Weather Conditions That Caused the LA Fires Made 35% More Likely By the Climate Crisis

Human-induced planetary warming made the weather conditions that caused the Los Angeles fires 35% more probable, according to a report published on Tuesday by the research organization World Weather Attribution.

The report from WWA, which performs attribution studies that examine how the climate emergency impacts extreme weather events, further fleshes out the public understanding of wildfires that broke out in and around the Los Angeles region in early January. Those fires collectively burned tens of thousands of acres of land, killed 28 people, and destroyed more then 16,000 structures, according to the California Department of Forestry and Fire Protection.

Damage estimates indicate that the wildfires, which have placed strain on the California insurance industry, are one of the costliest natural disasters in U.S. history.

Southern California is no stranger to wildfire and can experience large fires year-round, according to the report's 32 researchers, who hailed from the United States and Europe.

In summer, fires in the region are promoted by low fuel moisture—the measure of the amount of water in a fuel, such as vegetation. In winter, the strong Santa Ana winds can drive fast-burning fires, but their ability to fuel fires in mid-winter is usually nullified by the onset of the region's rainy season, which begins in October-November, the report explains.

According to the researchers, summer 2024 was one of the warmest on record for the Los Angeles region. "As the cool, wet season approached, the typical onset of the rainy season did not arrive. However, the Santa Ana winds arrived, coinciding with very dry fuels," they wrote. There was also more fuel for the recent Los Angeles fires to burn because above-average precipitation during the winters of 2022-23 and 2023-24 had encouraged vegetation growth.

The report's researcher relied on the Fire Weather Index (FWI)—"a composite fire-risk index that accounts for longer-term drying conditions as well as wind and humidity driven conditions that can drive wildfire spread on a given day"—and found that "extreme" FWI conditions that drove the L.A. fires are expected to occur on average once every 17 years now that the globe is 1.3°C warmer relative to the preindustrial period.

That is an increased likelihood of 35% and an increased intensity of about 6% compared to a 1.3°C cooler climate.

To establish the role that the climate emergency has played in this trend, the researchers also combined this observation-based estimate with climate models, eight out of eleven of which showed an increase in extreme FWI conditions in January.

The researchers note that "while we have high confidence in the qualitative change, that the likelihood and intensity of the FWI has increased due to the human-induced climate emergency, the precise numbers have a wide range of uncertainty due to the model performance."

"Without a faster transition away from planet-heating fossil fuels, California will continue to get hotter, drier and more flammable," Clair Barnes, a co-author of the report and researcher at Imperial College London, told to CBC News.

The researchers also looked at changes to the timing of the dry season and found that the length of the dry season has increased by about 23 days since the global climate was 1.3°C cooler. This means that because of the burning of fossil fuels, the dry season and the Santa Ana Winds are increasingly overlapping—a recipe for more fire.

The group also found that the drought conditions leading up to the fires are now more likely to occur. Similarly dry seasons are 2.4 times more likely to happen compared to preindustrial times.