In 2017, an enormous landslide struck the Johnson River Valley in Canada’s Northwest Territories, dislodging house-sized chunks of permafrost and carving a 2-mile (3-kilometer) scar into the landscape. The scale of this event was unlike anything seen in the region before. Then, a year later, it happened again in the Redstone River Valley.
The 2018 event was smaller, but still massive. It moved about 160 million cubic feet (4.5 million cubic meters) of debris, roughly a quarter of the amount displaced by the 2017 landslide. Once again, huge slabs of frozen ground dislodged and careened down the slope—a stark contrast to the oozing slurry that’s typical of melting permafrost.
A new scientific investigation into these two extreme events suggests they belong to a new class of landslides that the researchers call “permafrost detachment slides.” The study, published in the journal Geophysical Research Letters, shows that these events could become more common as temperatures rise and wildfire dynamics change.
Upside-down permafrost melt
A team of researchers led by Joseph Young, who completed the study as a PhD student at the University of Alberta, used satellite images, drone surveys, field samples, geophysical measurements, and computer modeling to characterize the 2017 and 2018 landslides and investigate how they happened.
Both were located in the discontinuous permafrost zone of the central Mackenzie Valley, where the continuously frozen ground layer is typically 66 to 131 feet (20 to 40 meters) thick and close to melting at 30.2 degrees Fahrenheit (-1 degree Celsius). Between 1980 and 2020, the permafrost warmed about 0.5 degrees F (0.3 degrees C) per decade. And in 1995, severe wildfires scorched the areas where the two landslides occurred.
Based on their findings, Young and his colleagues believe rising surface temperatures and flash heating from wildfires kickstarted small permafrost movements that created cracks in the ground, allowing water to trickle down deep into the earth and melt the frozen soil from below. This “bottom-up” thawing is atypical for permafrost, which usually melts from the top down, creating a muddy slurry at the surface.
When the base of the permafrost melts, the entire frozen layer can become unanchored and rapidly slide downslope in huge, cohesive blocks. That’s exactly what happened during the 2017 and 2018 landslides. What’s more, it can take 20 years for water trickling down through those upper cracks to reach the bottom of the permafrost layer, so it makes sense that these events occurred about two decades after the 1995 wildfires.
An unstable future
The researchers say they have identified hundreds of similar landslides in permafrost layers less than 131 feet (40 meters) thick and warmer than 30.2 degrees F (-1 degree C) across the Mackenzie Valley. While they have yet to damage any infrastructure in this remote region, the exceptionally large 2017 and 2018 events did dam rivers, creating new lakes.
As climate change pushes temperatures higher and leads to larger, hotter, and longer-lasting wildfires, Young and his colleagues expect these landslides to occur more frequently across Canada and Alaska. Future events will offer new opportunities to study this emerging hazard and understand how it is reshaping this rapidly changing region.
