This work has benefited from the financial and documentary support of the Association Paul Helbronner (APH). The authors would particularly like to thank Sylvie Marcé for encouraging the use of the P. Helbronner collection for academic purposes. The authors would also like to thank the two reviewers including Antoine Rabatel (OSUG-IGE) for their constructive comments and Prof. Monique Fort for her editorial work.
This article is a tribute to Sébastien Ibanez, researcher in Alpine ecology and lecturer at the Savoie Mont-Blanc University, who was passionate about mountain environments and who recently died in an avalanche.

Introduction

1Glaciers play a major role in the Earth’s physical system by influencing global sea levels (e.g. Jacob et al., 2012), regional freshwater availability (e.g. Drenkhan et al., 2023), surface energy balance (e.g. Autin et al., 2022), local climate (e.g. Sauter et al., 2026) and local ‘natural’ hazards (e.g. Islam et al., 2025). Glaciers – particularly mountain glaciers – are also reliable indicators of the current climate crisis due to their responsiveness to variations in air temperature (Kotlarski et al., 2023; Al-Yaari et al., 2023).

2Globally, the rate of glacier mass loss has accelerated significantly in recent decades. The World Glacier Monitoring Service (WGMS) reports, for example, that the annual rate of loss based on 41 reference glaciers worldwide has risen from -0.171 m water equivalent/year in the 1980s to -0.889 m/year for the 2010s. Since 1976, glaciers have lost 9,179 ± 621 Gt of water (187 ± 20 Gt per year), contributing to an average rise in global sea level of more than 25 mm (+0.5 mm per year); 41% of this loss occurred over the last decade, including 6% during the record year of 2023 (Dusaillant et al., 2025). According to the GlaMBIE initiative (2025), glaciers worldwide lost 273 ± 16 Gt of mass per year between 2000 and 2023, with an increase of 36 ± 10% between the first (2000-2011) and second half (2012-2023) of the period. Since 2000, glaciers have lost approximately 5% of their global volume, with European mountains showing the highest regional retreat (39%).

3In the European Alps, the trend is indeed rather dramatic. The area covered by glaciers has fallen from 4,244 km² at the last maximum of the Little Ice Age, around 1850 (Paul et al., 2020), to 1,806 km² in 2015 (-57%), and their volume from approximately 280 ± 43 km³ to 100 ± 17 km³ (-64%) (Reinthaler and Paul, 2025). At the same time, at least 1,938 glaciers have completely disappeared, with entire catchments becoming ice-free (Reinthaler and Paul, 2025). Over the last four hydrological years (2021-2025), Swiss glaciers have lost 15.5% of their total volume according to GLAMOS (https://www.glamos.ch).

4These data highlight the urgent need to conduct detailed studies of the behaviour of all types of mountain glaciers in order to better understand how they respond to climatic forcing and to anticipate the associated impacts (water resources, natural hazards, environmental changes). In the French Alps, large glaciers such as the Mer de Glace or the Argentière glacier (Mont-Blanc massif; Vincent et al., 2019) and the smaller ones of Gébroulaz (Vanoise massif), Sarennes and Saint-Sorlin (Grandes Rousses massif; Six et al., 2023) and Blanc (Écrins massif; Bayle, 2020) are monitored as part of the Glacioclim observation service (Vincent et al., 2007). Smaller glacial masses found on steep slopes, such as hanging glaciers (Vincent et al., 2015) or ice aprons (Ravanel et al., 2023), have, however, been the subject of little research, often of recent date, whilst other types, such as ice-snow fillings in couloir (masses of ice and snow occupying the bottom of a couloir) have never been addressed.

5This study focuses on the evolution of various types of ice bodies in certain areas of the Mont-Blanc, Vanoise and Écrins massifs (French Alps) over the last century. It draws on an exceptional collection of panoramic photographs taken by Paul Helbronner between 1905 and 1920 and compares them with recent photographs in order to quantify the loss of glacier surface area and document changes in morphology and type, fragmentation processes and the formation of new lakes. The aim is to highlight and characterise the evolution trajectories of these Alpine glaciers in the current context of global warming.

6Paul Helbronner (1871-1938) was a French geodesist, topographer and mountaineer, and a pioneer in mapping the French Alps. Between 1903 and 1928, he led 22 geodetic campaigns covering the French Alps and Corsica, culminating in the publication of the monumental Description géométrique détaillée des Alpes françaises (A detailed geometric description of the French Alps) in 12 volumes and two albums of panoramic photographs. These photographs, taken from peaks and summits, constitute a valuable historical reference for reconstructing the shape of glaciers at the beginning of the 20^th century. Indeed, the use of his iconographic work in the field of glaciology allows for comparisons over a relatively long period (a century), a key element in describing trends.

7The approach adopted in this study involved selecting and digitising a series of panoramic photographs from Helbronner’s work, then comparing them with recent aerial photographs (2022-2023) from the French National Geographic Institute (IGN). A monoplotting method applied to three glaciers complements the use of a geographic information system (here QGIS) for all the glaciers. These two methods have made it possible to reconstruct past glacial boundaries for comparison with contemporary ones. A typology of ice bodies (valley glaciers, slope glaciers, cirque-and-slope glaciers, ice aprons, couloirs) was developed to provide a detailed understanding of the dynamics specific to each morphology, in terms of both fragmentation and retreat. Particular attention was also paid to the formation of new lakes.

8First, we describe the iconographic source and the methodology used. We then present the results obtained for the selected areas in the three massifs, before analysing the dynamics dependent on the morphology of the ice masses. Finally, we discuss the climatic and environmental implications of these glacier changes.

Data and Methods

The Place of Photography in the Work of P. Helbronner and Selected Documents

9Photography plays a central role in the work of Paul Helbronner, who, at the beginning of the 20^th century, undertook the vast project of producing a precise topography of the French Alps (Couteaux, 2000). While his work relied primarily on geodetic triangulation, the thousands of photographs he took at high elevation served as an important complement to his measurements (Guilhot, 2005; Léon, 2015). He thus systematically documented peaks, valleys and glaciers (Fig. 1), providing an essential visual basis for the cartographic precision of his Description géométrique des Alpes françaises. His relationship with photography was twofold: scientific and documentary. Scientifically, the images served as a support for his surveys and made it possible to record the state of the observed terrain, including the glaciers. This role proves particularly valuable today, as Helbronner’s photographs are among the few detailed views of the glaciers of the French Alps taken from high up in the mountains in the early 20^th century, a time when few photographers still dared to venture there (Garimoldi and Ginouvès, 1995). From a documentary perspective, Helbronner also attached particular value to the image itself. His panoramic photographs, some of which are vast in scale, bear witness to his desire to preserve a visual record of the mountains. He thus leaves behind a unique record of the Alpine landscapes prior to their modern-day transformations (including the glacier advances of the 1920s and 1980s). Helbronner’s photographic legacy now constitutes a major heritage, held mainly at the Musée Dauphinois in Grenoble (France), and a valuable historical resource for studying glacier retreat (see Nussbaumer and Zumbülh, 2012; Scaioni et al., 2018).

10The 12 volumes (each containing between 500 and 1,000 pages, with fold-out panoramic photographs and draws) and two photographic albums of the Description géométrique des Alpes are the result of 25 years of topographical work (1903-1928) carried out across the entire French Alps (de Margerie, 1943). In it, P. Helbronner synthesises, with a wealth of photographic illustrations, the trigonometric observations, levelling surveys, calculations and descriptions carried out during his vast triangulation project.

11We have selected the first two volumes to study the Mont-Blanc, Vanoise and Écrins massifs (Fig. 2), the most glaciated in the French Alps (Gardent et al., 2014). Furthermore, these two volumes include photographic overviews in the appendices. These enabled the photographs (replicas of those included in the volumes) to be digitised flat, which would have been impossible with the volumes themselves due to the risk of serious damage. A review of these volumes did not reveal any written material relevant for this study.

12Volume I, Chaîne méridienne de Savoie [campagnes 1907-1908] (Helbronner, 1910), lays the foundations for the entire work. It comprises a detailed methodological introduction, a coherent and modern overview of French Alpine geodesy, and a systematic description of several massifs in the Northern French Alps, with the precise positioning of numerous peaks and geodetic points. Volume II, Massifs du Chablais et du Faucigny, versant français du massif du Mont-Blanc [campagnes 1914-19-20-21] (Helbronner, 1930) continues the study. These two volumes form the descriptive and methodological basis of the entire work and already cover a significant portion of the highest massifs of the French Alps with exceptional precision.

13We digitised and used 12 panoramic photographs (Table 1; Appendix 1) following a selection process that excluded images showing glaciers that were too incomplete, too distant, or of insufficient quality. Covering a wide range of elevations and aspects, they were compared with IGN orthophotos from 2022 for the Vanoise and Écrins massifs and from 2023 for the Mont-Blanc massif. We also used digital elevation models (DEM) from 2020 (Savoie and Haute-Savoie) and 2024 (Hautes-Alpes) from the RGE Alti database (IGN) and HD LiDAR point clouds (IGN).

Table 1. The 12 panoramic photographs by P. Helbronner selected for this study

Volume / Photo - Massif	Date	Location where the photograph was taken	Area photographed (basin, summit or main glacier)
I / 01 - É	14 July 1905	Aig. du Goléon	La Meije, Le Rateau, Girose glacier
I / 05 - V	09 Sept. 1907	Dent Parrachée	Dôme de l’Arpont
I / 09 - V	05 Sept. 1907	Bellecôte	Mont Pourri, La Grande Casse
I / 11 - V	15 Aug. 1907	Aig. Rouge	Mont Pourri*
II / 13 - MB	22 Aug. 1920	Aig. des Posettes	Tour glacier*
II / 14 - MB	22 Aug. 1920	Aig. Rouges massif	Aig. du Grépon, aig. du Plan
II / 15 - MB	29 July 1907	Le Brévent	Bossons and Taconnaz glaciers
II / 19 - MB	31 Aug. 1920	Tour Noir	Aig. Verte, Argentière glacier
II / 20 - MB	17 Aug. 1920	Aig. Gds Montets	Argentière gl. (upper part, Fig. 1)
II / 24 - MB	17 July 1920	Aig. du Moine	Géant, Talèfre, and Leschaux gl., Aig. du Tacul*
II / 25 - MB	07 Aug. 1920	Aig. du Midi	Mont Blanc, Vallée Blanche
II / 26 - MB	06 Aug. 1920	Pte Helbronner	Géant gl., Aig. Marbrées

É: Écrins, V: Vanoise, MB: Mont-Blanc. *Glacier reconstructed using monoplotting.

Study Sites

14Located on the border between France (Haute-Savoie and Savoie), Italy (Aosta Valley) and Switzerland (Valais), the Mont-Blanc massif (Fig. 2) is the highest mountain range in Western Europe, with mont Blanc (4,806 m a.s.l.) as its ‘roof’ and a very high proportion of area ice covered. Its glaciers have long attracted the attention of researchers, notably that of Joseph (naturalist), Henry and Charles (topographers) Vallot, who studied several glaciers including the Mer de Glace, the largest glacier in the French Alps (26.2 km² in 2023). C. Vallot (1922) thus recorded changes in the length of several glaciers in the Chamonix valley between 1894 and 1921, and reported a glacier advance at the beginning of the 20^th century, with a marked advance of the Bossons and Taconnaz glaciers in 1919 and 1920. Eugène Viollet-le-Duc (1876), on his side, examined the ‘accumulation of snow’, snowfields, the action of glaciers on rock, moraines and the ‘advance of glaciers’. Recent studies, however, show a significant retreat of these glaciers over the past few decades: between the end of the Little Ice Age (134.3 km²) and 2022 (88.7 km²; Rabatel and Klee, 2023), the glacial area of the massif in France has decreased by 34%. Between 1905 and 2018, the Argentière glacier (Fig. 1) and the Mer de Glace lost 34 and 45 m water equivalent respectively, equivalent to an average ice thickness reduction of 38 and 50 m across their entire surface (Vincent et al., 2019). These losses represent 25 and 32% respectively of their average thickness since the beginning of the 20^th century (Vincent et al., 2009, 2019).

15With around a hundred peaks over 3,000 m a.s.l., including Grande Casse (3,855 m a.s.l), the highest point in Savoie, the Vanoise massif still contains a large ice cover, the first summary report on which was produced by Vivian and Bocquet (1973). The glacier-covered area of the massif has decreased by 74% between the end of the Little Ice Age (238.5 km²) and 2022 (62.2 km²; Rabatel and Klee, 2023). The loss accelerated during the 20^th century, reaching 52.8% between 1967-1971 and 2022 (Rabatel and Klee, 2023). Fluctuations in the Gébroulaz Glacier (Vivian and Chinal, 1969) have been partially documented since 1730 and scientifically monitored since the early 20^th century, initially by the Water and Forestry Administration and, since 1990, by the Institute of Environmental Geosciences (IGE) as part of the National Observation Service Glacioclim (Six et al., 2024).

16With its steep terrain and more than 150 peaks exceeding 3,000 m a.s.l., including Barre des Écrins (4,102 m a.s.l.), the Écrins massif (Hautes-Alpes and Isère) is also still heavily ice-covered. The first systematic studies date back to Jacob’s (1927) surveys of fluctuations in the Oisans glaciers between 1905 and 1911, which showed widespread retreat prior to the advance of 1918-1921 (Allix, 1927). Today, retreat is accelerating there too: following a loss of area of around 3% per decade up to the 1970s (Gardent et al., 2014) and the minor advance of the 1970s-80s (Rabatel et al., 2002), it reached -34% between 2003 and 2022 (Rabatel and Klee, 2023). In total, the massif had lost 69% of its glacier surface area by 2022 (53.7 km²; Rabatel and Klee, 2023) compared to the end of the Little Ice Age (171.1 km²). The Blanc glacier, the largest in the massif (Vivian, 1967), lost more than 10 m water equivalent between 1981 and 2005 (Rabatel et al., 2008), and more than 8 m water equivalent between 2022 and 2025 (Écrins National Park data).

Typology of Ice Bodies

17The response of glaciers to the climate crisis varies greatly depending on their morphological characteristics and thermal regimes, as highlighted in the works of Allen (1998), Gardent et al. (2014) and Kaushik et al. (2022). Gardent et al. (2014), for example, showed that the glaciers most affected by melting between 1967-1971 and 2006-2009 were small glaciers, facing east, south-east and south, and located on steep slopes. The study of Alpine glacier dynamics therefore requires a differentiated analysis based on glacier types (Romshoo et al., 2022), particularly as glacier retreat often leads to glacier fragmentation, increasing the number of small glaciers – a sign of the profound transformation of glacier organisation. In order to study these contrasting trajectories, and because the World Glacier Monitoring Service (WGMS) typology is not well suited to the Alpine environment (Linsbauer et al., 2021; Attaullah et al., 2023), a glacier typology inspired by Kaushik et al. (2022) and comprising 8 categories/types (Fig. 3) is proposed:

• valley glaciers (VG), such as the Mer de Glace, characterised by a long glacier tongue with a temperate base (i.e. at 0°C) at the bottom of a valley with a slope angle of less than 15°, fed by one or more cirques;

• cirque glaciers (CG), such as the Vanoise glaciers, situated in semi-circular depressions, the width of which is greater than or equal to the length and/or with a slope angle of less than 15°;

• cirque-and-slope glaciers (CSG), such as the Chardonnet glacier (Mont-Blanc massif), which combine an upstream cirque with a downstream extension on a slope steeper than 15°;

• slope glaciers (SG), such as the Bossons glacier, with a slope steeper than 15° and lacking an upstream cirque;

• ice caps (IC), such as that of aiguille Verte (Mont-Blanc massif), forming a summit dome whose cold-based ice (i.e. < 0°C) is stuck to the bedrock;

• hanging glaciers (HG; Pralong and Funk, 2006), such as part of the Taconnaz glacier (Mont-Blanc massif; Vincent et al., 2015), with a cold base on a slope with an angle greater than 35-40° and a front forming seracs;

• ice aprons (IA; Ravanel et al., 2023), such as Le Linceul on the north face of Grandes Jorasses (Mont-Blanc massif; Ravanel et al., 2025), small ice masses on a slope steeper than 40°, with an area of less than 0.1 km², with a cold base, several meters thick, bounded downstream by a bergschrund if they overhang another glacier;

• ice-snow fillings in couloir (FC; hereinafter ‘couloirs’), masses of ice (and snow) occupying the bottom or even the entire length of a fault (generally V-shaped) or erosion couloir.

18This typology extends that of Kaushik et al. (2022) with two new types: (i) cirque-and-slope glaciers to distinguish between the fragmentation dynamics of slope glaciers with or without an upstream cirque; (ii) couloirs, which to our knowledge are absent from the scientific literature, to account for the couloirs and gullies climbed or skied by mountaineers (Arnaud et al., 2024). The classification of glaciers according to this typology was carried out for the two periods 1905-1920 and 2022-2023.

19For this work, ice bodies connected to a major glacier downstream were considered part of that glacier, with the exception of cold-based ice bodies (ice aprons, ice caps and couloirs), which are almost immobile. Furthermore, an inventory of all current water bodies – generally absent in 1905-1920 – with a surface area greater than 100 m² (the threshold used for the French National Inventory of Water Bodies, INPE) was carried out to account for this key indicator of the transformation of recently deglaciated areas.

20All the data collected from the 12 panoramic photographs – for each ice body (type, front elevation, aspect, debris cover, area loss, lakes, etc.) – has been incorporated into a table comprising 202 ice bodies. An identification code (e.g. 2SG1 for volume2_photo20_slope-glacier1) allows each glacier to be linked to the photographs by P. Helbronner that we have annotated. This table forms the basis for the statistical processing and analysis carried out in RStudio and, for certain glaciers, by monoplotting (see §2.4).

Assessment of Past and Present Glacier Surface Areas

21The outlines of the current glaciers were mapped using a GIS (in this case, QGIS) on orthophotos from 2022-2023, enabling their surface area to be measured (see Lardeux et al., 2016). Delimiting debris-covered areas is challenging, and their boundaries were identified on the basis of field visits for some glaciers and by studying the surface morphologies of debris-covered areas for others. These orthophotos were then tilted using Géoportail 3D and Google Earth to match the viewing angles of early 20^th-century glacier photographs. Stable morphological features (moraines, boulders, fractures in the bedrock) visible in both historical and current images enabled the repositioning of the fronts and margins of the ancient glaciers in QGIS, so that their area could then be measured. Given the uncertainties inherent in the method and in order to limit interpretative bias, losses in glacier area were categorised into seven classes: <20%, 20-40%, 40-60%, 60-80%, >80%, ‘total disappearance’, and ‘value unavailable’.

22Changes in the surface area of three glaciers (the Tour glacier and the hanging glacier on the north face of aiguille du Tacul in the Mont-Blanc massif, and the hanging glacier on Mont Pourri in the Vanoise massif) have been quantified more accurately using monoplotting. This is a photogrammetric method in which uncorrected oblique photographs are aligned with a 1-m-DEM. In practice, the camera, the image and the DEM are aligned such that a line extending from the centre of the camera and passing through a selected point in the image plane intersects the ground surface at the corresponding real-world point (Bozzini et al., 2012; Scapozza et al., 2014). P. Helbronner’s oblique photographs were georeferenced to a DEM using control points. The IGN HD LiDAR point clouds were rasterised using CloudCompare, then imported into QGIS. The moniQue plugin, developed specifically for glaciers (Fajardo Turner, 2025), enables the position of the historical camera to be determined using at least 15 identifiable control points on a photograph and the DEM. Once this calibration was complete, the glacier boundaries visible in the old photographs were traced and then projected onto the DEM. Measuring the historical areas then allowed them to be compared with contemporary areas. Only three of Helbronner’s 12 photographs allowed this method to be applied. The other photographs show glaciers that are too distant or too snow-covered, or their resolution is of insufficient quality to identify reliable control points.

A Century of Glacier Retreat

Changes in Ice Cover in Five Specific Sectors

23Glacier changes have been mapped in detail in five areas: the north face of Mont Pourri (3,779 m a.s.l.) and the eastern side of Dôme de l’Arpont (3,599 m) in the Vanoise massif, the accumulation zone of the Argentière glacier (Fig. 1), the Tour glacier and the north face of aiguille du Tacul (3,444 m) in the Mont-Blanc massif.

24For Mont Pourri (1907-2022; Fig. 4; Appendix 2), diachronic analysis using monoplotting highlights the disappearance of the slope glacier, representing a loss of glacier surface area of nearly 228,000 m². Glacier retreat has resulted in the formation of two ice aprons and a couloir, and the disappearance of an ice cap and an ice apron that were present in 1907. The Turia and Grand Col glaciers are now covered by debris.

25In the early 20^th century, Dôme de l’Arpont area (1907-2022; Appendix 3) was home to the vast Mahure valley glacier and the broad Arpont slope glacier. Their fragmentation gave rise to numerous smaller ice bodies: the valley glacier split into a cirque glacier and a valley glacier, and 4 ice aprons emerged. The large slope glacier fragmented to produce a smaller slope glacier as well as 5 ice aprons. 23 glacial lakes (ranging from 125 to 35,725 m²) developed in the glacially overdeepened basins.

26In the accumulation zone of the Argentière glacier (1920-2023; Appendix 4), the upper section of the valley glacier has fragmented into 3 cirque-and-slope glaciers disconnected from the main glacier and 4 ice aprons. The retreat and thinning of the Argentière glacier have led to the formation of a couloir. The former cirque-and-slope glacier has also split into a slope glacier and 2 small cirque glaciers. All the couloirs present in 1920 have disappeared, and the confluence between the Tour Noir and Argentière glaciers is now hidden beneath a debris cover.

27The retreat of the Tour glacier’s tongue (1920-2023; Appendix 5) and the reduction in its slope angle have transformed this cirque-and-slope glacier into a cirque glacier. The ice apron has completely disappeared. The loss of 1.63 km² of surface area (approx. 21% of the original area) has led to the formation of 7 glacial lakes (ranging from 100 to 2,800 m²) in overdeepenings.

28Finally, on the north face of aiguille du Tacul (1920-2023; Appendix 6), the hanging glacier has lost 57% of its surface area (approx. 78,200 m²), illustrating the rapid deterioration of some of these glaciers. All the ice bodies located downstream appear to have disappeared, although ice may be present locally beneath the rock debris at the foot of the face.

Transformation, Fragmentation and Area Loss of the Original 202 Ice Bodies

29A statistical analysis was carried out on the ice bodies documented using P. Helbronner’s panoramic photographs. The aim was to quantify losses in surface area and to explore potential relationships between different variables (types of glacier, aspect, elevation, etc.) in order to gain a clear understanding of glacier trajectories.

30The initial dataset (1905-1920) comprises 202 ice bodies: 96 couloirs, 49 ice aprons, 15 cirque-and-slope glaciers, 15 ice caps, 9 hanging glaciers, 9 slope glaciers, 6 cirque glaciers and 3 valley glaciers (Fig. 5; Appendix 7).

31Between the two periods studied, the total number of ice bodies increased due to fragmentation, which resulted in 39 additional glaciers, bringing the total to 241 (+19.5%). There was a sharp increase in the number of ice aprons (from 49 to 126; +157%), a marked decrease in the number of couloirs (-61 entities; -63.5%), and smaller variations for the other types.

32Fragmentation is particularly pronounced for certain types (Fig. 6). It is extreme for the 3 valley glaciers, which give rise to 51 ice bodies encompassing all glacier types except ice caps. The 9 original slope glaciers produced 36 ice bodies, with partial preservation of the original types. The 9 hanging glaciers gave rise to 28 entities, a third of which hold their morphotypic identity, albeit with a smaller size. A complex fragmentation of cirque-and-slope glaciers gave rise to cirque, slope, cirque-and-slope glaciers and ice apron. Finally, the original ice caps, cirque glaciers and ice aprons remain largely unaffected.

33Analysis of area loss (Fig. 7) reveals significant changes: 48.5% of glaciers have disappeared entirely, 8.4% have lost more than 80% of their surface area, whilst only 1% have lost less than 20%. The extent of loss varies greatly depending on the type of glacier. Couloirs are the most vulnerable, with 82.3% having disappeared. One-third of cirque glaciers have lost between 40 and 60% of their surface area, another third more than 60%, and the remaining third have disappeared. 28.6% of ice aprons have disappeared, with a loss of area exceeding 80% for a large number of them. Cirque-and-slope glaciers are more robust, with no loss of surface area exceeding 80%, whilst slope glaciers show highly varied losses.

34The distribution of glacier surface area loss according to front elevation is not uniform (Fig. 8). Those with fronts located between 3,200 and 3,400 m a.s.l. are the most affected, accounting for 71% of all lost glaciers, all types combined. Between 3,400 and 3,600 m a.s.l., 66.7% of glaciers have disappeared and 8.3% have lost more than 80% of their surface area. Above 3,600 m a.s.l., all ice bodies (which are relatively small in size) have lost more than 80% of their surface area. Conversely, ice bodies with fronts located at elevations below 2,400 m a.s.l. – i.e., the larger ones – show surface area losses of less than 60%.

35The loss of surface area varies depending on the aspect of the glaciers. 73% of glaciers facing south-west have disappeared, and 14% have lost more than 80% of their surface area. For south aspect, 70% of glaciers have completely disappeared, 10% have lost more than 80% of their surface area, and the remaining 20% have lost between 60 and 80% of their area. Losses are high in the NE and NW aspects, with 50% having disappeared. In contrast, only 8% have disappeared in the north-facing aspect, the only aspect where the loss of surface area can be less than 20%.

36Finally, 43 new lakes have appeared in depressions created by glacial retreat: 20 (46.5%) have an area of 100 to 500 m², 22 (51%) between 500 and 5,000 m², and only the Arpont lake (Vanoise) exceeds 5,000 m² (35,700 m²).

Discussion

Between Fragmentation and the Disappearance of Ice Bodies: A Rapid Reshaping of Alpine Glacier Landscapes

37Our results confirm the widely documented glacier retreat in mountainous regions during the 20^th and early 21^st centuries (Zemp et al., 2015; Dussaillant et al., 2019; Linsbauer et al., 2025). They highlight the restructuring of the glacial system in the high Alpine mountain, beyond losses in length (retreat of the fronts), thickness and surface area. The disappearance of nearly half of the ice bodies recorded in P. Helbronner’s photographs and the significant fragmentation of larger glaciers reflect a distinct and symptomatic trend of the current rapid deglaciation (Paul et al., 2004, 2020; Fischer et al., 2015).

38Fragmentation, particularly in the case of valley glaciers, slope glaciers and cirque-and-slope glaciers, is part of a process of loss of ‘glacial connectivity’ (Diolaiuti and Smiraglia, 2010; Kääb et al., 2021), i.e., a loss of continuity between glaciers. Their widespread thinning, which is even more significant than the retreat of the fronts (Vincent et al., 2018), leads to the exposure of numerous rock outcrops and the occurrence of ice collapses at points of steep slope breaks (as in the case of the Argentière glacier in the 1990s-2000s), which progressively isolate the accumulation zones from the tongues. This mechanism is particularly effective in complex morphological settings, where the coexistence of cirques and tongues on steep slopes promotes rapid fragmentation (Pelfini and Smiraglia, 1992; Galluccio and Scotti, 2022). Furthermore, the development of dark rock surfaces and the resulting change in albedo further accentuates the local rise in air temperatures and thus ice melt (e.g. Dumont et al., 2012).

39The dynamics around Dôme de l’Arpont and the Argentière glacier accumulation zone illustrate this trend, which has already been observed on other Alpine glaciers (e.g. Forni, Fellaria, or Scerscen in the Lombard Alps; Diolaiuti and Smiraglia, 2010). Fragmentation does not lead to stable features: it results in a proliferation of small glaciers, whose vulnerability is heightened by a high surface area-to-volume ratio (Granshaw and Fountain, 2006; Tennant and Menounos, 2013). Given comparable specific ablation, these small glaciers retreat more rapidly than large ones, which accelerates their disappearance (Gardent et al., 2014; Huss and Fischer, 2016).

40Fragmentation is a transitional stage in the retreat of glaciers, preceding the complete disappearance of many ice bodies (Kaushik et al., 2022). Couloirs provide a striking illustration of this trajectory: their disappearance rate of over 80% reveals their dependence on snow conditions and their extreme sensitivity to temperature (Huss et al., 2017; Beniston et al., 2018).

41The emergence of numerous glacial lakes is another indicator of this transition (Carrivick and Tweed, 2013; Shugar et al., 2020). Our observations are consistent with other recent inventories in Switzerland (Mölg et al., 2021) and the Himalayas (Wang et al., 2020), which show a predominance of small lakes in glacial overdeepenings (Haeberli et al., 2016). These lakes, although small in surface area, reflect a functional shift in high-mountain landscapes, from ice-dominated systems to post-glacial rocky and lacustrine environments, which are potentially unstable and sometimes give rise to new hazards (Kapista et al., 2017; Emmer et al., 2022).

42Finally, an analysis of topographical factors confirms the central role of aspect and elevation in the evolution of glaciers. However, whilst the increased vulnerability of south- and south-west-facing glaciers is well documented (Evans, 1977; Rabatel et al., 2013; Gardent et al., 2014), the high rates of loss at high elevation highlight the importance of size (ice thickness in particular) and glacier type. The over-representation of small, cold-based bodies at these altitudes increases the rate of disappearance, highlighting the need for a cautious interpretation of the elevation-melt relationship (Paul and Haeberli, 2008; Zemp et al., 2015).

New Data on Little-Known Ice Bodies

43Our study highlights the evolution of ice bodies that have long been overlooked by research, in particular ice aprons and, to an even greater extent, ice-snow fillings in couloir. These features, which are generally absent from glacier inventories (Raup et al., 2007; Cogley, 2009), nevertheless contribute to our understanding of deglaciaition in the Alpine massifs.

44The sharp increase in the number of ice aprons observed over a century should not be interpreted as a positive sign, but rather as an expression of a transitional process in glacier morphotypes. They correspond either to remnants of cold-based ice bodies previously connected to larger glaciers, or to accumulations of perennial ice made visible by the retreat of the surrounding ice (Guillet and Ravanel, 2020; Kaushik et al., 2022; Ravanel et al., 2023). Their thinness and small surface area, and their exposure to solar radiation in early summer when the sun warms the north-facing slopes, explain their high rate of disappearance, as already highlighted by Ravanel et al. (2023).

45The evolution of couloirs represents an even more original contribution. Their absence from the scientific literature contrasts with their importance in certain Alpine activities (mountaineering and steep-slope skiing) and their sensitivity to climatic conditions (Arnaud et al., 2024). Extremely responsive to temperature and snowpack variations, the ice-snow fillings in couloir are among the first ice bodies to disappear. Their very rapid retreat results from a combination of climatic, morphological and dynamic factors. Under equivalent climatic conditions, small ice bodies disappear much more rapidly than large ones (Granshaw and Fountain, 2006; Huss and Fischer, 2016). They are in fact directly dependent on snowfall as, unlike cirque or valley glaciers, couloirs lack an accumulation basin or an autonomous glacial mechanical regime. Their supply relies mainly on direct snowfalls trapped by the enclosed topography and, for less steep couloirs, on avalanches from adjacent slopes. They therefore function more like névés than glaciers. Rising temperatures, which raise the snow-rain limit, reduce the duration and thickness of the snowpack (Haeberli and Beniston, 1998; Beniston et al., 2018), compromising the annual refill of these couloirs. Winter snow accumulation no longer compensates for the increasing summer melt, leading to their rapid and often irreversible disappearance, despite the possibility of renewed snowfalls during the winter season. Furthermore, the couloirs have an extremely unfavourable geometry with a high surface area-to-volume ratio (Granshaw and Fountain, 2006; Huss and Fischer, 2016) due to their thinness (ranging from meters to tens of meters) and their surface area in contact with the atmosphere and rock faces. This geometry results in high sensitivity to ablation via heat conduction from the warmed rock faces, via radiation reflected and emitted by the rock walls, and via meltwater runoff on the rock walls. The warming of rock walls and the ‘adjacent’ cryosphere plays a key role: couloirs are in fact thermally coupled to the rock walls that flank them and which warm as summer temperatures rise (Peleg et al., 2025), permafrost degrades (Gruber and Haeberli, 2007; Noetzli et al., 2024), and the insulating effect of snow in early summer is lost (Magnin et al., 2017). This lateral heat input towards the ice or snow trapped in the couloirs explains why some couloirs disappear rapidly despite what would appear to be a favourable aspect (N or NE). Finally, ice-snow fillings in couloir are particularly affected by boulder falls and rockfalls (Ravanel et al., 2017): the mechanical ablation reduces the volume and increases the surface area available for energy exchanges, and thus thermal ablation. Finally, their often-steep slope angle prevents any sustainable refill of the ice mass, as fresh snow is frequently swept away. Unlike large glaciers and similar to ice aprons, couloirs do not flow, or flow very little: they therefore have no positive dynamic feedback able to compensate for mass loss. Once a critical thickness threshold is crossed, the disappearance of couloirs is generally sudden and irreversible.

Conclusions

46This study highlights the contribution of P. Helbronner’s photographic work – and, more generally, of historical photography – to the analysis of long-term glacial dynamics in the Alps. His panoramic photographs from the early 20^th century constitute an exceptional archive that provides a rare baseline, predating the recent acceleration in global warming, and enabling a diachronic analysis spanning a century. In his Description géométrique détaillée des Alpes françaises, he had noted that “It is, moreover, permitted to anticipate the value that this body of documentation may one day hold, particularly from a glaciological perspective”.

47The comparison of glaciers between the beginnings of the 20^th and 21^st centuries highlights the level of glacier retreat in the Alps. 48.5% of the 202 ice bodies identified between 1905 and 1920 have completely disappeared, particularly certain types such as ice-snow fillings in couloir, where 82% of such features have vanished. At the same time, the fragmentation of larger glaciers has increased the total number of ice bodies (+19.5%), with a proliferation of small-scale features, which are often transient and more vulnerable to the effects of the climate crisis.

48Over a century, we have observed a systemic transition in Alpine glacial environments, characterised by a loss of glacial connectivity, the development of marginal ice bodies (notably ice aprons) and post-glacial landscapes, with 43 new lakes forming in the areas studied; the potential risks associated with these developments must be considered.

49The inclusion of ice-snow fillings in couloir in the glacier typology adapted to the Alpine context proposed in this article represents a conceptual advance in better accounting for the diversity of glacial evolution trajectories. Current climate change does not elicit a uniform response from glaciers. These morphotypic responses depend on their size, geometry, aspect and thermal regime. In this respect, small cold-based ice bodies (ice aprons in particular) signal critical environmental thresholds well before the disappearance of valley or slope glaciers. This study thus highlights the need to fully integrate these little-known ice bodies into research, in order to better understand the past dynamics and the current and future trajectories of high-mountain environments in the context of the climate crisis.