By Navin Singh Khadka BBC News in Nepal

No one is keeping an eye on the glacial lakes

They claim the situation is not being adequately monitored; the last major studies having been done in the 1990s.

Swelling glacial lakes would increase the risk of catastrophic flooding.

In the long term, the glaciers could disappear altogether, causing several rivers to shrink and threatening the survival of those who depend on them.

"It is high time we did field studies to assess the situation or else a big natural catastrophe could hit us anytime," said Arun Bhakta Shrestha, from Nepal's Department of Hydrology and Meteorology.

Rising temperatures

There are 3,300 glaciers in the Nepalese Himalayas and 2,300 of them contain glacial lakes. These lakes are quietly growing because of rising temperatures, but a sufficiently close eye is not being kept on them, campaigners say.

We urgently need to update our glaciological data Dr Bhakta Shrestha

Nobody knows how many are close to bursting, and no steps have been taken to establish early warning systems for the villages downstream.

A burst lake would cause flash floods which could sweep away people, houses, roads and bridges in Nepal, Bhutan, Bangladesh and India.

Such disasters have already happened more than a dozen times around Nepal in the last 70 years.

A glacial lake burst in Khumbu, Nepal, in 1985, killing at least 20 people. It also washed away a hydropower station, a trekking trail and numerous bridges.

Despite the real threat, no systematic on-the-ground research has taken place since the mid-1990s.

Between 1970 and 1989, Japanese researchers discovered most of the glaciers in the Khumbu region had retreated 30-60m. In Nepal's Dhaulagiri region, field studies until 1994 showed the same trend.

And Nepal's most studied glacier in Tsorong Himal underwent a 10m retreat between 1978 and 1989.

For now, there is reliance on satellite data. This even shows some glaciers are stable or advancing, particularly in the west and north.

"We urgently need to update our glaciological data," said Dr Bhakta Shrestha, "otherwise we won't have any warning when disaster strikes."

Rivers run dry

British geologist John Reynolds, of Reynolds Geosciences Limited, agrees.

"There has to be a fresh look at the entire issue because we may be running a risk of great magnitude," he said.

In the long term, researchers fear that a warmer climate may switch the problem from too much water in the region to too little.

A burst lake would cause flash floods, which could sweep away whole villages

Nearly 70% of discharge to the Ganges is from Nepalese rivers, which means that if Himalayan glaciers dry up so will the Ganges downstream in India.

"In some rivers, the flow may go down by as much as 90%," said Syed Iqbal Hosnain, of the University of Calicut, India.

Although the evidence for the future impact of climate change is accumulating, scientists say there is not enough field research to provide empirical proof about the gravity of the crisis.

"Once we get the figures in, we will know the real scenario," said Chandra Gurung, WWF-Nepal. "Perhaps then we can get a plan."