ROCK faces on high mountains could start crumbling at alarming rates if summers get hotter as global climate models predict. Higher temperatures could destabilise rocks by thawing the permafrost beneath them.
In Europe, concern about disintegrating rock faces was heightened last year when at least 50 people died in the Alps as a result of falling rock. Many mountain paths, including classic routes on Mont Blanc, were closed because of the danger of rockfalls, and nearly 100 climbers had to be rescued from the normal route on the Matterhorn when part of the ridge collapsed.
Because there was no unusual snowfall or rainfall to trigger these incidents, geologists suspected that thawing of the permanently frozen interior of the rocks could be the reason. Now, a computer model developed by Stephan Gruber of the University of Zurich, Switzerland, and his colleagues adds weight to the idea that it was melting permafrost that devastated the Alps last year.
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During the summer of 2001, Gruber鈥檚 team abseiled onto 22 different rock faces in the Alps, drilled small holes and inserted devices to record the temperature of the rock. After a year they were able to recover 14 of these devices and used the stored data to fine-tune their computer model, which shows how variations in climate, including changes in air temperature and solar radiation, affect the temperature of the rocks. Then they modelled the temperature of a rock face sloping at 70 degrees, at seven different elevations between 2000 and 5000 metres, working backwards from 2003 to 1982.
鈥淥ur model suggests that higher summer temperatures will heat some rock faces to such an extent that the permafrost, which glues the cracks and joints together, will melt and decrease the stability of the rock face,鈥� says Gruber. The high temperatures of 2003 would have been enough to do this (Geophysical Research Letters, vol 31, pL13504).
Most vulnerable are north-facing rock walls, because they do not normally face the extreme fluctuations of temperature and solar radiation that south-facing rocks experience. Knife-edge ridges are also vulnerable because they are heated from both sides.
In the past month, at least four massive rock formations have come crashing down in the Italian Dolomites. Local scientists have argued that natural thermo-elastic erosion is largely to blame. This happens when daytime temperatures melt the snow and ice, which then seeps into cracks in the rock, expanding and dislodging the rocks when it freezes at night. But given the size of the rockfalls, including nearly 1000 cubic metres in one case, Gruber argues that it is an unlikely explanation. The rocks must have cracked several metres below the surface. 鈥淒ay-night cycles don鈥檛 reach [down] that far,鈥� he says. 鈥淏ut it could well be connected to last year鈥檚 extreme summer.鈥�
For slopes shallower than those modelled by Gruber, the early melting of winter snow cover can significantly affect the permafrost because the snow protects it by reflecting the sun, according to Marcia Phillips at the Swiss Federal Institute for Snow and Avalanche Research in Davos. 鈥淭he longer the snow stays into the spring, the better,鈥� she says.
A quarter of Earth鈥檚 land surface is permanently frozen, with some of this permafrost in mountain ranges. If global mean temperatures rise by as much as 1.3 掳C over the next 20 years, as predicted, 鈥渕ountain ranges from the Himalayas to the Andes will all be affected if permafrost starts to thaw,鈥� says Lorenz King, a geographer at the University of Giessen, Germany. And Michael Davies, from the University of Dundee, UK, says: 鈥淓uropean mountain ranges like the Alps and Pyrenees are particularly vulnerable because they are relatively warm and their permafrost is only marginal.鈥�
Davies鈥檚 own work is showing that rock faces could become unstable well before the permafrost melts completely. He has been spinning frozen rock slopes made in the lab in a massive centrifuge, to see what makes rock faces fail. 鈥淥ur centrifuge model allows us to speed up global warming and see how the rock face responds,鈥� he says. Ice normally acts like glue and helps to hold rocks together, but Davies鈥檚 experiments have revealed that as ice begins to warm up it loses its strength, making the cracks and joints in a rock face less stable than if there was no ice there at all.
鈥淭his means temperatures don鈥檛 have to rise above freezing to make a rock face become unstable,鈥� he says. 鈥淕lobal warming could have an effect sooner than we thought.鈥�