Trips to Chacaltaya were a favourite - this picture was taken in 1990

A favourite outing of ours was to pile into our sturdy old jeep, wind our way up a track that only just clung to the mountainside, and finally arrive at the foot of a majestic glacier called Chacaltaya.

Family photos from the time show sunburnt, smiling faces against a backdrop of a long, snow-covered glacier.

Chacaltaya was famous then for being the world's highest ski run at 5,300 metres (17,400 feet). I remember seeing exhausted but exhilarated skiers crashed out on the floor of a nearby ski lodge, gasping for breath.

Dryly-written articles in scientific journals have traced the rapid demise of the glacier as a result of rising air temperatures. Reading them is like picking your way through a long obituary.

Chacaltaya could be as much as 18,000 years old, but it has lost 80% of its area in the last 20 years.

It has become an icon of the effects of global warming, and a laboratory for predicting what is to befall other Andean glaciers.

Monitoring

Retracing the journey in 2007 turned out to be somewhat unexpected.

The day we chose to return was perfect for climate change sceptics. A recent heavy snowfall had left the top of the track impassable by jeep, so we had to walk through deep snow.

We keep having to revise downwards our projections of when Chacaltaya is going to disappear completely

Edson Ramirez

"Days like this are completely misleading," said Edson Ramirez, a 37-year-old quietly-spoken Bolivian, who is his country's leading expert on glaciers.

"It's the wet season now. The snow will melt quickly and run off the glacier."

Sure enough, a few days later I received by e-mail a photo taken by members of Mr Ramirez's team on 1 March of what was left of the glacier. All that could be seen of what was once a 500-metre long glacier are just two separate areas of ice.

Even on the day of our visit, you could see the clear outline of the two patches of glacier under the snow, measuring about 60 by 20 metres. Not far away stood a forlorn ski lift, not used since 1998.

"We keep having to revise downwards our projections of when Chacaltaya is going to disappear completely," said Mr Ramirez, who has been monitoring the glacier since 1995.

"Not long ago we thought it was going to be 2015, now we think it could be this year or next."

Impact feared

Mr Ramirez is not alone in stressing the dramatic acceleration of the glacial melt since the 1980s.

Tuni Condoriri provides drinking water to El Alto and parts of La Paz (Photo: Edson Ramirez's team)

Back down in La Paz, the head of the national climate change programme, Oscar Paz, said: "These glaciers are our water stores. One of our great concerns is the future of our drinking water supplies."

Chacaltaya may be a symbol of what is happening to small Andean glaciers, but it does not provide the water to La Paz and neighbouring El Alto, home to nearly two million people.

For that you have to travel about an hour down more bone-jarring tracks to the spectacular range of mountains called Tuni Condoriri, named after their appearance like a condor hunched ready to pounce.

A reservoir under the range provides about 80% of the drinking water to El Alto and large parts of La Paz.

Bolivian and international glaciologists are shifting their attention here, in part because of fears over the impact of glacial melt on Bolivia's long-term development.

Various measurements show the area of the 15 original glaciers in the range has reduced by more than a third from 1983 to 2006. Five glaciers may have already disappeared completely.

'Hurt'

The glaciers are particularly important during the dry seasons as they slowly release water.

"We don't know exactly how much of the water supply comes from glacial melt but it could be as much as 60%," says Mr Ramirez.

Most worrying is when the glaciers completely disappear, which Mr Ramirez puts in the range of 2025 to 2050.

Even now Bolivia has a pressing need for more water. The arrival of several thousand migrants from rural areas to El Alto every year means demand for water will increase as the supply remains the same.

Mr Ramirez's model predicts that as early as 2009 there will more demand than water available in the reservoirs.

Many agree that the long-term drop in water from glacial melt exacerbates the urgent necessity to capture more of the precipitations during the wet season, probably by building dams.

Mr Paz clearly thinks it wrong that desperately poor countries like Bolivia should have to pay the cost of adapting to global warming when they are not to blame.

In the meantime, Mr Ramirez and the team I travelled with were coping with the imminent loss of Chacaltaya by resorting to graveyard humour.

"Maybe we should put the last bit of the glacier in a freezer for posterity," they said.

But there was no doubt about their sorrow. "It really hurts," they agreed. "We have had the privilege of seeing their beauty. The next generations will not."