Two days after a good snowstorm, Chamonix guide Denis LeRoy leads a 10-mile descent through Vallée Blanche, one of the best off-piste ski trips in the world. The sun shines brightly on the face of the glacier as we make turns in untracked knee-deep powder. It's so pristine that it's easy to ignore the signs of trouble: Rocks keep rattling down from the moraines, which rise on both sides of the run. A few hundred yards later, the snow turns to dirt, and LeRoy slides to a stop. "Every year the level of the glacier drops," he says. "The mountain faces are thawing and breaking off."
The glacier on the surface of Chamonix's crown jewel, Mont Blanc, which helps support a number of iconic rock formations, has lost 275 feet of depth in its lower stretch over the last 25 years. Even more serious, the mixture of ice and rock set deep within the world's tallest mountains, known as permafrost, is melting as well. When permafrost thaws, cracks in the mountains refill with water, and the ensuing cycle of freezing and melting slowly pushes the rock apart. "It's like a frozen turkey in that the surface warms more quickly than the center," says Antoni Lewkowicz, the president of the International Permafrost Association. "And very little can be done about the subsequent debris flows."
The French mountain town of Chamonix – famous for its backcountry skiing, ice climbing, and wingsuiting – is known as the world's death-sport capital. But in recent years, another major hazard has been its crumbling topography. In 2003, a rash of rockfalls closed a popular climbing route on Mont Blanc – one climbing website called it the worst year on record for mountain travelers – and since then things have only gotten worse. In 2005, another major route on Chamonix's towering Bonatti pillar fell off. The following year, a chunk of the nearby Eiger mountain sent 900,000 cubic yards of stone roaring into the valley. Last March, boulders broke free on the Pas de Chèvre slope – just up the hill from where LeRoy led his group – killing one skier and seriously injuring another.
The situation seems particularly bleak in the Alps, where more people visit mountains at high altitude, but similar melt is happening around the world. In the U.S., permafrost is thawing in the highest reaches of the Sierra Nevada, the Rockies, and mountain ranges throughout Alaska. Mark Williams, a geography professor at the Institute of Arctic and Alpine Research at the University of Colorado, has studied permafrost in the mountains around Boulder for more than 20 years. "There is much less permafrost now, and we attribute that to warming air temperatures," he says. "You get a very large increase in rockfall, and that, in turn, has a very large impact on hiker and climber safety."
In Chamonix, officials are searching for ways to predict and prevent rock slides. Motion sensors have been placed on the pillars of the cable car on Aiguille du Midi, which takes visitors to a spire high above the valley. Similar systems are now being installed on a number of other lifts. Guides like LeRoy have helped to build reinforcements on overhanging boulders and steep slopes, and constantly assess the integrity of the area's many trails and climbing routes. Still, the warmer it gets, the more likely it is that rocks will fall. "We need to change our behavior," LeRoy says. "Mont Blanc is a mirror."