Tsunami of dirt: What caused Nepal's deadly glacial flood?

More than 150 lives were lost and hundreds are still missing after a massive wave of water, ice, mud, and rock thundered down the Bhote Koshi River valley in Nepal, more than 170 kilometers away from its source. The route serves as a crucial link between Kathmandu and Lhasa. The disaster struck with little notice, as footage of the debris-filled flash flood is both shocking and not entirely unexpected.

Experts in glaciers have observed their accelerated retreat worldwide due to melting, resulting in potential disasters. Initially, the landslide triggering the flood was mistaken for an earthquake, registering a magnitude of 4.4 on seismographs. Early estimates suggest the debris wave could have reached heights of up to 30 meters.

Although glacial lake outburst floods have occurred in Nepal before, early analysis suggests that was not the case in this instance. Satellite imagery indicates the landslide may have removed substantial sections of a glacier, leading to its collapse. While the precise cause of the glacier's collapse remains unknown, worldwide glaciers are generally retreating as they melt, potentially resulting in subsequent floods and debris flows.

A smaller flood in the same Nepalese district last year claimed nine lives, while a melting glacier in Switzerland triggered a massive debris flow that obliterated the village of Blatten earlier this year. In Georgia, my home country, a landslide and subsequent flood in a mountain resort last year killed 33 people. However, this incident stands out as one of the most devastating to date.

The high-elevation glaciers of the Himalayas react differently to climate change compared to those at lower elevations or warmer regions. Thawing and degrading permafrost can weaken steep Himalayan slopes, making rockfalls, landslides, and other ground movements more probable. Should an earthquake, rockfall, or other disturbance occur, it could trigger a rapid downslope movement of rock, soil, and debris.

No casualties occurred when the Swiss glacier collapsed because early warning systems were already in place, evacuating residents before the event. As temperatures rise, such disasters may become more frequent or severe in certain regions. Consequently, early warning systems will prove increasingly vital for villages and towns near glaciers or thawing permafrost.

These systems can monitor sudden river level rises, intense rainfall, and seismic waves to alert authorities before disasters strike. However, constructing and maintaining effective early warning systems entails considerable investment, posing a challenge for lower-income and developing nations. The Green Climate Fund recently announced funding for enhanced monitoring systems in Nepal.

Despite this, meltwater from Himalayan glaciers sustains rivers vital to billions residing in Nepal, Pakistan, India, and China. As the climate warms, increased glacier melt can temporarily boost river flows, providing floods in some areas. Yet, as glaciers continue to diminish, their contribution may eventually wane, resulting in less water availability, particularly during the dry season.

Reducing greenhouse gas emissions, investing in early warning systems, and enhancing monitoring in vulnerable mountain regions can aid communities in preparing for such disasters, potentially saving lives and improving chances of adaptation to a changing climate.