engleski

Accelerated loss of cave ice in SE Europe related to heavy summer rains

The recent IPCC Special Report on the ocean and cryosphere in a changing climate highlighted the worldwide shrinking of the cryosphere, with ice sheets, mountain glaciers, snow cover and Arctic sea ice all losing mass. Small glaciers are most impacted by this recent melting, climate scenarios suggesting losses up to 80 % by the end of this century for glaciers in several regions, including Central Europe. The cited IPCC report does not include information on the dynamics of perennial ice accumulations in caves. Whereas ice loss in surface glaciers is mostly due to melting related to rising temperatures, cave ice ablation is mostly due to drip water delivering heat to the ice and secondary to rising temperatures. Consequently, whereas the projected increase in air temperature in mountain areas would result in an enhanced mass loss for surface glaciers, the same rising temperatures might only marginally affect ice mass balance in caves. Most of the caves hosting perennial ice are found in Central and South-Eastern Europe, a region that experienced some of the fastest loss of glacier ice over the past decades, but for which climate models suggest a mixed response to the general global warming, with a general decrease in annual precipitation, increase in winter precipitation and increase in the frequency and intensity of extreme summer precipitation events. In this context, we present here the response cave glaciers in Eastern and South-Eastern Europe to the weather conditions in winter 2018–2019 and summer 2019. We have investigated ice caves in Greece, Croatia, Romania, and Slovenia and analyzed the ice mass balance in response to the climatic conditions in the 2018–2019 winter season (one of the coldest and wettest in the region) and the spring and early summer of 2019 (one of the wettest). In 2018, we initiated a research program aimed to preserve the climate memory of vanishing Eastern Mediterranean subterranean glaciers. This included the monitoring of cave climate, ice level changes and of the mechanisms by which the climatic information recorded by the stable isotope composition of precipitation is preserved in cave ice. Field trips were conducted regularly in the investigated caves since July 2018. In the Scărişoara Ice Cave (Romania), heavy snowfall in the 2018–2019 winter and subsequent melt in spring 2019 led to rapid infiltration of large volumes of water inside the cave, resulting in ca. 5 cm of ice being formed in the still undercooled cave environment. Between May and July 2019, the ice level in SIC dropped by ca. 35 cm, due to the continuous infiltration of warm water from the surface. This catastrophic melt event spread across the entire upper surface of the 3000 m2 ice block, resulting in the loss of ca. 1050 m3 of ice. In the Chionotrypa Cave (Greece), a gradual decrease of the ice volume was evident since 2014, reaching a maximum between July 2018 and September 2019. The ice level at the bottom of the entrance shaft fallen ~3 m and receded ~1 m from the cave walls between July 2018 and September 2019. A rough estimate suggests that >600 m3 of cave ice was lost during this period. In the Crna Ledenica (Croatia) observations in June 2019 show that following the cold and wet 2018–2019 winter large amounts of fresh snow accumulated below the entrance shafts. However, heavy summer rains and subsequent infiltration of warm water resulted in the complete melting of these ice crusts by September 2019. In some parts of the cave, the ice thickness dropped by up to ~2 m, and the total area covered by perennial ice and snow dropped by ca. 200 m2, resulting in a loss of about 150–200 m3 during summer 2019. In the Ledena jama v Paradani (Slovenia) the ice surface lowered by 220 centimeters since 2013, of which 41 centimeters were lost between June 2018 and June 2019. This heavy ice loss in 2019 was the result of low snow accumulation in winter and heavy summer thunderstorm events resulting in large volumes of warm water reaching the ice surface. Summarizing these observations, two conclusions result: 1) in the summer of 2019, perennial cave deposits throughout SE Europe experienced extensive melting, losing ice volumes that correspond to several years or even decades of accumulation ; and 2) the region was battered by heavy late-spring and early summer rains, unprecedented over the past several decades. In contrast, during summer 2019, Western Europe experienced record high temperatures. This precipitation pattern was likely the result of enhanced meridional transport of Mediterranean moisture towards SE Europe, while W Europe was under atmospheric blocking conditions. Climatic models suggest that underongoing general warming, blocking conditions are set to become more frequent, thus resulting in higher than average precipitation in parts of Europe, while others are going through extreme heatwaves. This atypical distribution of precipitation might lead to the complete loss of cave glaciation in S Europe, taking away with it also the paleoclimatic information stored underground.

cave ice, ablation, precipitation

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