In summary
Key moisture measurements are only 2% to 5% of average, leaving dusty soils. And the recent swing from wet to dry is among the most extreme on record. This combination of climatic conditions crossed into a danger zone, priming much of Southern California for wind-whipped fires.
As much of Los Angeles smolders, wind warnings return and fire crews stand guard, scientists say almost unprecedented climatic conditions throughout Southern California led up to the disaster.
Last summer was one of the hottest on record, and the extreme swings between wet and dry conditions over the past two years has been unusually severe. Two rainy winters — which promoted heavy growth of brush — have been followed by near-zero rainfall for the past eight months and counting.
This pattern of weather whiplash, likely exacerbated by climate change, hasn’t been seen in Southern California since 1992-1993, and before that, 1907-1908. “We find only three instances where an anomalously dry start to the wet season follows back-to-back wet water years,” a team of UCLA researchers wrote in a report released on Monday.
Soil moisture levels across much of the region from Santa Barbara to San Diego hover between just 2% and 5% of average — leaving dust where there should be mud.
Also, an important measure called “vapor pressure deficit” has exceeded norms. Calculated from a combination of temperature and relative humidity, it reflects the ability of air to draw moisture from the landscape.
“The way to think about vapor pressure deficit is that it is the drying power of the air,” said John Battles, a UC Berkeley forest ecology professor.
Readings from Jan. 8 show an extreme deficit across much of inland Southern California. Such conditions can draw much of the moisture from living plants, so fires become almost unstoppable once they start.
“When it’s that dry, wind has ultimate power,” said UC Merced climatology professor John Abatzoglou.
In Malibu Canyon, local gauges recorded 53 degrees Fahrenheit and relative humidity of 36% on Jan. 4. Three days later, on the day that the Palisades and Eaton fires began, the air temperature was 64 degrees while the relative humidity had dropped to 13%, more than doubling the vapor pressure deficit.
These levels are “literally off the charts,” Battles said.
This combination of conditions crossed a dangerous threshold, priming the landscape throughout much of Southern California for high risk of wind-whipped fires. Across seven counties, drought has sapped the air, soil and vegetation of moisture.
The National Weather Service issued a warning Tuesday of critical fire weather or red flag warnings from the Mexican border to San Luis Obispo County. The alert predicted gusts up to 50 mph, humidity of a lip-splitting 10%, and virtually no chance that rain would relieve the conditions anytime soon. This comes on the heels of the third hottest summer in coastal Southern California since at least 1895.
The threat goes far beyond Los Angeles, affecting much of Southern California. Across Orange County “current live and dead fuel moistures remain at or below established critically low thresholds,” said Sean Doran, a public information officer with the Orange County Fire Authority. He called Tuesday’s fire danger level in Orange County “extreme.” The county has dried-out canyons, near residential areas, full of ultra-flammable chaparral and sage scrub.
Officials and researchers routinely weigh samples of vegetation, dehydrate them and weigh them again. This allows them to calculate the “live fuel moisture” percentage, which tells them how flammable the landscape is.
These measurements and related data are critical to firefighters, who monitor them regularly so they can gauge the risk of a fire erupting and determine which tools, vehicles and equipment are needed to fight the blazes, explained Scott McLean, a Cal Fire public information officer.
Last May, the live fuel moisture content of Santa Monica Mountains chamise — a prominent chaparral plant — was a wet and heavy 143%. That means that the weight of the water in the plants was almost 1.5 times the weight of its woody material. (A reading of 100% means equal parts water and plant mass.)
By November, live fuel moisture in the same region had dipped to just over 60%.
Even more recently, on Jan. 7, measurements from Santa Barbara vegetation showed levels of 61% — substantially below the 77% average for this time of year. That means their water weight was less than two-thirds of their plant material.
“Once the live fuel moisture hits around 60%, that is the critical danger zone,” said UC Merced’s Abatzoglou, explaining that below this level, vegetation loses much of its resistance to fire.
To put it another way, he said, at and below about 60%, “the live fuels behave and burn more like dead fuels.”
“Once the live fuel moisture hits around 60%, that is the critical danger zone…The live fuels behave and burn more like dead fuels.”
UC Merced climatology professor John Abatzoglou.
Abatzoglou cited research from 2009 suggesting that a critical threshold between vegetation that can and cannot support a large fire lies around 79% — which would put current conditions much deeper into the danger zone.
Dead vegetation, baked by the sun for months or years, is also perilously dry. “By January 7th of 2025, dead-fuel moisture was 6th lowest on record for that date,” the UCLA team wrote in Monday’s report.
Brush clearing wouldn’t help much, experts say
While President-elect Donald Trump has claimed on social media that incompetent state leadership led to the wildfires and hindered efforts to tame the flames, experts say there is little that could have prevented the disaster.
The extremely dry conditions have been aggravated by winds gusting up to 100 miles per hour — what UCLA climate researcher Daniel Swain recently likened to using an atmospheric blow dryer on bone-dry terrain.
Alexandra Syphard, a senior research ecologist at the Conservation Biology Institute and an adjunct professor at San Diego State University, said the extreme conditions have rendered humans powerless, at least in the nearterm, to subdue wildfire threats.
“I do not believe there is anything that wildland management could have done to qualitatively or substantially alter the outcome of these fires,” she said.
“I do not believe there is anything that wildland management could have done to qualitatively or substantially alter the outcome of these fires.”
Alexandra Syphard, Conservation Biology Institute research ecologist
While thinning trees or conducting controlled burns can reduce fire dangers in some forests, the same approach does not work in the areas of Southern California dominated by chaparral, Syphard said. These areas are too vast to clear brush, encompassing thousands of square miles.
She said such clearing tends to increase fire danger in chaparral landscapes by killing off both mature plants and the natural seed bank in the soil, triggering a long-lasting conversion to grasslands, which she says create an “explosively flammable” duff layer each summer and fall.
The best preventative strategies for reducing fire danger in a chaparral landscape, Syphard said, are to “create very strategically placed fuel breaks that enable safe firefighter access” as well as to “rethink where homes are constructed and how to make houses more resilient.”
Reducing greenhouse gas emissions, though it’s not likely to provide any immediate relief of the dangers facing millions of Californians living in or near flammable landscapes, is another difficult but necessary solution, experts say. Global warming is conditioning the already arid Southwest to burn.
As much as 88% of the increasing average vapor pressure deficit in the western United States is linked to human-caused warming, according to a 2021 UCLA paper. Compared to the 1980s and 1990s, the number of days with an extreme vapor pressure deficit nearly doubled in the first two decades of this century, the researchers found.
“These are global conditions playing out … There’s very little California can do to reshape these weather patterns.”
John Battles, UC Berkeley forest ecology professor
And with rates of global emissions increasing in spite of international pledges to reduce them, this increasing aridity is only going to get worse.
“This change in risk requires urgent and effective societal adaptation and mitigation responses,” the UCLA scientists wrote.
The new UCLA report noted that linking weather anomalies to climate change “requires deep analysis.” But the authors were confident about one potential connection: “The clearest way in which climate change may have intensified the January 2025 wildfires is the anomalously warm summer and fall of 2024,” they wrote.
With or without a climate change link, the extremes seen in Southern California over the past two years have been exceptional, including a hurricane-driven cloudburst in August 2023, an extraordinarily wet February last year that delivered an average of almost half an inch of rain daily, and a dry streak that is quickly catching up with 1962-1963 as the longest in the region’s history.
Battles, at UC Berkeley, said the likely role of climate change in the weather extremes that are clobbering California makes direct human intervention almost negligible, and better planning key to safety.
“These are global conditions playing out … There’s very little California can do to reshape these weather patterns,” he said.
“With the climate making things drier, we need to think about how we transition into a new state, and how we deal with wildfire and development and public safety. These are not hard science questions, but they’re super hard policy questions.”
