Could Climate Change Reshape Avalanche Danger in the Sierra Nevada? Scientists Say It’s Complicated

The death toll from the avalanche this week near Lake Tahoe makes it California’s deadliest in modern history — eight people died, and one is still missing. These snowy white landslides are natural during winter in the Sierra Nevada. But climate change is altering winters globally, raising questions about its impact on these mountain rumbles.

Backcountry skiers, guides, and researchers like Richard Bothwell know the Tahoe area like the back of their hand. He’s skied the backcountry peaks and valleys of the Sierra Nevada for three decades and is the head avalanche director for the Outdoor Adventure Club. The Bay Area organization offers professionally guided outdoor trips, including backcountry skiing.

Bothwell is heartbroken over the deaths this week from the avalanche: “It’s a bad day for the backcountry community writ large. It’s a bad day for the guiding community.”

But the fact that an avalanche took place this week wasn’t a surprise. January was practically snowless. What was left turned almost sugary; it rained at some point, and an icy top formed on that snow. Then this week, a big dump of snow fell on top of that icy crust. It was just sitting there, ready to slide off and trigger a powerful avalanche.

“During any gap [in]wintertime, it’s relatively common that the snow surface weakens, and that’s what we experienced,” said David Reichel, executive director of the Sierra Avalanche Center.

Reichel said his group has no real idea how many avalanches barrel down mountainsides in the Sierra Nevada each year because there’s no sensor system to detect them. Researchers know whether an avalanche has stormed down a hill when someone clocks it and reports it.

When someone reports an avalanche, the center and others like it across the country will rate its destructive size. They currently list the Tahoe avalanche as a D-2.5, with the size of a football field and the force to kill or bury a human.

The Sierra Avalanche Center also forecasts dangerous conditions using a separate five-point scale ranging from low to extreme. The center rated the danger on the day of the avalanche as high.

Avalanches occur every winter in the Sierra Nevada, but is human-caused climate change increasing their size or frequency?

“It’s super complicated,” said Benjamin Hatchett, an earth system scientist at Colorado State University who grew up backcountry skiing around Tahoe and researches snow.

Hatchett said that when it comes to climate change, “the fingerprints are everywhere.” But the conditions that led to the Tahoe avalanche are meteorological, not climatological. The rapid change to wet, cold weather brought by winter storms pushed down from the Gulf of Alaska.

“I see no evidence for climate change to play a role, certainly not a first or second order, probably not even further down the list than that,” Hatchett said. “And that’s kind of going back to the setup of the storm.”

In some parts of the world with year-round snow, Hatchett said, there is a signal that climate change could be increasing avalanche danger.

“In large glaciated mountains like the Alps, the Himalaya, and the Andes, the answer is very likely yes because of a warming environment that’s destabilizing snow and ice,” Hatchett said.

But in areas with seasonal snowpacks, like the Sierra Nevada, Hatchett said there isn’t a clear answer.

“This is something we expect to see more of in the future, but we don’t have strong evidence for that happening now,” Hatchett said. “There are absolutely ways that a warming world will statistically change things, and that goes back to the way this winter started with a lot of rain instead of snow. That to me is a signal of a warming world.”

But Hatchett said he sees another issue. When winter snow comes later, like this year, people can get antsy to get outdoors and ski. But when warnings, like about avalanche danger, are issued. He urges extreme caution.

“Thinking about that more strongly could save lives in the future,” Hatchett said.

Hatchett recognizes that holding off is a hard decision, but he said the recent tragedy just might show it’s worth it not to head out into a storm. And more and more people are having to make that hard decision as the sport has become more popular. Especially after the pandemic spurred a surge of interest in these kinds of outdoor adventure sports, said Brenda Giese, a backcountry ski trip leader for the Sierra Club San Francisco Bay Chapter.

“People want a change from the downhill ski resorts because there are more people there now and they’re willing to take these risks,” Giese said.

She’s also worried that the influx of people in the backcountry and the growing atmospheric potential for bigger and more intense storms could put more skiers in danger in the long run.

“There were severe storms in the past, but they weren’t as frequent,” Giese said. “And there are just more people out there.”