The Alpine glaciers, source of some of Europe's biggest rivers, have been in retreat for more than a century, but the loss of ice has speeded this year as temperatures have soared.

"The rate of ice melt is some three or four times the usual amount," said Charly Wuilloud, head of the department of natural dangers at the Valais state forestry department.

Some 9,000 feet above the Rhone Valley in southern Switzerland, at the junction of the Ferpecles and Mount Mine glaciers, the temperature is an unusually warm 59-68 degrees Fahrenheit.

Sporting sunglasses and a short-sleeved shirt more typical of beach wear, Wuilloud pointed to the rush of melt water streaming from the ice wall of the Ferpecles glacier.

The so-called equilibrium line, the point at which any fresh snow or rain falling will turn to ice and not melt or run off, is some 984 to 1,312 feet higher up the mountain this year than usual this summer, he added.

At one time, the 4-miles-long Ferpecles and Mount Mine joined to form a forked tongue of ice stretching down into the valley.

Lines gouged into the mountain side tens of yards above the valley floor show the height to which the ice once reached.

COLD SNAP

Scientists say Europe's glaciers have been shrinking since the 1850s, initially as a result of a natural warming of the Earth following a 250 year cold snap.

But the process has picked up pace over recent decades -- particularly since the 1970s -- under the impact of global warming fueled -- many scientists believe -- by high emissions of greenhouse gases.

According to the United Nations' International Panel for Climate Change (IPCC), the average temperature of the earth rose 0.5 Celsius during the 20th century and could rise several times that rate over the next 100 years.

Back in the 1990s, even before this year's blistering summer, geologists at the Zurich-based World Glacier Monitoring Service (WGMS) forecast that the glaciers would shrink to just 10 percent of their 1850 size by the end of the 21st century.

By 1970, they had already declined to around half and were seen losing a further 50 percent by 2025, according to geography professor Wilfried Haeberli of the University of Zurich.

The latest estimates are that 25-30 percent has already gone. "It looks like our prediction was a little bit optimistic. It is going faster than we thought," he told Reuters.

SAHARA DUST

Glaciers have long been seen as one of the most sensitive detectors of climate change, with the impact showing up first in the thickness of the ice rather than its length, which can take years to respond.

This year a number of factors have combined to intensify the rate of melt, including a freak weather event in November when a cloud of dust was blown north from the Sahara Desert.

As glacier surface ice melted with the coming of spring, the dust was exposed again, helping give the ice a grayish appearance that reduced reflection and increased the amount of sunlight absorbed -- hence the melt.

Although studies for this year are not yet finished, Haeberli said that the Alpine glaciers could have slimmed down some 6.6 feet or more -- an exceptional loss of thickness.

This would be 10 times the average annual melt over the length of the 20th century and some four times that of the two decades between 1980-2000 when global warming was already making its presence felt.

"This looks like a record year," he said. "There is no doubt that it has been exceptional."

RISK TO RIVERS

For those living in glacial valleys, the thinning of the glaciers when combined with heavy rain brings the danger of flash floods like the one which killed 13 people in the Gondo valley of Switzerland in 2000.