Here’s what one team of scientists thinks happened in Nepal’s deadly flood wave

By Laura Paddison, CNN

(CNN) — The catastrophic August flooding on the Nepal-China border was influenced by a complex cascade of factors, many of which were worsened by human-driven climate change, according to a new scientific analysis by more than 20 international experts.

The region is still grappling with the aftermath of the devastating rock-ice avalanche that tore through narrow mountain valleys in late August, wiping out towns, roads and bridges, killing at least 1,300 people and leaving more than 5,000 missing.

As rescue attempts transition into recovery operations, and the country begins to reckon with the mammoth task of rebuilding, scientists all over the world have been working to understand what caused the tragedy.

Here’s what we already knew: on August 26, a vast chunk of mountain rock and glacier ice around 2,000 feet wide sheared off Langtang Lirung mountain, plunging roughly 7,000 feet into the river below, releasing the same amount of energy as a magnitude 5.2 earthquake.

It fell with enough force to melt the ice almost instantaneously. The torrent also picked up moisture-laden sediment and likely crashed into buried ice on the valley floor, adding to a raging flood wave that thundered more than 20 miles downstream at devastating speed.

It was “an unprecedented disaster” in the Himalayas, said Walter Immerzeel, a mountain hydrologist at Utrecht University in the Netherlands.

But why did the avalanche happen in the first place?

The team of glaciologists and climate scientists with World Weather Attribution, or WWA, a network which examines the role climate change plays in extreme weather events, released its analysis Wednesday in a comprehensive attempt to answer the question.

The disaster was not triggered by a single extreme weather event, but rather by a slew of processes acting together, compounded by decades of climate warming caused by fossil fuel pollution, according to the report, which used peer-reviewed methods but is not yet itself peer reviewed.

The scientists found the mountain may have been preconditioned for collapse by a 7.8 magnitude earthquake in April 2015, which triggered a rock and ice avalanche from the south side of the mountain, potentially weakening the slope. Since then, the mountain has experienced increased rates of landslides, the report found.

The scientists then looked at the role climate change could have played and found several processes acting together.

Climate change is pushing up the “zero degree” line, the point in the atmosphere where temperatures hit 0 degrees Celsius (32 Fahrenheit), by more than 320 feet a decade in the Himalayas.

Rising temperatures are weakening the permafrost in Nepal’s high mountains — the mix of ice, rock, sand and soil that can act like glue — especially around the elevation where the collapse happened, the report found.

Thawing permafrost “reduces the rock strength and allows more water to penetrate into the slope,” Immerzeel said.

Glacier change is another important factor. The Langtang-Lirung Glacier has undergone substantial thinning and shrinking over recent decades, particularly after 2010, making the slope less stable and producing meltwater able to seep into fractures in the rock, the report found.

The researchers also looked at the weather preceding the collapse. Last fall saw exceptional amounts of snow in the region in October and early November. The following July and August were the warmest on record locally, which could have created large amounts of meltwater, further weakening the slope.

Using historical data and climate models, the researchers found that July and August temperatures in the region are now around 1.5 degrees Celsius (2.7 Fahrenheit) warmer because of climate change. And the region’s annual temperatures have increased by 2 degrees Celsius (3.6 Fahrenheit), significantly above the global average.

The report found less of a clear link between global warming and increased snow and rainfall, due in part to the challenges of measuring and monitoring across the complex, rugged Himalayas.

Overall, however, “there is absolutely no doubt that human-induced climate change played a role here in the preconditioning of the disaster,” said Friederike Otto, professor of climate science at Imperial College London.

The scientists concluded that this collapse was fundamentally different in its size, speed and complexity from anything the region has experienced before. “No existing early warning system could have provided sufficient lead time,” they wrote.

It is “one of the types of events that are absolutely outside of the limits of what we can possibly adapt to,” Otto said. Unless the world rapidly cuts fossil fuel pollution, she added, “we will inevitably see more disasters of this scale.”

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