Lake Palcacocha in Huaraz, Peru, in May 2022.

Lake Palcacocha in Huaraz, Peru, in May 2022. (Angela Ponce/for The Washington Post)

The Shishpar glacier in northern Pakistan started rapidly thawing during a record heat wave last spring. The melted snow and ice flowed into a nearby ice-dammed lake until water levels grew too high, triggering a large flash flood that wiped out a key bridge and battered a downstream village.

This is not a unique event. Across the world’s iciest regions, communities live with the looming threat of inland tsunamis — massive walls of water moving quickly and forcefully from melting glaciers, known as glacial lake outburst floods.

A study published Tuesday in Nature Communications found that around 15 million people live in danger of such glacial flooding. More than half of those at risk are in only four countries: India, Pakistan, Peru and China.

“It sounds quite dire, but it doesn’t need to be,” said Tom Robinson, a co-author of the study and risk researcher at the University of Canterbury in New Zealand. “With thoughtful investment and careful planning, we can avoid these [flooding disasters].”

Glacial lake outburst floods have existed for as long as glaciers, but the risk has greatly increased in recent decades due to climate change, Robinson said in an interview. A warming planet is melting glaciers more quickly, draining additional water into nearby lakes and causing a bigger flooding event when a breach may occur. The number and total area of glacial lakes worldwide have increased by about 50% since 1990, previous research shows.

Glacial lake outburst floods “pose a globally significant hazard, and that hazard appears to be getting significantly worse with climate change,” Robinson said.

While not all glacial floods are caused by heat waves like the one that triggered the Shishpar flood, climate change is also making extreme heat more common. A study showed that climate change made last year’s record heat in India and Pakistan 30 times more likely to occur and elevated temperatures of the heat wave by about 1.8 degrees Fahrenheit compared to preindustrial times.

Global populations have also skyrocketed, particularly downstream of many glacial lakes. In 1942, a glacial lake outburst near Huaraz, Peru, killed more than 1,000 people. A breach on that glacial lake today, which many fear, could affect more than 100,000 people living downstream.

Taking into account lake conditions and the number of people within 30 miles downstream, the study released Tuesday assessed the areas with the highest danger from glacial lake outburst flooding. They also looked at the level of human development, determined by the United Nations, and a corruption index from nonprofit Transparency International for each location, which could influence how resources are allocated in flood prevention and response.

They found that high mountain Asia, home to the Himalayas, ranked the most dangerous, with 9 million people exposed to 2,211 lakes. Pakistan and China had the highest risk globally, with nearly 2 million and 1 million people exposed, respectively.

The High Arctic, such as Greenland, had the lowest danger. Even though Greenland had the highest number of glacial lakes worldwide, very few people live there and are in harm’s way. The Pacific Northwest, particularly in Canada, also had a large number of glacial lakes, although the population is not as vulnerable.

The most surprising part of the study came in the Andes, Robinson said. Peru ranked third globally in danger risk. Danger overall in the Andes comes in a close second behind high mountain Asia, but the study found the area receives much less attention from the scientific world.

The number of glacial lakes across the Andes increased by 93% over the past two decades, compared to 37% in high mountain Asia. Yet the researchers found the Himalayas accounted for 142 studies in English language journals between 1990 and 2015. Fewer than 100 studies on glacial lake outburst flooding in the Andes have been published in similar journals since 1979, the study said.

“The focus over the last few years on high mountain Asia is good,” said Robinson. “But that shouldn’t come at the expense of other places with really high danger as well, such as the Andes.”

Juan Torres, a risk researcher at National Institute for Research on Glaciers and Mountain Ecosystems in Peru, said he and his colleagues have published many Spanish-only studies in four sub-basins in the region and plan to publish more in the upcoming year. He agrees that the area is not well-researched in general and wants more collaboration between his organization and other universities around the world.

“Fortunately, this type of hazard can be identified and at least we could know where it will occur,” Torres, who was not involved in the new study, said via email. “We would need to carry out many more studies to cover the largest number of lagoons potentially dangerous due to overflow.”

Dirk Hoffmann, a former director of the Bolivian Mountain Institute who has published studies on Andean glaciers, said some parts of the region, such as the Cordillera Blanca region, have also been better researched than other parts, such as the high Andes in Bolivia — and that there must be more study of potentially dangerous glacial lakes.

It is important to identify “possible future sites of those lakes (as global warming worsens, glaciers will continue to melt, new lakes will be forming),” Hoffman, who was also not involved in the study, wrote in an email. “If the aim is to prevent damage or even death from [glacial lake outburst floods], the only way is to look at concrete local conditions and realities.”

Despite the potential risks, the researchers say glacial flood disasters can still be averted with proper preparations and responses. For instance, early warning systems in Nepal and Bhutan alert people further downstream to move to safer spot once a glacial lake breach occurs. Although early warning systems aren’t too helpful for those who live closer to the lake, said Robinson.

Also in Nepal, Bhutan and Peru, governments and researchers have implemented mechanisms to lower the glacial lake levels to prevent breaching. Researchers have proposed to do another draining at Lake Palcacocha in Peru, the site of the 1942 glacial outburst, but the cost is a whopping $4 million.

Study co-author Caroline Taylor said more effective land planning downstream of glacial lakes can also help by preventing people from building in vulnerable locations in the first place. However, this would be complicated for many existing communities.

“None of these options will work on their own, and what is appropriate and works in one location may not work in another,” said Taylor, a glaciologist at Newcastle University. “It’s really now about looking at the local-level and finding the appropriate measures for the threatened populations.”

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