Millions of mountaineers flock to Switzerland each summer to conquer a breath-taking array of vertical challenges such as the 4,478-metre-high (14,777-foot) Matterhorn.

But sometimes the perilous trek up a glacier ends in tragedy, with at least nine people this month alone plunging to their doom.

"When the weather is warm, the danger of rock slides or being hit by a rock is much greater than when it is cold," said Fred Salamin, a 30-year-old mountain guide in the valley of Anniviers, southwest Switzerland.

Some 47 people have died in Swiss climbing accidents so far this season, in addition to those killed by avalanches -- a fairly average toll according to the experts, who note that fewer deaths occur when the weather is cooler.

"Lots of people on climbing holidays have limited time, so many go even when the conditions are not too good," said Hans Jaggi, an official in charge of the mountain rescue service at the Swiss Alpine Club.

Muggy weather, thunderstorms and scorching sunshine melt snow and ice along mountain trails, rendering some unsuitable for climbers, he explained.

"You have many accidents related to ropes when someone slips and pulls his partner down with him," said Jaggi.

On August 1, four Austrian mountaineers, who were roped together while descending the Taeschhorn glacier in the south, slipped and fell to their death in Switzerland's deadliest climbing accident in recent years, police said.

Ignorance and insufficient training remain the main causes of disaster on the mountains, said Salamin, who has been a climber since the age of 15 and instructed others in the sport for the past decade.

At the same time, adverse weather conditions also play a part, he said.

Average temperatures are rising at an ever-increasing rate, causing glaciers to melt and the icy soil that holds rocks in place on mountain tops -- known as permafrost -- to thaw, said climatologist Martin Beniston, who works for the UN's Intergovernmental Panel on Climate Change.

Temperatures rose by one degree Celsius from the start of the 1900s to about 1980 and jumped by another degree Celsius in the next 20 years alone, he said.

Experts expect temperatures to gain a further four-to-six degrees by the end of this century.

"If temperatures are going to warm as we expect them to do then what is certainly likely to happen is that permafrost and glaciers will melt even more rapidly than before," Beniston told AFP.

This may prompt "enhanced slope instability and risks of people hiking or climbing... receiving a few tonnes of rocks on their head."

Jaggi and Salamin said climbing conditions in Switzerland have visibly changed in the past few decades due to the retreating glaciers.

"The chance of rock falls is growing because the weather is warmer," said Jaggi, while Salamin noted that climbers have been forced to take alternative routes as those used just 40 years ago are too dangerous.

The prospect of more tracks becoming inaccessible would fail to deter people from finding new slopes to scale, enthusiasts said.

"I think the impact of global warming on mountain climbing in general will be minimal," said Christopher Goddard, an English lawyer and ice-climbing fan who is due to go on a mountaineering holiday in the Alps next week.

"The top climbers have already moved to scale the thousands of unclimbed peaks in Tibet and Greenland, away from the falling rocks and tourists of the Alps."

SPACE.WIRE