Unprecedented loss of surface and cave ice in SE Europe related to record summer rains in 2019

Glaciers worldwide are shrinking at an accelerated rate as the climate changes in response to anthropogenic influence. While increasing air temperature is the main factor behind glacier mass loss, changing atmospheric circulation 20 patterns and the distribution of precipitation also plays a role, though these are not as well understood. Furthermore, while the mass balance of surface glaciers (from large polar ice sheets to small alpine glaciers) is relatively well documented and continuously monitored, little to nothing is known about the response of cave glaciers (perennial ice accumulations in rockhosted caves) to atmospheric warming. In this context, we present the response of cave and surface glaciers in SE Europe to synoptic conditions in summer 2019. Our investigation shows that extreme precipitation events occurring between May and 25 July 2019 led to catastrophic loss of ice at levels unprecedented during the last century. As climate models predict that such extreme precipitation events are set to increase in frequency and intensity, the presence of cave glaciers in SE Europe and the paleoclimatic information they host may be lost in the near future. Moreover, the same projected continuous warming and increase in precipitation extremes could pose an additional threat to the Alpine glaciers in southern Europe, resulting in faster than predicted melting. 30

7.5x5 m), one middle (5x4 m) and one southern (11x6 m). These wide entrances ensure a constant snow accumulation at the 100 bottom of a 60 m long, 25 m wide chamber, as well as cold air circulation during winter (HBSD, 2019, last access: 25 May 2020). The snow, firn and ice deposit occupies an area of about ~450 m 2 , its thickness ranging from 12 m below the northern and middle entrances to 0.5 m in the more distal parts of the cave. The upper part of the deposit is covered by fresh snow and organic matter collapsing from the surface during winter. Deeper in the cave, the snow is metamorphosed to firn and finally layered ice with embedded rock particles and organic matter. The mass balance of the ice deposit is controlled by the 105 circulation of cold air between the cave's chamber and the four entrances, snow accumulation and water inflow during warmer, wetter periods (from spring to early autumn).
Velika ledena jama v Paradani is located on Trnovski gozd karst plateau in western Slovenia (45°59′19.70 ′′N, 13°50′40.24′′ E). The cave is 6534 meters long and 858 meters deep with the main entrance located at 1135 m a.s.l. The entrance opens at the bottom of a doline and leads to a series of interconnected halls, with the first and second containing the perennial ice 110 block. This block has a layered structure, including clear ice alternating with detritus derived from the surface, and it changes from firn to congelation ice with increased distance from the entrance. The maximum depth of the ice block is unknown, but is estimated to an average of 3 meters, suggesting a maximum volume of 8,000 m 3 (Mihevc, 2018). The main ice growth periods are in winter (as snow accumulates in the entrance doline) and spring, when snowmelt water freezes to form congelation ice. The main ice loss occurs in summer and autumn following heavy infiltration of rainwater. 115 In addition to these underground glaciers, we have also studied Europe's two southernmost glaciers, Snezhnika and Basnki Suhodol, located in the Pirin Mountains (Bulgaria) below the northern cliffs of Vihren (2914 m) and Kutelo (2908 m) peaks (Gachev et al., 2016). Snezhnika glacier (41°46′09′′ N; 23°24′12′′ E) is located on an eastward-facing slope and lies at 2440-2490 m a.s.l., whereas Banski Suhodol (41°46′09′′ N; 23°23′40′′ E) faces north and lies at 2620-2700 m a.s.l. Both glaciers are a legacy of the Little Ice Age (Hughes, 2009) and occupy less than 1 ha (Grunewald and Scheithauer, 2010) with recent 120 geophysical investigations revealing maximum ice thickness of 14 m and 17 m, for Snezhnika and Banski Suhodol, respectively (Onaca et al., 2019).

Methods
In 2018, we initiated a research program aimed to preserve the climate memory of vanishing Eastern Mediterranean subterranean glaciers. Ice levels in caves were measured against fixed points on the cave walls and/or mass balance changes 125 were estimated using photogrammetry.
In Scărișoara Ice Cave, ice level changes were recorded monthly using a dual approach. Firstly, we measured that distance between the ice surface and a fixed point in the rock ceiling directly above the ice and secondly, we measured the surface ice level changes against a marker embedded in the ice. The results of the first measurement record the sum of changes at the surface as well as bottom of the ice block, whereas the later registers only the changes at the surface of the ice block. 130 Subtracting the latter from the first enables us to disentangle changes at the surface likely resulting from the influence of https://doi.org/10.5194/tc-2020-287 Preprint. Discussion started: 26 October 2020 c Author(s) 2020. CC BY 4.0 License. external climate from changes at the bottom, the later influence by long-term basal melting, independent of climatic conditions. In Chionotrypa Cave (Falakro Mountain), annual ice level fluctuations were intermittently recorded at the end of the ablation period over the past five years. Changes in ice volume are continuously monitored in Ledena jama v Paradani since September 2009, with the method of wall to ice distance measurements made twice annually. 135 To analyze in detail the surface changes of Snezhnika and Banski Suhodol glaciers, two UAV (DJI Phantom 4 Pro drone equipped with 20 megapixel camera) surveys were performed at the end of the ablation season (October 2018 and September 2019). High-density point clouds and high-resolution ortophotos produced from the drone surveys were further processed with Agisoft Photoscan Professional software using the Structure from Motion (SftM) algorithm to generate a digital elevation model (DEM) with 22.5 cm pixel -1 resolution and an orthophoto-mosaic with 2.8 cm pixel -1 resolution for 140 Snezhnika glacier, and a DEM with a 18.7 cm pixel -1 resolution and an orthophoto with a 4.6 cm pixel -1 resolution for Banski Suhodol glacier.
As most of the investigated caves are located far from meteorological stations, or existing station data covers inappropriate time periods for this study, we extracted meteorological data from the E-OBS dataset (Cornes et al., 2018). In order to link climatic parameters with large scale circulation patterns, we have computed the anomalies of the mean air temperature (TT) 145 and geopotential height at 500 mb (Z500) and we analyzed the conditions of snow cover and extreme precipitation (monthly maximum consecutive 5-day precipitation, RX5day) for both the ice accumulation (December 2018-February 2019) and ablation (May-July 2019) periods. The Z500 file was extracted from the NCEP/NCAR 40-year reanalysis project (Kalnay et al., 1996). The snow cover data were provided by MODIS/Terra Snow Cover Monthly L3 Global 0.05Deg CMG, Version 6 (Hall et al., 2006). The Highest 5-day precipitation amount (RX5day) was computed based on the EOBS-v20e data set 150 https://doi.org/10.5194/tc-2020-287 Preprint. Discussion started: 26 October 2020 c Author(s) 2020. CC BY 4.0 License.

Meteorological data 195
Meteorological data (Fig. 5) show that 2019 was exceptionally wet in SE Europe. At all investigated sites, precipitation amounts exceeded the long-term  average by more than 150 %. Following a relatively dry early spring In December 2018, the geopotential height anomalies at 500 mb level were characterized by a wave-train like structure with negative Z500 anomalies (indicative of a low-pressure system) over the central North Atlantic Basin, positive Z500 anomalies (indicative of a high-pressure system) over the central part of Europe extending northwards, and negative Z500 anomalies over the eastern part of Europe (Fig. 6a). The low-pressure system over Eastern Europe, which was carrying moist 205 air from the Black Sea together with cold and dry air from the north, led to snowfall events over the high altitudes in the Carpathian Mountains and some parts in Ukraine (Fig. 6d). Overall, December 2018 was warmer than normal over the whole European region with some exceptions in the Alpine areas and Bulgaria (Fig. 6g).
In January 2019, the large-scale atmospheric circulation was characterized by a dipole-like structure with positive Z500 anomalies centered over the central North Atlantic basin and negative Z500 anomalies centered over Europe (Fig. 6b). This 210 dipole-like structure led to several episodes of snowfall in the Alps and the Carpathian Mountains, due to the advection of northerly cold air from the Arctic region coupled with moist and warm air intrusions from the Atlantic basin. In January 2019 most of Central and Eastern Europe were snow covered (Fig. 6e) and the Alpine region was up to 5 °C colder than normal (Fig. 6h).
In February 2019, the prevailing large-scale atmospheric circulation was reversed compared to the previous month. A low-215 pressure system prevailed over the central part of the North Atlantic basin, while Europe was under the influence of a highpressure system (Fig. 6c), resulting in dry and warm weather (Fig. 6i). Snow was present only over the mountain regions in the Alps and the Carpathian Mountains (Fig. 6f). Overall, February 2019 was exceptionally warm in the northern and eastern part of Europe (European State of the Climate, 2019).
In May 2019, the central and south-eastern parts of Europe were characterized by below average air temperatures. The 220 prevailing large-scale atmospheric circulation featured a Rossby wave guide with negative Z500 anomalies over the eastern U.S. coast, positive Z500 anomalies over the central North Atlantic basin and western part of Europe, negative Z500 anomalies over the eastern part of Europe and positive Z500 anomalies over Russia (Fig. 7a). Concurrently, Italy, the Alpine regions, Croatia and the northwestern part of Romania experienced extreme rainfall (Fig. 7b). Rainfall records were broken over small areas in the Alps and Ukraine (Fig. 7c). Most of the intense and record-breaking rainfall was recorded over the 225 regions were the investigated glaciers are located (Fig. 1).
In June 2019 most of the European continent was under the influence of a high-pressure system (Fig. 7d) that led to the advection of warm and dry air from Africa and subsequent development of record-breaking heat wave over the south-eastern and central parts of Europe (European State of the Climate, 2019). June 2019 was dry over large areas of Europe, with some small exceptions over the Alpine region and the southeastern region (Fig. 7e), with record rainfalls in Bulgaria and Greece 230 limited to relatively small areas (Fig. 7f). The precipitation in the southern part of Europe was mainly occurring during heavy thunderstorms, resulting in one-day rainfall amounts of ~50 mm/day locally.
As in June 2019, in July 2019 most of Western Europe was under the influence of a high-pressure system (Fig. 7g) and was very dry over large parts of Europe (particularly the western half). Wetter conditions, with enhanced rainfall, were recorded in South-Eastern Europe (including Croatia and Slovenia). The rainfall over these regions was mainly due to heavy 235 thunderstorms (Fig. 7h).

Discussion
All ice caves in the investigated region are located well below the 0 °C isotherm. Contrary to high-altitude glaciers, underground glaciers have both the accumulation and ablation zones in the same location, in most cases at the bottom of vertical shafts where snow and ice accumulates in winter but also melts in summer, thus making them extremely sensitive to 240 both short and long term climatic changes. At all these three locations precipitation amounts between May and July 2019 exceeded the multiannual mean (Fig. 5), mainly during extreme thunderstorm events (Figs. 7b,e,h). These extreme events delivered large volumes of warm water directly on the surface of glaciers, leading to rapid melting. In the case of Scărișoara Ice Cave (Romania) and Ledena jama v Paradani (Slovenia), where the main ice blocks are not located directly below the caves' entrance shafts, similarly extreme 255 summer thunderstorms (Figs. 5 and 7) resulted in high volumes of water entering the caves through fissures in the host rock, leading to enhanced melting through heat delivered to the surface of the glaciers by percolating water (Perșoiu and Onac, 2019). On Mt. Olympus (Chionotrypa cave), summer 2019 was warm and dry (Fig. 7). In this cave, the surface of the ice is https://doi.org/10.5194/tc-2020-287 Preprint. Discussion started: 26 October 2020 c Author(s) 2020. CC BY 4.0 License. just 6 m below the cave entrance (Pennos et al., 2018) and thus the glacier responds to climatic variability in a manner similar to surface glaciers. The thermal inversion layer inside this shallow entrance shaft was easily destroyed during the 260 prolonged warm spell, triggering he rapid melt of the surface and sides of the glacier. A similar behavior was observed in the case of the surface of Snezhnika and Basnki Suhodol glaciers, where, in addition to the heat delivered directly by rainwater, melting was enhanced by the warm and moist conditions resulting from turbulent heat fluxes near the surface of the ice (Marks et al., 1998;Pomeroy et al., 2016). The relative area loss of Snezhnika glacier was ~ 0.94 % a -1 between 1959 and 2008, similar to the average global value of 1 % a -1 (Vaughan et al., 2012), but increased to 1.86 % a -1 between 2008 and 265 2018 and 1.93 % a -1 between 2018 and 2019. For Basnki Suhodol glacier, the relative area loss between 2018 and 2019 was close to that of Snezhnika glacier at 17.2 % a -1 . Similarly, perennial snow patches on Mt. Olympus, remnants of glaciers from the last glacial cycle , began to disintegrate in 2019 under the prolonged heat wave. These cases mirror recent findings from SW Europe, where Moreno et al. (2020) have shown that glaciers surviving warm periods of the past 2000 years are rapidly melting, being at the risk of disappearing within the coming decade(s). 270 Cave ice deposits are similarly prone to rapid disintegration once melt-water channels start to develop on their surface, channels that 1) drain away water before it would freeze to form new layers of ice (Persoiu and Pazdur, 2011) and 2) enhance melting and fragmentation leading to rapid loss of ice (e.g., Lauritzen et al., 2018). The cumulative ice loss in summer 2019 for the four analyzed caves is ~1300 m -3 . This loss ad to the long-term worldwide melting of cave ice (Kern and Persoiu, 2013), but it is unprecedented over the past century (Fig. 8). Several studies (Kern et al., 2009;Persoiu and 275 Pazdur, 2011) of long-term links between ice mass balance and climate in caves have shown that cave glaciers have a nonlinear response to air temperature changes, extreme warming events playing an insignificant role in the melting of ice, with warm water infiltration and the length of the warm season being by far the most important factors causing melting.
All surface glaciers in southern Europe are out of balance with present-day climatic conditions, but the slow melting occurring at their termini results in gradual re-equilibration with local climatic conditions (Zekollari et al., 2020). However, 280 recent rapid warming leads to an increase in the altitude of the 0 °C isotherm (Rottler et al., 2019) thus further increasing the imbalance between glaciers and climate and enhanced melting. Our results suggest that, adding to the melting under increased temperatures, heavy summer precipitation events result in enhanced melting of both cave and surface glaciers.
With increasing temperatures, the altitudinal rise of the 0 °C isotherm (Rubel et al., 2017) would bring more glaciated terrain under warming conditions, and thus yet more susceptible to heat transfer during heavy summer thunderstorms and extreme 285 summer heat waves. Accelerated warming of the Arctic (Holland and Bitz, 2003) would result in meridional amplification and slower propagation of the Rossby waves, leading to an increase in the frequency of blocking conditions and associated extreme events (Francis and Vavrus, 2012;Liu et al., 2012;Screen and Simonds, 2014). The increased frequency, duration and intensity of both heat waves (e.g., Spinoni et al., 2015) and heavy rainfall events (Púčik et al., 2017;Rädler et al., 2019) would thus lead to a higher ablation rate of surface and cave glaciers than that expected from increased temperatures alone. 290 Especially vulnerable are cave glaciers, already located in areas subject to both warming and extreme summer https://doi.org/10.5194/tc-2020-287 Preprint. Discussion started: 26 October 2020 c Author(s) 2020. CC BY 4.0 License. thunderstorms, and surface glaciers close to the 0 ° isotherm, thus resulting in the loss of ice faster than predicted by the most recent estimates (IPCC, 2019;Paul et al., 2020).

Conclusions and outlook
We have investigated the response of cave and surface glaciers to extreme summer rain events in SE Europe during 2019 and 295 unraveled unprecedented ice mass loss over the observational period (the last 120 years). Surface glaciers in the Northern Balkan Mountains lost on average nearly one fifth of their surface, a rate at which mountains in SE Europe could be glacier free by the end of the decade (AD 2030). The ice mass loses are related to enhanced melting resulting from outsized amounts of water reaching the caves and delivering large amounts of heat directly to the ice. The synoptic conditions leading to these extreme events were induced by the persistence of blocking conditions over Western Europe leading to extreme heat waves 300 in southern Europe and record-breaking amounts of rainfall. While cave and surface glaciers in mountains across Europe are sensitive to increasing temperature, our observations show that extreme summer rains result in rapid melting and disintegration of ice bodies, rendering them even more sensitive to temperature changes. As climate models suggest future changes in the dynamics of Rossby waves leading to more extreme events, disappearance of both surface and cave glaciers in SE Europe (and elsewhere) will occur earlier than predicted. 305 The ongoing and predicted loss of recently accumulated ice threatens the possibility to accurately reconstruct past climate variability using various proxies harbored in cave ice. In order to generate (semi)quantitative reconstructions it is crucial to collect ice that grew during the instrumental period (the last ~50-100 years) and compare the reconstructed variables with measure ones. The accelerated melting of ice is quickly reducing the possibility to perform such studies, thus leading to the additional loss of invaluable climatic information.