This file picture taken May 11, 2009 shows the Khumbu Glacier at the Everest Khumbu region, of the longest glaciers in the world.(AFP)

LUKLA, Nepal (AFP) — Over two decades, Funuru Sherpa has watched the lake above his native village of Dengboche in Nepal's Himalayas grow, as the glacier that feeds it melts.

The 29-year-old, who runs a busy Internet cafe for tourists visiting the Everest region, remembers his grandfather telling him that 50 years ago the lake did not exist.

"Before, it was all ice," he told AFP in the eastern Himalayan town of Lukla, in the shadow of Mount Everest.

"This is proof that the glaciers in the high Himalayas are melting. And that must be because the temperatures have gone up."

Scientists say the Imja Glacier above Dengboche is retreating by about 70 meters a year, and the melting ice has formed a huge lake that could devastate villages downstream if it bursts.

The trend is not new. Nepal's International Centre for Integrated Mountain Development (ICIMOD), which has studied the Himalayas for three decades, says many of the country's glaciers have been retreating for centuries.

But ICIMOD glaciologist Samjwal Ratna Bajracharya said this was now happening at an alarming speed, with temperatures in the Himalayas rising at a much faster rate than the global average.

"Our studies of the past 30 years show that the temperatures (in the Himalayas) are rising up to eight times faster than the global average. Melting is taking place higher and faster," Bajracharya told AFP.

"The melting of glaciers and formation of glacier lakes is a key indicator of the temperature rise. And lately, we have seen massive ice melt."

Nepal has more than 2,300 glacial lakes and experts say at least 20 are in danger of bursting.

At almost one square kilometer, the Imja lake is the country's second biggest, estimated to hold 36 million cubic meters of water, and is considered the biggest flood threat.

It is a subject close to the heart of Nepalese mountaineer Apa Sherpa, who has climbed Everest a record 19 times.

In 1985 Apa Sherpa lost his house and farm when the Dig Tsho glacial lake burst, causing a giant wave to flow down the mountain.

Seven people were killed by the flood, which swept away bridges and houses and destroyed a new hydropower station.

"For me, climate change is personal," said the climber, who dedicated his latest Everest expedition to raising awareness of the impact of climate change on mountain communities.

"There's probably no one who can relate to this issue in the way that I can."

Information about how many people would be affected by a glacial lake bursting remains limited, but experts say the floodwaters could reach as far as Nepal's southern planes and beyond.

Environment secretary Uday Raj Sharma said last week the bursting of the Imja lake would be like a "Nepalese tsunami," comparing it with the 2004 Indian Ocean disaster in which around 220,000 people died.