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A new study by researchers from Scripps Institution of Oceanography at the University of California San Diego and colleagues combs through the many factors that can promote wildfire, and concludes that in many, though not all, cases, warming climate is the decisive driver.

The study, led by Columbia University's Lamont-Doherty Earth Observatory, finds in particular that the huge summer forest fires that have raked Northern California recently have a strong connection to arid ground conditions brought on by increasing heat. It suggests that wildfires could grow exponentially in the next 40 years, as temperatures continue to rise.

The study notes that average summer temperatures in the state have risen 1.8 ℃  (3.3 ℉) since 1896, with three-quarters of that increase occurring since the early 1970s. From 1972 to 2018, the area burned annually has shot up fivefold, fueled mainly by a more than eightfold spike in summer forest fires.

“We’ve worked out the causes for trends that have been observed and connected them more clearly to anthropogenic climate change,” said Scripps climate scientist Alexander Gershunov, a co-author of the paper. “There are a lot of reasons why fire activity changes, not all related to climate change, but in this paper we wanted to quantify how much of it is due to climate change.”  

The researchers say the summer forest fire increases are driven by a simple mechanism: when air heats up even modestly, it causes more moisture to evaporate from soils and vegetation. Thus fires start more easily, and can spread faster and farther. During the fall, and in non-forested coastal areas, more complicated dynamics are at work and the results are less clear; but the researchers project that climate-driven aridity, related in part to declining fall precipitation, is likely to play a growing role there as well.

"It's not a surprise to see that climate has this effect in forests, but California is so big and so variable, there is no one-size-fits-all explanation for how climate might affect wildfires across the board," said the study's lead author, Park Williams, a bioclimatologist at Lamont-Doherty. "We have tried to provide one-stop shopping to show people how climate has or, in some cases, hasn't affected fire activity."

Williams and colleagues have already shown in a 2016 study that heightened temperatures and resulting aridity on the ground has doubled the area burned in forest fires over wider areas of the West in past decades. The new study appears in the journal Earth's Future.

The premise that warmer air draws moisture from the ground level — a phenomenon known as vapor pressure deficit — is already well established. However, many confounding factors can shift fire risk up or down, and so it is not always possible to measure the effects of vapor pressure deficit.

In California, human infrastructure is sprawling into forests, introducing more chances for people to both cause fires and suffer from them. And a century of efforts to suppress virtually all fires has led to a buildup of flammable materials in many forests. On the other hand, fragmentation of forest landscapes by human intrusion may in some cases limit the spread of fires. Rainfall and snow can vary year to year, sometimes adding to fire risk, sometimes subtracting. In the long-term, snow accumulation is declining and the shrinking snowpack melts earlier, contributing to long-term drying and flammability of montane forests. Meanwhile, coastal areas dominated by shrubs or grasses, where large fires are human-caused and fanned by dry gusty winds, are impacted in more complex ways by climate change. Here, observed trends are more subtle and are less clearly attributed to the changing climate.

The researchers combined data from many sources, some of it going back more than 100 years. They found that growing temperature-induced vapor pressure deficit accounted for nearly all the growth in forest fires from 1972-2018. In 2017, a modern state record was set for the largest individual wildfire (more than 285,000 acres) and the most destructive (5,636 structures burned, 22 people killed). The year 2018 saw a new record for total annual area burned (almost 1.7 million acres), and the 2017 records were broken for the biggest individual fire (the Mendocino Complex fire, which took out 464,500 acres) and the most destructive: the Camp Fire, which burned 18,804 structures and killed 85 people. The Camp Fire torched almost the entire Northern California community of Paradise.

"The ability of dry fuels to promote large fires is non-linear, which has allowed warming to become increasingly impactful," says the study. "Human-caused warming has already significantly enhanced wildfire activity in California, particularly in the forests of the Sierra Nevada and the North Coast, and will likely continue to do so in the coming decades."

That said, the authors note that the effects of climate are highly seasonal, and can vary depending on vegetation type, topography and human settlement patterns across California's highly diverse landscape. They found that summer fires did not increase in many non-forested areas dominated by grasses or shrubs. This, they say, was probably due to a combination of intense firefighting and prevention efforts, and reduced vegetation due to drought. In fall, destructive fires have grown, but because the dynamics of this season are complex, the effects of warming climate are not as obvious — at least not yet.

The researchers say fall fires are driven in large part by powerful winds sweeping from the highland interior, as well as the amount and timing of precipitation, both of which tend to pick up around this time of year. These factors wax and wane from year to year, perhaps masking the effect of overall warming. But researchers detect a delayed start to winter rains, which has resulted in recent wildfires occurring later in November and December. That effect is indeed just starting to show up in fall, and is likely to become more evident in the future, said Williams.

The study was coauthored by John Abatzoglou of the University of Idaho; Janin Guzman-Morales of Scripps; Jennifer Balch of the University of Colorado; Dennis Lettenmaier of the University of California; and Daniel Bishop of Lamont-Doherty.

– Adapted from Lamont-Doherty Earth Observatory