Atmospheric Rivers in California’s Ancient Past Exceeded Modern Storms

Clarke Knight studies just how far back in history, massive atmospheric river storms wreaked havoc on California.

As she reviewed her recent findings on a computer at her then-home in Menlo Park, the power went out. The cause? An atmospheric river in February of last year.

“It was kind of an ironic moment to be thwarted by the very thing I’m trying to understand,” said Knight, a USGS research geographer who studies paleoclimatology — the effects of weather on Earth in the past.

By looking 3,200 years into the past, Knight extended atmospheric river knowledge significantly: twice in three millennia, atmospheric river activity exceeded anything in modern instrumental record keeping, deluging the state with widespread rainfall beyond what current Californians have ever experienced.

The 32 massive storms that drenched California last year pale in comparison to some of the storms in the state’s past. Climate scientists argue Knight’s data established a new baseline for understanding intensifying storms in today’s warming world because of human-caused climate change.

“Understanding what happened when we didn’t have this additional layer of climate change is important to consider as a baseline for what to expect,” she said.

Knight and her colleagues extracted around 15-foot-deep sediment samples from the bottom of Leonard Lake, an almost entirely undisturbed lake in Mendocino County. Atmospheric rivers often hit the lake, causing sediment layers to settle on the lake floor, cementing things like titanium and silica into place. Using radiocarbon dating to determine the age of those organic materials, Knight compared that signal with current records. Once unearthed, the cores provided a more precise long-term history of atmospheric rivers in California.

“We have provided some of the first direct physical evidence of atmospheric storms in California’s history that had not been previously known,” she said. “[It is] about 20 times longer than the information we had previously.”

‘It sets the baseline’

Widespread meteorology records in California began in the late 1940s, and for the longest time, historians viewed the wettest and most disastrous rain event in California as the Great Flood of 1862 — which killed at least 4,000 people and cost more than $3 billion in today’s dollars.

Cody Poulsen, who studies atmospheric rivers at the Scripps Institution of Oceanography, said Knight’s findings are one missing puzzle piece in our understanding of future weather patterns in a warming climate.

“It sets the baseline in the sense that it provides a logical connection regarding the importance of atmospheric rivers,” he said. “This study creates a sobering result that the things that we think are extreme, amplified via global warming and climate change, could be more extreme.”

Knight’s study does have limitations. First, it focused only on one lake. Poulsen said that samples from lakes across the state are needed to have a more comprehensive view of atmospheric rivers’ effect on California in the past.

“This is just the tip of the spear,” he said.

Also, the study doesn’t resolve individual storms or water years. Instead, each data point holds around 10 years of information, “which in our field is extremely high resolution,” Knight said.

John Chiang, a UC Berkeley professor who studies atmospheric science, said Knight’s new record doesn’t accurately predict future storms in California.

“That being said, it does set a baseline in that this is a first of its kind to reconstruct the atmospheric activity in the past,” he said. “This data doesn’t corroborate the exact physics of what we think will happen in a future climate. Those variations occurred in the past when we didn’t have humanity.”

‘The findings bolster our current efforts’

Knight also selected Leonard Lake because water managers operate large regional reservoirs. More than half of the water delivered to that watershed along the Russian River comes from atmospheric rivers.

Knight hopes to expand her work to similar lakes across the coastal range and said learning from history “sets us up for a better conversation about risks.” She also would like her study to cause the state and water managers to “reassess the ability of existing infrastructure to handle these events.”

Michael Anderson, the state’s climatologist, is excited about the study because it takes computer model projections of future weather and turns them into “tangible” observations showing what happened in the past.

“That creates a stronger motivation to keep up the work we’re doing,” he said. “Our system is built to manage floods up to a certain size. Beyond that, the system can be overwhelmed.”

Anderson said the study is helpful in understanding “what makes extreme storms happen,” but more data is needed as the state prepares its reservoirs and waterways for extreme storms.

Along the Russian River in Sonoma and Mendocino counties, water managers track atmospheric rivers using radar units dispersed across mountaintops, flights during storms and the release of water from reservoirs when a big storm approaches.

“I think the findings bolster our current efforts to plan for the extremes that we’ve already been doing,” said Chris Delaney, principal engineer at Sonoma Water.

He said the agency might use Knight’s study in future planning because the new information means extreme events could extend beyond what the agency can handle.

“What we think is a 100-year event or a 500-year event now is probably not accurate if you were to look at the much longer period of climate like this study has done,” Delaney said.

Nathan Baskett, a hydrogeologist for Sonoma Water, said that having this new historical information about atmospheric rivers allows the agency to prepare for what could happen in the coming decades as the world continues to warm.

“It’s nice to see that they have actual physical evidence of it,” he said. “From where I’m sitting, the more data, the better because I think that having that kind of data helps us project for the future.”