Fire weather and climate change

In Ventura County this week, the local National Weather Service station in Oxnard has issued the following warnings:

On Sunday, a Red Flag Warning (for high heats and offshore winds)
For the week, a Critical Fire Weather warning (for high heat, low humidity)
for the last three days, an Excessive Heat Warning

Do these “fire weather”warnings seem increasingly common? That’s because they are, in fact, more common — about twice as common as in the previous climatology of the 80’s. Here’s the story on the subject I published this week in the Ventura County Reporter:

As new wildfires in Shasta, Napa and Sonoma counties in Northern California exploded in flames this past weekend, forcing evacuations and destroying vineyards and homes,  a “Red Flag warning” was issued on Sunday for the mountains of Ventura and Los Angeles County. The National Weather Service warned of winds gusting up to 40 miles per hour, relative humidity falling into the single digits, and temperatures expected up to 105 degrees. A “critical fire weather” warning for Ventura County was extended through this week due to “an extended period of hot, dry conditions, along with offshore breezes and plume dominated fire potential.”

Screenshot of National Weather Service front page for Ventura County area, Sept. 28, 2020: www.weather.gov/lox/

This is to be expected across California in fall now, scientists say. “Fire weather” conditions have become far more common in the 21st century, according a study released in late August, in which a team of climate scientists from Stanford, UCLA, Columbia and other research institutions showed that the sort of conditions that foster massive wildfires have doubled since the 1980s. 

Were the warnings this week in Ventura County an example of the underlying change in California climate?

Yes, according to Daniel Swain, a climatologist with UCLA and the National Center for Atmospheric Research and one of the authors of the study.

“The upcoming weather pattern is indeed exactly the kind of fire weather pattern we find is occurring more frequently due to climate change: unusually warm and dry conditions co-occurring with an offshore wind event in the context of already record or near-record dry vegetation,” he said. “The main climate signal comes through the ever-increasing dryness of vegetation, which is itself mainly a production of warming temperatures.”

Ventura County vegetation levels in early September fell to “critical” danger levels of 60 percent or less, according to a statement from the Ventura County Fire Department. The Ojai area already stands below 60 percent, in contrast to this time last year, when vegetation moisture levels were at about 71 percent. 

National Weather Service alert. Sept. 30, 2020.

The study, “Climate Change is Increasing the Likelihood of Extreme Autumn Wildfire Conditions,” shows that 1950-79, the South Coast region that includes Ventura County recorded five to six days a year in which the Fire Weather Index was at an outlying extreme, with hotter, drier, and windier conditions than 95 percent of the days recorded. From 2006 to 2020, with projections extending to 2035, the South Coast region registers about 10 days of these extreme fire weather conditions a year. In years to come, that trend will intensify, bringing a total of at least two weeks of extreme fire weather a year to our region, depending to some extent on whether greenhouse gas emissions continue at the current rate, or decline with hoped-for reductions in the burning of fossil fuels. 

Swain said that one of the motivations for the study was to test a claim that Southern California — which has fewer forest environments, and more chaparral — might be less vulnerable to climate change than forested regions in Northern California. But the research found that climate change is bringing the same dangerous “fire weather” conditions to the entire state. 

“It was a bit surprising to us that historical warming and drying has already produced such a large increase in extreme fire weather days, but that’s what the data shows!” Swain said. “And since the vast majority of major fire ignition and much of the spread of established fires occurs on such days, this has major practical implications.” 

This year has seen five of the six largest wildfires in California history, totaling over 3.6 million acres burned, 7,630 structures destroyed, and 26 people killed, according to CalFire.