Melting glaciers in Himalayas threaten catastrophic floods

The giant glaciers of the Himalayas are melting so quickly that within five years dozens of glacial lakes could burst their banks and kill tens of thousands in their path, a report today from the United Nations Environment Programme (Unep) warns.

Researchers who have been monitoring the glaciers for 15 years are deeply alarmed by the speed with which many lakes are filling as glaciers shrink under the impact of global warming.

Raphstreng Tsho glacial lake in Bhutan, for example, was 1.6 kms (about one mile) long, 0.96 kms wide and 80 metres deep in 1986. Nine years later it was 1.94 kms long, 1.13 kms wide, and 27 metres deeper.

Surendra Shrestha, of Unep's Early Warning Division, said, "Our findings indicate that 20 glacial lakes in Nepal and 24 in Bhutan have become potentially dangerous as a result of climate change.

"We have evidence that any one of these could, unless urgent action is taken, burst its banks in five to 10 years with potentially catastrophic results for people and property hundreds of kilometres downstream. These are the ones we know about. Who knows how many others, elsewhere in the Himalayas and across the world, are in a similarly critical state?"

The Tsho Rolpa lake in the Dolakha district of Nepal covered 0.23 square kms in the late 1950s. But today, with the melting of a nearby glacier, it has swollen to six times the size.

Pradeep Mool, a remote sensing expert with the International Centre for Integrated Mountain Development , said: "A flood from this lake could cause serious damage down to the village of Tribeni, 108 kms downstream, threatening about 10,000 human lives, thousands of livestock, agricultural land, bridges and other infrastructure."

The warnings add a new dimension to the understanding of the multiple environmental hazards posed by climate change. And while the deaths of nearby residents and the destruction of homes and farms will be the most immediate effects of such glacial lake outburst floods, or glofs, as they are known, they are by no means the only hazard.

Klaus Toepfer, the executive director of Unep, said: "It is not just the risk to human lives, agriculture and property that should worry us. Mountains are the world's water towers, feeding the rivers and lakes upon which all life depends.

"If the glaciers continue to retreat at the rates being seen in places such as the Himalayas, many rivers and freshwater systems could run dry, threatening drinking water supplies as well as fisheries and wildlife. We have another compelling reason to act to reduce emissions of carbon dioxide and other greenhouse gases."

There was a serious glof at the Dig Tsho glacial lake in Nepal in August 1985. The outburst of water destroyed 14 bridges and caused $1.5m damage at a nearby hydroelectric plant, but researchers say the threat is now far more widespread.

At Tsho Rolpa, engineers have installed a network of sensors and sirens to give people living downstream enough warning to escape. They are also working to lower the lake's water level by 30 metres, to postpone the disaster. But Unep scientists warn that to go beyond such pilot projects to tackle all the problem lakes will cost an enormous sum. Mr Shrestha said: "Solving this problem is going to be costly because glacial lakes are situated in remote areas which are difficult to reach."

Mr Toepfer added: "Mountains were once considered indomitable, unchanging and impregnable. But we are learning that they are as vulnerable as the world's oceans, grasslands and forests."