
by negative feedbacks. Because of this imbalance, the glacier is not able to
recover ice losses during periods with favorable conditions (high
accumulation and/or little ablation in the frame of the 1983–2014 period).
This hypothesis is strongly supported by our detailed TLS measurements from
the last four years. In particular, these TLS data showed that two
consecutive anomalously positive years (2012/13 and 2013/14) did not allow
the glacier to recover the losses from a negative year (2011/12). Thus, the
glacier thinning during this three-year period was -1.93±0.4 m (-0.58±0.36 m w.e. yr-1), approximately one-quarter of the loss
during 1981–1999, and during 1999–2010. The AAR for 2011–2014 was 16 %,
and during a warm and dry year the ice thinning affects almost the whole
glacier (AAR = 3.5 %), indicating the lack of a persistent accumulation
zone. Pelto (2010) observed that this is a symptom of a glacier that cannot
survive. There can be years with mass gain, but mass loss occurs in most
years and the retained snowpack of positive years is lost in negative years,
such that there is no cumulative accumulation. Thus, the behavior observed
for the Monte Perdido Glacier during the study period is very likely
explained by very negative mass balance in some years, as can be seen in
Fig. 2. For example, air temperatures were very high in 2003, 2005 and
2012, and in the latter two years the high air temperatures were accompanied
by low winter precipitation. The feedback from decreased albedo and
increasing glacier slope may also have played a key role in the recent
acceleration of glacier thinning. Together, these findings indicate that the
future of the Monte Perdido Glacier is seriously threatened, even under
stationary climatic conditions. A ground-penetrating radar (GPR) survey of
the lower glacier in 2010 reported a maximum ice depth close to 30 m in the
westernmost part of the lower glacier (unpublished report), suggesting that
large areas of this glacier may disappear within the next few years. This
process may be accelerated by negative feedback associated with the recent
rise of rocky outcrops in the middle of the glacier and the thin cover of
debris, both of which may accelerate glacier ablation by decreasing the
albedo and increasing the emissivity of long-wave radiation. The highly
consistent spatial pattern of ice thinning in the last 30 years suggests
that the westernmost part of this glacier will disappear first; the
easternmost part will survive longer as a small residual ice mass because of
greater snow accumulation during positive years and a lower rate of
degradation. When the glacier is restricted to this smaller area, it is
likely that its rate of shrinkage will decrease, as observed for other
Pyrenean glaciers (López-Moreno et al., 2006).
The future long-term monitoring of the Monte Perdido Glacier should provide
important information on the year-to-year response of its mass balance to a
variety of climatic conditions, and will allow detailed analysis of the role
of positive and negative feedbacks in this much-deteriorated glacier. Thus,
this glacier may serve as a model for studies of the evolution of glaciers
in other regions of the world that have similar characteristics.
Acknowledgements
This study was funded by two research grants: “CGL2014-52599-P, Estudio del manto de nieve en la montaña española, y su respuesta a la variabilidad y cambio climático”
(IBERNIEVE-Ministry of Economy and Competitivity), and “El glaciar de Monte Perdido: estudio de su dinámica
actual y procesos criosféricos asociados como indicadores de procesos de cambio global” (MAGRAMA
844/2013). The authors are grateful for the support provided by the
“Dirección General de Conservación del Medio Natural (Government of
Aragón)” and to the staff of the Ordesa and Monte Perdido National Park
during our field campaigns.
Edited by: E. Berthier
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