When a "Himalayan tsunami" roars down from the rooftop of the world, water from an overflowing glacial lake obeys gravity. Obliterating everything in its path, a burst is predictable only in its destructiveness.



"There was no meaning in it," one person who withstood the waters in India's Himalayas told a Public Radio International reporter. "It didn't give anyone a chance to survive."



Christian Huggel, a professor at the University of Zurich in Switzerland who specializes in glaciology and geomorphodynamics (the study of changing forms of geologic surfaces), said thousands of cubic meters of water moving down a mountain "is really quite destructive and it can happen suddenly."



That water comes from glacial lake outburst floods, or GLOFs, which are increasing in frequency as climate change increases the rate of glacial melting. This catastrophic lake drainage occurs wherever there are glaciers in places such as Peru and Alaska.



The most devastating GLOFs occur in the Himalayan regions of India, Pakistan, Afghanistan, Bangladesh, Bhutan, India, Myanmar, Nepal and the Tibetan Plateau. When combined, the area has the third-largest accumulation of snow and ice after Antarctica and the Arctic.



Melting glaciers



In the Himalayas, climate change melted glaciers by a vertical foot and half of ice each year from 2000 to 2016, according to a study released in June's Science Advances by Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.



That is twice the rate of melting from 1975 to 2000.



Local people have noticed the change. In a 2016 interview from the Everest basecamp, Dr. Nima Namgyal Sherpa told VOA that in the past, the glacial streams in the mid-Everest region started flowing in May, but the Sherpas now see the flow beginning in April.



That melted snowpack seeps down to fill mountainside indentations to form glacial lakes. As global warming accelerates the melting, the lakes are expanding, as is their number and threat, monitored in some areas with automated sensors and manual early warning systems by army and police personnel with communication gear.



"Bigger lakes may increase the risk of catastrophic dam failure," Joseph Shea, a glacier hydrologist at the University of Saskatchewan in Saskatoon, Canada, told the magazine Science.



The retreating ice of the Pastoruri glacier is seen in the Huascaran National Park in Huaraz, Peru, Aug. 12, 2016. The melting of glaciers has put cities like Huaraz at risk of what scientists call a “glof,” or glacial lake outburst flood.

Today, there are more than a thousand glacial lakes on the Tibetan Plateau, with more than 130 larger than 0.1 square kilometer in Nepal alone. The lakes threaten the livelihoods and lives of tens of thousands of people who live in some of the world's most remote areas.



On June 12, 2016, a GLOF near Mount Everest sent 2 million cubic meters of water toward the Nepalese village of Chukhung, which lost just one outhouse to the torrents, in part because scientists warned residents in the area about the approaching danger.



Weeks later, on July 5, a GLOF near the village of Chaku registered on seismometers, which had been installed after an earthquake the year before, as a "huge pulse of energy," Kristen Cook, a geologist at the GFZ German Research Center for Geosciences, told EOS, an online site that covers earth and space science news.



Examining satellite images, Cook and her colleagues found the GLOF moved boulders as large as 6 meters in diameter.



Early warning systems



This year, on July 7, a GLOF early warning system of weather monitoring stations and river discharge sensors saved lives in Pakistan's Golain Valley, which has more than 50 glaciers and nine glacial lakes.



The event destroyed villages, roads and bridges, but there were no reported deaths. A shepherd located upstream from the valley called authorities to report the burst, which gave communities downstream as much as an hour to evacuate.



"Our standing crops [and] apple and apricot orchards have been completely destroyed," Safdar Ali, whose shop was heavily damaged as the water swept away livestock, stored grain, irrigation channels and micro hydropower plants, told Reuters.



"I see no loss of human life this time as a positive," Amanullah Khan, assistant country director for the U.N. Development Program (UNDP) told Reuters. "It shows our training has been a success."



The UNDP program, which helped establish flood protection systems in the area starting in 2011, has installed small-scale drainage systems and mini-dams, and taught people in the remote region survival skills, such as simple first aid, because the arrival of skilled emergency help can be delayed by the rugged topography.



The International Center for Integrated Mountain Development (ICIMOD) and other international groups are setting up early warning systems for glacial lakes in Nepal.



Local governments are taking preventive measures, such as removing loose rocks and debris that make the bursts of water even more destructive. Authorities are also draining glacial lakes to reduce the amount of water released by a breach, and they are discouraging settlement in GLOF hazard zones.



"If the lakes burst above the villages up in the Everest area, up between 12,000 to 13,000 feet, there are villages all the way downstream and they will wipe [away] some of these villages," said Norbu Tenzin Norgyal, whose father, Sherpa Tenzin Norgyal, summited Mount Everest with Sir Edmund Hillary in 1953. "The danger is real."