ANCHORAGE (NYTIMES) - Climate change has increased the risk of a huge landslide in an Alaskan fjord that could cause a catastrophic tsunami, scientists said Thursday (May 14).

Warming temperatures have caused the retreat of a glacier that helps support a steep, mile-long slope along one flank of a fjord in Prince William Sound, about 96.5km east of Anchorage. With only a third of the slope now supported by ice, the scientists said, a landslide could be triggered by an earthquake, prolonged heavy rain or even a heat wave that could cause extensive melting of surface snow.

While the slope has been moving for decades, the researchers estimated that a sudden, huge collapse was possible within a year and likely within two decades.

"It could happen anytime, but the risk just goes way up as this glacier recedes," saidDr Anna Liljedahl, an Alaska-based hydrologist with the Woods Hole Research Centre in Massachusetts, who was part of the team.

Alaska's Department of Natural Resources, after being briefed on the findings, issued a statement Thursday afternoon warning that "an increasingly likely landslide could generate a wave with devastating effects on fishermen and recreationalists."

Computer modeling showed that a collapse of the entire slope - roughly estimated to be 500 million cubic metres of rock and dirt, or several hundred times the volume of the Hoover Dam - could cause a tsunami that would start out at several hundred feet high.

About 20 minutes later, when it reached Whittier, a town at the head of another narrow fjord 48km away, the wall of water could still be 9m high and cause extensive destruction.

"As a hazard, it's really worrisome," said another researcher, Dr Hig Higman, who studies geological hazards and runs an organisation called Ground Truth based in Seldovia, Alaska.

Dr Liljedahl said that although their findings have yet to be peer reviewed, "we realised we needed to let people know." She said the researchers hoped that money would be made available for near real-time monitoring of the slope that could provide a warning if a landslide and tsunami occurred.

The fjord, Barry Arm, and other nearby waters are frequently visited by tourist and fishing boats, and the surrounding land is popular with hunters. In good weather, potentially hundreds of people could be in the area.

Although it has a year-round population of only several hundred, Whittier is typically a disembarkation point for thousands of cruise ship passengers headed inland to Anchorage and points north.

Tsunami-inducing landslides are rare but have occurred in Alaska and elsewhere. Perhaps the most famous occurred July 9, 1958, in Lituya Bay on Alaska's south-east coast, when a nearby earthquake caused 30 million cubic metres of rock to slide 60m into the narrow bay.

Scouring of vegetation on a hillside opposite the slide showed that the tsunami reached a maximum height of 520m - essentially a giant splash that is the highest tsunami ever documented. The water then rolled up the bay in a wave that was still about 22m high when it reached the mouth, swamping several fishing boats and killing two people.

More recently, a 2015 landslide at Taan Fjord in a remote area west of Yakutat, Alaska, spawned a tsunami that was initially more than 182m high. And a landslide on the west coast of Greenland in 2017 created a wave, initially more than 91m high, that destroyed much of a nearby fishing village and killed four people.

A slide at Barry Arm would potentially be much bigger in terms of the energy involved. "It's in a whole different class than we've ever even studied after the fact, much less before it happens." Dr Higman said.

The researchers, from 14 organisations and institutions - including Ohio State University, the University of Southern California and the Anchorage and Fairbanks campuses of the University of Alaska - only began studying the Barry Arm area about a month ago, as part of a Nasa-financed project to study land-mass movement across the North American Arctic.

Dr Higman was aware of several areas in Alaska that had landslide risk. But it was his sister, Ms Valisa Higman, an artist, who alerted him to the potential hazard at Barry Arm. Aware of her brother's work, she was on a tourist boat in the arm when she saw the slope, which looked to her as if it had fractures, suggesting that it was slowly sliding. She took some photographs and sent them to her brother.

Dr Higman studied satellite imagery and determined that the slope had indeed been sliding over time. Additional analysis showed that the rate of movement at times had been high: Between 2009 and 2015, the landslide moved downhill about 182m, leaving a large scar.

Chunli Dai, a researcher at Ohio State, showed a connection between the sliding and the movement of the nearby Barry Glacier, which, like glaciers all over the world, has been melting and retreating as the climate warms.

The researchers say that permafrost, or permanently frozen ground, may exist in the area and may be helping to keep the slope stable. Climate change has also been causing permafrost to thaw in many parts of the world; so if it were thawing at Barry Arm that could contribute to the landslide risk. But Dr Higman said he doubted permafrost played much of a role there.

The violent shaking of an earthquake can cause a slope to collapse, and Alaska is among the most earthquake-prone areas of the planet. Whittier, in fact, was heavily damaged by a tsunami during the 1964 Alaska earthquake, the second most powerful ever recorded.

But gravity can cause a slope to fail as well, especially if it becomes saturated by water during times of heavy rainfall or a heat wave that melts surface snow, with the water acting as a lubricant, making it more likely for the land to be pulled downhill by gravity.