Nawa Jigtar was working in the village of Ghat, in Nepal, when the sound of crashing sent him rushing out of his home. He emerged to see his herd of cattle being swept away by a wall of water.

Jigtar and his fellow villagers were able to scramble to safety. They were lucky: 'If it had come at night, none of us would have survived.'

Ghat was destroyed when a lake, high in the Himalayas, burst its banks. Swollen with glacier meltwaters, its walls of rock and ice had suddenly disintegrated. Several million cubic metres of water crashed down the mountain.

When Ghat was destroyed, in 1985, such incidents were rare - but not any more. Last week, scientists revealed that there has been a tenfold jump in such catastrophes in the past two decades, the result of global warming. Himalayan glacier lakes are filling up with more and more melted ice and 24 of them are now poised to burst their banks in Bhutan, with a similar number at risk in Nepal.

But that is just the beginning, a report in Nature said last week. Future disasters around the Himalayas will include 'floods, droughts, land erosion, biodiversity loss and changes in rainfall and the monsoon'.

The roof of the world is changing, as can be seen by Nepal's Khumbu glacier, where Hillary and Tenzing began their 1953 Everest expedition. It has retreated three miles since their ascent. Almost 95 per cent of Himalayan glaciers are also shrinking - and that kind of ice loss has profound implications, not just for Nepal and Bhutan, but for surrounding nations, including China, India and Pakistan.

Eventually, the Himalayan glaciers will shrink so much their meltwaters will dry up, say scientists. Catastrophes like Ghat will die out. At the same time, rivers fed by these melted glaciers - such as the Indus, Yellow River and Mekong - will turn to trickles. Drinking and irrigation water will disappear. Hundreds of millions of people will be affected.

'There is a short-term danger of too much water coming out the Himalayas and a greater long-term danger of there not being enough,' said Dr Phil Porter, of the University of Hertfordshire. 'Either way, it is easy to pinpoint the cause: global warming.'

According to Nature, temperatures in the region have increased by more than 1C recently and are set to rise by a further 1.2C by 2050, and by 3C by the end of the century. This heating has already caused 24 of Bhutan's glacial lakes to reach 'potentially dangerous' status, according to government officials. Nepal is similarly affected.

'A glacier lake catastrophe happened once in a decade 50 years ago,' said UK geologist John Reynolds, whose company advises Nepal. 'Five years ago, they were happening every three years. By 2010, a glacial lake catastrophe will happen every year.'

An example of the impact is provided by Luggye Tsho, in Bhutan, which burst its banks in 1994, sweeping 10 million cubic metres of water down the mountain. It struck Panukha, 50 miles away, killing 21 people.

Now a nearby lake, below the Thorthormi glacier, is in imminent danger of bursting. That could release 50 million cubic metres of water, a flood reaching to northern India 150 miles downstream.

'Mountains were once considered indomitable, unchanging and impregnable,' said Klaus Tipfer, of the United Nations Environment Programme. 'We are learning they are as vulnerable to environmental threats as oceans, grasslands and forest.'

Not only villages are under threat: Nepal has built an array of hydro-electric plants and is now selling electricity to India and other countries. But these could be destroyed in coming years, warned Reynolds. 'A similar lake burst near Machu Picchu in Peru recently destroyed an entire hydro-electric plant. The same thing is waiting to happen in Nepal.'

Even worse, when Nepal's glaciers melt, there could be no water to drive the plants. 'The region faces losing its most dependable source of fresh water,' said Mike Hambrey, of the University of Wales.

A Greenpeace report last month suggested that the region is already experiencing serious loss of vegetation. In the long term, starvation is a real threat.

'The man in the street in Britain still isn't sure about the dangers posed by global warming,' said Porter. 'But people living in the Himalayas know about it now. They are having to deal with its consequences every day.'

Additional reporting: Amelia Gentleman and Felix Lowe