They are a playground for summer skiers, a challenge for mountaineers, and their dazzling whiteness under a deep-blue sky is of other-worldly beauty. But glaciers are also crucial for Alpine water balance — and they are a unique indicator of global warming.

Since the Industrial Revolution, glaciers in the Alps have decreased by more than one-third in size and by a half in volume. This dramatic retreat of supposedly eternal ice has now been visualized by a couple of German photographers who are also environmental scientists.

Wolfgang Zängl and Sylvia Hamberger collected hundreds of historic postcards of glaciers in Germany, Austria, Switzerland, Italy and France. They carefully determined the month and time of day when each picture was photographed or painted, and identified the artists' viewing points. Then they took similar shots of the landscapes as they appear today. The pictures shown here are of the Rhône glacier in Vallais, Switzerland.

The results are now on show in the Alpine Museum in Munich, Germany. The exhibition “Glaciers in the Greenhouse”, which runs until 16 January 2005, also provides some numbers and diagrams to illustrate the problem of global warming in the twentieth century. But its visual centrepiece, the pairs of old and new glacier photographs, depict much more shockingly what climate change is doing to the Alps.