the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Mar 2025
Unprecedent cave ice melt in the last 6100 years in the Central Pyrenees (A294 ice cave)
Abstract. Ice caves are understudied environments within the cryosphere, hosting unique ice deposits valuable for paleoclimate studies. Recently, many of these deposits have experienced accelerated retreat due to global warming, threatening their existence. The A294 cave contains the world’s known oldest firn cave deposit (6100 years cal. BP), which is progressively waning. This study presents 12 years (2009–2021) of monitoring data from A294, including temperature measurements both outside and inside the cave, meteoric precipitation, and ice loss measurements by comparing historical cave surveys (1978, 2012, 2019), photographs, and ice measurements within the cave. Our findings indicate a continuous increase in cave air temperature (~1.07 to 1.56 °C over 12 years), increases in the Thaw Index, and a decrease in the number of freezing days (i.e., days below 0 °C) as well as in the Freezing Index. Calculated melting rates based on cave surveys and measurements show significant variations depending on the cave sector, ranging from ~15 to ~192 cm per year. The retreat of the ice body is primarily driven by an increase in winter temperatures, the rise in rainfall during the warm seasons, and the decrease in snowfall and snow cover duration. The ice stratigraphy and local paleoclimate records suggest unprecedented melting conditions since this ice began to form 6100 years ago. This study highlights the urgent need to recover all possible information from these unique subterranean ice deposits before they disappear.
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Status: open (until 14 Apr 2025)
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RC1: 'Comment on egusphere-2025-8', Anonymous Referee #1, 21 Mar 2025
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This study presents a detailed analysis of ice loss in A294 Ice Cave over the last 12 years (2009–2021), with implications for climate change impacts on cave ice deposits. The authors use a combination of temperature monitoring, precipitation data, and ice loss measurements from historical cave surveys, offering valuable insights into the unprecedented nature of recent melting. The authors successfully place their results within the broader framework of climate reconstructions and ice retreat trends in the Pyrenees.
Suggestions for areas of improvements:
1) The temperature reconstruction method using quantile-based gap-filling should include a validation step against external datasets beyond Góriz and La Renclusa stations.
2) More details on uncertainties associated with ice retreat rates would be helpful. For example, specifying potential sources of error in survey comparisons (e.g., instrument precision, human factors) would strengthen confidence in the retreat estimates.
3) Some sections, particularly those discussing temperature trends and ice retreat, could benefit from a clearer distinction between observed data and modeled inferences. For instance, in section 5.1, the link between external temperature anomalies and ice melting rates could be more explicitly quantified. Including R-squared values or additional statistical measures would strengthen the argument.
4) Linking the cave’s ice retreat to regional hydrology or ecosystem impacts could provide further relevance beyond cryosphere studies.
Figures & visual presentation
Figure 3: The cave survey should have clearer legends to indicate differences in retreat rates across different sectors of the cave. In panel (d) the ice limit for 2011 is shown but not for 2012. Why there are no plan views for all ice deposit limits shown in (d)?
Figure 4: I suggest using “Twelve years …” instead of “12 years…"
Figure 8: (a) The caption is unclear. Light blue bars are visible in other months than April to November. The dark blue lines are intended to represent the number of rainy days (April to November), but the curve includes data from months outside this period. The same applies to the total rainfall. Why are the Y-axes titled “April-November” when the plots show wider intervals? Are the last two plots (nr of rainy days and total rainfall) depicted in Fig. 8a necessary, given that they are also shown in Fig. 8b and in panel (a) there is a lack of correlation?
Figure 9: Enhance text visibility in panels (e) and (f) by employing larger white font.
Figure 11 is highly informative, but it is quite dense. Can you think of some changes that might improve readability? For (h), orange (Fall) is not the best color as is almost impossible distinguish from red.
Minor editorial issues:
Lines 80-82: Several additional factors influence cave temperature during the “closed” phase, not solely the heat exchange during winter. The sentence requires revision for clarity, considering the information provided in lines 82-85.
Line 183: The (dd/mm/aa) format is used, but at line 187, authors use the (dd/mm/yy) format. Choose and maintain one format consistently.
Line 594 (Acknowledgements): should be “… foe for ..” or it should only be “for”
References: There are minor formatting inconsistencies in the list, which should be checked for uniformity.
Citation: https://doi.org/10.5194/egusphere-2025-8-RC1 -
RC2: 'Comment on egusphere-2025-8', Anonymous Referee #2, 31 Mar 2025
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In this paper, the authors indicate, based on several sets of data, observations and inferences, that cave ice melt in the central Pyrenees (Spain) is unprecedented over the past ~6000 years. The “message” of the paper can be broken down into two sections: 1) melting of ice during the past decades and 2) unprecedented melting during the Holocene. The authors use cave climate monitoring data to decipher the main factors responsible for ice melting (and/or accumulation) and use these in combination with mapping of the extent of ice during the past ~45 years to reconstruct the general retreat of ice. Further, the authors combine these observations with data from previous studies (aided by two additional 14C ages) to show that the melting that occurred in modern times is unique in the history of the ice cave. I find the first part well supported by data, measurements and associated discussions, but cannot say the dame about the second part. Support for this thesis is given mainly by circumstantial observations, mostly at the end of section 5.2. As the manuscript stands now, the title and conclusions are not supported by the data and the discussions. I suggest the authors restrict their analysis to the modern (i.e., post 1978 melting) – there findings fit well with similar data from the Alps and the Balkans and give strong support to the usage of cave ice as indicators of melting cryosphere. The part dealing with the Holocene, however, is not well constrained and would require much better support to be considered for publications. I will restrict my comments to this section, as it is the one where most of the criticism would go.
So, first, reconstructing the extent of past ablation in a sedimentary sequence is difficult, as it implies usage of an equation with two unknowns: accumulation and ablation. It is impossible to derive a well-constrained (age-wise) melting period, based on observations of what is left behind. The authors say that no periods of extended melting occurred in the past, based on the putative absence of debris layers, but these are clearly identifiable in the figures and also the same authors (Sancho et al., 2018) identified several such periods based on the presence of unconformities in the ice deposit (mentioned also in lines 224-228 in the current manuscript). This is also indicated in the current manuscript (lines 155-156), the authors identifying changes in the internal structure of the ice sequence (which would contradict their later statements). While it is not clearly stated in the manuscript, it seems, based on the photos, that while the ice filled most of the cavity, the current retreat happens in a series of steps that combine lateral melting (retreat from the walls) followed by collapse of the overhanging flat surface, and again retreat. This retreat (10+) would have definitely destroyed possible layers indicating melting that would have formed in the past. Second, the new 14C ages seems to be derived from layered ice deposit located 15 m above the main one. It is not clear what relationship exists between the two, but if they belong to the same ice mass, 15 m of missing ice must have melted away sometimes in the past, thus far exceeding the current melting. Another support for the unprecedented of the melting is given by the comparison of the two age models. It is not clear how this data supports the hypothesis (and why is tucked away in the supplementary material). The two sedimentary logs look quite different so it is difficult to understand what the authors wanted to say – perhaps that there is no vertical unconformity (lines 485-487) inside the ice mass? This is just absence of evidence.
To sum it up, there is no clear evidence for the uniqueness of melting beyond 1978, but rather a weakly supported hypothesis that requires multiple inferences and no data to support them. I suggest the authors to concentrate on the first part of the study, and use the data they have collected to better constrain the link between climate (cave and external) and ice melt (for example, the discussion in lines 395-400 is confusing) and reshape the manuscript to better convey their message of rapid, current ice loss in the cave. This would better benefit the wider cryospheric community.
Citation: https://doi.org/10.5194/egusphere-2025-8-RC2
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