Ice caves as an indicator of winter climate evolution: a case study from the Jura Mountains Marc Luetscher, 1,2 * Pierre-Yves Jeannin 1 and Wilfried Haeberli 2 ( 1 Swiss Institute for Speleology and Karstology (SISKA), CP 818, CH-2301 La Chaux-de-Fonds, Switzerland; 2 Glaciology and Geomorphodynamics Group, Geography Department, Universityof Zurich, Winterthurstr. 190, CH-8057 Zurich, Switzerland) Received 9 September 2004; revised manuscript accepted 3 February 2005 Abstract: Subsurface ice fillings were first described in the Jura Mountains at the end of the sixteenth century. In order to assess the impact of climate change on low-altitude cave ice a detailed inventory has been drawn up and more than 50 objects have been identified. Comparisons between older cave maps, photographic documents and present-day observations outline a negative trend in ice mass balances, a trend that increased at the end of the 1980s. As most of these ice cavesact as cold air traps, this negative mass balance is mainly attributed to higher winter temperatures and to reduced snow precipitation at low altitudes. The equilibrium line altitude of ice caves is believed to have increased several hundred metres between AD 1978 and 2004. Photographic comparisons and proxy records in some of the caves studied provide evidence of a rapid mass turnover. Ice ages range between less than a few decades and a millennium. Climatic records in these ice fillings will therefore present only short time series compared with other cave sediments. However, indications of former ice fillings have been found in different caves of the Jura Mountains and outline their potential role as palaeoclimatic markers. Key words: Ice cave, winter climate, Jura mountains, mass balance, freezing index, palaeoclimate. Introduction During the twentieth century, climate changes induced a negative trend in the mass balance of most glaciers in the alpine belt (Braithwaite, 2002; Hoelzle et al., 2003). Summer air temperatures and precipitation regimes (implicitly taking into account net radiation and albedo variations) have been recognized as being among the most important factors influencing glacier mass balances (e.g., Paterson, 1994). There- fore, glacier fluctuations are commonly considered to represent key factors in the early detection of enhanced greenhouse effects on climate (Kuhn, 1980; Haeberli et al., 1999). The mass balance of subsurface ice fillings, which can be found in low- altitude regions and more particularly in rock-hosted caves, is by far less well known. Long recognized for their natural interest, such features have been defined in English literature as ‘ice caves’ (e.g., Ford and Williams, 1989), which must not be confused with ‘glacier caves’ that result from channelled water circulation in glaciers (e.g., Smart, 2004). This paper focuses on the way ice masses in caves have evolved during the last few centuries and attempts to link their evolution to external climatic parameters. If this link can be established, ice caves could be used for palaeoclimatic reconstructions. Ice caves have been described in many parts of the world at altitudes where the mean annual air temperature is several degrees above 08C (e.g., Harris, 1982; Maire, 1990). Early investigations (e.g., Thury, 1861; Balch, 1900) provided quali- tative descriptions of the processes at the origin of these sporadic permafrost occurrences (Haeberli, 1978). Several authors pointed out the importance of cave air circulation for the cooling of such systems in temperate regions (e.g., Bock, 1913; Luetscher and Jeannin, 2004a). In winter, because of the density difference between ‘warm’ cave air and ‘cold’ outside air, significant air circulation (up to 20 m 3 /s) takes place throughout the cave. This air flow enables substantial heat exchanges, leading to the freezing of any available infiltration water as well as to the conservation of intrusive snow accumulation and to evapo-condensation processes (i.e., sublimation and/or hoar frost deposits, for further details see also Hill and Forti, 1997; Yonge, 2004). Thus, a new layer of ice is formed each winter at the top of the ice volume, leading to its stratification. Conversely, heat exchange with the external atmosphere is considerably reduced *Author for correspondence: SISKA, CP 818, CH-2301 La Chaux-de- Fonds, Switzerland (e-mail: marc.luetscher@isska.ch) The Holocene 15,7 (2005) pp. 982  /993 # 2005 Edward Arnold (Publishers) Ltd 10.1191/0959683605hl872ra