Nepal disaster was a mountain chain reaction

Scientists say earthquake damage, melting ice and record warmth combined to create an unprecedented flood wave

The devastating flood wave that tore through Nepal's Himalayas was not caused by one freak event.

Instead, scientists say it was the result of a terrifying chain reaction that developed over years — and was finally unleashed when a huge section of mountain collapsed.

More than 20 international scientists have examined the disaster for World Weather Attribution, concluding that a combination of geological damage, shrinking ice, thawing permafrost and exceptional warmth helped prepare the mountain for collapse.

The analysis has not yet been peer reviewed, but the researchers used established attribution methods to investigate what happened.

At least 1,300 people are reported dead and more than 5,000 remain missing as Nepal continues the enormous task of recovery.

It started years before the flood

On August 26, an estimated 2,000-foot-wide section of rock and glacier ice broke away from Langtang Lirung mountain.

It plunged roughly 7,000 feet into the valley below, releasing energy comparable to a magnitude 5.2 earthquake.

The impact melted ice almost instantly and generated a torrent of water, rock and sediment which thundered more than 20 miles downstream.

But scientists believe the mountain may already have been weakened.

The devastating 7.8-magnitude earthquake that struck Nepal in 2015 triggered an earlier rock-and-ice avalanche on the mountain's southern face. The new analysis suggests that damage may have left the slope increasingly vulnerable to further landslides.

Then the climate changed the mountain

The researchers identified several additional factors.

Temperatures around the collapse zone have risen substantially, while the Himalayan “zero-degree” line — the altitude where temperatures reach freezing — has been climbing by more than 320 feet every decade.

That matters because permafrost acts like a natural glue holding rocks and soil together.

As it thaws, water can penetrate cracks and weaken the rock.

At the same time, the Langtang-Lirung Glacier has been thinning and shrinking, particularly since 2010. The loss of ice and increased meltwater may have further destabilised the mountainside.

Then came an extraordinary weather sequence.

The region experienced unusually heavy snowfall in October and November last year, followed by the warmest July and August on record locally.

Scientists estimate that climate change has made July and August temperatures in the region around 1.5C warmer than they would otherwise have been.

A disaster beyond existing warnings

The scientists' conclusion is not that climate change alone caused the collapse.

Rather, they describe a series of processes interacting with one another — geological damage, glacier retreat, thawing permafrost, snow accumulation and exceptional warmth — until the mountainside finally failed.

That distinction matters.

The report found a much less certain connection between climate change and changes in rainfall and snowfall because reliable weather measurements are particularly difficult across the remote Himalayas.

But the researchers say there is strong evidence that human-caused warming contributed to the conditions that weakened the mountain.

Nothing could have stopped this one

Perhaps the most sobering finding is what happened after the collapse.

The scientists concluded that the event was so large, fast and complex that existing early-warning systems could not have provided enough time to respond.

This was not simply a flood that rose gradually.

It was a mountainside collapsing thousands of feet almost instantaneously, creating a destructive wave that raced down a narrow Himalayan valley.

And that leaves Nepal facing a grim reality.

Some disasters can be forecast.

Others can be prepared for.

This one, the scientists say, was effectively outside the limits of what current warning systems can handle.

Source: CNN / World Weather Attribution