In 2014, a landslide in Oso, Washington, sent 270 million cubic feet of mud through a neighborhood, killing 43 people and destroying 49 homes. The slide covered a 1-square-mile area, and it is the deadliest landslide in U.S. history.

An engineering report found that recent logging may have increased the amount of water on the slope, but the report didn’t single out any one factor as a cause of the slide.

Now, research from Washington State University is showing that landslides on logged forests will become more widespread as the Northwest’s climate changes. The study, modeled on clear-cut lands in the Olympic Peninsula, anticipates the climate of 2045 based on current models, and concludes that there will be a 7 to 11-percent increase in the amount of land that is highly vulnerable to landslides.

Climate change is expected to bring warmer and wetter winters to the Pacific Northwest. It could also bring more frequent extreme rainfall events.

“Logged landscapes become more susceptible to landslide activity under climate change,” said State of Washington Water Research Center Associate Director Professor Jennifer Adam.

The study found that vulnerable slopes tend to be at elevations over 1,600 feet, with slopes or 40 or more degrees, and have talus or sandy soils.

The study “is telling people, if you are cutting trees on this slope, it might be okay today. But in the future, it might not be, so plan according to that,” said study lead author Muhammad Barik. “If you do logging in this area without considering future projections, it might become susceptible to landslides.”

The hydrologic model the researchers used considered variations in topography, soil, land cover, and subsurface moisture. They also factored in meteorological data, a digital landslide database, and satellite imagery.

“The combination of warming, precipitation, and less snow means more liquid precipitation, which will then sit in the soil and keep it wet and unstable,” said Adam.

The researchers used two greenhouse gas emission scenarios from the United Nations’ Intergovernmental Panel on Climate Change. The scenario predicting the lowest emissions saw an average 7.1 percent increase in the area highly susceptible to landslides. The scenario predicting the highest emissions increased the risk by 10.7 percent.

“We’re giving you a tool to see into the future,” said Barik. “Most of the landslide studies are based on historical data. Here, along with historical data, we also used climate models so you can look at future projections.”

Hopefully, the data from Washington State University will sound an alarm and get land managers thinking in the long term about what trees to harvest and from where. It may even promote the wider use of land management scenarios like planting trees to replace the ones that have been harvested in order to reduce the risk of future landslides.

This data could also be useful for other forested regions all across the world that are going to get warmer and wetter under climate change.

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