The last couple years have seen devastating and record-setting wildfires in California, leaving many in the region to wonder what to expect in the future. Elsewhere in the US West, research has found that fires were increasing due to a combination of climate change and other human activities, which exacerbate both the fires and the damage they cause. But California is a different beast from much of the West and requires its own analysis.

A new study from a team led by Park Williams and John Abatzoglou—also the scientists behind a recent study of western US fires—uses government records of California wildfire areas going back to 1972, along with weather data and climate model simulations. The work breaks California into four different regions based on vegetation. The coast is split into a forested northern section, separate central and southern shrublands, and the forested Sierra Nevada rounding out the list.

Big changes

Overall, the average area burned by fires each year in California has increased by a factor of five since 1972—a remarkable increase. However, this is mostly due to an even larger increase in the forested parts of the state, as the central and southern coastal regions haven’t really seen an increase.

California has gotten warmer over this time period, and at a rate that matches climate model simulations of human-caused warming. There isn’t a clear trend in precipitation, though mountain snowpack has declined with rising temperatures. But the temperature change alone can have a huge impact on fire conditions. Hotter air sucks more moisture out of the soil and out of the vegetation that the fire burns.

The summer fire trend in the forested Sierra Nevada and northern coastal region correlates well with this warming-driven atmospheric drying. In fact, the area burned by fires increased exponentially with the drying—not surprising given the physics involved. In the analysis, this was the single biggest factor, accounting for most of the increased area burned.

In the central and southern coastal regions, the correlation with weather is pretty weak since there hasn’t been an increase in summer burn area. This is probably related to the nature of this shrubbier and more populous landscape, where the spread of fires can actually be halted by gaps in the vegetation (or rapidly controlled by firefighters). There was, however, a relationship between fires there and precipitation in the previous year—rain causes a burst of growth that subsequently dries out, setting up the potential for rapidly spreading fires.

Getting particular

So what about the recent fall fires around Los Angeles and the Bay Area? This type of fire is particularly variable from one year to the next, as they rely on a particular confluence of events. In fall and winter, strong downslope winds (called Diablos in the north and Santa Anas in the south) develop that can stoke fires like a bellows. But by the time these winds arrive, the rainy season has usually begun, greatly reducing fire risk. What happened in both 2017 and 2018 is that the rains were late, leaving tinder-dry vegetation in the path of the winds. In those conditions, a major fire is almost inevitable, but the conditions are unusual.

Trends in those weather conditions are less clear. Climate models do project later starts to the rainy season with continued warming (a little of which has already been seen), as well as an increase in swings between wet and dry years. But they also show a possible reduction in fall winds. Warming temperatures, though, have led an increase in the number of fall days where vegetation is dry enough to make the fire risk high.

The study acknowledges other human impacts on fire—from the history of fire suppression that caused fuels to accumulate to developments that create gaps in the vegetation even as they put more homes in harm’s way—but can’t separate them out in this case. They did find that their correlations for 1972-2018 were the same if you compared the first half of this time period to the second half. That at least tells you that those other human activities have had a pretty constant influence.

Overall, the researchers found that climate change looks to already be a major factor in increased summer forest fires in California. “Importantly,” they write, “the effects of anthropogenic warming on California wildfire thus far have arisen from what may someday be viewed as a relatively small amount of warming. According to climate models, anthropogenic warming since the late 1800s has increased the atmospheric vapor-pressure deficit by approximately 10 percent and this increase is projected to double by the 2060s. Given the exponential response of California burned area to aridity, the influence of anthropogenic warming on wildfire activity over the next few decades will likely be larger than the observed influence thus far where fuel abundance is not limiting.”

Earth’s Future, 2019. DOI: 10.1029/2019EF001210 (About DOIs).