Changing climate expected to increase landslide risks in Alaska
December 6, 2023
As Wrangell continues to deal with the landslide that killed six people, Alaskans face a long-term challenge: How to prevent tragedies in the future as mountainous regions of the state become more unstable.
“These landslides affecting Alaskans are going to keep happening, and we need to get out in front of them,” said Gabriel Wolken, manager of the climate and cryosphere hazards program at the Alaska Division of Geological and Geophysical Surveys.
The Nov. 20 landslide in Wrangell was the third deadly and rain-triggered landslide in Southeast in the past eight years. A 2020 landslide in Haines killed two people and caused extensive property damage. A 2015 landslide in Sitka killed three people.
With the changing climate, Wolken said, “we would expect increases in temperature to bring about increases in precipitation and increases in extreme events,” leading to more landslides.
Rick Thoman, a scientist with the Alaska Center for Climate Assessment and Policy at the University of Alaska Fairbanks, gave a similar warning.
The rain-triggered landslides in Southeast Alaska are “certainly climate-change related,” he said. Warmer oceans mean a warmer atmosphere that results in more water being poured over the landscape, including in more extreme events, he said.
“These extreme precipitation events, whether it’s a single storm or it is a wetter season, this is exactly what we would expect in a warming world,” he said.
The storm that triggered the Wrangell slide — along with several others in Southeast that day — dumped more than 3 inches on the community, Thoman said. While that is not an unusual amount of rain for Wrangell, it followed an unusually wet fall that had already saturated the ground, he said. And the storm brought high winds that were unusual and may have contributed to the landslide risks, he said.
Even without climate change, Wolken said, landslides are ever-present in Southeast. The region, covered by mountains and a rainforest, with topography carved by glaciers and permafrost and with occasional earthquakes, is by nature susceptible to shallow slides known as “debris flows” that result from the separation of surface soil and rock from the layer above bedrock, he said.
But climate change, by increasing the amount of water that loosens debris, adds to the vulnerability, according to Wolken and other scientists.
Precipitation in naturally rainy Southeast Alaska increased by 8% in a seven-decade period, and statewide the increase was even more, at 17%, according to research by UAF scientists. The trend is on pace to continue in the future, University of Alaska Southeast scientists said. Also on trend to increase are extreme rain events like the record rainfall that led to the fatal Haines landslide in 2015, according to UAF scientists.
Though Southeast Alaska is a landslide hotspot, there have been deadly slides in mountainous areas around the world and in other parts of the United States. The 2014 Oso landslide in Washington state that killed 43 people was caused at least in part by heavy rain, according to the U.S. Geological Survey.
Events have prompted action in Congress. A bill passed in 2020 and co-sponsored by Alaska’s two U.S. senators, established the National Landslide Hazards Reduction Program within the USGS.
Southeast Alaska is a global hotspot for another type of landslide triggered by thaw of high-altitude permafrost, melt of glaciers that buttress bases of mountains or a combination of those forces.
Numerous thaw-triggered landslides in the high mountains of northern Southeast Alaska and adjacent parts of Canada have cleaved off massive amounts of debris, sometimes into marine areas, where they created local tsunamis.
Among them was a 2015 mountainside collapse in a remote part of Wrangell-St. Elias National Park and Preserve that was the largest non-volcanic landslide on record in North America and caused a localized tsunami in a site called Taan Fjord that reached to about 600 feet up the opposite mountain slope, making it one of the highest recorded anywhere in the world in the past century.
None of those huge slides affected people, but there are reasons to worry about future events, Wolken said. “While these are happening in remote locations, the impact of some of these events can be felt many, many miles away. It’s not as if the isolated events will stay isolated always,” he said.
One site with potential for such a major event is in Southcentral Alaska’s Prince William Sound. At a place called Barry Arm, about 60 miles southeast of Anchorage and 30 miles northwest of the coastal town of Whittier, unstable mountain slopes could collapse catastrophically, causing a major tsunami.
The Division of Geological and Geophysical Surveys and other agencies, along with University of Alaska scientists, are cooperating on a focused monitoring program to track rock movement at Barry Arm. That work is “providing us with an incredible amount of knowledge about how large rock slope instability in fjord situations” where permafrost is thawing and glaciers are retreating, Wolken said.
Different types of landslides — those that are induced by thaw and move relatively slowly — have damaged or are threatening to damage important Alaska infrastructure.
A continuing slide, caused by melt of a rock glacier and thaw of underlying permafrost, has forced a yearslong closure at the midpoint of the sole road that runs through Denali National Park and Preserve. The Pretty Rocks slide is the most serious of nearly 150 landslides that have been identified along the corridor of the park road.
Fixing the landslide-damaged Pretty Rocks section of road is an ongoing and complex project, involving a specialized bridge and permafrost-protection devices that are estimated to cost about $100 million.
In the Brooks Range region above the Arctic Circle, thawing permafrost has sent slow-moving slides of mixed soil, rock and vegetation creeping toward the Dalton Highway, the only road to the North Slope oil fields. Already, the state Department of Transportation has had to reroute a section of road to avoid those slides, known as “frozen debris lobes.” University of Alaska Fairbanks scientists and state highway officials continue to monitor the movements of those lobes.
The rain-triggered disasters like the Wrangell landslide, in contrast to the continuous slides farther north, are sudden and extremely difficult to predict.
“It’s this terrible middle-of-the-night kind of thing, and we don’t know when or where it might happen,” Thoman said. With more precipitation and more intense storms, it adds up to “a very scary situation,” he said.
In some landslide-prone areas of the world, there are sophisticated systems to protect communities. Those involve instruments that constantly measure slope movements, soil conditions and local weather conditions.
“That all takes extensive upfront research, extensive money to maintain instruments for those types of operations,” said Barrett Salisbury, a state geological hazards manager. “And then when those systems are effective, they’re effectively destroyed and need to be rebuilt. It’s a huge effort to do that. But it is possible.”
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