The Himalayan ice avalanche that killed at least 12 Nepalese guides earlier today happened in a place where both the sun and the Earth conspire to increase risk and where many other lives have been lost in past years.
The Khumbu Icefall, where the avalanche reportedly occurred, is basically a glacier that's in a constant state of flux as it grinds down one of Mount Everest's flanks. On the surface, it consists of giant blocks of ice that are very unstable and shift regularly -- sometimes catastrophically. When that happens, the river of ice becomes more like a stampede of ice boulders.
The difference is that ice has the ability to self-lubricate when it falls or flows, Turnbull said. This drops the friction inside the flow and makes it move more easily. The self-lubrication comes from the collision of ice particles, which causes small amounts of short-lived melt water on their surfaces -- the same way ice skate blades create a tiny area of pressurized melt water that makes skating possible.
"When an icefall falls to pieces, it's with big chunks of ice," said the University of Nottingham's Barbara Turnbull, a fluid dymanicist with a special research interest in ice and snow. "So it's more like a rockfall."
But even without that lubrication, the house-sized blocks of ice in the Khumbu Icefall are primed to fall because the glacier itself is buckling and turning over steep, uneven ground. This creates constant stress in the ice, which means avalanches have a hair trigger.
Primed as it is to fail, there are at least a couple of things that set an avalanche in motion in an icefall, Turnbull explained. One trigger, also seen in snow avalanches, is people. As small a thing as a person tramping on the surface of such an unstable area can be enough to compress and shift some layers of snow or ice, causing a small amount of ice to melt, and the avalanche begins, Turnbull said.
Another very well known trigger is the sun. When it rises in the morning and shines on the glacier, the sunlight starts melting some of the ice. That meltwater percolates down and weakens the glacier, making it even more unstable. This is also a common trigger for snow avalanches, Turnbull said.
This danger of the warm sun is why Everest climbers always try to get through the Khumbu Icefall before sunrise.
Other common triggers for snow and ice avalanches are rainfall, earthquakes and even animals. These are unlikely to have been triggers this morning at Khumbu Icefall.
A factor that may be playing a role is climate, which as changed a lot since the first Everest expeditions almost a century ago. Warmer temperatures and changes in snowfall just add another layer to the already complex picture of the Khumbu Icefall.
"Global warming isn't helping," Turnbull said.
Worldwide, most mountain glaciers are receding due to human-caused climate change, according to climate scientists and glaciologists. Those in the Himalayas are no exception, although there are regional variations there, according to Ann Rowan of the British Geological Survey.
"We see acceleration of mass loss since the 1990s in the eastern Himalaya glaciers," said Rowan.
That's the part of the Himalaya that includes Everest. It's also the part that's most affected by the Indian monsoons, she said.
So with all of this instability on so many levels, why do people still traverse the deadly Khumbu Icefall? It's simply matter of it being the least dangerous of some very dangerous options, Turnbull said. It's the lesser evil, but evil nonetheless.