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 yr cal BP), which is progressively waning. This study presents 12 years (2009–2021) of monitoring of A294 cave, 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, more rainfall during the warm season, and a decrease in snowfall and snow cover duration. The ice stratigraphy and local paleoclimate records suggest unprecedented melting conditions since this ice began to form about 6100 years ago. This study highlights the urgent need to recover all possible information from these unique subterranean ice deposits before they disappear.