Fire Weather Index Maps

Frequency of autumn days with Fire Weather Index exceeding the 1979-2018 95th percentile. 

There is a statistical historical trend showing that hotter and drier California fire seasons have dramatically increased wildfire risk, according to a new report from university researchers that takes a look at the human-caused effects of the change.

There has been a "strong and robust" warming trend of about 0.3 degrees Celsius, and a "modest" decline in precipitation of 12.03 millimeters each decade, over the period of 1979 to 2018 in California, according to the academic report. Observations as well as climate model simulations suggest that the likelihood of Northern and Southern California experiencing extreme fire-risk conditions each fall has increased since the mid-20th century.

California's wildfires have increased along with fire-prone conditions that include strong offshore winds, dry vegetation, warm weather and later onset of fall precipitation. The study's authors observed changes in the magnitude of weather factors that enable extreme wildfire conditions and used climate model simulations to determine whether the changes are attributable to human-caused climate change.

"Our climate model analyses suggest that continued climate change will further amplify the number of days with extreme fire weather by the end of this century, though a pathway consistent with the UN Paris commitments would substantially curb that increase," the report says. "Given the acute societal impacts of extreme autumn wildfires in recent years, our findings have critical relevance for ongoing efforts to manage wildfire risks in California and other regions."

The work was performed by seven researchers from Stanford University; the Institute of the Environment and Sustainability, University of California, Los Angeles; The Nature Conservancy of California; Capacity Center for Climate & Weather Extremes, National Center for Atmospheric Research in Boulder, Colorado; the Management of Complex Systems Department, University of California; the Department of Geography as well as the College of Natural Resources, University of Idaho; and Lamont-Doherty Earth Observatory of Columbia University.

The frequency of fall days with risk in the extreme, 95th percentile of fire weather has more than doubled in California since the early 1980s, the report says. It also found an increase in the climate-model-estimated probability of those extreme fall conditions since roughly 1950.

The five warmest years on record in the state occurred between 2014 and 2018, with pronounced warming in late summer and early fall over the past century. This has increased the likelihood of drought, decreased mountain snowpack, and increased vegetation moisture stress and forest mortality. The bottom line is an extension of California's fire season.

The state's snowpack fluctuates greatly, but so far this year is down from 2019 (see related story).

The increase in deadly wildfires in recent years is not under dispute. The report discusses other practical realities like fires that occur in fall exposing vulnerability in firefighting resources that are often deployed during the core summer firefighting season, such as personnel, vehicles and aircraft, which can become unavailable as winter approaches. Historically, wildfires have decreased in fall, creating a resource mismatch when it comes to fighting fires.

The report notes that both the Camp Fire and the Woolsey Fire occurred during dry offshore downslope wind energy events, known in each place locally as the Diablo and Santa Ana winds. The hot summer in 2018 and the late arrival of rainfall in mid-to-late November created a predisposition to extreme fire danger conditions.

The researchers analyzed changes in fall temperature, precipitation and daily fire weather statistics "with a particular emphasis on the simultaneous co-occurrence of extreme conditions in northern and southern portions of the state."

But included were several caveats, including that the increases in wildfire probability the report quantified are based on links with the Fire Weather Index model, not on simulations of wildfire frequency. It also noted that the gridded datasets that enables analysis of historical and projected changes in extreme fire weather potential are imperfect approximations of real-world conditions.

"We also emphasize that climate change is only one of several factors driving California's multi-year wildfire disaster," the report says. Nearly 88 percent of fires and 92 percent of burned area from autumn wildfires in California are human-caused, "highlighting human ignition sources as key contributors."

However, the number of wildfire ignitions has declined over the past several decades. The study also did not quantify the impact of increased suburban and urban building in the wildland-urban interface or the contribution of historical land-use practices to increased wildfire risk.

The final opinion seems to be that human-caused climate change has increased wildfires, but such conclusions and comparisons might not be as simple as first meets the eye.

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