LAST BASTION OF THE ICE AGE

"A man who keeps company with glaciers comes to feel tolerably insignificant by and by," wrote W H Davies, author of A Tramp Abroad. Though he was thinking of the Alps, his words could apply equally to the Perito Moreno glacier, in Patagonia, southern Argentina.

Dr Charles Warren knows that feeling: he has been there, to study its white mountains of gleaming ice, as they inch invisibly and relentessly towards the blue waters of the Lago Argentino, three times since 1992. He investigates glaciers in his research for the department of geography at the University of St Andrews, and he echoes Davies's words. "You feel incredibly small, and in the presence of the raw power of nature. It's a great adrenalin surge."

Unlike many other glaciers, the temperate forests nearby mean that the surroundings are not especially cold. "You walk through something that looks as verdant as Surrey, step out to its edge and there's this enormous blue-green tongue of ice across the lake."

It looks like a peaceful place, but it is not. There is a constant crunching and grinding from the confrontation of unseen pressures deep below the surface of the ice. Occasionally there is a distant cracking sound as pieces of the snout of the glacier break off and tumble with a roar into the freezing water. This process, known as "calving", happens day and night, and yields mini-icebergs that float in the lake until they melt or drift away. "It sounds like artillery fire at times," recalls Dr Warren. "It used to wake us up at night in our camp."

The casual observer will have plenty of opportunities to feel insignificant. The first arises from the sheer scale. The snout is almost 5km wide and 70m high - tall as an 18-storey tower block. It is 30km long, stretching back to the southern Patagonian icecap, which is 360km long. This, with the northern Patagonian icecap, forms the third largest accumulation of ice in the world (after Antarctica and Greenland).

Despite its slow-moving nature, the glacier can be dangerous. When calving takes place, and a piece lands in the lake, the underwater concussion can create huge waves that can wash over vantage points and wash away sightseers (brought there by a regular coach trip). "I've had boats stolen by waves, and had to spend nights in the open as a result," says Dr Warren.

It's dangerous living on the land nearby too: every three or so years since 1935 the glacier's advance has dammed the lake, causing the water level behind it to build up - sometimes to a depth of 36m. Eventually the pressure is so immense that the ice dam explodes. The resulting cascade floods adjacent farmlands, and people have drowned.

But the last such time was 1988, and Dr Warren had hurried out there in 1992, expecting to see another explosive flood. It didn't happen - which he thinks may indicate that Perito Moreno is about to go into an abrupt (for a glacier) retreat. Presently, though, Perito Moreno remains unusual, because unlike the vast majority of the rest of the world's glaciers - the repositories for 97 per cent of the world's fresh water supplies - it is not receding.

Glaciers play a vital part in the delicate balance of sea and river levels. Water evaporates from the oceans to be deposited as rain, snow or hail on mountains, where it often becomes locked into a glacier - to remain there perhaps for decades, while the body of the ice moves at a few millimetres per year down the mountains towards the seas. When glaciers melt, the climatic balance is disturbed, since it means either that sea levels are rising, or that there is more water vapour in the lower atmosphere (where it traps heat, warming the planet further) - or both. In the 18th and 19th centuries, the world endured a "mini-Ice Age", and the glaciers grew; now that process has gone into a sudden reverse, and the glaciers are shrinking at an order of magnitude faster than ever before.

Glacial recession, which has been happening for the past century, is usually taken as a clear sign of climate change caused by greenhouse gases in our atmosphere. In the European Alps, glaciers have shrunk by about 50 per cent in volume and up to 40 per cent in surface area since the middle of the 19th century, according to the UN World Glacier Monitoring Service. Other parts of the world - the mountains of Central Asia (where 163 of 224 glaciers are retreating), New Zealand (where glaciers have retreated by more than three kilometres this century), Sweden (where they have been retreating for the past 50 years) - are suffering too.

And South America's glaciers are disappearing too. In the past 40 years the south Patagonian icefield has lost 500 square kilometres of its previous area of 13,500 square kilometres. While this is only a four per cent fall, it is still significant.

Yet Perito Moreno, by virtue of its stable size, is different. Nobody knows why it is not shrinking, though theories abound. Experts have cited varying causes, like the huge amount of precipitation in the region: the Andes receive 400cm of rain and 500cm of snow every year, with some parts of southern Chile getting rain or snow 328 days a year. However, neighbouring glaciers receive the same sources, yet they have started receding. Ameghino, which is fed from the same icefield, has retreated by 4km in the past 20 years.

"Calving glaciers, which end at the sea or a lake, are different from those which simply end in a valley," says Dr Warren. "The debit and credit equation, to decide whether it grows or recedes, is more subtle. In a valley glacier, you have the credit - rain and snow - and the debit - melting. But in a calving glacier, the debit is complicated by the production of icebergs. That becomes more important than climatic factors. The key factor for a calving glacier becomes the depth of the water that the snout is in. The greater the depth of water, the faster icebergs will be produced, and the faster it will retreat."

Perito Moreno, he thinks, is delicately poised: if it begins to retreat even slightly it will move back into deeper water, setting off a process that will make it retreat increasingly quickly. Nobody is certain whether the climate has tipped far enough in that direction to cause what Dr Warren calls a "catastrophic retreat". The only way to find out will be to revisit it: the next damming of the lake is due in 1998. !