The cause of the powerful floods that carved a devastating path across Nepal and Tibet, engulfing towns, roads and bridges, is still unclear, but as the planet warms, Nepal has become a hotspot for climate disasters.
Vast bands of ice are melting, making rocky mountain faces unstable, reducing solids and increasing fluidity. When glaciers turn to water and form huge lakes at high mountain elevations, summer melt can release these lakes downstream. On top of that, climate change is increasing extreme rainfall that can cause deadly flooding.
Scientists are still working to determine exactly what caused the disaster, but here’s what we know (and what we don’t know) so far.
Possibly: Flood caused by landslide
At 8:37 a.m. local time, a magnitude 4.4 earthquake struck north of Kathmandu along the Nepal-China border, the U.S. Geological Survey reported.
Around the same time, an “avalanche” (more like a landslide in this case) of ice and rock destroyed the mountainside and flowed, often through deep canyons, into the Rende Khola River, a tributary of the Bhote Kosi River, which flows from China to Nepal.
Whether an earthquake caused the landslide or whether the earthquake-like shaking was caused by the landslide itself is still being investigated, but a group of scientists investigating the cause believe it is the latter.
Daniel Sugar, a geologist at the University of California who was part of the group that reviewed the satellite images, said the reported earthquake was “probably seismic energy released by a landslide.”
The group believes a huge chunk of the glacier was sheared off and fell into the valley. The high volume of water is also thought to have caused rock landslides, creating a rapidly moving mixture of ice, rock and sediment.
The landslide likely “triggered a rapid rise in water levels downstream,” said Saswata Sanyal of the International Center for Integrated Mountain Development in Kathmandu, a regional climate agency.
“These are cascading hazards,” Sanyal said, adding that what happens on the iceberg could quickly flood towns downstream.
Scientists say landslides can be caused by a variety of factors, but climate change is often a contributing factor. Nepal, located in the rapidly warming Himalayas, is home to thousands of glaciers, which are melting and becoming unstable as temperatures rise.
Nepal is particularly vulnerable to “glacial lake outbursts,” where vast lakes formed by melting glaciers fill up and break through the land and ice they dam up, sending water and debris flows down steep mountainsides.
“These glacier dams are no different than the dams that were built,” Tom Robinson, a senior lecturer at the University of Canterbury in New Zealand, previously told CNN. “If you take the Hoover Dam, for example, there’s a huge lake behind it, and all of a sudden you remove the Hoover Dam, that water has to go somewhere, and it’s going to flow down the valley in huge flood waves.”
Recent studies show that the rate of ice loss from glaciers across the Hindu Kush Himalayan region, including Nepal, has doubled since 2000, posing significant risks to the approximately 2 million people living downstream.
Similar floods in the same area last year were found to have been caused by the bursting of a glacial lake in neighboring Tibet, but experts have not suggested what role, if any, the bursting played in the disaster.
Some experts believe that this is highly unlikely. “So far, satellite images have shown that there are no large lakes upstream of the flood path, so a glacial lake outburst can be ruled out,” Wolfgang Schwangardt, a professor of geomorphology at Germany’s Frei University Berlin, told CNN.
Recent weather conditions may not have had a significant impact on flood disasters in Nepal.
No widespread torrential rain has been observed near the region over the past week, according to a CNN Weather analysis of satellite and weather station data.
However, summer is the wettest period of the year in the region due to monsoon storms, some of which have brought rain to the region over the past week. The combination of seasonal rain and snowmelt in recent months could lead to even richer river flows.
