Many unanswered questions remain surrounding last month's massive, deadly landslide in northwest Washington, but geologists are fairly certain that higher-than-average rainfall played an important role.
Experts with the U.S. Geological Survey say precipitation in the region surrounding Oso during February and March was 150 to 200 percent of the long-term average, likely helping to destabilize the 10 million cubic yards of sediment that buried 30 homes on March 22. According to Seattle television station KING 5 News, 33 people have now been confirmed dead in the aftermath.
The landslide was unusual because it surged 0.7 mile from the base of the slope, a distance three times farther than experts had predicted.
"It traveled exceptionally far and exceptionally fast, and that's why it was exceptionally destructive," said Jonathan Godt of USGS's Geologic Hazards Science Center in Denver, who was sent to Washington to assess the damage.
The rains that drizzled over the Puget Lowland region over the past several months saturated the soil in the landslide area, Godt explained, weakening the material holding the slope together.
Additionally, the area's geology makes it particularly prone to landslides. Godt described the land as "young, poorly consolidated glacial outwash," meaning that it is made up of sediments around 15,000 years old that are not nearly as solid as other formations.
However, these factors together still don't explain the speed and long travel distance of the Oso mudslide, Godt explained, and geologists will be studying the event for months to come.
"We have some ideas, but I would say they are no more than that at this stage," he said.
Will future extreme rainfall affect future landslides?
It's important to note that even though rainfall in the Oso area was higher than normal, the Office of the Washington State Climatologist has reported that this level of precipitation is not unprecedented.
"There's no reason to think that this event had anything to do with global warming," said Cliff Mass, a professor of atmospheric sciences at the University of Washington. "We had very heavy March precipitation ... but it wasn't like we had some extreme, short-term precipitation."
Reports by the University of Washington's Climate Impacts Group project that the western portion of the state could see more frequent extreme, high-precipitation events by midcentury, but geologists and climatologists have not definitively linked this to an increased likelihood of future landslides.
While he allowed that heavier rainfall in the future would certainly not decrease the frequency of the disasters, "from a landslide perspective, it's very difficult to sort out the climate signal, if there was one, from other signals," Godt said.
Other experts agreed. Eric Salathe, an assistant professor at University of Washington, Bothell, and a part of the university's Climate Impacts Group, said that "the pieces are there, and it is something that we should certainly be concerned about."
But, he added, "It's hard to go from sort of a generic statement of when it rains, it's going to rain more heavily ... to saying that the risk of landslides will increase in a similar way."
'We are not adapted to our current climate'
David Montgomery, a professor of Earth and space sciences at the University of Washington, added that while brief but heavy downpours could contribute to future landslides, they would look different from the one that took place in Oso.
"High intensity rainfall tends to trigger shallow landsliding," Montgomery said in an email. "Large, deep-seated landslides (like the Oso one) tend to be triggered by longer duration periods of higher precipitation -- like a record setting monthly rainfall, or several years or seasons of above average precipitation."
"So the more rain we get the more slides we may expect, but how the rain is delivered will affect the type of slides one would expect to see being triggered," Montgomery added.
Montgomery and other experts stressed that the kind of efforts needed to prevent similar tragedies in the future need to happen now, climate change or no climate change.
"We can get extreme precipitation even in the current climate, so it's clear to me that we need to better address these hazards, no matter what is happening to the climate," Montgomery said.
Said Mass: "The real story is the fact that people are living where they shouldn't be. We are not adapted to our current climate -- we have people living in floodplains and next to steep slopes that have failed many times."
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