California’s main wildfire season is starting earlier in the year, and human-caused climate change is a major reason, new research finds.
The onset of summertime fire activity in large parts of the state has crept into spring by up to two months since the early 1990s, according to a study published Wednesday in the journal Science Advances.
The change has been especially pronounced in the Cascade Range in Northern California, the coastal mountains of Central California, and coastal Southern California from Monterey to San Diego.
Officials and disaster managers in the state now often speak of fire as a year-round hazard, instead of a seasonal threat.
The study rules out two factors that might theoretically be behind the shift: buildups of vegetation and changes in the number of fires ignited, either accidentally or on purpose, by humans.
The more important drivers, the researchers found, are the effects of greenhouse warming, including earlier and faster snowmelt and a warmer atmosphere that pulls more moisture out of soil and vegetation.
When blazes get going earlier in the year, “there’s more time for the environment to be primed for large fires that can come later in the season when it’s really hot,” said Gavin D. Madakumbura, a postdoctoral climate researcher at the University of California, Los Angeles, and the study’s lead author. “The vegetation has more time to dry out.”
The findings suggest that, as nations continue to burn fossil fuels and heat up the planet, Californians should expect peak fire activity to keep moving up in the calendar, though how much earlier it can get is an open question, Dr. Madakumbura said.
This year has already made fire history in California because of the destructive blazes that killed at least 31 people and destroyed thousands of homes around Los Angeles in January. But the state’s peak fire season, which starts in summer and runs through October, has had a slow start. June and July were unseasonably cool across much of the state. Thick fog has hugged the coast of Northern and Central California, keeping the vegetation moist.
California is so vast and diverse that the characteristics of wildfires vary across different parts of the state. That’s why Dr. Madakumbura and his colleagues examined each of the state’s 13 ecological regions separately, using records from the U.S. Forest Service of fires that burned between 1992 and 2020.
To see whether changes in vegetation accounted for shifts in fire-season timing, the researchers separated the blazes in each region by the type of landscape on which they ignited: forest, grassland, shrub land, farmland or developed land.
Changes in land management practices have caused some forests and other vegetated areas of the American West to become dense, overgrown and highly flammable. Yet even after controlling for differences in landscapes, Dr. Madakumbura and his co-authors still found that fire seasons showed similar trends toward earlier start dates. There had to be something else that was driving these trends across land of all kinds.
Was it changes in human-caused ignitions? As human settlements expand into fire-prone places, you might expect that more fires would be started by people and infrastructure such as power lines, and could affect the timing of wildfire season.
But that’s not what the data shows. Dr. Madakumbura and his colleagues found that the frequency of human-caused fires fell or was flat across California over the past three decades. Other researchers have documented similar declines.
In the end, Dr. Madakumbura and his colleagues found that the earlier onset of fire season was best explained by climate factors. Previous research has suggested that the warming climate could be causing California fire season to grow longer at the back end, too.
A longer fire season might not translate directly into more destruction from wildfires, said Alexandra Syphard, a research ecologist at the Conservation Biology Institute, a nonprofit science organization. It does, however, mean there are more chances each year that the ingredients will come together for a catastrophic blaze, Dr. Syphard said: dry fuels, strong winds, a spark.
Amy Graff contributed reporting.