The austere beauty of the Himalayas is belied by the fragility of its lakes in the face of climate change. The danger is not only to the stability of the environment but to the people who live downstream of these lakes, right in the path of flooding. 

"Sustained glacier melt in the Himalayas has gradually spawned more than 5,000 glacier lakes that are dammed by potentially unstable moraines," report University of Potsdam researchers in Proceedings of the National Academy of Sciences. "When such dams break, glacier lake outburst floods (GLOFs) can cause catastrophic societal and geomorphic impacts."

The lakes form from glacier melt, flowing down mountains and pooling in crevasses. Pushed along by a glacier, accumulations of dirt and rocks held together with ice, called moraines, act as a barrier. However, if this ice melts, the barrier gives way and the water can result in flooding. 

To make a projection of glacier melt in the Himalayas, the team carried out 5.4 billion simulations using topographic and satellite data. They found that 5,000 lakes in the Himalayas have moraines that are unstable in the face of rising temperatures. To add to the cautionary report, they note that the lakes with the largest volume of water are also those with the highest risk of glacial lake outburst floods. 

With the threat of climate change looming large, they say these future hazards needs to be taken into account, especially given increasing trends in population, buildings, and hydropower projects in Himalayan headwaters. 

"Regional projections for the lower Indus, Ganges, and Brahmaputra rivers hold that flood frequencies will rise noticeably in the 21st century, putting the livelihoods of 220 million people at risk," they write. "In Himalayan headwaters, such prognoses have disregarded episodic, but potentially destructive, floods from the sudden emptying of moraine dammed lakes."

Currently, monsoonal floods are among the most destructive natural hazards in the region and nearby mountain ranges. In the future, with little warning, communities downstream may experience greater glacier lake flooding, suffering losses to human life and livestock. The purpose of the simulation, say the team, is to minimize future harm as "GLOFs have gained growing attention in the Himalayas, where these disasters have had the highest death toll worldwide."

Previous studies have noted that about two-thirds of Himalayan glaciers could disappear in the next decade. This doesn't bode well for existing lakes and their stability, particularly those in the Eastern Himalayas – a hotspot at three times the risk of GLOF hazards than the surrounding region.