The relentless storms that hit California from Dec. 27 to Jan. 16 caused extreme flooding and extensive damage in most of the state, killing at least 22 people. A series of storms hit back to back, soaking the state in the midst of California’s driest three-year period on record.
“If anybody doubts that climate is changing, then they must have been asleep for the last couple of years,” President Joe Biden said in California on Jan. 19, after witnessing the destruction left behind by the storms.
He later added: “For example, places that were ravaged by past wildfires are now at a higher risk of landslides. Extreme weather caused by climate change means stronger and more frequent storms, more intense droughts, longer wildfire seasons — all of which threaten communities across California.”
There is a good scientific basis to think that storms, including the type that struck California, are generally becoming more extreme due to climate change. But climate scientists told us it’s too soon to know whether climate change had a role in this particular event, and if so, to what degree.
“We are not entirely sure,” Julie Kalansky, a climate scientist at the Scripps Institution of Oceanography at the University of California, San Diego, told us in an interview. “It’s an active area of research.”
Daniel Swain, a climate scientist at the University of California, Los Angeles, told us all extreme weather events are the result of multiple complex and interrelated processes happening across time and space. Therefore, climate change is not “the singular cause” of the storms. But did it affect the storms’ intensity?
“Here, the answer is probably yes, climate change thus far has likely increased both the intensity and likelihood of seeing such an intense period of precipitation in California,” he wrote in an email. “But then the question becomes: to what degree?”
Here is what we know so far.
What kind of storms hit California?
California was hit by a series of nine atmospheric rivers, which the National Oceanic and Atmospheric Administration describes as “naturally occurring air currents” that can create extreme rainstorms and flooding. The atmospheric rivers were accompanied by a bomb cyclone, a mid-latitude storm or weather system that rapidly intensifies.
Atmospheric rivers are long and narrow corridors in the lower atmosphere that transport water vapor from the tropics to the poles — “like rivers in the sky,” as NOAA explains. When these columns of vapor move inland from oceans and over mountains, the water vapor cools and creates heavy precipitation in the form of snow or rain. Their contribution to the water supply is crucial: A few of them provide, on average, 30% to 50% of the U.S. West Coast’s annual precipitation.
But stronger atmospheric rivers, which carry greater amounts of moisture pushed along by stronger winds, can cause damage when they hit and stall over lands that are prone to flooding — as seen in the recent storms. Intense atmospheric river sequences have the potential to create a catastrophic “megaflood,” according to research.
Atmospheric rivers were only defined in the 2010s, but they are not new, as F. Martin Ralph, a research meteorologist and director of the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography, wrote in Scientific American.
These kinds of storm systems slam western coasts across the globe several times a year, but they can also reach as far inland as Yellowstone National Park in the U.S., which lies mostly in northwest Wyoming, he explained. Atmospheric rivers can grow up to 2,000 miles long, 500 miles wide and two miles deep, Ralph wrote, adding that they transport on average “enough vapor to equal 25 times the flow rate of the Mississippi River where it pours into the Gulf of Mexico.”
It’s not uncommon for atmospheric rivers and bomb cyclones to occur together, and they feed off of one another. Around 80% of atmospheric rivers are accompanied by an extratropical cyclone, research shows. The cyclones can enhance the winds of an atmospheric river, while atmospheric rivers provide ideal conditions for a cyclone to intensify. A bomb cyclone is a mid-latitude cyclone that intensifies very quickly because of a dramatic drop in pressure in a single day, usually a result of cold and warm air colliding.
How does climate change impact atmospheric rivers?
Climate modeling studies show that, in general, in a warmer climate atmospheric rivers become more intense, leading to an increase in heavy precipitation. According to a recent study, climate change “has already doubled the likelihood of an event capable of producing catastrophic flooding” in California. But although the effects of climate change in atmospheric rivers have been studied using different approaches, uncertainty remains.
Most of the climate change impact on the intensification of atmospheric rivers is caused by what’s called the “thermodynamic effect,” Swain, the climate scientist at UCLA, told us. That is, he said, “the fact that the atmosphere can hold exponentially more water vapor” with each degree of temperature increase.
“A good rule of thumb is that a 1C increase in temperature increases the water vapor holding capacity of the atmosphere … by ~7%,” he said.
According to the most recent report from the Intergovernmental Panel on Climate Change, it is “unequivocal” that the atmosphere, ocean and land have warmed due to human influence. The group concluded, based on an evaluation of evidence quality and agreement, that there is “high confidence” a warmer climate increases the amount of moisture in the atmosphere, making wet seasons and events wetter. There is also “high confidence” heavy precipitation will follow the rate “of about 7% per 1°C of global warming.”
“Given that global warming is increasing the amount of water vapor, it does seem reasonable to suggest that climate change may be making these storms stronger,” Travis A. O’Brien, assistant professor of earth and atmospheric sciences at Indiana University, Bloomington, told us in an email.
“Indeed, climate model studies of atmospheric rivers and global warming … suggest that atmospheric rivers become ‘stronger’ (more water vapor transport) in a warmer climate and are generally associated with higher precipitation amounts,” O’Brien added.
Atmospheric rivers are measured in what’s called integrated water vapor transport, Kalansky, from the Scripps Institution of Oceanography, explained. That includes both how much water is there and the wind that transports the vapor, she said.
What climate models are showing, she said, is that in a future warmer world, atmospheric rivers will contribute more than other storms to California’s rainfall total annually, and that extreme atmospheric river events will become more extreme — and that’s mostly explained by the increase in water vapor.
Swain told us the thermodynamic contribution is likely responsible for 80% of the projected change in atmospheric river intensity and the projected extreme precipitation increases. The remainder is more uncertain, he said, but wind and pressure patterns could be important factors.
But there are still many basic things scientists don’t know about atmospheric rivers and the ways they will respond to a warming climate.
“In climate models, there is a robust increase in global mean precipitation; however, how the response of ARs contributes towards this change is still uncertain and depends on many more factors than increased moisture alone,” reads a review article on the responses of atmospheric rivers to climate change published in Nature in 2020.
A recent case study, for example, suggested that not all atmospheric rivers are affected to the same degree by climate change. The study simulated a specific atmospheric river storm that hit Northern California in two waves in 2017 under past, present and future climate scenarios. While both waves of the storm dropped more precipitation because of warming, the second wave dumped more. Precipitation amounts for the first and second waves were about 11% and 15% higher, respectively, under present-day warming, the study found. Those amounts increased to an additional 21% and 59% boost in precipitation, respectively, under late-21st century warming.
It’s not clear whether there will be more or fewer atmospheric rivers in a warmer climate. Most studies, O’Brien said, “do indicate an increase in the frequency of atmospheric rivers, but also some indicate a decrease or no change for western North America.”
Part of the issue, O’Brien found in a 2021 paper, is that researchers are not always consistent in how they define an atmospheric river.
For California specifically, Swain said that while it’s uncertain, the “preponderance of evidence is for fewer” atmospheric rivers overall in a warmer future. But, he said, “there is also strong evidence that the *strongest* atmospheric rivers in California (like those being experienced during this storm sequence) are very likely to be stronger and to produce more precipitation as the climate warms.”
“There is a quite a bit of evidence pointing in this direction at this point,” he added.
Can we say whether or how much climate change impacted this particular series of storms?
Not yet, climate experts say.
“I don’t think we have evidence to show the degree these events are connected to climate change,” Duane Waliser, chief scientist at NASA’s Jet Propulsion Laboratory, told us in an email.
Waliser, who has studied atmospheric rivers and climate change’s effects on them, told us it would require more study to quantify an estimate of the effects of climate change on the storms. “[U]ntil that happens, a statement along these lines would be complete speculation,” he wrote.
O’Brien agreed. He said it is impossible to make a “formal statement” about the effect of climate change on these storms without a detection and attribution study.
Detection and attribution studies “can help determine whether a human influence on climate variables (for example, temperature) can be distinguished from natural variability,” according to a federal report on climate science. They are important, O’Brien said, because events like this can, and did, happen before climate change.
“[T]here have not yet been any D&A studies on atmospheric rivers in general, let alone this storm,” he said, referring to detection and attribution studies. But he added that he expects there will be one coming out in about the next six months.
Kalansky said the storms do fit into what the climate models are showing. California weather, which is already highly variable and volatile, is and will become more extreme. Projections also show in a warming climate, because the air can hold more moisture, there is the potential that atmospheric rivers will drop more rain or snow, as we’ve explained. But to know whether or not that’s the case with these storms, more studies are needed, she said.
What was unique about this winter’s atmospheric river storms, she said, was that they came one after the other.
“The fact that they are coming back to back, to back, to back, has been really impactful,” she said in a phone interview.
Without proper attribution studies, it’s hard to say if that was part of California’s natural climate variability or not.
“It may,” she said, stressing the word may, “it may have been fueled by climate change. … But it’s too soon, at least in my opinion, to be able to say whether or not, without doing some more studies.”
Are there any estimates?
On Jan. 4, in the midst of the storms, Michael Wehner, a senior scientist in the Computational Research Division at Lawrence Berkeley National Laboratory, tweeted what he called a “conservative attribution statement.”
“Anthropogenic climate change will cause the rain in today’s west coast … to be about 5% heavier,” he said, linking to a study he co-authored that was published in 2022.
O’Brien told us the study on which Wehner is basing his estimate “isn’t exactly” a detection and attribution study. What it did is look at historical storms and ask what would they look like in a future climate.
“His statement of the 5% number is based on the numbers that they found in that study: that storm-total precipitation increases by about 5-10% per degree C of warming,” he said. “So he’s doing a bit of inference with that statement rather than doing a careful D&A study. That said, I suspect that when a formal D&A study is done, it will produce results consistent with his statement.”
Swain, the climate scientist from UCLA, told us that estimate is “a reasonable lower bound.” His best guess, he said, would be 10%, or something in the range of 5% and 15% heavier rainfall due to climate change.
Martin Hoerling, a research meteorologist in the NOAA Physical Sciences Laboratory, told us in an email that because the air can hold more water vapor as a consequence of warming, “[t]here is a scientific basis to expect, for identical weather patterns today versus in the 19th Century, that a rainstorm would yield about 5% more precipitation today.”
But he added that prolonged rains like this one did occur in the 19th century. For example, he said, the wettest 15-day period on record reported in downtown San Francisco, with 19 inches of rain, occurred in 1862. The second, with 13.5 inches, occurred in December 1866, he said. This winter’s storm represented the third, with 12.37 inches.
While Wehner “is correct to offer an important reminder of how rain events are becoming more extreme, historical records when examined carefully provide no less important reminders that nature (without human modification) can deliver remarkable rains alone,” he added.
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