Ice in caves – a threatened climate archive in the Alps

Austria hosts some of the largest ice caves on this planet. These underground glaciers are not only top tourist attractions; they also preserve ancient precipitation frozen in ice. Unfortunately, little is known about how these ice deposits grew and decay and how precisely they store climate information. This project examines a special type of ice caves found in the Eastern Alps, i.e. vertical, sag-type caves that act as traps of cold winter air and snow. Preliminary studies suggest that these caves are uniquely suited to preserve a centennial-, if not millennial-scale record of winter precipitation. The aim of this project is to reach beyond the instrumental period and to obtain palaeoclimate data since the onset of the so-called Little Ice Age, a cool and varied period which lasted from approximately 1250 to 1850 AD. This research is timely given the rapid decrease in ice volume in most alpine ice caves in the last couple of decades, threatening the survival of this potentially unique archive in the Alps in a warming world.

Ice deposits in caves are a heavily under-researched environmental archive whose existence is seriously threatened by global warming. The Austrian Alps host some 1200 such ice-bearing caves, eight of which were thoroughly studied in this project. The focus was on subvertical caves which act as traps of winter snow and cold air and may contain up to tens of meters of snow, firn and ice. In the first part of the project we established the age of these ice deposits using radiocarbon dating of wood fragments enclosed in the ice. Our results show that the oldest preserved ice dates back to the fourth millennium before Christ, while the majority of the ice was deposited during the so-called Little Ice Age. Wide-spread melting of cave ice occurred during medieval times. Recent decades and years have seen a drastic reduction of ice volume in these caves, unprecedented in the context of the last six millennia. The second part of the project examined the modern meteorology of three selected ice caves. The data show that the winter is the most important season controlling the annual mass balance of snow and ice in these caves. During summer these cold traps are largely decoupled from the outside atmosphere. The increase in winter air temperatures and the general decrease in winter snow precipitation in recent years have led to a steady decline in snow, firn and ice volume in these caves. Small and medium-sized alpine ice caves will likely became ice-free in the near future.