# Pyrenean glaciers are disappearing fast: state of the glaciers after the extreme mass losses in 2022 and 2023

Eñaut Izagirre<fnref n="1" />,<fnref n="2" /> ID · Jesús Revuelto<fnref n="1" /> ID · Ixeia Vidaller<fnref n="1" /> ID · César Deschamps-Berger<fnref n="1" /> ID · Francisco Rojas-Heredia<fnref n="1" /> ID · Ibai Rico<fnref n="2" /> ID · Esteban Alonso-González<fnref n="3" /> ID · Simon Gascoin<fnref n="3" /> ID · Enrique Serrano<fnref n="4" /> ID · Juan Ignacio López-Moreno<fnref n="1" /> ID

Received: 20 September 2023 / Accepted: 31 October 2024 / Published online: 29 November 2024
© The Author(s) 2024

## Abstract

Given rapid glacier thinning and retreat observed in the Pyrenees in recent decades, an updated glacier inventory and continuous mass balance assessments are important to understand the ongoing variability and changes of these very small glaciers (<0.5 km<fnref n="2" />). The mass balance years 2021/22 and 2022/23 were characterised by prolonged extreme heat waves and reduced snow duration that severely affected the Pyrenees, which also impacted their glaciers. This paper reviews the criteria for classifying ice bodies as glaciers or ice patches, presents the latest high-resolution glacier inventory for the Pyrenees, and quantifies the mass losses caused by the extreme climate conditions in 2022 and 2023. The glacierised area was determined by manual mapping of high-resolution (0.2 m spatial resolution) aerial orthomosaics acquired by unmanned aerial vehicles (UAVs) and aerial orthophotos (0.25 m spatial resolution) for the few glaciers not surveyed by UAVs. 3D point clouds, also obtained from UAV flights, were used to update the results for the change in surface elevation (glacier thickness) and mass balance between 2020 and 2023. For the Pyrenees, the total glacierised area in 2023 is 143.2 ± 1.8 ha in 15 different glaciers and 8 ice masses were degraded to ice patches according to our criteria. The resulting area change between 2020 and 2023 is -94.8 ha, representing a -39.8% decrease of the glaciarised area from 2020 to 2023, increasing the annual ratio of area change from 2020 to 2023 by -8.7% yr⁻<fnref n="1" /> compared to the period 2011–2020 (-2.4% yr⁻<fnref n="1" />). The change in glacier thickness measured on 12 glaciers shows a decrease of -2.52 m yr⁻<fnref n="1" /> for the period 2020–2023, which represents a significant acceleration in glacier thickness loss compared to -0.80 m yr⁻<fnref n="1" /> for the period 2011–2020. The three glaciers (Infiernos, Monte Perdido and Aneto) on which annual geodetic measurements were carried out showed slightly higher glacier thickness losses (-0.91 m yr⁻<fnref n="1" />) in the first mass balance year (2020/21) than in the previous decade (2011–2020), while the losses in the last two mass balance years (2021/22 and 2022/23) were three to four times higher (-3.42 m yr⁻<fnref n="1" /> and -3.07 m yr⁻<fnref n="1" /> respectively) and exceeded the record values.

**Keywords** Pyrenean glaciers · Unmanned Aerial Vehicle (UAV) · Glacier inventory · Heat waves · Glacier mass balance

Communicated by Wolfgang Cramer

<figure><img src="image_1.png" /></figure>

Eñaut Izagirre
eizagirre@ipe.csic.es; enaut.izagirre@ehu.eus

## Introduction

As global glacier mass loss accelerates in the early twenty-first century (Grunewald and Scheithauer 2010; Huss and Hock 2018; Hugonnet et al. 2021), scientific and societal interest in mountain glaciers has increased substantially (Huss et al. 2017; Adler et al. 2019). These important components of the cryosphere are known to provide high-confidence evidence of variability in climatic conditions and recent climate change (Haeberli et al. 2007; Marzeion et al. 2014; Beniston et al. 2018), as their evolution depends on the mass balance between winter snow accumulation and melting during the warmest period (Braithwaite and Hughes 2020). The recent acceleration in glacier mass loss observed

<footnote n="1">¹ Instituto Pirenaico de Ecología, Consejo Superior de Investigaciones Científicas (IPE-CSIC), Avenida de Montañana 1005, 50059 Saragossa, Spain</footnote>

<footnote n="2">² Department of Geography, Prehistory and Archaeology, University of the Basque Country UPV/EHU, Vitoria-Gasteiz, Spain</footnote>

<footnote n="3">³ Centre d'Etudes Spatiales de La Biosphère CESBIO, Université de Toulouse, CNRSCNES/IRDINRA/UPS, Toulouse, France</footnote>

<footnote n="4">⁴ Department of Geography, GIR PANGEA, University of Valladolid, Valladolid, Spain</footnote>
worldwide is significant in temperate mountain regions and in the Arctic compared to other regions (Pelto 2010; Hugonnet et al. 2021).

In the Pyrenees, the last remaining glaciers have encountered a strong decline in area (-23.2%) and volume from 2011 to 2020, with a mean glacier mass balance of about -0.59 m w.e. yr⁻¹ for the same period (Vidaller et al. 2021). Pyrenean glaciers are considered very small glaciers (<0.5 km²; Huss and Fischer 2016) and their evolution is primarily controlled by regional climate forcing, although they are also influenced by local topographic factors and geomorphological processes (Vidaller et al. 2021). In contrast to other alpine glaciers, which are important sources of water in other mountain regions (Fountain and Tangborn 1985; Beniston 2003; Huss et al. 2017; Miles et al. 2021; Gascoin 2023), the Pyrenean glaciers hardly contribute to river runoff in this region (López-Moreno et al. 2020b). Nevertheless, the ice loss from very small glaciers such as the Pyrenean glaciers has a clear impact on local erosion rates (Riihimaki et al. 2005), alpine stream biodiversity and macroinvertebrate communities (Snook and Milner 2001; Brown et al. 2007), or the microbiology of these iconic landscapes and surrounding downstream areas.

The Pyrenean glaciers are in a highly critical situation and could disappear in the coming decades (López-Moreno et al. 2020b; Vidaller et al. 2021). Their committed decline in area and thickness (Christian et al. 2018), means the loss of invaluable environmental and climatic archives that they contain (Moreno et al. 2021), and the disappearance of an outstanding landscape heritage of the Pyrenees (Olalde et al. 2023). To this end, the shrinkage of Pyrenean glaciers often leads to a transition from very small glaciers (i.e. ice bodies with movement) to ice patches with melting of non-moving remnant ice masses (Serrano et al. 2011; Hughes 2018; López-Moreno et al. 2020b), which are stagnant and therefore do not transfer mass from an accumulation to an ablation area (Ødegård et al. 2017). Recent regional studies conclude that the ice on the largest glaciers in the Pyrenees, such as the Monte Perdido glacier, has not been constantly renewed in recent decades (Moreno et al. 2021), as the accumulation zone is almost negligible, especially after very hot summers such as in 2022 (Vidaller et al. 2023). Therefore, although glacier retreat is progressing rapidly, it is still difficult to distinguish very small glaciers from remnant ice patches, as the process of disintegration and disappearance is not yet complete.

An important aspect of ongoing climate change is the occurrence of more frequent and more intense heat waves, which affect not only the core of the summer but also shoulder seasons (Christidis et al. 2015; Vogel et al. 2020; Senevi-ratne et al. 2021), and are expected to increase in duration in mountainous regions by the end of the century (Molina et al. 2020). Higher temperatures during the spring and summer

seasons melt the snow cover on mountain glaciers earlier, causing a darker, bare ice surface to melt faster and longer, increasing mass loss at the end of the mass balance year (Di Mauro and Fugazza 2022; Pelto et al. 2022). In recent decades, prolonged extreme heat events have become more common, with significant implications for glacier mass balance. For instance, 2022 was characterized by prolonged extreme heat events across most of the Eurasian continent and North America (Barriopedro et al. 2023; C3S 2023; Lu et al. 2023). These summer heat waves affected alpine glacier regions in Europe such as the Alps, demonstrating that heat waves are important for extreme melt events on alpine glaciers (Cremona et al. 2023).

In the Pyrenees, the summer of 2022 was extremely warm compared to the observations of the twentieth century and the last 700 years (Serrano-Notivoli et al. 2023). The latest extreme heat waves occurred in mid-June and mid-July 2022, setting new records across the region with a temperature anomaly of over +3 °C (reference period: 1981–2010) on the southern slope of the mountain range (AEMET 2022). Copernicus and Ebro Basin authorities indicated that the 2021/22 winter season in the Pyrenees showed a strong precipitation deficit (rainfall and snowfall), with the lowest snow depths and snow cover areas since at least 2001 (Copernicus 2023; ERHIN 2023a). Following 2022, global temperatures reached exceptionally high levels in 2023 (Ripple et al. 2023), especially between June and November, when they were declared the warmest record since 1850 (ECMWF 2024). In the Pyrenees, 2023 was characterized by extreme drought in terms of snow cover and its duration and consistently warm temperatures from the beginning of March to the end of November, but without reaching the extreme records of 2022 (ERHIN 2023b; AEMET 2024).

To better study these extreme melt mass balance years, novel methods such as the use of repeated photogrammetry by unmanned aerial vehicles (UAVs) allow identifying glacier area and surface elevation changes at high spatial and temporal resolution (Leigh et al. 2019; Revuelto et al. 2021a; Vidaller et al. 2023). In particular, the latest available detailed inventory of glacier area in the Pyrenees by Vidaller et al. (2021) for late summer 2020 provides an opportunity to analyse the impact of the 2022 heat waves and the extreme drought and persistently warm temperatures in 2023 on these glaciers. Therefore, we conducted an extensive campaign of UAV imagery and analysed the changes in glacier area and surface elevation between 2020 and 2023. In addition, we improved some of the glacier outlines from 2011 and 2020 using the extent of debris cover over dense 3D point clouds from UAV flights. We also improved the delineation of glacier outlines by taking advantage of the extremely reduced snow cover of glaciers in 2022 and 2023, and developing a new characterization criteria to distinguish very small glaciers from ice patches using high-resolution imagery. The

<figure><img src="image_2.png" /></figure>
impact of the extreme heat waves of the summer of 2022 and extreme drought and persistently warm temperatures in 2023 on the mass balance of some selected glaciers is also examined in more detail between 2021 and 2023. Finally, we discuss the importance of the last two extremely warm mass balance seasons (2021/22 and 2022/23) for the regional glacier evolution of the last remaining Pyrenean glaciers.

## Study area

Pyrenean glaciers remain in the most favourable cirques of the highest massifs in the central Pyrenees (Fig. 1), above or around the annual 0 °C isotherm, which is between 2,700 and 3,000 m a.s.l. (Jomelli et al. 2020; Serrano 2022), although some of them still exist below 2,700 m a.s.l. due to favourable topoclimatic control (low solar radiation, snow avalanches, etc.) and geomorphological processes (e.g.

rockfalls) that favour the supply of supraglacial debris (Vidaller et al. 2021; Serrano 2022). Mean annual precipitation exceeds 2,000 mm up to 3,000 mm on north and northwest facing slopes in the highest central massifs (Serrano-Notivoli et al. 2019), with winter and spring being the wettest seasons (Buisan et al. 2015), favouring the duration of snow cover in most of the highest massifs in the Pyrenees during several months of the year (Gascoin et al. 2015; López-Moreno et al. 2020a; Bonsoms et al. 2022).

These highest massifs, which, with the exception of Mont Valier (2,838 m a.s.l.), have peaks over 3,000 m a.s.l., host the southernmost glaciers in Europe (Grunewald and Scheithauer 2010; Marti et al. 2015; Vidaller et al. 2021). They are known to be particularly vulnerable to climate warming due to their very small size (<0.5 km²), relatively low elevation (between 2,400 and 3,200 m a.s.l.), and southern location (all below 43°N latitude) (Grunewald and Scheithauer 2010). Indeed, Pyrenean glaciers have been

<figure><img src="image_3.png" /><figcaption>**Fig. 1** (a) Geographic location of the central Pyrenees (shown as a black rectangle on the map of southwestern Europe) with the Pyrenean glaciers still present in 2023 (blue dots) and the glaciers that degraded to ice patches between 2011 and 2023 (green dots); no UAV data are available for the glaciers marked with a purple asterisk in two different years, so it is only included in the analysis of glacier</figcaption></figure>

area change. The red polygon shows the 1,900 km² area used for the ERA5-Land extraction of the climate data. (b) Distribution of Pyrenean glaciers from west to east in relation to their maximum and minimum altitude (blue polygon) and the highest peaks of the massif (black line); the glacier area and its aspect are shown at the mean altitude of each glacier (modified after Serrat and Ventura 1994)
steadily losing surface area and thickness in the last decades, indicating that they are in significant disequilibrium with the current regional climate (Vidaller et al. 2021). In particular, specific mass balance studies of changes in surface elevation and ice thickness conducted on Ossoue, Monte Perdido, Maladeta and Aneto glaciers reveal the critical situation of Pyrenean glaciers (Chueca et al. 2007; Marti et al. 2015; Gascoin and René 2018; López-Moreno et al. 2016, 2019; Martínez-Fernández et al. 2023; Vidaller et al. 2023), which are prone to disappearing in the next decades (López-Moreno et al. 2020b). The glaciers of Maladeta (ERHIN programme) and Ossoue (Association Moraine) have also been surveyed for many years (since 1991 and 2002 respectively) using the traditional glaciological method.

# Data and methods

## Data acquisitions

To ensure a complete inventory of glaciers in the Pyrenees, we continued the work of Vidaller et al. (2021) for the period 2011–2020 and extended it for the most recent years (2020–2023). To this end, we conducted an intensive fieldwork campaign taking advantage of cloudless and windless days to survey the remaining glaciers in late summer 2021, 2022 and 2023 with three different UAVs selected according to the logistical requirements of the sites (see Table 2 in the supplementary online information).

The images captured by the UAVs were then processed using Structure from Motion (SfM) software to derive 3D point clouds. In this work, we used Agisoft Metashape Professional (v1.6.3, https://www.agisoft.com/) to process each SfM project. The processing flow and parameters were set according to the official Agisoft guidelines (Agisoft 2018), and the SfM algorithm was used to generate the final dense point clouds, from which orthomosaics and digital elevation models (DEMs) with very high-resolution (0.2 m and 0.4 m, respectively) were derived.

The surface of Monte Perdido glacier was surveyed in early October 2021 and 2022 with a long-range terrestrial laser scanner (TLS, RIEGL LPM-321), based on previous measurements by López-Moreno et al. (2019). In 2022, a UAV flight was also carried out to validate this technique with the TLS technique used on this glacier since 2011. Following the good results of the TLS-UAV comparison for the mass balance year 2021/22 (not shown), this glacier was observed only with the UAV in 2023.

The remaining glaciers that were not surveyed with UAVs in 2021 and 2022 were only considered in terms of their extent and changes in glacier area. For this purpose, the high-resolution (0.2 m) orthophotos of the Instituto Geográfico Nacional (IGN Spain, PNOA 2021) and the Institut

National de l'Information Géographique et Forestière (IGN France, BD ORTHO® 2021) in 2021 were used. These glacier outlines were updated to the extent in 2022 using PlanetScope satellite images with a spatial resolution of 3.7 m (Planet 2022), selecting only cloud-free images at the end of the 2022 ablation season. In 2023, all remaining Pyrenean glaciers were surveyed with UAVs.

## Glacier mapping and surface elevation change calculation

All glacier outlines in the 2023 inventory were manually digitised using QGIS software (v3.28) based on the corresponding UAV orthomosaics and DEMs for each glacier. The negligible snow cover in late summer 2023 was crucial to accurately determine the extent of debris cover and glacier margins. To ensure consistency of mapping, the delineation process was performed by only one user (as suggested by Racoviteanu et al. 2009), but to avoid subjective interpretations (Paul et al. 2013), the digital glacier outlines were cross-checked by different experts with experience in mapping Pyrenean glaciers.

In this study, the digital extents of the 2011 and 2020 inventories (Vidaller et al. 2021) were also cross-checked and improved, as we found slight errors in the extent of debris cover of some glaciers (e.g. Infiernos, Gabietous, Llardana and Maladeta W).

For each glacier, we obtained the glacier area and surface (3D) in hectares (ha) in different years, which is frequently commuted due to their very small size (< 50 ha, López-Moreno et al. 2016; Rico et al. 2017; Vidaller et al. 2021). To calculate the area uncertainties of the orthomosaics derived from the 2021, 2022 and 2023 UAV flights, we applied the same procedure as in Rabatel et al. (2011) by considering the root of the quadratic sum of four different sources of error (pixel size, geometric correction, manual delineation and marginal snow cover) and multiplying by the perimeter of the glacier outline. The largest error is related to geometric correction and residual snow cover in 2021, while the orthomosaics were snow-free in 2022 and 2023, so this problem was overcome in the final delineation. For the PlanetScope images, we considered a buffer of ± 1 pixel (~ 4 m), a value reported by Tarca et al. (2023) for small glaciers (< 1 km²) in the Alps with the same satellite sensor.

To calculate glacier surface elevation changes, we used the same methodology as in Vidaller et al. (2021), where all 3D point clouds from the 2021, 2022 and 2023 UAV surveys (including all glaciers surveyed with the DJI Mavic Pro 2, and taking into account that the SenseFly eBee-X and DJI Mavic 3E observations were directly acquired in RTK and PPK geolocation accuracy) were co-registered using an iterative closest point (ICP) algorithm (Rajendra et al. 2014) in CloudCompare software (Girardeau-Montaut 2016). In
case of the Monte Perdido glacier, the TLS-derived 3D point clouds from 2021 and 2022 also followed same comparison protocol. This way, surface elevation changes relied on TLS technique in the mass balance years 2020/21 and 2021/22, whereas 2022/23 relied on UAV-derived 3D point clouds. As the area surveyed by TLS was lower than the total glacier extent (which was covered by UAV in 2022 and 2023), for 2022/23 two surface elevation changes were computed; one for the TLS area, to be comparable with previous acquisitions and one for total glacier extent, which will allow future comparisons in the coming years. The surface elevation change for the period 2020–2023 was calculated from a TLS-UAV comparison as both techniques have equivalent uncertainties (Revuelto et al. 2021b).

Glacier surface elevation differences (change in glacier thickness hereinafter) between the 3D point clouds of 2020 and 2023 and for the mass balance years 2020/21, 2021/22 and 2022/23 were determined using the CloudCompare M3C2 tool (James 2017). These differences are perpendicular to the glacier surface and were rasterized (0.4 × 0.4 m) for further analysis and calculated over the most recent (and smallest) glacier surface in 2023. Finally, the specific annual mass balance was calculated assuming a glacier ice density conversion factor of 850 ± 60 kg m⁻³ (Huss 2013), considering the most recent surface and area-weighted to the mean glacier-wide mass balance rate.

## Characterising very small glaciers and ice patches

Leigh et al. (2019) proposed a scoring system to identify very small glaciers based on features visible on high-resolution aerial imagery. Using their criteria, we were able to validate all Pyrenean glaciers remaining after the summer of 2023, but the distinction between these very small glaciers and their transition into ice patches was still missing. Therefore, to facilitate the identification of glacier degradation processes and the evolution of very small glaciers into remnant ice patches, we introduce new identification criteria with a set of features that are present on the ice surface and visible on high-resolution aerial imagery (Table 1). The new identification criteria extends earlier work by Serrano et al. (2011), who proposed that ice patches are ice masses without movement by flow or internal motion and that the absence/presence of activity, not size, is the determining factor in distinguishing between an ice patch and a glacier (Kuhn 1995).

Nevertheless, classification of very small glaciers and/or ice patches can be difficult if the ice masses show a high degree of degradation (López-Moreno et al. 2016), and multitemporal high-resolution data are required to assess this transition. Therefore, a minimum glacier surface area (3D) of 2 ha (0.02 km²) is also considered as an important numerical indicator, applied to the very small glaciers of the

Pyrenees. Finally, the state of fragmentation of each glacier in relation to its extent in 2011 and 2020 is also considered (Vidaller et al. 2021), which in some cases and after identifying some of the degradation features (Table 1), shows how former glaciers have degraded into remnant ice patches (Fig. 2).

## Temperature and precipitation anomalies in 2020/21, 2021/22 and 2022/23

Air temperature at 2 m above the surface, total precipitation and snow water equivalent (SWE) were extracted from ERA5-Land pixels at daily time steps over a rectangle of 1,900 km² covering almost all glaciers of the central Pyrenees (Fig. 1). The extraction was performed using Google Earth Engine, which provides ERA5-Land aggregated at daily time steps from its native hourly resolution (Gorelick et al. 2017; Muñoz-Sabater 2019).

ERA5-Land is a global climate reanalysis provided at 9 km spatial resolution from 1950 to present and updated with a few months delay. The accuracy of the ERA5-Land data has not been specifically assessed in the central Pyrenees, but is known to be spatially variable as it depends on the density of the assimilated data (Muñoz-Sabater, et al. 2021). It has been found that ERA5-Land data at low and medium elevations have a lower error than ERA5 data, but snow accumulation is generally overestimated (Muñoz-Sabater et al. 2021; Monteiro and Morin 2023). However, the interannual variability of ERA5-Land temperature and precipitation is good compared to observational data, which favours its use for climatological analyses (Gomis-Cebolla et al. 2023; Monteiro and Morin 2023).

Monthly cumulative precipitation, mean temperature and SWE were calculated over the area of interest for the period 1991–2020 to obtain a climatological distribution over a 30-year period. Monthly anomalies for the mass balance years 2020/21, 2021/22 and 2022/23 were then calculated and compared to the frequency distribution of monthly temperature, precipitation and SWE to quantify the intensity of monthly anomalies.

## Results

### Changes in area and total number of glaciers 2020–2023

In the entire Pyrenees, the total glacierised area mapped for 2023 is 143.2 ha divided into 15 individual glaciers (see Table 3 in the supplementary online information). The total uncertainty error of the glacier outlines for 2023 is 1.68 ha. This represents an uncertainty error of 1.2% of the total glacier area in 2023. In 2020, there were 23 glaciers
<table><caption>**Table 1** Criteria for identifying features that are present on the ice surface and visible (✓) or not visible (x) with high-resolution aerial imagery. They help to distinguish glacier degradation processes and serve as a reference for identifying their evolution into remnant ice patches</caption><thead><tr><th>Feature</th><th>Observed processes</th><th>Glaciological significance</th><th>Remote detection (Aerial photo, UAV...)</th></tr></thead><tbody><tr><td>1 Absence of crevasses</td><td>Ice lacks traction or extension crevasses, proving the absence of internal flow. The ice body behaves like a rigid block</td><td>This is the most important feature, especially when studying the evolution of the ice mass from the presence of crevasses to their absence</td><td>✓</td></tr><tr><td>2 Melting processes leading to collapse</td><td>Melting processes occur inside the ice mass due to air currents and lack of flow. The ice body hollows out and cavities form with a thinning of the roof, which then collapses and causes a sudden retreat of the glacier front</td><td>It is usually present, although it is often not visible on the surface, but only at the bottom. If they have already formed, they can be detected by aerial photography or UAVs. They can be recognised by circular cracks that appear before the collapse and can be identified using high-resolution DEMs</td><td>✓ / x</td></tr><tr><td>3 Water incisions in the ice, rectilinear or meandering, with water circulation through the substrate</td><td>Water flowing over the ice mass creates incisions in the ice that extend to and circulate through the substrate or ground moraine. In this way, the ice masses are separated from each other and do not deform or eliminate these incisions in the absence of flow continuity</td><td>This is very common, but when water circulates on ice or substrate. In unfavourable years, it is also visible on the upper part of the glacier</td><td>✓ / x</td></tr><tr><td>4 Disconnection of the accumulation area</td><td>The ice mass is separated from the accumulation area, stopping the supply and transformation processes of snow into snow and ice, as well as deformation and flow</td><td>This is not only the case with ice patches. There are glaciers that split but remain as fragmented glaciers, usually by mass or slope transfer, or by input from another (regenerated) glacier</td><td>✓</td></tr><tr><td>5 No false bergschrund or fracture in the upper part in contact with the wall</td><td>The false bergschrund, that is, the gap between the ice and the wall over the upper part, is not present, so that the ice rests directly on the wall. If landslides or rockfalls occur, the pressure of the ice during the rotational movement on the wall causes fractures</td><td>This is a clear indicator, but it does not always occur, and there are ice patches as well as snowfields that can have false bergschrunds</td><td>✓</td></tr><tr><td>6 Covering debris by falling rocks or moraine sediments</td><td>The ice is gradually covered with debris, partly by melt leaving englacial debris on the surface, and partly by falling rocks of moraine sediments surrounding the glacier. There are no contributions from the ice (shear crevasses, folds or fractures)</td><td>The distinction between a debris cover on the glacier and an ice-patch is tricky. The important point is that this debris cover mostly favours the preservation of the ice-patch</td><td>✓ / x</td></tr><tr><td>7 Fragmentation in relict ice bodies</td><td>Ice mass fragments into smaller bodies between cirques, walls, or thresholds, loses its dynamic connection (thrust, flow, deformation), and eventually become independent ice masses</td><td>This is very obvious in the smallest glaciers and ice-patches, but in larger glaciers their fragmentation does not mean that they become ice-patches</td><td>✓</td></tr></tbody></table>

with an area of 238 ha, which decreased to 18 glaciers and 172.6 ha in 2022. This corresponds to a total area loss of -94.8 ha (-40%) and -29.4 ha (-17%) since 2020 and 2022, respectively. For a better understanding of the changes in area of the Pyrenean glaciers, we calculated the total absolute and relative rates of glacier area change for the period 2020–2023, which are -0.35 ha yr⁻¹ and -8.55% yr⁻¹ respectively (Fig. 3).

Glaciers larger than 10 ha (> 0.1 km²) showed the largest area loss, with an average of -8.3 ha between 2020 and 2023 (Fig. 3), with Ossoue (-9.5 ha) and Maladeta East (-7.6 ha) glaciers standing out with more than a quarter of their area (-30.3% and -36.6%, respectively). In contrast, glaciers smaller than 10 ha (< 0.1 km²) lost on average -1 ha of their area in 2020 (see Table 3 in the supplementary online information). The area change becomes variable with a group
**Fig. 2** (a) High-resolution orthomosaic of the former Boum glacier (Perdiguero massif) that has degraded to an ice patch after the summer of 2022. Several features are visible on the ice surface (see Table 1), with the absence of the accumulation area and fragmentation into three distinct, smaller ice bodies evident: (b) former crevasses in the eastern part of the image have nearly disappeared and stratified debris bands are coalescing; (c) a circular crack (white circle) due to surface collapse resulting from melting processes, and the ice rests directly on the wall without a false bergschrund; (d) the ice is gradually covered with englacial and morainic debris due to continuous thinning of the ice surface

<figure><img src="image_4.png" /></figure>

of glaciers (e.g. Les Néous, Infiernos, Gabietous, Taillon, MUNIA, Llardana, Tempestades, d'Arcouzan) showing a relatively low area loss (-10.9% on average), and another group of glaciers (e.g. Oulettes, Petit Vignemale, Seil de la Baque) showing a relatively high area loss (-31% on average).

In 2023, 8 ice masses (Pailla W, Astazous, Barroude, La Paul, Portillon d'Oô, Boum, Maladeta W and Barrancs) are no longer classified as glaciers according to our criteria. Their surfaces show several features of glacier degradation processes (Table 1), which prove their lack of activity, as well as highly fragmented parts (e.g. La Paul and Boum; see Fig. 2), and their surfaces (3D) are smaller than 2 ha (e.g. Portillon d'Oô and Barrancs). Many of the glaciers smaller than 10 ha also showed persistent degradation processes and very weak movement characteristics in 2022 and 2023 (e.g. Infiernos, Oulettes, Gabietous, Taillon, MUNIA, Llardana), indicating their highly critical situation (Fig. 3).

## Glacier thickness changes 2020–2023

The glacier thickness changes and mass balance of a total of 12 of the 15 remaining Pyrenean glaciers analysed from 2020 to 2023 show significant glacier-wide thickness losses and accelerated thinning during the study period. The

area-weighted mean thickness change is -7.56 m, which corresponds to an average annual rate of -2.52 m yr⁻¹ (see Table 3 in the supplementary online information). For this period, the root mean square error (RMSE) of the co-registration process in areas outside the glaciers (stable terrain) is 0.27 ± 0.06 m.

Glaciers larger than 10 ha showed significant glacier thickness losses, with Ossoue recording a total loss of -9.1 m and an average annual loss of -3.0 m yr⁻¹, including maximum losses of over -20 m (Fig. 4). Monte Perdido, Maladeta East and Aneto also showed significant thinning, with total losses of -8.5 m (-2.8 m yr⁻¹), -7.6 m (-2.5 m yr⁻¹) and -7.8 m (-2.6 m yr⁻¹), respectively, between 2020 and 2023. Smaller glaciers (<10 ha) showed greater variability in the change in glacier thickness, with the smallest losses of -5.4 m (-1.8 m yr⁻¹), -5.5 m (-1.8 m yr⁻¹) and -6.2 m (-2.1 m yr⁻¹) in Infiernos, Oulettes de Gaube and Llardana, respectively. Larger losses are recorded in Petit Vignemale, Gabietous and Tempestades with -6.6 m (-2.2 m yr⁻¹), -7.2 m (-2.4 m yr⁻¹) and -7.5 m (-2.5 m yr⁻¹) respectively. Finally, the largest losses are observed in Seil de la Baque with -8.1 m (-2.7 m yr⁻¹) and Taillon, where a maximum total loss of -9.1 m is recorded, which is at the top of the annual losses with -3.3 m yr⁻¹ in the period 2020–2023.
<figure><img src="image_5.png" /></figure>

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<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

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►Fig. 3 (a) Absolute (top) and relative (bottom) rates of area change of all glaciers in the Pyrenees over four time periods from 1984 to 2023, shown as boxplot diagrams. The boxes represent the first and third quartiles; the horizontal black line is the median; the whiskers represent data extremes (5th and 95th percentiles) and the circles are outliers (data outside the 5th and 95th percentiles). (b) The mapped areal extent of all very small glaciers in the Pyrenees with outlines from 2011, 2020, and 2023 on very high-resolution aerial imagery from 2023 (UAV orthomosaics). The four largest glaciers are grouped as glaciers larger than 10 ha, and the remaining glaciers are grouped as glaciers smaller than 10 ha. On closer inspection of the individual UAV orthomosaics, many of the incipient glacier degradation processes listed in Table 1 can be seen on most glacier surfaces

Annual mass balance observations for the three glaciers studied (Infiernos, Monte Perdido and Aneto) showed increasing losses over time, with the 2020/21 mass balance year showing average losses of -0.91 m. These losses increased significantly in the following years, reaching an average of -3.42 m in 2021/22, and contributing to the overall negative trend of glacier-wide thickness loss.

In the last mass balance year (2022/23), strongly negative losses were recorded on most of the studied Pyrenean glaciers (13 of the 15 remaining), with an area-weighted mean glacier-wide thickness loss of -3.07 m. The individual glaciers showed different degrees of thinning (see Table 3 in the supplementary online information), but all recorded considerable losses of about -2.0 m in some of the smaller glaciers (<10 ha) such as Les Néous, Infiernos, Petit Vignemale and Gabietous to near -4.0 m in case of Taillon and Monte Perdido. The rest of the larger glaciers (>10 ha) showed a similar behaviour with losses slightly above the mean: -3.3 m in Ossoue and -3.2 m in both Maladeta East and Aneto.

Thickness change rates are on average negative for all studied glaciers and almost all pixels for the last two mass balance years (2021/22 and 2022/23), regardless of their elevation and location within the mountain range, indicating ice loss even in the higher parts of the accumulation areas and a thinning of the entire glacier surface.

## 2020/21, 2021/22 and 2022/23 in the central Pyrenees

Temperature, precipitation, and SWE anomalies for the mass balance years 2020/21, 2021/22 and 2022/23 compared to a reference period from 1991 to 2020 are shown in Fig. 5. In 2020/21, the anomalies are generally within the range of the climatological values and increase in absolute numbers for a single month at best. For example, September had a strong positive precipitation anomaly (+56%) and February had a strong temperature anomaly of +3 °C, but the month before and after had anomalies with opposite signs. As for the summer months, the year 2020/21 shows no anomaly in precipitation and temperature. The SWE shows a rapid evolution from relatively high values until February 2021 compared to the

climatology to low values in April 2021, due to low precipitation in March and April, and average temperatures.

The 2021/22 anomalies show that precipitation in winter and spring was well within the climatological normal values, although February and May were clearly negative (-54% and -37% respectively) while December was strongly positive (+85%). Temperature records were broken in May and July with an anomaly of more than +3 °C for both months, while June and August also had strong temperature anomalies of +3 °C and +1.7 °C, respectively. The 2021/22 temperature anomaly is remarkable both for its intensity (record value) and duration (four consecutive months), resulting in a longer melting period than ever before, as warm temperatures persisted at the beginning of the 2022/23 mass balance year (especially in October). High precipitation in December led to record SWE accumulation in December and January, but normal spring conditions led to normal SWE values in April.

The SWE anomalies remained notably negative throughout 2022/23. This is due to a warm start to the snow season with temperature anomalies of +3.09 °C, +1.82 °C and +1.79 °C for October, November and December, respectively. Precipitation were within the climatological normal between +20% and -20%, until a relatively dry period occurred in February, March and April with anomalies between -32% and -41%, together with temperature anomalies of +1.72 °C and +2.17 °C in March and April, respectively. Overall, the snow season was not favoured by a combination of warmth and drought, which was not compensated for by January with a near record cold anomaly (-1.76 °C) and +10% above-average precipitation. The mass balance year 2022/23 was also characterised by a long melting period, as the snow cover was already very low in April and the summer temperatures were persistently high with anomalies of +1.43 °C, +1.6 °C and +2.64 °C for July, August and September, respectively.

## Discussion

### Area changes 2011–2023 and mapping criteria

The results of the study on changes in area and total number of Pyrenean glaciers from 2020 to 2023 show clear trends in glacier retreat. The total glacierised area mapped for 2023 of 143.2 ha represents a decrease compared to 302.4 ha in 2021/1, 238 ha in 2020 and 172.6 ha in 2022. This represents a significant loss of -159.2 ha (-52.6%), -94.8 ha (-42.9%) and -29.4 ha (-21.3%) since 2011, 2020 and 2022 respectively. Taking into account the 1984 glacier inventory of 814 ha (Serrat and Ventura 1993), a total area change of -670.7 ha between 1984 and 2023 was calculated, which corresponds to a total area loss of -82.4% since 1984. It is worth noting that the total glacierised areas reported by Vidaller
<figure><img src="image_12.png" /><figcaption>**Fig. 4** Spatial distribution of glacier thickness change within the 2020 extent for individual glaciers from 2020 to 2023. The black lines show the glacier extent in 2023, and each number refers to its position in Fig. 1. Since the 3D views show a frontal view of the glaciers, each</figcaption></figure>

image is labelled with an arrow indicating the north direction. *The elevation change of the Monte Perdido glacier surface is affected by the topographic shading of the TLS, which prevents the observation of the lower right part of this glacier

et al. (2021) for 2011 (293.9 ha) and 2020 (229.2 ha) differ slightly (3.3%) from our revised results. This is due to major difficulties in mapping in 2011 and 2020, as the high-resolution aerial images had some snow on the glacier margins; we were able to correct these slight errors after producing snow-free high-resolution orthomosaics in 2022 and 2023, which improved the glacier delineation and allowed us to draw on these earlier inventories.

The results clearly show how quickly the remaining Pyrenean glaciers are retreating, especially the larger glaciers (> 10 ha). Glaciers such as Ossoue and Maladeta East stand out with significant area losses (-30.3% and -36.6% respectively) between 2020 and 2023. They also show new fragmented small glacier parts that are separated from the main body and are classified as relict ice bodies as, unlike ice patches, they tend to disappear within a few years. Interestingly, smaller glaciers (< 10 ha) also show varying degrees of area loss, with most of them experiencing relatively low losses (-10.9% on average), while a few others show higher

rates of area change (-31% on average). This variability highlights the complex nature of glacier responses to climatic conditions and the importance of topoclimatic factors as they become smaller (DeBeer and Sharp 2009; Hughes 2018; Carturan et al. 2013, 2020).

Analysis of changes in glacier area in the Pyrenees from 2020 to 2023 reveals a significant acceleration in glacier retreat compared to previous periods based on previously published inventories with individual glaciers, namely 1984–2008, 2008–2016 and 2011–2020 (Rico et al. 2017; Vidaller et al. 2021). Although the absolute median rates of glacier area change for the periods 1984–2008, 2008–2016 and 2011–2020 show a higher variance, the corresponding relative median rates for these periods are similar: -2.52% yr⁻¹, -2.39% yr⁻¹ and -2.01% yr⁻¹, respectively. Due to the shorter time period of 2020–2023, the absolute median rate is not representative, while the relative median rate of -8.55% yr⁻¹ is significantly higher than the previous values. This dramatic increase indicates that the glaciers in the

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>**Fig. 5** Distribution and anomalies of precipitation, temperature (2 m above surface) and snow water equivalent (SWE) from ERA5-Land reanalysis data for the hydrological years of 2020/21, 2021/22</figcaption></figure>

and 2022/23 compared with a reference period from 1991 to 2020 (monthly cumulative precipitation, mean temperature, and SWE) as histograms (a-c) and boxplot diagrams (d-f)

Pyrenees are experiencing unprecedented shrinkage, which has been exacerbated by two consecutive extreme climate years.

In relation to the glacier mapping criteria used in this study go beyond the standardised methods of the GLIMS project or the Randolph Glacier Inventory (Racoviteanu et al. 2009; Pfeffer et al. 2014). While these guidelines are useful to achieve a complete coverage of mountain glaciers worldwide and are based on minimum area thresholds (e.g. > 1 ha), they lack guidance on how to distinguish very small glaciers from ice patches or perennial snowfields (Leigh et al. 2019). The advantages of using high-resolution imagery for mapping very small glaciers benefit accurate glacier mapping by providing a high level of visible detail and small-scale structures such as crevasses, exposed glacier ice under snow or debris cover, or even abrupt changes in surface topography of glacier margins (Fischer et al. 2014; Paul et al. 2017; Leigh et al. 2019).

The new criteria presented in this study provide new tools to identify glacier degradation processes and to distinguish very small glaciers from ice patches; for this purpose, a minimum number of 2 to 3 features is proposed, including an area below the minimum glacier surface (3D), which is set at 2 ha in the case of Pyrenean glaciers. For the 8 ice masses that are no longer classified as glaciers according to our criteria; the most common features are the total absence of crevasses (e.g. La Paul, Portillon d'Oô, Barrancs), extensive melting processes leading to collapse (e.g. Pailla W, Barroude, Boum), deep water incisions in the ice (e.g. La

Paul, Boum) and multiple fragmentation in relict ice bodies (e.g. Pailla W, Boum, Maladeta W). These ice patches join the list of glaciers degraded to ice patches presented by Vidaller et al. (2021) for the period 2011–2020 (Pailla East, Maupas, and Coronas).

## Impact of 2020/21, 2021/22 and 2022/23 climatologies on Pyrenean glaciers

The climatic anomalies observed in the central Pyrenees in the mass balance years 2020/21, 2021/22 and 2022/23 represent extreme and unprecedented conditions that have had a profound impact on changes in glacier thickness and area. In particular, the anomalies in the mass balance year 2021/22 are characterised by their intensity and duration, with record-breaking temperature anomalies of more than +3 °C in spring and summer (Serrano-Notivoli et al. 2023). The prolonged heat waves during this period led to an extended melt season and contributed to a significant thinning of glaciers and mass losses already observed on the Aneto glacier (Vidaller et al. 2023) and also in other European mountains such as the Alps (Berthier et al. 2023; Cremona et al. 2023; Voordendag et al. 2023). In addition, the combination of warm summer temperatures and drought during the mass balance year 2022/23 had a significant impact on snow accumulation and persistence, further exacerbating the negative trends of glacier thinning and mass loss.

In this context, the glacier thickness changes for the studied Pyrenean glaciers from 2020 to 2023 show a concerning
trend of increasing glacier thinning and mass loss for both larger and smaller glaciers. The strongly negative mean glacićier thickness change observed for the period 2020–2023 (-2.52 m yr⁻¹) represents a strong acceleration of glacier thickness loss compared to the -0.70 m yr⁻¹ reported by Vidaller et al. (2021) for the period 2011–2020, or the value updated in this study for the same period (-0.80 m yr⁻¹). In addition, the three available observations (Infiernos, Monte Perdido and Aneto) for the mass balance year 2021/22 indicate an extreme glacier thickness loss during the summer of 2022, breaking all previous records with an average of -3.44 m yr⁻¹ in a single mass balance year. Annual glacier thickness losses of more than -3 m yr⁻¹ have never been observed in the Pyrenees since geodetic measurements have been taken, as the largest records to date were observed at Monte Perdido in the mass balance years 2011/12, 2014/15 and 2015/16 with an average of -1.8 m, -1.7 m and -2.5 m respectively (López-Moreno et al. 2016, 2019). In a long-term context, the change in glacier thickness (converted to mass) measured in Ossoue shows two periods of significant ice loss, 1924–1948 and 1983–2013, interrupted by a stable period (1948–1983) and the most recent period 2013–2017, which shows the largest glacier thickness loss of -1.55 m yr⁻¹ on average (Marti et al. 2015; Gascoin and René 2018). The latter is in line with the glacier thickness loss reported by Vidaller et al. (2021) for Ossoue, which is -1.2 m yr⁻¹ for the period 2011–2020.

In summary, these extreme climatic anomalies impacted the Pyrenean glaciers and led to exceptional values of thickness change for all studied glaciers in the period 2020–2023. To better interpret our results and compare them with other field-based measurements, we converted the glacier thickness change values into glaciological mass balance values, which are summarised in Fig. 6.

The glacier mass losses measured on the Maladeta East glacier are -2.72 m w.e. yr⁻¹ in the last mass balance year 2022/23 and -2.14 m w.e. yr⁻¹ on average for the study period 2020–2023 (see Fig. 6 in the supplementary online information). These values are consistent with the values of -2.66 m w.e. yr⁻¹ (2022/23), the second highest value after 2021/22 of the entire series (1991–2023), and -2.54 m w.e. yr⁻¹ on average (2020–2023), which were determined using the traditional glaciological method and based on 7 ablation stakes distributed over the glacier surface (ERHIN 2024). In the case of the Ossoue glacier, the measured glacier mass losses are -2.81 m w.e. yr⁻¹ in 2022/23 and -2.59 m w.e. yr⁻¹ on average for the period 2020–2023 (see Fig. 6 in the supplementary online information), while values of -4.52 m w.e. yr⁻¹ (2022/23), which is the highest value of the entire series (2002–2023), and -3.6 m w.e. yr⁻¹ on average (2020–2023) were determined using the traditional glaciological method and based on 5 ablation stakes distributed over the glacier surface (Association Moraine, personal communication).

The slight differences between the values could be related to the small number of stakes used and the choice of a specific glacier ice density conversion factor (850 ± 60 kg m⁻³), which in the case of the geodetic measurements is based on published literature (Huss 2013; Vidaller et al. 2021), and we acknowledge that the uncertainty range may not be sufficient.

The extreme increase in glacier mass loss observed in three different glaciers during the 2021/22 mass balance year, which continued in most of the studied glaciers (13 of the 15 remaining) in 2022/23 (see Fig. 6 in the supplementary online information), was the reason for the sharp declines observed between 2020–2023 on all studied glaciers (12 of the 15 remaining), leading to degradation of the very small glaciers to ice patches in the six reported cases. Our results show that the occurrence of two consecutive mass balance years (2021/22 and 2022/23) with extreme climatic anomalies has put the remaining Pyrenean glaciers in a highly critical situation, with a significant loss of glacićier thickness and mass (three to four times greater). This shows that years with unfavourable climatic conditions have a negative impact on the preservation of glaciers in their interannual evolution (Pelto 2010; Christian et al. 2018; Zekollari et al. 2020; Cremona et al. 2023), and that they reduce the topoclimatic control of the glacier mass balance in the Pyrenees. Most of the remaining glaciers are also subject to incipient degradation processes, which have intensified in the last two negative mass balance years 2021/22 and 2022/23.

## Future prospects of Pyrenean glaciers

With Pyrenean glaciers already in the highest elevation and protected cirques of the highest massifs (Rico et al. 2017; Serrano 2022), these very small glaciers are in clear dis-equilibrium and will continue diminishing and degrading from very small glaciers to remnant ice patches, retreating rapidly and eventually disappearing earlier than expected (Martínez-Fernández et al. 2023). By the end of the summer of 2023, all glaciers studied had largely reduced their extent and thinned considerably even in the higher parts of their accumulation areas in the last two mass balance years (2021/22 and 2022/23), and after a decade of strong area decline (Vidaller et al. 2021), they are now significantly smaller and changed in shape compared to 2011. In some cases (e.g. Infiernos and Gabietous), increasing debris cover from rockfalls and snow avalanches is helping to reduce ice losses, but most of these glaciers are in the final stages of their transition to remnant ice patches as incipient glacier degradation features (e.g. melting processes leading to collapse, water incisions and increasing debris cover) have been observed on many of the remaining glacier surfaces (e.g. Infiernos, Oulettes, Gabietous, Taillon, Munia, Llardana). In
addition, glacier retreat (e.g. Aneto) is exposing new over-
deenpings and depressions where small mountain lakes are
forming and will continue to form in the future (Vidaller
et al. 2023), which at the same time considerably enhances
melting at the front of the glacier (Otto 2019).

Given the current state of rapid thinning and retreat of Pyrenean glaciers, accelerated by the impact of extreme mass balance years such as 2021/22 and 2022/23, and the available ice thickness measurements on the three largest glaciers (Aneto, Vidaller et al. 2023; Monte Perdido, López-Moreno et al. 2019; and Ossoue, Marti et al. 2015), their preservation is delicate. Considering the decrease in snow cover and increased energy availability for snow ablation in the mountain range (López-Moreno et al. 2020a; Bonsoms et al. 2022) and the short-term climate projections for the Pyrenees advancing a general increase in temperature extremes (Amblar-Francés et al. 2020), the fate of all Pyrenean glaciers is that they will be degraded to ice patches.

This transition has been already observed in other Medi-
terranean mountain ranges, such as the Apennines and
Dinaric Alps (Pecci et al. 2008; Gachev 2020), and the rapid
rate at which glaciers shrink in their terminal phase has also
been observed in other glaciated mountains such as the last
tropical glaciers in Colombia, the Quelccaya ice cap in Peru,
East Africa (Mt. Kenya, Mt. Kilimanjaro and Rwenzori
Mountains) and Puncak Jaya in Papua (Rabatel et al. 2018;
Yarleque et al. 2018; Permana et al. 2019; Thompson et al.
2021). Even in the absence of extreme years, it is likely that
the last glaciers in the Pyrenees will disappear or become
remnant ice patches in the next few decades (Vidaller et al.
2021), but it could be imminent if conditions similar to
2021/22 and 2022/23 are repeated. However, further analysis
combining detailed observations and modelling approaches
is needed to more accurately determine the timing of extinc-
tion (Maussion et al. 2019; Vincent et al. 2023).

Given the possible disappearance of the Pyrenean gla-
ciers, and even if they do not have a direct impact on global
Earth dynamics (e.g. sea level rise) or at the regional level
in terms of river water supply, there are other environmental
consequences at the local and regional level (López-Moreno
et al. 2020a, b). Directly, the increase in temperature not
only leads to a decrease in glacier thickness and area, but
also affects the decompression of rock faces after deglacia-
tion and other elements of the high mountain cryosphere
such as permafrost, which thaws and leads to greater insta-
bility on slopes and walls, e.g. rockfalls (Mourey et al.
2019; Serrano 2022). The indirect impacts on these fragile
ecosystems can lead to the extinction of little-known extre-
mophiles, the colonization of new species and changes in
high mountain ecosystems, as well as an irreversible loss of
cultural heritage (Orlove et al. 2008; Olalde et al. 2023). The
Pyrenees are therefore a "last chance tourism" destination to
observe the disappearing glaciers in southern Europe, and

future mountaineering and climbing activities will not only
have to deal with the existence of glacier-free mountains,
but also with a more unstable and vulnerable high mountain
environment.

Conclusions

This study, based on very high-resolution UAV data, shows
that the remaining glaciers of the Pyrenees continue to
shrink rapidly and that extreme heat and drought severely
affect their surface and thickness in the last two mass bal-
ance years (2021/22 and 2022/23).

While the mass balance year 2020/21, which was
observed in the three glaciers studied, had a similar glacier
thickness loss rate (-0.91 m yr⁻¹) as the period 2011–2020
(-0.8 m yr⁻¹), glacier thickness losses of -3.4 and -3.1 m
on average were observed in the last mass balance years
2021/22 and 2022/23, respectively. Both years were char-
acterized by an earlier than usual snowmelt and exception-
ally warm conditions from May to September (2021/22) and
June to September (2022/23). This makes it clear that certain
years with low snow accumulation and high summer temper-
ature cause extreme melting of the Pyrenean glaciers, which
could mean their complete disappearance or their transition
to remnant ice patches for the smallest glaciers. From 2020
to 2023, 8 out of 23 glaciers were classified as ice patches
according to the new criteria for identifying glacier degrada-
tion processes. Currently, the 15 existing glaciers cover only
143.2 ha and are in an extremely vulnerable state, with many
of them classified as highly critical. In this way, the Pyrenees
will become one of the next mountains in the world to be
completely deglaciated.

**Supplementary Information** The online version contains supplementary material available at https://doi.org/10.1007/s10113-024-02333-1.

**Acknowledgements** This work was partly supported by the Interreg-POCTEFA OPCC ADAPYR project, the "Instituto de Estudios Altoaragoneses" *IceDuration* project, and the BBVA research foundation "Melting Ice: La desaparición de los últimos glaciares Pirenaiocos" project. E. Izagirre and I. Rico have been supported by the Grant PPGI19/02 (UPV/EHU) and the Consolidated Research Group IT1678-22 (Basque Country Government). Ixeia Vidaller is enrolled in the PhD programme at the University of Zaragoza (grant no. FPU18/04978). The authors are grateful for the support and permits given by the authorities of the "Parque Nacional de Ordesa y Monte Perdido" and "Parque Natural Posets-Maladeta" in Spain and "Parc National des Pyrénées" in France. The authors would like to thank the Guardia Civil, in particular the "Unidad Aérea (UHEL)" of Huesca, for their support during the Aneto field campaigns. Finally, the authors would also like to thank Wolfgang Cramer and four anonymous reviewers for their detailed reviews, which contributed significantly in refining the final manuscript.

**Author contributions** The idea of this work arose from a working meet-
ing between the CryoPyr research group (IPE-CSIC) and collaborators
(Hydro-Environmental Processes research group at UPV/EHU and GIR-PANGEA research group at UVa). EI, JR and JIL-M designed the study and wrote the paper. EI, JR, IV, CD-R and FR-H processed the analyses and prepared the figures. All co-authors contributed to and approved the final version.

**Funding** Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature.

**Data availability** Data produced in this analysis are available on request.

## Declarations

**Conflict of interest** One of the co-authors is a member of the editorial board of *Regional Environmental Change*. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

**Open Access** This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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