What we know — and don’t know — about what caused the Nepal flood wave

By Laura Paddison, Angela Fritz, CNN

(CNN) — The cause of the powerful torrents of floodwater that carved a devastating path through Nepal and Tibet — swallowing towns, roads and bridges — is not yet clear, but Nepal is a hotspot for climate disasters as the planet heats up.

Vast swaths of ice are melting and rocky mountainsides are destabilizing, becoming less solid and more fluid. As glaciers turn to water and create massive lakes in high mountain elevations, summertime melting can release these lakes downstream. On top of it all, climate change is leading to more extreme rainfall that can trigger deadly flooding.

Scientists are still working to determine exactly what caused the disaster, but here’s what we know (and don’t know) so far.

Likely: A landslide-triggered flood

At 8:37 a.m. local time, a 4.4 magnitude earthquake was reported by US Geological Survey to have struck along the Nepal-China border, north of Kathmandu.

Around the same time, an “avalanche” of ice and rock — more like landslide in this case — tore down a mountainside and into the Lhende Khola River, a tributary of the Bhote Koshi River, which flows from China into Nepal, often through deep gorges.

Whether the quake triggered a landslide, or the earthquake-like shaking was caused by the landslide itself, is still being investigated, but a group of scientists investigating the causes believes it was the latter.

The reported earthquake “was probably seismic energy released by a landslide,” said Daniel Shugar, a geologist at the University of California, who’s part of the group which has pored over satellite images.

The group believes a huge chunk of glacier sheared off and fell into the valley. Because of the volume of water, they also suspect a rock landslide also happened, creating a rapidly moving mix of ice, rock and sediment.

This landslide likely “released a sudden surge downstream,” said Saswata Sanyal, of the International Centre for Integrated Mountain Development in Kathmandu, a regional climate organization.

“These are cascading hazards,” Sanyal said, and what happens in the icy mountains can quickly flood towns downstream.

While landslides can be caused by a number of factors, climate change often plays a role, scientists say. Nepal, in the rapidly-warming Himalayan region, is home to thousands of glaciers which are melting and destabilizing as temperatures rise.

Unlikely: A glacial lake outburst

Nepal is particularly vulnerable to “glacial lake outbursts,” where the vast lakes of water formed by melting glaciers become so full that they burst through the land or ice that dams them, sending water and debris cascading down steep mountainsides.

“These glacial dams are no different to constructed dams,” Tom Robinson, a senior lecturer at the University of Canterbury in New Zealand, previously told CNN. “If you take the Hoover Dam, for instance, you’ve got a massive lake behind it, but if you suddenly remove the Hoover Dam, that water has to go somewhere, and it’s going to come cascading down a valley in massive flood waves.”

Ice loss rates from glaciers across the Hindu Kush Himalaya, a region which includes Nepal, have doubled since 2000, according to recent research, with enormous risks for the nearly 2 million people who live downstream of them.

A similar flood in the same region last year was traced to a glacial lake outburst in neighboring Tibet, but experts have not indicated what role, if any, an outburst may have played in this disaster.

Some experts think it’s very unlikely. “So far, satellite imagery suggests that no major lakes are upstream of the flood path, indicating that a glacial lake outburst can be excluded,” Wolfgang Schwanghart, a professor for geomorphology at the Freie Universität Berlin in Germany, told CNN.

Least likely: Heavy rain

Recent weather conditions likely did not play a major role in the Nepal flood disaster.

Widespread intense rainfall hasn’t been observed near the area over the past week, according to a CNN Weather analysis of satellite and weather station data.

However, summer is the wettest time of year in the region because of monsoon-fueled storms, some of which brought rain to the area over the past week. The combination of those seasonal rains and snowmelt in recent months would likely make rivers run fuller.

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CNN’s Simone McCarthy contributed reporting.