Meltdown in the mountains

Mankind has hit the defrost button. By 2035 huge glaciers high in the mountains may no longer exist. Thousands of local people live in fear of drowning in the melt water. Can science do anything to help?

High in the Himalayan mountains of Nepal, engineers will this April resume a battle to empty a lake that threatens to drown 7,000 people. The lake, Tsho Rolpa, has formed at the tip of the Trakarding glacier, northeast of Kathmandu. It is one of dozens of potentially lethal lakes forming in these remote Himalayan valleys as global warming melts the glaciers behind them.

High in the Himalayan mountains of Nepal, engineers will this April resume a battle to empty a lake that threatens to drown 7,000 people. The lake, Tsho Rolpa, has formed at the tip of the Trakarding glacier, northeast of Kathmandu. It is one of dozens of potentially lethal lakes forming in these remote Himalayan valleys as global warming melts the glaciers behind them.

Glaciers cover around one-sixth of the Himalayas. Taken together with Tibet to the north and the Karakorum to the west, this region contains most of the surviving snow and ice outside the polar regions - thousands of cubic kilometres of frozen water, much of it dating back to the last ice age.

But each summer now, more ice and snow melts than is replenished by the monsoons. The glaciers are shrinking. The story of the extent of their demise has been slow to get out. Up here, far from roads and power lines and science labs, much of the information on the state of these glaciers has been anecdotal. But, as scientists begin to collate data, a picture is emerging of meltdown on the roof of the world.

Syed Hasnain of the Jawaharlal Nehru University in New Delhi presented the most detailed survey yet to a meeting of glaciologists in Birmingham last August. His four-year study for the UN's International Commission on Snow and Ice concluded: "Glaciers in the Himalayas are receding faster than in any other part of the world. If the present rate continues, the likelihood of them disappearing by 2035 is very high."

It was a devastating and largely unexpected finding. Only five years ago, glaciologists on the UN's Intergovernmental Panel on Climate Change said that, even under pessimistic assumptions about global warming, the region's glaciers "should continue to exist into the 22nd century".

The melting of glaciers is emerging as one of the least ambiguous signs of climate change. Amid arcane arguments about the meaning of yearly fluctuations in the weather, it is hard to argue with the wholesale melting of some of the largest glaciers in the world. Mankind, it seems, has hit the defrost button. And while glaciers are thawing out from Peru to the Alps, from Kenya to New Guinea, nowhere is the meltdown faster than in the Himalayas.

Their eventual disappearance is a potential catastrophe for the hundreds of millions of people in southern Asia, who depend on the summer melt even more than the monsoon rains to irrigate their crops and provide drinking water. But for the people in the mountains the threat is more immediate. For, as the glaciers melt, much of the water collects behind unstable natural dams of debris, known to generations of geographers as terminal moraine, which was left behind in valley bottoms when the glaciers were at their longest.

Unlike permanent lakes, the glacial lakes behind these moraines often have no outflow. The further the glaciers retreat, the larger the lakes grow and the greater the pressure on the moraines - until the force of water or a sudden overflow washes away the dam and sends a wall of water rushing downstream.

These lakes and the occasional catastrophic failure of their moraine dams are a natural feature of the Himalayas, where glaciers have been in gradual retreat since the end of the "little ice age" more than 300 years ago. But the pace of current global warming is much greater. Most of the region's glaciers for which there is data have been in ever more rapid retreat for two or three decades now - making the lakes bigger and the breakouts more frequent, says Hasnain.

And more people than ever before are living or holidaying in valleys immediately below the precarious glacial lakes. Some of the most scenic lakes admired by trekkers in the heart of the Himalayas are in fact disasters waiting to happen.

John Reynolds, a geologist based in Mold, Clwyd, is the chief technical adviser on glacial lakes to the Nepalese government. "Half a century ago, there was a lake burst about once a decade," he says. "Now they occur about every three years. By 2010 they will likely be an annual event."

The most damaging recent burst occurred 15 years ago at Dig Tsho in the Khumbal Himal region of Nepal. A rush of water up to 15 metres high swept down the valley for some 90 kilometres, drowning villagers and destroying a new hydroelectric plant.

In 1994, Luggye Tsho burst in northern Bhutan: 50 million cubic metres of water swept with extraordinary force down a narrow valley; 27 people drowned; bridges, flour mills and farmland were devastated. A fortified monastery and temple at Punakha, 100 kilometres from the lake, were ruined. River records show that even 200 kilometres downstream the flood wave was still more than two metres high. Such elemental force, if unleashed in a more heavily populated valley, could cause a major catastrophe.

Reynolds has compiled detailed surveys of glacial lakes in both Nepal and Bhutan. He estimates that there are dozens of such lakes in Nepal, mostly in the east of the country, with many more forming. The Nepalese government has compiled a short-list of seven lakes in imminent danger of collapse, but many others are unmapped.

"A major issue here is that Nepal and its neighbours such as Bhutan want to build giant hydroelectric power schemes in these valleys to sell power to their heavily populated neighbours. The Himalayas could be the power house of Asia," says Reynolds. But it is not safe to build large dams when these lakes hang, like a sword of Damocles, above them, he says.

Glaciologists play a macabre game trying to predict which glacial lake could burst next. A favourite is the Imja lake in Nepal's Sagarmatha national park, home of the Sherpas and one of the most popular trekking destinations. It began to form in the 1960s, and now holds 30 million cubic metres of water. A Japanese glaciologist who has studied the lake, Tomomi Yamada of Hokkaido University, believes it is likely to burst within five years. Downstream lies one of the most heavily populated valleys in Nepal, with Sherpa villages and trekkers' lodges alike under threat.

But attention this summer is focused on Tsho Rolpa, the largest glacial lake in the Himalayas. It started to form back in the 1950s, but could have only a short time left unless emergency works are completed before this summer's melt season, says Reynolds. The lake is approaching four kilometres long and contains more than 100 million cubic metres of water - all held back by a moraine dam 150 metres high. "The moraine has degraded significantly in the seven years we have been watching it," says Reynolds.

That is hardly surprising. Reynolds and a team from the Department of Hydrology and Meteorology in Kathmandu have used ground-penetrating radar to map the moraine dam. This has revealed that much of the dam is not made of rock and earth, but of giant slabs of ice kept cool by the cover of debris. "Some of the ice blocks are 30 metres thick," says Reynolds. But this ice, too, is now melting. As the blocks fracture and melt, lake water is pushing its way through the dam. "The risk of an explosive failure becomes ever higher," he says.

The Nepalese government has put in a remote early-warning system. Sensors below the dam should set off sirens in villages downstream, giving 7,000 residents a few minutes' notice of a wall of water heading their way. This is the only major glacial lake in the Himalayas to have an early-warning system.

An engineering solution is vital. Last autumn Nepalese engineers airlifted construction equipment to Tsho Rolpa, which is 4,500 metres up and 85 kilometres from the nearest road. When work resumes in April, after a winter lay-off, they hope to complete a concrete-lined channel, three metres deep and 100 metres long, in the top of the Tsho Rolpa's dam in time to let water out before the peak melt water flows in August.

The channel should remove 17 per cent of the lake's water and relieve the pressure on the dam. But it will not make the lake permanently safe. "The next phase should be to lower lake levels by a further 15 metres," says Reynolds. "But right now there is no technical plan and no money to do it."

He says donors are reluctant to provide cash for such projects. "The problems posed by these lakes seem very intangible - until it is too late," he says. "Technically there is no reason for anyone to get killed by one of these bursts, but right now an awful lot of people are at risk unnecessarily. For most of the lakes the question is not whether they will burst - only when."

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