By Richard Wilson At the Imja glacier lake in eastern Nepal

This is a lake that should not exist. It is 6,000 metres above sea level, a kilometre long and 100 metres deep.

Large glaciers around the world in both hemispheres have been retreating over the last 100 years Chris Folland Hadley Centre for Climate Research

Twenty-five years ago it was a glacier.

But since then, temperatures in what is one of the world's largest ice fields have risen year after year. And the world's leading climatologists think they know why.

"It's an important piece of evidence that the climate is actually warming," said Chris Folland, at the UK's Hadley Centre for Climate Research.

"What we've seen is that the large glaciers around the world in both hemispheres - South America, Europe, the Himalayas - have been retreating over the last 100 years."

'Wall of water'

The lake is held in place by a wall of frozen rock known as its terminal moraine.

I think the responsibility lies with everyone Gana Shyam Gurung WWF Nepal

The ice that binds it together is melting and it is inevitable that sooner or later this natural dam will burst, releasing a massive wall of water down the valley.

According to Utan Rai, a local Sherpa, the glacier is getting ever smaller in size.

"The lake is becoming bigger and bigger," he says.

Settlements below the lake could be swept away

Below the Imja glacier is the most densely populated Sherpa valley in Nepal.

The only way to get here is on foot and everything is carried in and out on paths that criss-cross the precipitous river gorges.

When the dam breaks, it will be a local disaster that the Nepalese blame on the industrialised West.

"I think the responsibility lies with everyone," says Gana Shyam Gurung of the World Wide Fund for Nature.

He believes that greenhouse gas emissions are to blame.

Catastrophe looms

Every Himalayan glacier is in retreat. And it seems this is a global trend.

The glacier is disappearing at an alarming rate

Proving a link between environmental cause and effect is always very difficult.

So a glacier here in the Himalayas is in retreat. Is it some sort of local weather effect?

Is it pollution from the West causing global warming, heating up our atmosphere?

All the while the Imja continues to break up and shrink.

Even if it is a global trend, the consequences here will be a local catastrophe.