By Jonathan Amos
BBC News Online science staff
One of South America's leading natural tourist destinations, the San Rafael Glacier in Chile, is retreating at an alarming rate, say UK scientists.
The dense, blue ice of the San Rafael Glacier
Located in a World Heritage Site, the glacier draws thousands of visitors each year to marvel at the way icebergs calve into the sea from its front wall.
But Dr Neil Glasser and colleagues say rapid melting is now under way because of historically high air temperatures.
They warn that if the glacier withdraws on to the land, tourism will suffer.
"This glacier is not only in a World Heritage Site, it is also in a Unesco biosphere reserve and huge national park," Dr Glasser, from the University of Wales, Aberystwyth, told BBC News Online.
"If the glacier retreats further up valley, it will cease to calve icebergs into the Laguna San Rafael, and one of the reasons why this area attracts so many tourists will be largely gone."
The San Rafael Glacier is part of the Northern Patagonian Icefield.
It is one of the fastest-moving glaciers in the world, flowing at 17m a day.
Falling from an altitude near to 3,000m right down to sea level, it is driven on by gravity and the mass of prodigious quantities of snowfall high up in the Andes.
Now, Glasser and Aberystwyth colleague Dr Krister Jansson, together with Dr Stephan Harrison from Oxford University, have been able to show that the glacier's front wall stands 1km further back in the water compared with the early 1990s.
Calving activity off the 70m-high vertical ice cliff has been dramatically reduced, too.
"We first went there 13 years ago.
"People put paint marks on the rock wall where the glacier was then; they even built a lookout post directly over the front of the glacier in 92," Dr Glasser said.
Tourists visit the site to see icebergs break off
"This year, the glacier is nowhere near this point - it's about a kilometre back from where it was.
"We've looked at the precipitation records closest to this area and they show no obvious change over the last 100 years, but they do have a rise in temperature recorded."
Scientists concede their historical data on the extent of glaciers - across much of the world, not just in South America - is patchy. However, they argue a consistent pattern of recession is beginning to emerge with many ice bodies from the Arctic to the tropics.
At San Rafael, the glacier's position was recorded once in the late 1800s as being more than 10km further out into the sea than it is now.
And moraine, the sediments dumped by the glacier, about 12km from the present ice front are currently being dated by the UK team - but are expected to be 3,000-5,000 years old.
"So it seems this glacier was relatively stable for 3,000-5,000 years and then suddenly, in the last 100 years, it came back.
Jansson and Glasser have been going to the laguna for over a decade
Dr Harrison added: "In recent years, the glaciers of the Northern Patagonian Icecap have been melting rapidly as a result of global warming, and the San Rafael Glacier has mirrored this retreat.
"The Patagonian icefields are losing ice more rapidly than any other comparable ice masses on Earth and we must see this as the inevitable consequence of global climate change."
Last year, US researchers working in the Patagonian icefields reported similar concerns. The Nasa-led study, published in the journal Science, looked at ice loss in 63 areas, comparing data from three decades.
The researchers found ice was lost at a rate sufficient to push up ocean waters by 0.04mm per year during the period from 1975 through to 2000.