https://www.latimes.com/environment/story/2024-04-25/atmospheric-rivers-
could-pound-california-with-more-extreme-rain

For years, scientists have said that atmospheric rivers can either make
or break the water supplies of thirsty California cities and farms.

For the last two winters, a steady succession of these giant �rivers in
the sky� have dumped record-breaking and drought-busting precipitation
across the state, while simultaneously causing catastrophic floods,
landslides, and dangerous blizzards.

But now, new research has found that these recent atmospheric rivers
pale in comparison to some of the monster storms that battered ancient
California � a sobering revelation that suggests to some experts that
the state could be revisited once again by such cataclysmic storms.

�Our findings show that atmospheric river activity exceeds what has
occurred since instrumental record keeping began,� said Clarke Knight, a
U.S. Geological Survey research geographer and the lead author on the
study that detailed � for the first time � atmospheric river activity
dating back 3,200 years. �This is important because it suggests the
latent potential of our area to experience storms beyond those that we
have seen today.�

Although few people had even heard of atmospheric rivers just a couple
of decades ago, research into the mammoth vapor trails has proved
critical to California water planning and public safety.

The study�s findings do not bode well for a state whose flood
infrastructure was severely strained last year, when a train of
atmospheric rivers breached numerous levees, flooded communities and re-
filled once dry Tulare Lake. The findings also up the ante for state
efforts to capture stormwater as climate change causes more
precipitation to fall as rain instead of snow and ushers in a new era
of more frequent and prolonged drought.

Climate California logo
How climate change is disrupting California's storms
Here�s a look at how humanity�s heating of the planet affects
California�s storms, snowpack and more.

� California�s storms are projected to become more extreme �

� How the latest storms fueled California�s snowpack recovery �

� Tracking water supplies: How much water does the state have stored? �

� Read more Climate California coverage �

Knight and her fellow researchers arrived at their conclusion after
analyzing ancient layers of mud from Leonard Lake in Mendocino County.
The team was able to determine when more sediment had been pushed into
the lake, indicating periods of higher precipitation.

Then, using data for atmospheric rivers over the last 60 years, the
researchers found a �strong correlation� between their sediment findings
and modern storms, allowing them to model that link through the rest of
the mud layers to reconstruct historical atmospheric river activity,
Knight said. Their research was published Thursday in the journal
Nature.

The research provides the most historical context to date for the
state�s rainfall variability, and found that the region �consistently
registered extreme precipitation over a 3,200-year period.�

Knight said this new hydrologic data can better inform climate modeling
and projections, providing a historical record 20 times longer than
what�s been available.

Although the team�s research focused on Northern California � where the
state typically sees the the most atmospheric rivers � she said it�s
fair to conclude that the southern half of the state would have seen
similarly extreme rainfall in its ancient climate given the widespread
effects of large atmospheric rivers.

Previous research has shown that the average atmospheric river
transports more than twice the flow of the Amazon River. The prospect
of even larger storms hitting California is a concerning one, experts
say.

Daniel Swain, a UCLA climatologist who was not involved in the USGS
study, said the paper provides �direct physical evidence� of
atmospheric river activity more extreme than anything seen in recent
California history � well beyond the Great Flood of 1862, which
reconfigured the state�s landscape.

He said the research �re-emphasizes the perils of assuming that the
extremes we saw in the 20th century are representative of the kinds of
extremes that are possible in this part of the world.�

�It�s an indication that � even if we didn�t have to contend with
climate change � we should still be circumspect about the risks that
are posed by extremes because we know that the climate system ... can
throw big, bad things at us periodically,� Swain said. �I don�t find
that at all reassuring.�

The continuing climb of global average temperature due to humanity�s
burning of fossil fuels also threatens to exacerbate matters.

�Adding energy into the system through greenhouse gas emissions is
basically like shaking the soda can ... and adding a little bit more
energy into the system, allowing these extremes to be a little bit more
extreme,� said Cody Poulsen, a graduate student researcher at the
Scripps Institution of Oceanography�s Center for Western Weather and
Water Extremes, who also was not involved in the Nature study.

Swain has posited that every degree increase in global temperature
increases the risk of an �ARkStorm Scenario� � originally projected as a
once-in-a-thousand-years megaflood event. But these new USGS findings
may indicate that bad-case-scenario modeling isn�t extreme enough, he
said.

For a state that is grappling with more frequent and severe periods of
drought, the last two wet winters have come as a rare bounty. However,
many Californians may be surprised to learn that these two wet seasons
fall within the realm of natural variability. They may also be surprised
to learn that this year has delivered more atmospheric rivers than the
previous year, which caused far more damage and disruption.

Recently, researchers confirmed that 51 atmospheric rivers hit the West
Coast during the 2023-24 rainy season � significantly more than the 38
atmospheric rivers that hit during the 2022-23 rainy season, according
to new data from the Center for Western Weather and Water Extremes.

In California specifically, 44 atmospheric rivers made landfall from
October through March, up from 31 during last year�s rainy season, said
Chad Hecht, a center meteorologist.

But even though there were more atmospheric rivers this rainy season,
fewer of the storms measured strong or extreme on the center�s strength
scale compared to the season before that.

�It�s not the quantity, it�s the quality,� Hecht said.

For example, 12 strong, extreme or exceptional atmospheric river storms
hit California between October 2022 and March 2023. These heavier storms
tend to bring news-making rain and snow. This season, however, the state
recorded only five.

�If you compare it to last year, ... this [water] year was a couple of
strong storms, but it�s a lot more weaker,� Hecht said. �But the
abundance of weak-to-moderate [atmospheric rivers] kind of helped keep
us on trajectory to hit that normal [precipitation levels].�

As of this month, records for both statewide precipitation and the
snowpack across the Sierra Nevada stood at about 105% of average for
this time of year � which Hecht called shockingly close to average.

�This year was abnormally normal,� Hecht said. �We like to talk about
California being the land of extremes, where it�s either extremely dry
or extremely wet. This year was abnormal because it was fairly close to
normal through April 1,� the date that typically marks the end of
California�s wet season.

However, Southern California has seen a more anomalous water year, with
its yearly rainfall well over 140% of average across many coastal areas,
according to the California Water Watch.

Hecht said one strong, slow-moving atmospheric river in early February
had an outsized effect on the region�s rainfall, and he noted that many
areas were also hit by thunderstorms during what he called �overly
productive� weak atmospheric river storms.

The systems aren�t typically accompanied by thunderstorms, but several
systems were this season, driving locally historic rainfall and flash
flooding in several areas, including San Diego and Oxnard.

Hecht said it�s not immediately clear why so many atmospheric rivers
this season included thunderstorms, but he said higher ocean surface
temperatures � a signature of the El Ni�o weather pattern � could have
helped spur the unstable convective pattern.

Even with many water measurements pointing to an overall average water
year thus far, federal officials recently issued a major disaster
declaration for nine counties after the deadly February atmospheric
river storms.

Knowing that further rainfall extremes are possible, Swain said he hopes
state officials can better prepare for emergencies, or at least better
understand the possible risks.

�If we don�t correctly estimate the risk to begin with ... it�s awfully
hard to have an accurate discussion about costs and benefits of any
particular intervention,� Swain said.

But, he noted that climate change is still expected to further stretch
those natural extremes.