TCThe CryosphereTCThe Cryosphere1994-0424Copernicus GmbHGöttingen, Germany10.5194/tc-9-1773-2015Evolution of Ossoue Glacier (French Pyrenees) since the end of the Little Ice AgeMartiR.renaud.marti@gmail.comGascoinS.https://orcid.org/0000-0002-4996-6768HouetT.RibièreO.LafflyD.CondomT.MonnierS.SchmutzM.CamerlynckC.TihayJ. P.SoubeyrouxJ. M.RenéP.Géographie de l'Environnement (GEODE), UT2J/CNRS, Toulouse, FranceCentre d'Etudes Spatiales de la Biosphère (CESBIO), UPS/CNRS/IRD/CNES, Toulouse, FranceLaboratoire d'étude des Transferts en Hydrologie et Environnement (LTHE), Université Grenoble-Alpes, Grenoble, FranceInstituto de Geografia, Pontificia Universidad Católica de Valparaiso, Valparaiso, ChileInstitut Polytechnique de Bordeaux (IPD), Pessac, FranceMilieux Environnementaux, Transferts et Interactions dans les hydrosystèmes et les Sols (METIS), UPMC/CNRS/EPHE, Paris, FranceUniversité de Pau et des Pays de l'Adour (UPPA), Pau, FranceMétéo France, Direction de la Climatologie (DCLIM), Toulouse, FranceAssociation Moraine, Luchon, FranceR. Marti (renaud.marti@gmail.com)8September201595177317956February201517April201531July201528August2015This 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/9/1773/2015/tc-9-1773-2015.htmlThe full text article is available as a PDF file from https://tc.copernicus.org/articles/9/1773/2015/tc-9-1773-2015.pdf
Little is known about the fluctuations of the Pyrenean glaciers. In this study, we reconstructed
the evolution of Ossoue Glacier (42∘46′ N, 0.45 km2), which is located in the
central Pyrenees, from the Little Ice Age (LIA) onwards. To do so, length, area, thickness, and
mass changes in the glacier were generated from historical data sets, topographical surveys,
glaciological measurements (2001–2013), a ground penetrating radar (GPR) survey (2006), and stereoscopic satellite images
(2013). The glacier has receded considerably since the end of the LIA, losing 40 % of its length
and 60 % of its area. Three periods of marked ice depletion were identified: 1850–1890,
1928–1950, and 1983–2013, as well as two short periods of stabilization: 1890–1894, 1905–1913, and
a longer period of slight growth: 1950–1983; these agree with other Pyrenean glacier
reconstructions (Maladeta, Coronas, Taillon glaciers). Pyrenean and Alpine glaciers exhibit similar
multidecadal variations during the 20th century, with a stable period detected at the end of the 1970s
and periods of ice depletion during the 1940s and since the 1980s. Ossoue Glacier fluctuations generally concur with climatic data (air temperature,
precipitation, North Atlantic Oscillation, Atlantic Multidecadal Oscillation). Geodetic mass balance over 1983–2013 was -1.04 ± 0.06 w.e.a-1
(-31.3 ± 1.9 m w.e.), whereas glaciological mass balance was -1.45 ± 0.85 m w.e. a-1
(-17.3 ± 2.9 m w.e.) over 2001–2013, resulting in a doubling of the ablation rate in the last
decade. In 2013 the maximum ice thickness was 59 ± 10.3 m. Assuming that the current ablation rate remains constant, Ossoue Glacier will disappear midway through the 21st century.
Introduction
The Pyrenees are a mountain range in southwestern Europe spanning ∼430km from the Bay of Biscay (Atlantic Ocean) to the Mediterranean
Sea. According to regional climate model projections, the thickness and
duration of its snowpack could decline over the 21st century
. However, analysis of snow depth observations over
1985–1999 in the Spanish Pyrenees showed contrasting trends, with increasing
snow depth above 2200 m elevation and decreasing snow depth below
2200 m. Tree-ring time series from living
trees and in situ relict samples, collected at elevations of
2200–2450 ma.s.l., have allowed the reconstruction of 1260–2005
summer temperatures in the Pyrenees. The data confirmed warming in the
twentieth century . For recorded data, the longest
meteorological time series in the French Pyrenees began in 1882 at an
astronomical observatory located on the Pic du Midi (2862 ma.s.l.).
A mean annual temperature increase of 0.83 ∘C was observed over
1882–1970 with a significant decrease in the mean annual diurnal temperature
range (2.89 ∘C per century), mainly due to a significant increase in
the daily minimum temperature
. Recent
work on data homogenization within the framework of the Pyrenean
Climate Change Observatory depicts a uniform warming for the massif
over the last 60 years, and highlights a significant warming
signal from the 1980s onwards .
The Pyrenees hosts the southernmost glaciers in Europe, all below 43∘ N
latitude. Their small sizes (<1km2), relatively low
elevations, and southern locations make them particularly vulnerable
to climate warming . Pyrenean glaciers are
strongly out of balance with regional climate and are quickly retreating
. While Pyrenean glaciers are in jeopardy, little is known about their evolution since the end of LIA.
Their comparisons with other mountain range glaciers (e.g. Alps) are rare, and hampered by fragmented data sets .
Due to the paucity of meteorological measurements, especially at high altitude, Pyrenean climate
proxy records are useful to complete past climate fluctuations at
secular scales. A part of this is that glaciers are considered robust
climate proxies ; their reconstruction may provide further independent evidence that the climate is changing.
More generally, however, retreat of Pyrenean glaciers could affect local
ecosystems by diminishing the beta diversity in Pyrenean streams
. Furthermore, natural patrimony and the visual perception of the
high mountain landscape could also be irrevocably affected .
Ossoue Glacier (42∘46′ N, 0.45 km2) is the second
largest glacier in the Pyrenees. In comparison with that of other Pyrenean
glaciers, the evolution of Ossoue Glacier is well documented, with
observations starting at the end of the 19th century. These include
historical data sets, topographic maps, aerial images, and stake
measurements.
The objective of this paper is to reconstruct the evolution of Ossoue Glacier based on the available data to provide further information:
on the evolution of a Pyrenean glacier since the end of the LIA
on the comparison between Pyrenean and Alpine glacial evolution
on the potential climate drivers of Ossoue Glacier
on the likely evolution of Ossoue Glacier in the near future.
The first section gives a brief review of studies on glaciers in the
Pyrenees (Sect. ). From there, and after describing the site of Ossoue Glacier (Sect. ), the data, and the methods used (Sect. ),
we propose a reconstruction that includes various
glacial metrics (i.e. glacier length, area, and mass balance) for Ossoue Glacier
(Sects. ). The combination of these
metrics allows us, for the first time, to depict a consistent evolution of the glacier since the LIA.
We compare the timeline of Ossoue Glacier fluctuations with that of Pyrenean and Alpine glacial reconstructions (Sect. ) and with the
meteorological time series (temperature, precipitation). We also mention
a possible connection with the North Atlantic Oscillation (NAO) and the
Atlantic Multidecadal Oscillation (AMO) (Sect. ). We
use ground penetrating radar (GPR) measurements collected in 2006 to estimate
the ice depth in the upper part of the glacier and to estimate the evolution
of the glacier thickness in the coming decades (Sect. ). The
implications and the limitations of the results are discussed in section
, and summarized in section .
Glacier studies in the Pyrenees
The last favourable period to glacier development in the Pyrenees was
the Little Ice Age (LIA), which occurred between the 14th and 19th
centuries . LIA climate cooling in the Pyrenees led
to the formation and advancement of glaciers in 15 massifs, in which
there are up to 100 cirques . In the middle
of the 19th century, after the respective advance and recession phases, the Pyrenean
glacier fronts reached positions close to their maximum LIA extent. At
that time, the area of Pyrenean glaciers is estimated to have been slightly
over 20 km2. Since then, their area
covered 8 km2 in 1984 ,
6 km2 in 2004 and approximately
3 km2 in 2013 .
Due to their remote locations and small sizes, Pyrenean glaciers have
not benefited from long-term glaciological
studies . That said, early topographic measurements were made by
“Pyreneists”, alpinists who became enthusiasts in the exploration
and observation of the Pyrenees
. However, it was not until the
Commission internationale des glaciers (CIG) was created in
1894 in Zürich, which led thereafter to the present-day International
Association of Cryospheric Sciences (IACS)
, that the situation slightly improved. Its first president, the Swiss scientist
François-Alphonse Forel, promoted the organized monitoring of
glaciers in the Pyrenees for comparison to the evolution of the
glaciers in the Alps . Prince Roland
Bonaparte established and communicated to the Commission the first
regular observations of glacier frontal variations between 1874 and
1895 . Next, Gaurier monitored the glaciers over
the period 1904–1927, which was interrupted by World War I
. On the French side of the Pyrenees, Eaux et Forêts,
the French national agency in charge of forest and
water management, took over the measurements in 1932 and after
World War II, during the 1945–1956 period . At
the end of the 1970s, under the initiative of François Valla from
the Centre Technique du Génie Rural et des Eaux et
Fôrets and with the support of the Parc National des
Pyrénées, the first, to our knowledge, mass balance measurements in the Pyrenees were performed at Ossoue and Taillon glaciers between
1978 and 1984 (with only qualitative data taken in 1983 and 1984)
. This initiative led to the creation of the
Groupe d'Etudes des Glaciers des Pyrénées (GEGP),
a collaborative group comprising the Institut National de
l'Information Géographique et Forestière (IGN) and
researchers at Pau University. Two topographic maps, dated from 1948
and 1983 (and shown below), were produced by the GEGP
. However, this group lasted only a few
years, so that, for the period between 1957 and 2001, only raw terrestrial and aerial
images are available for reconstructing glacier front and area
variations. Since 2001, a group of volunteer glaciologists called the
Association Moraine have performed regular glaciological field
measurements . On the Spanish side, the institutional
program Evaluación de Recursos Hídricos Procedentes de
Innivación (EHRIN) has monitored Spanish glaciers since the
1990s. Since 1991, this program has collected an uninterrupted
glaciological mass balance time series of the Maladeta Glacier
still ongoing;.
In spite of all these efforts, observations of the Pyrenean glaciers
remain scarce and irregular. Hence, there are few available
reconstructions of glacier evolution since the LIA, and quantitative
studies are even rarer. A brief review of Pyrenean glacier evolution
is given in . On the Spanish side of the Pyrenees,
the ice-covered area has decreased by 74 % since the end of the LIA
. In comparison, Alpine glaciers lost 35 % of their
total area from 1850 until the 1970s, and almost 50 % by 2000 . Field
measurements completed by early maps, paintings, and terrestrial and
aerial photographs have allowed the reconstruction of the
fluctuations of the Taillon, Maladeta, and Coronas glaciers throughout
the 20th century
. The results of
these studies are consistent with general glacial recession since
the LIA. Each glacier experienced alternating periods of strong
recession with periods of stability or limited readvance. In
particular, there seems to be a common period of strong recession
after 1850, a period of readvance or stability between 1960 and 1980,
and a period of strong recession from the mid-1980s until now.
One of the main drivers of these glacial changes since the LIA is regional
temperature increase . Periods of low precipitation were
identified without evident trends; however, research has been lacking in
order to identify potential connections to larger-scale atmospheric patterns.
Generally, there are correlations with the NAO affecting the snow
accumulation in the Pyrenees during the second half of the 20th century, in
particular at high elevations . The
AMO was more recently identified as a possible driver of multidecadal
variations in river flow and precipitation in southwest France, including the
Pyrenees .
Local topo-climatic effects, such as avalanching, wind-drifted snow, or
shading, may significantly influence accumulation and ablation processes. In
the Pyrenees, these local influences are expected to have introduced spatial
disparities in ice shrinkages; in particular, to have promoted steep north
and northeast glacier cirques, located below the highest summits
.
Study site
Ossoue Glacier (42∘46′15′′ N, 0∘08′40′′ W) is
located in the central part of the Pyrenees, beneath the border pass of
Cerbillona. It belongs to the Vignemale Massif, which owes its name to the
eponymous highest peak of the French Pyrenees (3298 m a.s.l.)
(Figs. and ). The glacier is an east-facing cirque. Its bedrock
comprises metamorphic limestone ridges and quartzite rocks from the Devonian
period.
Meta-data of Ossoue Glacier topographic survey. Ci indicates the
contour interval (m) of the topographic maps. The last column refers to
random errors calculated for each type of metric measurements. For volumetric
measurements, we give the random error at the elevation associated with the
DEM.
MetricPeriodMethodSource characteristicsInstitutionEstimated error(surveyed by)Length1850Moraine observationGlacier depositsAssociation Moraine10 mvariations1885PhotointerpretationPhotographJ. Vallot10 m1889–1895Length measurements (field)–R. Bonaparte10 m1904–1928′′–L. Gaurier10 m1935–1953′′–Eaux et Fôrets5 m1957PhotointerpretationAerial imageIGN10 m1962; 1970′′PhotographJ. M. Grove; J. Jolfre5 m1982–1986; 1990′′′′GEGP; B. Clos3 m1995′′Aerial imageIGN3 m2001–2013Field observation–Association Moraine1 mArea1850Moraine contour digitalizationGlacier depositsAssociation Moraine10 havariations1851Glacier contour digitalizationEtat-Major mapFrench army20 ha1924; 1948′′Aerial imagesIGN4 ha1950; 1953Field measurements–Eaux et Fôrets5 ha1983; 1988; 1992Glacier contour digitalizationAerial imagesIGN4 ha2002–2011Topographic surveyGPSAssociation Moraine2 haHeight1881–1895Height measurements (field)Artificial caveH. Russell0.8 mvariations1901–1913′′′′L. Gaurier0.5 mat Villa1927; 1937′′′′L. Gaurier0.5 mRussell1945–1953′′′′Eaux et Fôrets0.5 m1983–1987′′′′GEGP0.5 m2002–2013′′′′Association Moraine0.5 mVolumetric1924Terrestrial photogrammetry (DEM)1:20 000 map; Ci = 20 mA. Meillon8.6 mmeasurements1948Airborne photogrammetry (DEM)1:2500 map; Ci = 2 mIGN; GEGP2 m1983′′1:2500 map; Ci = 2 mIGN; GEGP1.6 m2006Topographic survey (DEM)DGPS; base < 1 kmSissyphe-EGID1.5 m2006′′GPR; 50 Mhz antenna–6 m2013′′DGPS; base < 40 kmGEODE-CESBIO0.6 m2013Satellite photogrammetry (DEM)Pléiades stereo pairCNES1.8 m
Ossoue Glacier is the largest glacier of the French Pyrenees and had
an area of 0.45 km2 in 2011. It is characterized by a large
plateau on the upper part (mean elevation 3105 ma.s.l.,
elevation range 3030–3200 m). The plateau constitutes two-thirds of the overall area, and is located on a gentle slope
(8∘), while the lower part of the glacier has a steeper slope
(>20∘). Therefore, the elevation distribution along the
455 m elevation range is characterized by a relatively high
median value (3076 ma.s.l. in 2013). Ossoue Glacier has typical “alpine
morphology”, being significantly longer (1400 m) than it is wide
(400 m), and terminating in a double tongue.
Top: distribution of the present-day Pyrenean glaciers by mountain
massifs: 1. Balaïtous; 2. Infierno; 3. Vignemale; 4. Gavarnie Monte Perdido; 5. Munia; 6. Posets; 7. Perdiguère; 8. Aneto; 9. Mont Valier.
Bottom: Vignemale Glacieret: 1. Clot de la Hount. Glaciers: 2. Oulettes de
Gaube; 3. Petit Vignemale and Ossoue. Vanished glaciers: 4. Spanish
Montferrat; 5. Tapou; 6. French Montferrat. We note that the
vanished glaciers were oriented to the southwest and east. Clot de la
Hount is northwest-oriented and its area is less than 0.01 km2
(2011). North-oriented glaciers Oulettes de Gaube, 0.13 km2
(2011), and Petit Vignemale, 0.03 km2 (2011), were one unique glacier until 1888 . Coordinate system: UTM 31∘ N.
Ossoue Glacier is 150 km from the Atlantic coast and is thus
under the influence of the North Atlantic westerlies, which bring
abundant precipitation . The closest meteorological station
(Gavarnie, 11 km, 1380 ma.s.l.) recorded a mean annual
temperature of 7.68 C∘ and a mean precipitation of 1450 mm
over 1992–2012. The glacier is fed mainly during winter by direct
precipitation and wind-blown snow. Avalanching is most likely not
a significant source of nourishment for Ossoue Glacier. The
surrounding crest walls exhibit limited surfaces propitious for snow
interception. Thus, Ossoue Glacier carries little debris on its
surface, and topographic shading is quite limited. Dust particles are
frequently observed on the snow surface, which likely affects glacier
albedo and snowmelt in summer. The first day of the local hydrological
year is fixed on 1 October . The melting period
generally extends from the end of May to the beginning of October. We
thus consider the hydrological summer during JJAS. Moulins are often
observed during that period in the glacier upper area.
Ossoue Glacier volumetric variations (ΔVice in
km3) and associated cumulative geodetic mass balance
Bgeod.PoR (in mw.e.) and geodetic mass balance rate
Bgeod.a (in mw.e.a-1). We considered a mean density
of d=850± 50kgm-3 for 1948–1983 and 1983–2001, and 900kgm-3 otherwise. The term σgeod.total.a‾ refers to the annualized random
error.
Period of records1924–19481948–19831983–2013(PoR)(24)(35)(30)ΔVice-0.0324 km3+0.0044 km3-0.0219 km3Bgeod.PoR-34.1 mw.e.+4.8 mw.e.-31.3 mw.e.Bgeod.a- 1.42 mw.e.a-1+ 0.13 mw.e.a-1- 1.04 mw.e.a-1σgeod.total.a‾± 0.37mw.e.a-1± 0.07mw.e.a-1± 0.06mw.e.a-1
Ossoue Glacier mass balance time series measured by glaciological
methods (in mw.e.). Endw and
Ends refer to the end of winter and the end of summer,
respectively, in the floating-date system .
Bglac a means annual glaciological mass balance,
Bw refers to the winter mass balance, and Bs
refers to the summer mass balance.
Ossoue Glacier was irregularly monitored throughout the 20th century, but has
been quite well monitored since 2001 (Table ).
Data sets and methodsTopographic surveysEarly sources
As is usual in glacier reconstructions, our data come from various sources
(Table ). Moraines allow us to determine the glacier extent at
dates estimated to be close to the end of the LIA. The testimony of Henri
Passet establishes that the glacier reached the summit of the left lateral
moraine in 1865 . A photograph taken in 1885 by Joseph
Vallot provides evidence that Ossoue Glacier was still close to its moraines
at this date. The Etat-Major map edited in 1851 by the French army also
provides similar evidence. The map has an estimated accuracy of 15 m
in planimetry. Two elevation points located on the front of the glacier are
marked at 2458 and 2471 ma.s.l. Currently, both points are located
on the glacier moraine. At these locations, the present elevations are 2447
and 2491 ma.s.l. It is remarkable that the differences in elevation
are only 11 and 20 m, which gives us further confidence in the fact
that the glacier front was actually in contact with its moraine at the middle
of the 19th century, i.e. at the estimated end of the LIA in the Pyrenees
.
Length measurements were based on field observations reported by various
authors e.g. Commission internationale des glaciers
and complemented with estimations from photographs or aerial images.
The Villa Russell is a cave, accessible from the glacier at
3201 ma.s.l. It was extruded by Henry Russell and his employees in
1881 (Fig. , Table ). Vertical measurements
between the glacier surface and the cave threshold were made beginning in
1882.
We collected three paper topographic maps from 1924, 1948, and 1983
(Fig. and Table ). The map dated from 1924 is
a 1:20 000 scale topographic map with 20 m contour lines. It was
created by Alphonse Meillon, a Pyrenean topograph-alpinist from the
Club Alpin Français, and Etienne de Larminat, a military
cartographer . Its implementation involved both field
measurements and triangulation from photographs. Most of the photographs were
terrestrial photographs, but, in a unique collaboration, military aerial
photographs were also used to fill the information gaps .
The maps from 1948 and 1983 feature 2 m contour lines, and were
drafted by GEGP (Sect. ). Elevation contour lines were
generated by manual restitution from stereoscopic airborne photographs
. Both maps have a 1:2500 scale and were
projected in Lambert 3 (the official French coordinate projection system
until 2001).
We also collected summer aerial photographs, which date from 1924, 1948, and
1983, made available from the IGN in digital format. The latter two
photographs exhibit crevasse features that match the aforementioned
topographic maps, which indicates that they were the stereoscopic images used
to generate the contour lines in the first place. We used these photographs
to delineate the glacier outline and compute the glacier area, because we
found that the glacier outline on the topographic maps was either incomplete
or inaccurate. We also used the Etat-Major map (dated 1851) to compute the
glacier area. We preferred the outline derived from the moraine position to
that of this map in determining the glacier outline in and around the 1850s.
On the three digitized maps, contour lines were densely sampled to
generate close elevation points. Two (m) DEMs were generated by interpolation, based on a discretized thin plate spline
technique Anudem 5.3,.
Recent surveys
The length, area, and height (Villa Russell) measurements have continued in the 2000s.
To complete the historic DEM time series, two DGPS surveys (DGPS receivers
Trimble GEO XH 2008 and 6000) were performed on 3 September 2011 and
6 October 2013 (Table ). Post-corrections, based on
a 40 km distant base from the French geodetic permanent network (RGP), were
applied. Two (m) DEMs were generated from the elevation point canvas,
applying the same interpolation method previously mentioned. The estimated
random error on the DGPS DEM is 0.6 m.
The DEMs generated for 1924, 1948, 1983, 2011, and 2013 allowed us to
establish a geodetic mass balance over an 89 year period. Consecutive
DEMs were subtracted on a pixel by pixel basis. Volume changes
derived by differentiating DEMs is based on the following equation
:
ΔV=r2∑k=1KΔhk,
where K is the number of pixels covering the glacier at the maximum
extent, Δhk is the elevation difference at pixel k, and
r is the pixel size (2 m in this study).
Since we have very little information on the generation of the maps based on
terrestrial (1924) or aerial photogrammetry (1948, 1983), DEMs were assessed
on stable terrain following the technical recommendations given in
. A GCPs data set was generated from DGPS points,
collected on 23 October 2013, on the frontal margin of the glacier, i.e. on
a snow- and ice-free bedrock surface. DEMs were not horizontally shifted,
given the good absolute localization of the sources (5 m for 1924,
2 m for 1948 and 1983), and the limited surface cover outside the
glacier that would be needed to perform such an adjustment.
The annualized geodetic mass balance Bgeod.a was calculated
through the following formula :
Bgeod.a=ΔVPoRS¯.ρ.1N,
where S¯ is the average glacier area of the two survey dates assuming a linear change through time, and N is the number of years in the period of record (PoR).
Between 1948 and 2001, we used a mean density ρ of 850 kgm-3 with an uncertainty range of
±50 kgm-3 . Before 1948, we considered
a nearly absent firn zone. Since 2001, the glaciological measurements indicate that the glacier
summer surface is almost exclusively bare ice. In both cases, we considered a mean density of
900 kgm-3. For details on the error estimations, please refer to the Supplement.
Map designed by Meillon and Larminat, with focus on Vignemale
glaciers, 1933 edition (glacier data are from 1924).
Glaciological mass balances
Since 2001, Ossoue Glacier has been monitored by systematic winter and
summer mass balance measurements performed by Association
Moraine. These are available on the WGMS website (Id: 2867). The
direct glaciological method was used here
. The protocol was similar to
that used for the Saint-Sorlin and Argentière glaciers in the Alps
, and followed the technical
recommendations of the GLACIOCLIM observation network
. At eight sites
(Fig. ), the winter and annual mass balances were
determined by two specific methods: (1) the end-of-winter snow depth,
with respect to the previous summer surface, measured using snow
probes, and near-surface snow density, calculated by drilling
and weighing calibrated cores; and (2), the annual mass balance,
determined by inserting 10 m ablation stakes (five 2 m
sections) into the ice. Summer ablation measurements were repeated
once a month until a date close to the beginning of the next
hydrological year, according to the floating-date system
.
These point observations were spatially integrated using an area
extrapolation method. The glacier surface was divided into eight
polygons centred at each ablation stake. The polygon borders were
determined through empirical considerations based on field
observations, elevation aspect, and mean slope
(Fig. ). Further details are given in the Supplement. The winter mass
balance at a specific site k can be expressed as
bw.k=ρsnow.khsnow.k,
where ρsnow.k is the density calculated at site k, and
hsnow.k is the snow depth accumulated during winter on the
previous summer surface.
The glacier-wide winter mass balance Bw was obtained by
summing the contribution from each polygon:
Bw=∑kbw.kWk,
where Wk is the fractional surface area of the polygon k within
the glacier. The Wk values were
updated in 2006 and 2011 to reflect the evolution of the glacier
geometry.
The annual mass balance was calculated using the same spatial
integration method. If the field operator noted the disappearance of
the winter snow layer and the presence of older firn from a previous
year, a density of 600 kgm-3 was applied to that
layer. If ice was observed, a constant density
ρice=900kgm-3 was used. Lower density
values were not used because of the continuous glacier shrinkage
observed since the 1980s.
Meta-data of glacier variations from the literature used in this
study for comparison between Ossoue and others Pyrenean and Alps glacier
fluctuations.
Glacier nameID. number on figureMain metricDistance from OssouePublicationTaillon1Length variations30 kmMaladeta2Area variations80 kmCoronas380 kmSaint Sorlin4Glaciological mass balances550 kmGébroulaz5′′600 km′′Argentiéres6′′650 km′′Mer de Glace7′′650 km′′Sarennes8′′550 kmSwiss glaciers (30)9Geodetic mass balances>700km
These glaciological mass balance terms can be expressed in the
following equation :
Bglac.a=Bw+Bs,
where Bglac.a, Bw, and Bs designate the glacier-wide
annual, winter, and summer mass balances, respectively.
The summer balance Bs was calculated as the difference between the
two measured mass balance terms.
Correlation matrix (Spearman's ρs) between the
meteorological time series and Ossoue Glacier mass balances components after
removal of linear trends calculated in a fixed date
system,. Correlation values given in parentheses are based on
the monthly mean values. Significant correlations (p values < 0.05) are
marked with asterisks. The Gavarnie time series was not complete enough to
perform the correlations between the annual mass balance and the annual mean
temperature and precipitation over 2002–2011; we reported a no data value
(ND) in these cases.
Bglac.a.1 OctBs.1 OctBw.31 MayGavarniePic du MidiCRUGavarnieTarbesVariablesMass balance Temperature Precipitation Period of record2002–2013 1992–20121882–20131858–20131992–20121882–2012Bglac.a.1 Oct10.84*0.65*ND-0.57-0.66*ND0.74*Bs.1 Oct10.2-0.76* (-0.8*)-0.71* (-0.75*)-0.72* (-0.8*)––Bw.31 May1ND-0.380.150.71*0.72*
Location of the glaciers used for comparison with Ossoue Glacier
fluctuations.
The stake measurements performed from 1979 to 1985 followed similar protocol
as that described above, except that the glacier was divided into five
longitudinal sectors . All the details on the error
calculations are given in the Supplement.
Evolution of Ossoue Glacier area since the LIA. The glacier outlines
are superposed on a multispectral Pléiades orthoimage taken on
23 September 2013. UTM 31∘ N projection.
Comparison with other glaciers in the Pyrenees and the Alps
Ossoue Glacier metrics mentioned above were pieced together to detect the
main phases of glacier fluctuations. Metric variations were not considered
significant if they were within the estimated error range. We identified
periods of glacier accumulation or stabilization, periods of ice depletion,
and undetermined periods. We selected other glacier reconstructions from the
literature (Table and Fig. ).
For the Pyrenees, we considered the following glacier reconstructions (Sect. ):
Taillon (42.69∘ N,-0. 04∘ E, northeast-oriented, 2530–2800 m,
0.08 km2 in 2011),
Maladeta (42.65∘ N, 0.64∘ E, northeast-oriented, 2870–3200 m altitude
range, and 0.27 km2 in 2011),
Coronas (42.63∘ N, 0.65∘ E, southwest-oriented,
3100–3240 m and 0.02 km2 in 2011, ice patch since the 2000s)
These reconstructions were mostly based on length and area variations, but also used other sources when they were available (e.g. a short stake measurement time series in Taillon Glacier at the end of the 1970s).
In the Alps, we considered the main phases identified by:
and updated by on four well-studied glaciers: Saint-Sorlin, Gébroulaz, Argentière, and Mer de Glace
glaciers;
on Sarennes Glacier;
based on thirty Swiss glaciers.
Due to the morphological differences between the Alpine and Pyrenean glaciers, we considered only the Alpine glacier reconstructions which were based on mass variations (i.e. not on the glacier length or area).
Climatic data
To infer potential drivers for Ossoue Glacier variations, we used
several climatic data sets, including three mean monthly air
temperature data sets.
The closest meteorological station to Ossoue Glacier was located
at Gavarnie, in the same valley (1380 m elevation,
11 km east of the glacier, time series from January 1991 to
May 2012).
To cover the glacier reconstruction period, we also used the
temperature time series recorded at the Pic du Midi observatory
(42.93∘ N, 0.14∘ E; 2874 ma.s.l.,
30 km northeast of the glacier). The time series was
homogenized, and gaps were filled from January 1881 to May 2011
. We used raw data from 2011 to
October 2013.
We extended the Pic du Midi temperature time series with the
CRUTEM 4 data set (5 ∘× 5 ∘ gridded version) over the period 1858–1890 . The
mean annual difference between these data sets due to the elevation
difference between the stations was removed from the CRUTEM data set
(-13.07 ∘C).
All temperatures time series correlated well (Spearman's ρ>0.96,
p values < 0.05). Temperature time series were averaged over summer
periods (JJAS), winter periods (NDJFMA), and hydrological years
(beginning 1 October)
We also used two monthly precipitation data sets.
Monthly precipitation was recorded at the Gavarnie Valley
station simultaneously with the temperature (see above for station
location; period of record: January 1991 to May 2012).
Monthly precipitation was recorded over the period 1882–2013 at
Tarbes-Ossun Météo-France station (43.18∘ N 0.00∘ E,
360 m elevation, 50 km northeast of the
glacier). These data were homogenized until 2000 and used as raw
data since then .
Both precipitation data sets showed a significant correlation (Spearman's
ρ=0.56, p value < 0.05). We calculated annual and winter
precipitations (NDJFMA). October and May precipitation values were not
considered because the precipitation was as often liquid as solid in
these months, and thus presumably did not contribute significantly to
accumulation on the glacier.
We performed correlations between the meteorological data (temperature and
precipitation) and Ossoue Glacier mass balance measurements over the
2001–2013 period. To reconcile the different recording periods of the
meteorological and glaciological measurements, summer and annual mass
balances were linearly interpolated by using a fixed date system
1 October,. The mass balances terms Bw
were extrapolated to 31 May. Due to some missing data, it was not possible to
calculate the mean annual temperature and precipitation for several months at
Gavarnie over the period 2002–2012. Thus we did not correlate these data
with the annual mass balance. Mass balances and meteorological time series
were detrended using a linear least-squares fit prior to the correlation
analysis.
We also considered the Atlantic Multidecadal Oscillation (AMO) and the
North Atlantic Oscillation (NAO). Both indices represent fluctuations
in the North Atlantic climate and have been successfully used in
glacier–climate linkages .
For the NAO we used a winter (DJFM) index based on the monthly
1850–1999 Climate Research Unit (CRU) data set, completed by Tim
Osborn's 2000–2013 NAO Update . We
applied a 5 year moving average filter to smooth the signal.
For the AMO, we used the monthly index calculated from the Kaplan
sea surface temperature data set over 1861–2009 .
Geophysical surveys
A ground penetrating radar (GPR) survey was performed on 30–31 August 2006
in the upper area of Ossoue Glacier. The GPR apparatus used was a PulseEkko
100 (Sensors and Software Inc.) with 50 MHz unshielded antennas.
Three longitudinal profiles (W–E) running from the top along the slope
transition of the glacier, and four transversal (N–S) profiles, were
surveyed. From this data, a bedrock map was generated (see Supplement).
From the 2013 glacier DEM and the bedrock map, we generated a 2013 glacier ice thickness map (Fig. ).
To estimate the ice thickness maps in the next decades, we generated ice thickness projections based on a static approach
(Fig. ).
We interpolated, on a 4 m resolution grid, the mean point mass balances measured at the stake locations over 2001-2013 on the current
glacier outline (2011). The interpolation technique is the same used to generate the DEMs. This interpolation is consistent with the glaciological method.
The mean rate after interpolation is -1.5 m w.e. a-1, while the mean glaciological mass balance rate is -1.45 m w.e. a-1.
We made the assumption that this spatialized mass balance rate will remain constant in the next decades.
Based on this mass loss rate, we calculated the ice depth on the glacier plateau at decadal intervals from 2013.
As it is based on the superficial mass balance only, this method estimating the future ice depth does not take into account effects from basal and internal mass balances, nor the dynamics of the glacier.
Our reconstruction of Ossoue Glacier front shows
significant glacier retreat since the LIA, with intermittent
stationary phases (Fig. ).
From 1850 to 1889, Ossoue Glacier front retreated by 346 m
(-8.8 ma-1). During the following 15 years (1889–1904),
the front position was quite stationary, retreating by 11 m between
1892 and 1893 and by only 9 m between 1899 and 1904. In the 1904–1905 period, however, Ossoue Glacier front retreated by 23 m. The
following periods were characterized by stability (1905–1911) and
progression (1911–1927). In our data set, the glacier reached its most advanced
position of the 20th century in 1927.
Area variations
The area of Ossoue Glacier at the end of the LIA, based on moraine
locations, was 112.6 ± 10 ha. The glacier area extracted from
the Etat-Major map (dated near 1851) is 115 ± 20 ha.
Between the end of the LIA and 1924, the area of Ossoue Glacier decreased by
20 %. The area decreased by a further 10 % over the 1924–1948 period. During the 1948–1983 period, the front retreated by 315 m
until 1963, and then advanced by 156 m, although the changes in area
over this period were low (-3 %). Over 1983–2002, the area decreased by
17 % with a notable width reduction on the slope transition. In the early
2000s, the area of Ossoue Glacier was less than 50 % of its area at
the end of the LIA.
Changes in glacier geometry mainly occurred in the lower part of the
glacier (Fig. ). In the upper part, the glacier shape remained almost
unchanged until 1983. From 1983 to 2013, glacier width reduced
dramatically at the slope transition between the plateau and the
tongue of the glacier.
Elevations differences (m) on glacier (thin dashed line) and on
deglaciated margins (thick dashed line) based on differences between
consecutive DEMs. UTM 31∘ N projection.
Mass variations
Since 1924, Ossoue Glacier has lost a mean of 60 mw.e. over
the current glacier area (Fig. ). The two periods
of marked ice depletion, 1924–1948 and 1983–2013, were interrupted by
a stable period between 1948 and 1983 (Table ).
Between 1924 and 1948, the glacier lost -1.42 mw.e.a-1
(-34.1 ± 8.8 m.w.e.). The ice depletion signal was strongest in the
central part of the glacier (Fig. ).
Glacier surface elevation variations in mw.e. at Ossoue
stake locations from 2001 to 2013. For details of stake locations on the
glacier, see Fig. and the Supplement. Maladeta Glacier is
indicated by the dashed grey line.
The 1948–1983 period is the only period with observed positive geodetic mass
balance variation, with a rate of +0.13 mw.e.a-1
(+4.8 ± 2.6 mw.e.). However, a notable depletion was
observed on the tongue (Fig. ). The glacier advanced over
a very small area (1 ha) with a mean ice growth of
6.5 mw.e.. An area of higher accumulation is localized on the lower
part of the glacier, below the slope transition (Fig. ). At
the end of that period, using ablation stakes, François Valla and Henri
Pont measured mass gains of +0.81 mw.e. in 1978;
+0.26 mw.e. in 1979; +0.17 mw.e. in 1980; and
0 mw.e. in 1981 and 1982. In 1983 and 1984, they considered the
mass balances to be “negative” but did not provide quantitative information
.
Over 1983–2013, the glacier lost -1.04 ± 0.06 w.e.a-1
(-31.3 ± 1.9 m w.e.) .
A marked pattern of ice depletion occurs along a longitudinal
profile on the upper part of the glacier (Fig. ). This
phenomenon increased glacier convexity in that zone, which was once
named “Plateau des Neiges” in older maps.
Between 2001 and 2013, superficial mass loss on Ossoue Glacier given by the
glaciological method is 17.36 ± 2.9 mw.e.
(-1.45 mw.e.a-1) (Table and Fig. ). The strongest
mass losses were registered at the lowest elevated stakes (stakes 7 and 8).
The mass balance was negative every year since the stake measurements began
except in 2012–2013 with a value of +0.23 mw.e.. In 2008 the mass balance was only slightly negative.
Mains phases of variations
Taken together, all the metrics suggest a clear retreat of Ossoue Glacier since the end of the LIA (Fig. ).
Based on the dates of the survey reported in the available documents, we
identified several phases in the fluctuations of Ossoue Glacier:
1850–1890 (40 years, noted phase I in Fig. ). Between these dates, all the length variations are negative for all three
survey dates (1874,1885,1889). Between 1882 and 1889, however, Russell noted some thickening and thinning at Villa Russell, with high interannual variability.
1890-1928 (38 years, phase II). The length variations were null from 1891 to 1899, and within the range of errors between 1892 and 1893 (i.e. no significant variations).
Afterwards, length variations were negative until 1905. Between 1890 and 1894, Russell noted a period of stabilization at Villa Russell (II.a).
Then, several irregular thinnings were observed for the following survey dates (1895, 1898, 1902, 1904), interrupted by a thickening between 1898 and 1901.
From 1905 to 1911, no length variations were observed for the six survey dates. Between 1911 and 1921, the length increased, but the variation was not
significant. From 1905, the glacier thickened at Villa Russell until 1913 (II.b). The glacier thinned significantly at Villa Russell between
1913 and 1927. The length variation was positive, but not significant for between 1921 and 1927.
1928–1950 (22 years, phase III).
In spite of a large estimated error, the mass balance variation (1924-1948) was significant and markedly negative. Area variations were also negative between
1924 and 1948, and between 1948 and 1950. The glacier thinned at Villa Russell until 1950 (survey dates for significant variations: 1937, 1945, 1950).
The length variations were negative for all survey dates in that stage (1928, 1935, 1950).
1950–1983 (33 years, phase IV). The area variations were small and not significant between 1950 and 1953, and between 1953 and 1983. Negative length
variations were observed until 1962, and positive afterwards. The geodetic mass balance was slightly positive over the 1948–1983 period. At Villa Russell,
the glacier was thickening at the surveyed dates (1952, 1953, 1967). In 1953, the glacier reached the threshold of the Villa Russell, and was above the
threshold in 1967. Between 1967 and 1983, a slight thinning was observed. The glaciological mass balance was positive from 1978 to 1980, zero in 1981 and 1982,
and negative in 1983 and 1984 (qualitative assessment for these two dates).
1983–2013 (30 years, phase V). From 1983, all the significant length and area variations observed were negative. At Villa Russell, the only
positive variations were observed between 1987 and 1991, and 2007 and 2008. The geodetic mass balance was negative between the two dates of survey that define
that phase. The annual glaciological mass balances have all been negative since 2001, except for the hydrological year 2012-2013.
From the above considerations, we defined three periods of ice depletion,
when the metrics consistently indicate a negative trend: 1850–1890,
1928–1950, and 1983–2013 (noted I, III, and V in
Fig. ). By the same reasoning, three periods are
characterized by stability or slight growth: 1890–1904, 1905–1913, and
1950–1983 (noted II.a, II.b, and IV, respectively in
Fig. ).
Length (m), area (ha) and thickness (m) at Villa Russell and mass
changes (in mw.e.) of Ossoue Glacier. Glaciological mass balances of
Ossoue (orange) and Maladeta (blue) glaciers. The background colour indicates
the interpreted trend of the period, according to the metric variations (see
Sect. ). The ice depletion periods (I, III, V) are shown
in light grey. The periods of accumulation or stability (II.a, II.b,
IV) are shown in dark grey.
If we consider the 2013 glacier area as a common integration area for all the
periods, the absolute value of the geodetic mass balance increased over
1924–1948 (-35 mw.e.) and 1948–1983 (+6.1 mw.e.).
Comparison of Ossoue Glacier fluctuations with other Pyrenean and
Alps glacier reconstructions. The ice depletion periods are shown in red. The
accumulation or stable periods are shown in blue. Periods considered as
“undetermined” are shown in white. The meta-data relative to the glaciers fluctuations are
given in Table . For the localization of the
glaciers, please refer to Fig. . Roman numerals
above Ossoue Glacier fluctuations refer to the main periods of variation
identified (Sect. ).
Comparison with Pyrenean and Alpine glaciers variations
Pyrenean and Alpine glaciers exhibit similar multidecadal variations during
the 20th century. The ice depletion was particularly intense in the 1940s and
since the 1980s. The stable period detected at the end of the 1970s is also
evident in all the glaciers at both mountain ranges (Fig. ).
The Ossoue, Maladeta, and Coronas glaciers present consistent variations: ice
depletion in the 1930s and 1940s, transition in the 1950s, stable period from
the 1960s to the 1980s, and ice depletion since the 1980s. The Taillon
Glacier variations appear more specific, but coincide with the ice depletion
phase which had started in the 1980s. The Taillon Glacier and Ossoue Glacier
fluctuations present further consistencies: around 1910 (II.b), in the 1930s
(III), and around 1980 (IV). In the French Alps, two steady-state periods,
1907–1941 and 1954–1981, and two periods of recession, 1942–1953 and
1982–2013, were deduced from four glacier mass balance time series
. These periods are in good agreement with
Ossoue and Maladeta glacier variations. However, it seems that the glacier
retreat phase of 1928–1950 (III) identified from Ossoue Glacier data started
about a decade later in the Alps. In the Swiss Alps, Huss et al. (2010)
detected two short periods of mass gain (1910s and late 1970s) and two
periods of rapid mass loss (1940s and late 1980s to present). These
variations are also consistent with Ossoue Glacier variations (periods III,
IV, and V).
Comparison with meteorological time series
Correlations between Ossoue Glacier mass balance time series
(2001–2013) indicate that the annual mass balance is mainly dependent
on the summer mass balance, and that the winter mass balance has less
influence (Spearman's ρ=0.84 for summer mass balance and
ρ=0.65 for winter mass balance) (Table ).
The link between ablation and air temperature was verified at Ossoue
Glacier, as shown by the following significant (p value < 0.05) correlations (Table ):
between monthly summer ablation and monthly air
temperature time series over 2002–2013 (ρ=-0.8 for Gavarnie
valley station, ρ=-0.75 for Pic du Midi station, which is located
farther from the glacier, and ρ=-0.8 for the regional CRU time
series);
between mean summer air temperature and summer-wide mass balance
Bs (June–September) (ρ=-0.76 for Gavarnie,
ρ=-0.71 for Pic du Midi and ρ=-0.72 for the CRU time
series);
between annual mass balance and mean annual temperature (ρ=-0.66 for the CRU time
series).
The link between annual mass balance and mean annual temperature is weaker in
the Pic du Midi time series than in the CRU time series (ρ=-0.57 for Pic
du Midi). This may be due to the use of raw data in the Pic du Midi time
series, starting from 2011, or to the limited period for glaciological mass
balance records (annual mass balance measurements only began in 2001).
However, due to the good correlation between the CRU and the Pic du Midi
temperature data sets, we also considered that the mean annual temperature at
Pic du Midi is linked to Bglac.a over the longer 1890–2013
period. The elevation of the Pic du Midi station (2874 ma.s.l.) is
close to that of Ossoue Glacier front (2755 ma.s.l.); thus, we
principally used this data set to identify temperature trends over historical
periods.
Mean temperature (Pic du Midi, 2874 m), mean precipitation
(Tarbes, 374 m), and correlations between mean temperature and
precipitation time series and time (Spearman's ρ). Time ranges are based
on the interpretation of the glacier metrics (Fig. ).
Significant correlations (p values < 0.05) are marked with asterisks.
For 1858–1890, the mean temperature is based on the CRU data set.
Climatic time series: mean annual temperature at Pic du Midi
(beginning 1 October), annual precipitation at Tarbes, AMO mode and winter
NAO (DJFM) anomalies. The background colour indicates the interpreted trend of
the period, according to the metric variations (see
Sect. and Fig. ). The ice depletion
periods (I, III, V) are shown in light grey. The periods of accumulation or
stability (II.a, II.b, IV) are shown in dark grey.
Precipitation records at Gavarnie and Tarbes are significantly
correlated with the winter mass balances (ρ=0.71 for Gavarnie,
ρ=0.72 for Tarbes, which is located farther from the
glacier, Table ). The link between annual mass balance and annual
precipitation is significant in the Tarbes data set (ρ=0.74). Thus, we conclude that the Tarbes time series can be used to
identify trends in precipitation that are linked with Ossoue Glacier
fluctuations.
The mean annual temperature over the hydrological year (starting 1 October)
for 1858–2013 is -1.1 ∘C, and the mean summer
temperature (JJAS) is 5.3 ∘C. Both time series present a linear trend
over the period (ρ=0.38 for annual, ρ=0.28 for summer). This
correlation is stronger if we limit the period to 1882–2013 (ρ=0.54
for annual, ρ=0.51 for summer). The annual precipitation and the
winter precipitation (NDJFMA) over 1882–2013 are 1068 and
556 mm, respectively. No trend was observed in precipitation time series.
Map: bedrock depth as interpreted from GPR radargrams superposed
on a 2013 XS Pléiades image. Numbers 1 to 3: interpretations of longitudinal
radargram acquisitions. Numbers 4 to 7: interpretations of transverse
radargram acquisitions.
Map: estimated ice depth at Ossoue Plateau in the next decades.
These projections are based on the sum of the ice depth estimation (measured by
GPR, see Fig. 12) and the cumulative mass balance over the period of
projection. The latter is based on the interpolation values of the averaged
mass balance at stake locations over 2001–2013 (data per year and stakes are
given in Fig. 8). Spatial resolution is 4 m. East of the plateau limit
(dashed line), the ice depth was unknown in 2013.
Analysis of temperature and precipitation trends over the four long periods
stemming from the combination of glacier metrics (I, III, IV, and V,
Fig. ) reveals four significant trends
(Table ):
The mean annual temperature over 1858–1890 may have
continuously decreased. Over the same period, the mean summer
temperature (JJAS) is 0.5 ∘C higher than the mean summer
temperature over 1858–2013 (5.3 ∘C).
Both short periods of glacier accumulation (II.a and II.b) present annual (-1.3
and -1.4 ∘C) and summer (4.6 ∘C and 4.1 ∘C)
temperatures lower than that of the means over 1858–2013 (-1.1 ∘C
and 5.3 ∘C). The precipitation at Tarbes station over 1890–1994 and
1905–1913 is also higher than the mean over 1882–2013.
The annual precipitation trend over 1950–1982 is positive
(ρ=0.4) and its mean is equal to the mean precipitation over 1882–2013
(1068 mm). Winter precipitation is higher than the mean
recorded over 1882–2013 (586 mm). The annual mean
temperature (-1.4 ∘C) and mean summer temperature
(4.8 ∘C) are lower than the means over 1858–2013.
The last period considered (1983–2013) shows positive trends in
both mean annual and mean summer temperature, with the highest
registered mean temperatures (-0.4 ∘C for annual and
6.1 ∘C for summer).
Figure provides insight into the possible linkage between the
evolution of Ossoue Glacier and the regional-scale climate. The 1960–1980
period is characterized by a succession of negative phases in the NAO. This
coincides with a period of relative glacier growth or stability (positive
variations in various glacier lengths, areas, and mass balances). AMO warm
phases occurred during 1860–1880 and 1940–1960, and cool phases during
1905–1925 and 1970–1990 . The AMO index presents some
potential correlations with Ossoue Glacier variations: periods I and III of
ice depletion, in regards to the AMO warm phases; and periods II.b and IV, in regards to
the AMO cold phases (Fig. ).
Ice thickness maps
In 2006, the estimated mean ice thickness was
29.3 ± 6.3 m (max. 74.8 ± 10.2 m), giving
an estimate of 25 ± 6.5 m
(max. 59 ± 10.3 m) in 2013 (Fig. ). In
2011, another GRP survey indicated a maximum depth
of 45 m and an average depth of 30 m. Despite the
discrepancies in ice thickness, both studies suggest similar bedrock
morphologies. Moulins were explored over the 2004–2009 summers at the
Ossoue Glacier plateau. The depth of the explored moulins ranged from 30 m to 41.5 m.
Given that the bedrock was never reached according to the speleo-glaciologist,
the ice thickness obtained by GPR is consistent with these depth measurements.
Over the subsequent decades, the mean and the maximum ice depths at the
Ossoue Glacier plateau would rapidly decrease: 22 (mean) and 48 m
(max.) in 2023, 17 and 38 m in 2033, 11 and 27 m in 2043, and
only 3 and 7 m in 2053 (Fig. ).
Discussion
Using multiple data sets, we generated five independent time series of glacier
metrics (length, area, thickness at Villa Russell, and geodetic and
glaciological mass balances variations) to reconstruct the evolution of
Ossoue Glacier since the end of the LIA (Fig. ). The
metrics give a generally consistent chronology of glacier fluctuations since
the LIA, although there are some discrepancies. We should bear in mind that
the metrics do not directly reflect the same glaciological processes. The
time series of frontal variations offers the best temporal resolution of the
onset of glacier changes, but these changes are strongly influenced by ice
dynamics. Glacier motion is dependent on mass variations of the upper part of
the glacier, but the response time is largely unknown. Areal variations
depend on ice thickness at the edges only. Thickness variations registered at
Villa Russell are the result of accumulation and ablation variations at the
northern periphery of the glacier only, which could be prone to snow
drifting. Volumetric mass changes generated by the geodetic method are mostly
the result of the surface energy budget but also include internal and basal
mass variations, which remain difficult to estimate. Glaciological mass
balances reflect the link between energy and mass budget properly, but can
only be measured at a limited sample of points at the glacier surface.
However, these metrics are all sensitive to glacier mass changes, with
different time scales and response intensities. For instance, between 1948
and 1983, the mass balance was positive, yet frontal variations were negative
until 1963. This can likely be explained by the delay in the response time of
frontal response to glacier mass changes. In the case of Ossoue Glacier, it
is note-worthy that the metrics over the study period reveal a consistent
signal (Fig. ).
The evolution of Ossoue Glacier is consistent with the reconstructed
evolutions of other Pyrenean glaciers. Some discrepancies might be due to the
nature of the metrics used in the reconstruction, or due to the local
topo-climatic influences. Considering an accuracy of ±5 years,
the study was able to identify two common stable periods (1905–1930 and
1955–1985), as well as two periods of marked ice depletion (1850–1900 and
from the mid-1980s until now). The evidence of strong marked ice depletion
found in this study for Ossoue Glacier between 1924 and 1948
(-1.42 m m.w.e a-1) should be considered with caution
given the high uncertainties in the altimetry restitution process. However,
reconstructions of other Pyrenean glaciers over comparable periods tend to
corroborate this result. Between approximately 1928 and 1957, the length of
the Coronas Glacier decreased from 600 to 350 m, while its area
decreased from 19 to 8.6 ha and its equilibrium line altitude (ELA)
increased from 3065 to 3122 m. During the
1935–1957 period, the Maladeta Glacier lost 15 ha
(-0.68 haa-1) and its length decreased by 80 m. This
retreat is assumed to be due to a warm anomaly detected in the second half of
the 1940s . The Maladeta Glacier mass balance time series
(1991–2013) is in good agreement with Ossoue glaciological mass balance
time series over the 2001–2013 period (Fig. ). During
the 1990s, the Maladeta mass balance values were slightly negative. If we
compare the Ossoue geodetic mass balance during 1983–2013
(-1.04 mw.e.a-1) and the Ossoue glaciological mass balance
during 2001–2013 (-1.45 mw.e.a-1), we can deduce that the
1983–2001 Ossoue ablation rate was approximately
-0.76 mw.e.a-1. These results are consistent with the
variation of the mean annual mass balance of the Maladeta Glacier:
-0.2 mw.e.a-1 over 1991–2001 and
-1.03 mw.e.a-1, over 2001–2013 (Fig. ).
The comparison between Pyrenean and Alpine glacier fluctuations suggests that there is a common climatic driver governing
glacier fluctuations in both mountain ranges. Ossoue Glacier seems to be anti-correlated with the NAO. Similar results were reported by
and for glaciers in the southern
Alps. In addition, variations in the AMO index also appear relatively
similar to variations in the combined Ossoue Glacier metrics
throughout the 20th century (Fig. ). This result is consistent with previous
studies on the influence of the multidecadal internal variability of
the North Atlantic circulation on the Northern Hemisphere climate
e.g. and on Alps glacier fluctuations.
Variations of Ossoue Glacier metrics are in good agreement with
meteorological data: periods of ice depletion are generally characterized by
lower values of mean precipitation and temperatures (Table ).
The evolution of Ossoue Glacier may be partially explained by observed
trends, with a significant positive trend in 1950–1982 precipitation
(a stable period for the glacier) and a significant constant rise in mean
annual and summer temperature since 1983 (a period of depletion for the
glacier). The 1850–1890 and 1983–2013 periods are marked by ice depletion,
although the mean air temperature time series have opposite and significant
trends. Frontal variations and mean air temperature variations, over the
1850–1890 interval, point to a shorter period of marked ice depletion,
1850–1874, with lesser depletion over the 1874–1890 period. By the same
reasoning the selected 1928–1950 period of ice depletion may have been more
pronounced in the 1937–1950 “subperiod” than over the 1928–1937
“subperiod”. In the Alps, the 1942–1950 period is characterized by
extraordinarily high rates of mass loss .
The future evolution of Ossoue Glacier depends on climatic changes,
but is also constrained by the remaining ice volume. Assuming that Ossoue Glacier mass
balance follows the same trend as that recorded during 2001–2013, the glacier should
disappear in 40 years (Fig. ). We anticipate that the glacier may
split into two parts at the slope transition (Fig. )
in the near future. At this location the glacier may be particularly
thin, and there is an abrupt change in the glacier slope. The lowest
part may soon no longer be fed by the ice flow from the upper area and
could thus rapidly disappear. This separation would drastically change
the morphology of Ossoue Glacier from an active glacier to
a glacieret or ice patch. Such glacier fragmentation has been regularly
observed on Pyrenean glaciers, e.g. the neighbouring
Petit Vignemale and Oulettes glaciers.
However, future evolution of Ossoue Glacier based on interpretation of
Fig. was made under several strong assumptions: (i) basal and
internal mass balances were neglected; (ii) ice motion was neglected;
and (iii) the mass loss in the future decades will occur at the same pace
as during the last decade. In short, future work is necessary to better understand the effect of local
topography on the spatial variability of glacier mass balance. This
influence is expected to increase in the future as the glacier retreats
.
The reconstruction and the future evolution of Ossoue Glacier does present
large uncertainties, and the influence of the climate fluctuations on the
glacier metric variations are complex; however, considering the current
ablation rate, it seems doubtless that Ossoue Glacier will disappear halfway
through the 21st century. Its large, markedly convex plateau
(two-thirds of the present-day area) has allowed the accumulation of
a significant amount of ice at high altitude (3105 m) during
favourable periods. On the contrary, its eastern orientation and low shading may have a large
influence on the high rate of summer ablation (e.g. in comparison to the Maladeta Glacier).
Henceforth, due to the limited interval range of the
plateau (3030–3200 m, slope 8∘), any future rise of
the lower limit of the glacier (2755 m in 2013) would
drastically modify the responses of the metrics of Ossoue Glacier
to future climate fluctuations.
Conclusions
Ossoue Glacier is one of the southernmost glaciers in Europe. Using a
large archive of historical data sets and recent accurate
observations, we generate consistent time series of various glacier
metrics, such as length, area, elevation variations, and mass
changes since the LIA at high temporal resolution. The dominant trend
is a retreat over the 20th century, which was interrupted by two
stable short periods, 1890–1894 and 1905–1913, and a longer stable period, 1950–1983.
The evolution of Ossoue Glacier is in good agreement with those of other
Pyrenean glacier reconstructions (Maladeta, Coronas, Taillon glaciers),
suggesting the possibility of long-term high-elevation climate reconstruction
in the Pyrenees. The comparison between Pyrenean and Alpine glacial
fluctuations highlights similar multidecadal variations during the 20th
century. The ice depletion was particularly intense in the 1940s and since
the 1980s, while a stable period detected at the end of the 1970s is also
evident in all the glaciers from both mountain ranges. This result may
suggest that there is a common climatic driver governing glacier fluctuations
of both mountain ranges.
The time resolution of the generated metrics for Ossoue Glacier allows us to extract
consistent glacial changes over various periods. These periods appear to be
roughly in phase with hemispheric climate proxies, such as the North
Atlantic Oscillation and the Atlantic Multidecadal Oscillation. The
1960–1980 stable period may be partially explained by
anti-correlation to the NAO index. We found that the ablation rate may
have doubled in the last decade, likely as a result of the recent
climate warming. Ossoue Glacier fluctuations generally
concur with climatic data, suggesting that Ossoue Glacier is a good
regional climate proxy.
The eastern orientation and low shading of Ossoue Glacier make it
particularly vulnerable to climate fluctuations, although its
relatively high elevation has allowed the accumulation of
a significant amount of ice at high altitudes. In 2013, the maximum ice thickness was 59 ± 10.3 m.
Assuming that the current ablation rate stays constant, Ossoue Glacier will disappear midway through the 21st century.
The Supplement related to this article is available online at doi:10.5194/tc-9-1773-2015-supplement.
Acknowledgements
This manuscript was greatly improved thanks to the constructive comments of
Chris R. Stokes (Editor),
J. Ignacio López-Moreno (referee), and an anonymous referee.
The authors warmly thank all the volunteers who provided great help
during fieldwork, especially the members of the Association
Moraine (including Florian Pinchon, Antoine Simmonet, and Sabine Ayrinhac).
We acknowledge all our colleagues who helped collect the GNSS data in the
field (including Stephane Binet, Frédéric Blanc, Bruno Calvino, and
Vincent Cabot) and acquire the Pléiades images (including Claire Tinel
and Steven Hosford) within the framework of the CNES Pléiades thematic
commissioning phase and the Airbus Defence and Space “Pléiades User
Group”.
This work was supported by the Fondation Eau, Neige
et Glace through the project CLIM Ex-PYR (www.fondation-eng.org) and by the Région
Midi-Pyrénées and the University of Toulouse through the CRYOPYR project. We also thank Patrick Wagnon, who kindly sponsored the
CLIM Ex-PYR project, Etienne Berthier, and Christophe Kinnard for their useful comments that improved the manuscript.
Edited by: C. R. Stokes
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