It is strangely calming to watch the Imja glacier lake grow, as chunks of ice part from black cliffs and fall into the grey-green lake below.

But the lake is a high-altitude disaster in the making - one of dozens of danger zones emerging across the Himalayas as glaciers melt due to climate change. If the lake, at 5100 metres in Nepal's Everest region, breaks through its walls of glacial debris, known as moraine, it could release a deluge of water, mud and rock as far as 100 kilometres.

This would swamp homes and fields with a layer of rubble up to 15 metres thick, leading to the loss of the land for a generation. But the question is when, rather than if.

Mountain regions from the Andes to the Himalayas are warming faster than the global average under climate change. Ice turns to water; glaciers are slowly reduced to lakes.

When Sir Edmund Hillary made his successful expedition to the top of Everest in 1953, Lake Imja did not exist. But it is now the fastest-growing of some 1600 glacial lakes in Nepal, stretching down from the glacier for 2.5km and spawning three small ponds.

At its centre, the lake is about 600m wide and, according to government studies, up to 96.5m deep in some places. It is growing by 47m a year, nearly three times as fast as any other glacial lake in Nepal. 'The expansion of Imja lake is not a casual one,' said Pravin Raj Maskey, a hydrologist with Nepal's ministry of irrigation.

The extent of recent changes to Imja has taken glacier experts by surprise, including Teiji Watanabe, a geographer at Hokkaido University in Japan, who has done field research at the lake since the 1990s.

Watanabe returned to Imja in September, making the nine-day trek with 30 scientists and engineers on a US-funded expedition led by the Mountain Institute. He said he did not expect such rapid changes to the moraine holding back the lake. 'We need action, and hopefully within five years,' Watanabe said. 'I feel our time is shorter than what I thought before. Ten years might be too late.'

Unlike flash floods, a glacial lake outburst is a continuing catastrophe. 'It's not just the one-time devastating effect,' said Sharad Joshi, a glaciologist at Kathmandu's Tribhuvan University, who has worked on Imja. 'Each year for the coming years it triggers landslides and reminds villagers that there could be a devastating impact that year, or every year. Some of the Tibetan lakes that have had outburst floods have flooded more than three times.'

But mobilising engineering equipment and expertise to a lake 5100m up, and several days' hard walking from the nearest transport hub, is challenging in Nepal, one of the poorest countries in the world. And scientists and engineers still cannot agree on whether to rate Imja as the most dangerous glacial lake in the Himalayas, or a more distant threat.

There are other contenders for immediate action, with some 20,000 glacial lakes across the Himalayas, although many are concentrated in the Everest region. Bhutan has nearly 2700.

John Reynolds, a British engineer and expert on glacial lakes who has worked in Nepal, argues the international community has focused on Imja because of its proximity to Everest and trekking routes popular with Western tourists. He says there are other, more hazardous lakes elsewhere.

The Nepali government ranks Imja among the six most dangerous glacial lakes in the country largely because it is growing so quickly. More than 12 other such lakes are also seen as high risk. Imja, though fast-growing, is held in by a relatively wide moraine, which makes it secure in comparison to some others.

Watanabe concedes the geography of the lake could keep disaster at bay, at least in the next year or two. But, he says, there are signs an outlet channel at the bottom of the lake may be widening dangerously.

Reynolds said Nepal and the international community needs to think of a Himalaya-wide action plan. 'As the climate is changing more glacial lake systems are forming,' he said. 'The question is how to decide which are hazardous now and which are going to become hazardous in the future.'

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