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Explained: Study Says 15 Million Wordlwide At Risk Of Glacial Floods, What Are These Floods?

India ranks high in the threat list not so much because of the physical setup but because of 'a huge number of people downstream'.

An ISRO study of satellite images over 15 years shows over 75% of Himalayan glaciers in retreat
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Around 15 million people across the world are living under the threat of glacial floods, with Indians ranking high in the list, according to a study. 

More than half of these people are in just four countries: India, Pakistan, Peru, and China. The study is published in Nature Communications journal. 

A second study, awaiting publication in a peer-reviewed journal, catalogs more than 150 glacial flood outbursts in history and recent times. 

What are glacial floods?

The formal term for glacial flood is glacial lake outburst floods. These are sudden floods —outbursts— from lakes formed by melting glaciers. 

The European Geosciences Union (EGU) explains: "Since the early 20th century, thousands of glacier lakes have been forming behind natural dams in high mountains following glacier retreat. Researchers found that some of these lakes abruptly released pulses of water and sediment with disastrous downstream consequences. This abrupt release came to be called a glacial lake outburst flood: a low-frequency, high-magnitude event with major geomorphic consequences."

The EGU notes that that over 2,800 glacial floods have bene observed worldwide so far that have caused 12,000 deaths. It further notes that Central Asia is the most affected region in the world, followed by South America.

What have the recent studies found?

The second pre-print study notes that glacial floods are a threat Americans and Europeans rarely think about, but 1 million people live within just 6 miles (10 kilometers) of potentially unstable glacial-fed lakes. It catalogs over 150 such floods. 

One of the more devastating floods was in Peru in 1941 and it killed between 1,800 and 6,000 people. A 2020 glacial lake outburst flood in British Columbia, Canada, caused a tsunami of water about 330 feet (100 meters) high, but no one was hurt.

A 2017 glacial outburst flood in Nepal, triggered by a landslide, was captured on video by German climbers. Alaska's Mendenhall glacier has had annual small glacial outburst floods in what the National Weather Service calls “suicide basin,” since 2011, according to study lead author Caroline Taylor, a researcher at Newcastle University in the United Kingdom.

Heavy rains and a glacial lake outburst flood combined in 2013 in India to kill thousands of people. A 2021 deadly flood in India that was initially attributed to a glacial lake outburst wasn't caused by one, studies later found.

Glacial floods and climate change

Scientists say so far it doesn't seem like climate change has made those floods more frequent, but as glaciers shrink with warming, the amount of water in the lakes grows, making them more dangerous in those rare situations when dams burst.

“We had glacier lake outburst floods in the past that have killed many many thousands of people in a single catastrophic flooding event. And with climate change glaciers are melting so these lakes are getting bigger, potentially getting more unstable,” said study co-author Tom Robinson, a disaster risk scientist at the University of Canterbury in New Zealand.

Dan Shugar, a geoscientist at the University of Calgary who wasn't part of the two studies, said much of the threat depends simply on how many people live in a glacial flood zone.

Shugar said, “In a warming world we certainly expect more and larger glacial lakes. But the threat that these lakes might pose critically depends on where people are living and what their vulnerabilities might be."

Robinson said what's different about his study is that it's the first to look at the climate, geography, population, vulnerability and all these factors to get “a good overview of where in the world is the most dangerous places'' for all 1,089 glacial basins.

Pakistan tops the list, India also ranks high

At the top of the list is Khyber Pakhtunkhwa basin in Pakistan, north of Islamabad.

“That's particularly bad,” Robinson said. “Lots of people and they're very, very vulnerable” because they live in a valley below the lake.

The trouble is that scientists are focusing too much attention on the Pakistan, India, China and the Himalayas, often called High Mountain Asia, and somewhat ignoring the Andes, Robinson said.

The second and third highest risk basins are in Peru's Santa basin, and Bolivia's Beni basin, the paper said.

After the deadly Andes flood in the 1940s that region “was sort of a leader” in working on glacial flood outburst threats, but in the last decade or so, High Mountain Asia has taken over because of the high population, said University of Dayton geology professor Umesh Haritashya, who wasn't part of the studies.

India ranks high in the threat list not so much because of the physical setup but because of “a huge number of people downstream.”

Three lake basins in the United States and Canada rank high for threats, from the Pacific Northwest to Alaska, but aren't nearly as high as areas in Asia and the Andes with few people in the danger zone. They are in Alaska's Kenai Peninsula — distinct from the Mendenhall glacier near Juneau — northeast Washington and west central British Columbia.

“This ranking is a good checklist for further research,” said Oliver Korup of the University of Potsdam in Germany, who co-authored the list of glacial lake outburst floods. 

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(With AP inputs)

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