These floods, officially called Glacial Lake Outburst Floods (GLOFs), are caused by rising temperatures in the mountains. Where glaciers end, they deposit rock debris that was carried away in the ice, forming what is known as a moraine. Sometimes the meltwater from the glacier gets caught behind this debris, creating a lake.
Across the world, glacial meltwater feeds mountain lakes, and as glacial ice retreats due to global warming, many of these bodies of water are becoming larger and more unstable. Rainfall, landslides, earthquakes or increasing water pressure can cause a moraine to shift and break down, releasing a dangerous wall of water down the mountain.
“We can identify the risk hotspots, but we cannot predict when exactly a GLOF event will occur,” says Ashim Sattar, a Himalayan cryosphere scientist at the Indian Institute of Science. More than half of the 15 million people thought to be at risk from glacial lakes live in high mountain Asia, the high mountain regions surrounding the Tibetan Plateau. India and Pakistan alone account for over 5 million of the people at risk. Sikkim in particular is known for lake outbursts, but much of the Himalayan range has also experienced devastating lake floods in the past in Nepal and Bhutan.
The threat will only get worse in the coming decades. Glaciers “are very sensitive to a warming climate,” Sattar says. In a scenario where the world warms by an average of 1.5 degrees Celsius, glaciers in the high mountains of Asia will warm by 2.1 degrees, research estimates; Models predict that global warming will have an outsized impact on ice melting. Given that future warming is very likely to exceed 1.5 degrees, a lot of dangerous meltwater will accumulate in the coming years.
Outbreaks can happen without warning and are difficult to prepare for. There are 9,575 glaciers In the Indian Himalayas alone, ongoing melting has left more than 5,000 glacial lakes with “potentially unstable moraines“-Banks that can burst.
Monitoring the weather and water levels at a lake can provide an early warning of an outbreak, but setting up monitoring stations is time-consuming and expensive. Furthermore, such measures cannot prevent flooding. “As far as I know, sucking out the water or deliberately breaking the moraine dam is the only way to reduce the pressure [of a lake at risk of bursting]says Dhrupad Choudhury from the International Center for Integrated Mountain Development (ICIMOD) in Patan, Nepal. “The same principle is applied to dams during monsoon.”