London

Melting glaciers high in the Himalayan mountains are creating unstable lakes that threaten to burst their banks, endangering the lives of tens of thousands of people, according to a United Nations report.

Although not new, the frequency of such floods has risen in the past 30 years. The increase is blamed on global warming by the report, which was written by researchers at the United Nations Environment Programme (UNEP) and the International Centre for Integrated Mountain Development, based in Kathmandu, Nepal.

Ground surveys and satellite images have identified 44 lakes in Nepal and Bhutan as being in danger of bursting the natural dams constraining them within 5–10 years. Data from 49 monitoring stations across Nepal indicate that air temperatures in the area are on average 1 °C higher than they were in the 1970s.

Water works: operations under way at Tsho Rolpa lake, which has grown sixfold since the 1950s. Credit: REYNOLDS GEO-SCIENCES LTD

In Bhutan, some melting glaciers are retreating by 30–40 metres a year, according to the report, and in Nepal's Dolakha District the glacier feeding the Tsho Rolpa lake has retreated by up to 100 metres a year. The lake has grown sixfold since the 1950s, putting 10,000 people and a hydroelectric power station at risk.

Glacial lakes form behind moraines — dams of rocks, debris and ice that are left behind as the glacier retreats. These can quickly collapse; a small leakage as the ice melts is easily enlarged, and escaping water effectively eats the moraine from the inside out.

“Water was lapping the top of Tsho Rolpa's moraine and we didn't know how much time we had before it would burst,” says John Reynolds, a geophysical engineer at Reynolds Geo-Sciences, a consultancy based in Wales, who is advising the Nepalese government on Tsho Rolpa.

Initial piping to siphon off the water proved insufficient to deal with the rising levels at Tsho Rolpa, Reynolds says. So two years ago, an emergency drainage channel was cut through the moraine to lower the water level by 3.5 metres. This was a tough job, he points out — the lake lies at an altitude of 4,500 metres and is 60 kilometres from the nearest road. Materials and equipment had to be brought in by porters or helicopters.

The work, which was paid for by the Dutch government, reduced the lake's volume by one-sixth and has allowed part of the moraine to dry out, thereby stabilizing it. But now two large ice blocks in the moraine are melting and the water level needs to be lowered further.

Tsho Rolpa is just one of many lakes facing the same problem. Klaus Toepfer, UNEP's executive director, described the problem as “another compelling reason to act to reduce emissions of carbon dioxide and other greenhouse gases”.