The danger may be invisible but it is real enough. One such disaster remains in Saint-Gervais’s collective memory. In 1892, 80,000 cubic metres of water that had collected in a sub-glacial cavity burst through the ice “cork” that was holding it in. A torrential flow of water tore down rocks and trees in its path and buried Saint-Gervais in mud and debris, leaving 175 dead.
According to the current mayor, Jean-Marc Peillex, far greater damage would be caused now, “due to urbanisation and the large number of tourists visiting the glacier”. As many as 900 houses could be swept away.
The alarm was first sounded in 2007, when the thickness of the ice was measured by radar. “Nobody thought there might be water under the glacier,” Vincent said. “But the images showed something abnormal about 10 metres above the bedrock.”
In 2009 this was confirmed by proton nuclear magnetic resonance, a technique similar to a medical MRI scan. It proved that an enormous pocket of water – or possibly several pockets – was locked deep inside Tête-Rousse. The reason for the water collecting lies in climate warming. But paradoxically – grassroots science being more complex than theoretical models – this has led to a cooling of the lower part of the glacier. The probable process, as described by Vincent, is that the water from thawing in the upper part of the glacier trickles down on to the bedrock though micro-fissures until it finds an outlet.
In the case of Tête-Rousse, the warming observed over the past decades has reduced the thickness of the snow cover (the firn, which provides thermal protection), and to a greater extent in the lower part of the glacier than in the upper part.
As a result, during a recent cold snap, the thinner spur of ice below cooled more rapidly than the ice at the glacier’s summit (there being a difference of more than 2C between the two), resulting in the formation of a dam that blocked the water trickling down from above. However, being unable to find an outlet, the water has accumulated and now the pressure is rising – and threatening to burst like a pressure cooker.
A scientific report issued in July by three Grenoble laboratories concluded that it was necessary to pump the water out the sub-glacial cavity as soon as possible. A warning system, costing $640,000, was immediately set up. Two metal cables were placed across the glacier, which, if broken, would trigger a siren in the valley below. The nearest inhabitants have been informed about the 17 rallying points on high ground, and would have 10 minutes to reach the nearest one if the alarm sounds.
The pumping of Tête-Rousse began last month. Powerful boring machines and pumps were transported by helicopter to the glacier. The water will be pumped out within a month and gradually released. The whole operation will cost $2.5m, 80% of which will be paid for by the French government and the European Union.
Is that the end of the story? “In a year or two we will have to check if the pocket is filling up again,” says Vincent. “If that is the case, we will have to consider boring a permanent channel to drain the water.” Models show that the water collected in just two years.
Reservoir formation under glaciers is a rare phenomenon. But with global warming these risks are increasing, such as the collapse of surface ice and, with the receding permanent snowline, the formation of proglacial lakes whose natural barriers will give way, up there between earth and sky.
Keeping the lights on
In Chamonix, climate change is also a reality for EDF, the French electricity giant. The Mer de Glace glacier has been retreating fast in recent years and is threatening the sub-glacial water intake in the Les Bois hydroelectric power plant.
When this plant came on stream in 1973, the intake took place 200 metres under the ice. In spring 2009, it was out in the open, and, to make matters worse, covered by a mass of glacial rock and sediment following a number of storms.
EDF now has to maintain electricity production while carrying out the work needed to adapt to the new circumstances – and keeping the Les Bois plant “at the highest level of environmental integration”.
Not without reason: the 12km Mer de Glace is the longest French glacier and something of a national treasure.
The stakes for the Haute-Savoie region are considerable. The Bois hydroelectric plant produces 113m kWh per year, mostly during the thaw, which is the domestic consumption of 50,000 inhabitants, or a town the size of Annecy.
However, the glacier has been retreating at a rate of about 30 metres a year since 2003. “And the pace has increased in the past few years,” said an EDF official. At the Rochers de Mottets level, for instance, ice thickness has been falling by between eight and 10 metres a year since 2004.
“We anticipated this situation, and after some research, we decided to move the intake upstream in the glacier under 100 metres of ice, which won’t change anything to the scenery or the tourism business,” said EDF, before launching the $19m project. Work started in 2008 on an underground channel to divert the water permanently to the new intake area – no easy matter under such difficult geographic and climatic conditions. The installation is due to come on stream in the spring.
Meanwhile, a temporary solution was found by digging a channel a few dozen metres long to emerge below the glacier. That will provide sufficient water to feed the plant until 2011.
This article originally appeared in Le Monde