"It is shrinking quickly, just as we are standing here," the 48-year-old said, drops of water falling above him from the ice grotto's translucent entryway. "If anything, it makes me want to bring my daughter here to see it as soon as I can."

While it is hardly unusual for ice to melt in the summer sun, scientists are concerned that a heating-up of the world's atmosphere is causing Europe's glaciers to steadily disappear.

Swiss glaciers have lost more than 15 percent of their surface area in the last two decades, and could vanish almost entirely within a century if climate shifts are not moderated, said Max Maisch, a glaciologist from the University of Zurich.

As one of the only glaciers in Europe accessible by car, and with its man-made grotto that lets visitors walk underneath the ice mass, the Rhone Glacier has drawn many visitors looking to see for themselves how global warming is felt in the mountains.

"Tourists can observe a dying glacier here," Maisch said, standing next to the blue-tinged ice that once stretched a further 1.5 kilometres (1 mile) into the valley below.

No precise figures are available on how many people visit Switzerland's glaciers every year, because most are accessed and viewed freely, said Veronique Kanel of the Swiss tourist board.

QUANTUM CHANGES

But hundreds of thousands of people travelled on trains leading to Swiss glacier sites last year, she said, including 23,000 to the Rhone glacier and 562,000 on the Jungfrau railway line leading to the Aletsch glacier, the largest in the Alps.

Achim Steiner, executive director of the United Nations Environment Programme (UNEP), said Europe's melting glaciers provided a clear illustration of the dangers caused by carbon dioxide emissions from cars and industry.

"What we are seeing is stark. It is a reminder of the kind of quantum changes that we are about to witness as a consequence of global warming," he told Reuters in a telephone interview from UNEP's headquarters in Nairobi.

"The worrying part of it is not just that the glacier disappears. It is a chain reaction, and we still lack the capacity to understand the full consequence of these changes," he said.

Curt Bolliger, a 75-year-old Swiss national who lives in Mexico, first visited the Rhone Glacier in 1950.

"I was here many years ago, and you can be sure that it was a different place then. The ice went down, down, down. Now it is melting all over," he said after walking through the ice tunnel with his wife and daughter.

Recalling seeing the glacial ice stretch hundreds of metres further toward the nearby Gletsch village, Bolliger said he considered its disappearance highly disheartening.

"If we don't change anything about the ecology, and the world weather, our future generations will have lost a lot."