RESOLVING THE LEDGER
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Preliminary investigations have pointed to a collapsed glacier as the cause of the flash flood on the Nepal-Tibet border, highlighting the issue of rapidly melting Himalayan ice, scientists have told the BBC.
Initial reports of an earthquake led many to believe that a tremor had caused a landslide, which in turn caused the flood that has killed at least 177 people.
However, the US Geological Survey then clarified that the disaster was caused by a "glacial collapse", which is when a glacier or part of a glacier disintegrates.
The impact of the collapsed glacier hitting the bottom of a valley was so powerful that it registered a magnitude of 5.2, according to USGS data and scientists who have analysed the event.
Dr Simon Cook, a glacier expert with the University of Dundee, told the BBC his team had detected that the lower part of a glacier in Nepal near the Langtang Lirung peak - which is approximately 15km (9 miles) west of the site of the floods - had collapsed.
He estimated that the glacier collapsed in a northwesterly direction and then travelled westwards downstream.
An analysis , external by the International Centre for Integrated Mountain Development (ICIMOD), an intergovernmental scientific institution, found there could have been "an ice-rock avalanche originating from a high-altitude area".
This caused a "large volume of ice and rock debris" to enter the Lende Khola, a tributary of the Bhote Koshi river, which in turn generated a sudden surge in the river carrying water, sediment and large boulders downstream.
The centre found that water levels in rivers downstream rose by between seven and nine metres within a span of 30 minutes, and several water monitoring stations were washed away or damaged.
Mike Searle, a professor of earth sciences at Oxford University, said that this whole process - from landslide to flood - could have taken hours or even days.
"The sheer wave of debris suggests there could have been a landslide first; the water built up, dammed the river, and then it burst in one huge great flood," he told the BBC.
The topography of the area could have also exacerbated the flood. Searle pointed out that Langtang Lirung sits on the crest of the high Himalayas and there are "incredibly steep valleys" on both sides of the mountain, which could have funnelled the water at higher speeds.
It is still unclear what caused the glacier to collapse on Wednesday.
Searle said that Langtang Lirung, like the rest of the Himalayas, has been slowly rising due to long-term tectonic movement, where one plate has been pushing another plate up. "It's like putting a jack under the car, and then jacking it up."
"As the mountain is rising, the glacier gets steeper and steeper, and it's inevitable that parts of it fall off."
But usually these are smaller chunks which temporarily block the rivers, and are easily flushed out, he said. However, in this disaster, the whole glacier seems to have collapsed "in one go", which Searle said is "very unusual". "These floods happen all the time, but not at this scale."
He said Wednesday's event likely happened because of "two different processes - tectonics with the Himalayas uplifting, and then climate change with increasingly high temperatures and melting of glaciers.
"The whole thing is combining to form these catastrophic disasters that we are now seeing at a regular basis."
Scientists for years have warned of melting ice and permafrost, which is centuries-old frozen soil, in the Himalayas due to climate change.
One analysis released by ICIMOD , external earlier this year found that the Hindu Kush Himalaya glaciers are now losing ice at double the rate since 2000. This in turn exacerbates dangers such as flooding.
Dr Cook pointed out that there have been similar incidents of floods and landslides caused by glacier collapses in India, Switzerland and Italy in recent years.
In 2021, a large chunk of a Himalayan glacier collapsed in the valley of Chamoli in northern India, sending a cascade of debris and water downstream that killed 200 people. Scientists later estimated that the impact of the collapse was equivalent to 15 atomic bombs.
Even in Nepal, the same region saw flooding last year caused by a glacial lake in Tibet bursting.
Mohd Farooq Azam, a specialist with ICIMOD, said the increasing frequency of hazards in the cryosphere - or the frozen parts of the Earth - "is becoming more visible than ever before. The pace of change is so rapid."
While it is difficult to say whether any single event is caused by climate change, "when you look at the pattern of these events, you do think climate change could have an impact," said Dr Cook.
"The logic is that with a warming planet, you will see shrinking glaciers, lots of snow melt. The permafrost that is partially holding some mountainsides together is warming and thawing and degrading."
"So you are going to get more landslides, debris flows, glacial meltwater, lake outbursts, glacier collapses, because climate warming ultimately destabilises these high mountain environments."
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