Like much of the world, I watched and rewatched the videos of the catastrophic flood that have been filtering out of Nepal since Wednesday with horror and sadness.
But unlike many, I have an intimate knowledge of this part of the world, where I first traveled as an undergraduate geology student in 2005 to study how glaciers in the Khumbu region disintegrate into hazardous lakes. I’ve made six expeditions there over the last two decades.
Because of the remote location, scientists don’t know what precisely triggered Wednesday’s disaster — yet. What’s clear at this point is that a giant hunk of glacier peeled off the side of the mountain, tumbled down 4,000 vertical feet and slammed into a tributary of the Bhote Koshi River in China.
Originally misidentified as an earthquake, the mountain-shaking event registered as a 5.2 magnitude seismic event. Moments later, a wall of rocks and water blasted through the Gyriong Port building as the flood barreled down the valley.
The source of the water remains a mystery.
In many catastrophic floods in the Himalaya, glacier lakes are to blame. When Nepal’s glaciers were healthy, they bulldozed down mountains and pushed up piles of rocky debris hundreds of feet high to form features glaciologists call moraines.
As the planet has warmed and temperatures across the Himalaya have soared, glaciers have shriveled, and those moraines have trapped meltwater. That has created lakes that can grow until they replace the glacier feeding them — or until a disturbance causes them to drain catastrophically.
As I reported last year with the Times climate science reporter Raymond Zhong, a landslide smacked into a glacier in the Khumbu region in 2024, launching a flood wave over the moraine dam that sent 100 million gallons of water through the heart of Thame, a Sherpa village nearby. Luckily, no one died. But the flood destroyed the village’s school, medical clinic, several lodges and homes.
Data shows that glacial failures — the scientific term for a category of events that includes ice avalanches, glacier detachments and rock-ice avalanches — have happened dozens of times since 1900 in the China-Nepal border region that was the site of this week’s devastation.
While Wednesday’s flood had a massive landslide, there didn’t appear to be any glacier lakes below it, making it a bit of an anomaly, certain to be studied intensely in the coming years. Some scientists have speculated that the impact of a piece of glacier falling 4,000 feet could have pulverized the ice into water.
But we know that there will be more floods — and some may be far worse than this week’s catastrophe.
Unless the carbon dioxide emissions that are heating the planet are curbed, scientists project warming across High Mountain Asia, a broad region encompassing the Himalaya, Karakoram, Pamir, Hindu Kush and Tien Shan, will erase as much as 75 percent of the remaining glaciers by 2100.
Meltwater pooling behind moraines and in depressions exposed by the ice will transform thousands of glaciers into large lakes, each one a potential ticking time bomb for vulnerable communities downstream.
Over the last 20 years, I’ve watched this process in action. On my earliest trips to the region, I would scamper up the Ngozumpa, Nepal’s longest glacier, to my research sites, where I would step across a long, ribbon-shaped puddle, near the Sherpa village of Gokyo.
On return trips to Nepal over the next 20 years, I watched that puddle begin to gobble its way across the glacier. In 2025, when I returned to the region to report on glacier flooding hazards for The Times, it had become a lake, nearly 750 feet wide and 850 long.
The flood in Thame a few years ago showed that smaller lakes can be dangerous on their own, but they also play a critical role in creating larger lakes — and potentially larger flood hazards. The Ngozumpa, like many of Nepal’s glaciers, is blanketed by thick, rocky debris shed by adjacent mountains. Glaciologists once thought this debris would insulate ice from melting, but now we know that lakes can supercharge melt. Silt and mud trapped by meltwater turn lake water a dark gray color. And just as wearing a black T-shirt on a sunny day makes you warmer, that dark color heats up the water in the lakes to melt the surrounding ice.
Warm water can create caves that turn glaciers into Swiss cheese, rotting them from the inside out. When these tunnels collapse, they create new depressions that become lakes. And so, like tumors in a cancer patient, rapid lake growth can quickly consume a glacier.
The Imja glacier, another one I’ve visited many times during my research in the Khumbu, is a dramatic example. Aerial images from the 1950s show that the lake had but a handful of small lakes on the surface. By 1978, a single lake consumed nearly the entire glacier tongue. By 2021, the lower Imja glacier had been converted into a lake that was 1.7 miles long, 0.4 miles wide and more than 400 feet deep. The only thing holding it back is an unstable dam of ice, sand and rock.
Imja’s lake is one of the largest, and once one of the most dangerous, glacier lakes in Nepal. In 2016, the Nepali Army excavated an 11-foot-deep trench to lower the level of the water — and the magnitude of the flood that would be unleashed if it were to burst. An outburst in 1985 that flooded Thame had prompted the move. Thirty eight years later, I was there to document the effects of a second burst.
