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

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, shaking in the region was initially reported by US Geological Survey as a 4.4 magnitude earthquake along the Nepal-China border, north of Kathmandu. Around the same time, an “avalanche” of ice and rock — more like a 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. USGS later clarified that the shaking was not an earthquake, but was rather caused by this exceptionally powerful landslide itself, which registered as the equivalent of a magnitude 5.2 earthquake. In other words, an earthquake did not cause the landslide. The landslide caused an earthquake. It will take weeks to establish the exact causes of the flooding, but a team of international scientists believes a huge chunk of glacier sheared off and fell into the valley below. “What is hard to reconcile is the the gargantuan volumes of water that we see in these these absolutely horrific videos,” said Daniel Shugar, a geologist at the University of California, who’s part of the group. Their theory is that a rock landslide happened at the same time, creating a rapidly moving mix of ice and rock, which cascaded down the mountain. As it tumbled down, it also picked up sediment that was already water-logged because it’s monsoon season. Landslides can be caused by a number of factors, but 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. Warmer temperatures may have hastened the collapse that led to Nepal’s deadly flooding, with melting snow able to seep into cracks in the glacier and mountain rocks, weakening them, Shugar said. But it will take weeks, even months, to pin down exactly what happened, he added. 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 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.