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Study Bolsters Case That Climate Change Is Driving Many California Wildfires

Against a backdrop of long-term rises in temperature in recent decades, California has seen ever higher spikes in seasonal wildfires, and, in the last two years, a string of disastrous, record-setting blazes. This has led scientists, politicians and media to ponder: what role might warming climate be playing here? A new study 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 finds in particular that the huge summer forest fires that have raked the North Coast and Sierra Nevada regions 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.

Area burned by California wildfires in thousands of square kilometers, 1972-2018. Specific regions studied are at upper left. (Adapted from Williams et al., 2019)

The study notes that average summer temperatures in the state have risen 3.25 degrees Fahrenheit 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. 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. The result: fires start more easily, and can spread faster and farther. During the fall, and in non-forested areas, different dynamics may be at work and the results are less clear; but the researchers project that climate-driven aridity 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 Columbia University’s Lamont-Doherty Earth Observatory. “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 his 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 U.S. 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. And areas dominated by shrubs or grasses instead of trees may not react the same way.

A satellite image, August 6, 2018, shows smoke from multiple large California wildfires. The fires themselves, detected by thermal imagery, are in red. (NASA)

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). 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 leveled almost the entire forest 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. In summer, 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, which tends 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 that effect is indeed just starting to show up in fall, and is likely to become more evident in the future, says Williams.

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

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Park Williams
Park Williams
4 years ago

The effect of forest management (or lack there of) on wildfire in California is addressed repeatedly in the research article. In short, of course management matters. Fire suppression over the past century has caused vegetation to become artificially dense in many areas, particularly lower-elevation forests, and this is very likely to have allowed the response of wildfire to warming to be as strong as it was. More fuel -> more fire in hot years. However, it’s extremely likely that the increase in fire activity would have still been large even without this effect: (1) The relationship between forest-fire area and aridity was strong and unchanged over the past 5 decades despite increases in forest density during this time. (2) The strongest relationship between forest fire and temperature in the western US is found in the remote forests of the northern Rockys, where fire suppression over the past century has been minimal. That’s the region where the recent increase in forest-fire area has been strongest.

“Fire suppression over the past century allowed for artificial build-up of fuels in many regions that historically experienced frequent low-intensity fires, reducing fuel limitation as a constraint on fire activity and putting many areas into a so-called fire deficit (Minnich et al., 1995; Marlon et al., 2012; Higuera et al., 2015; Parks et al., 2015). Even under constant climate conditions, changes in California’s fire activity over the past century would be expected as populations increased and cities expanded into surrounding wildlands (Radeloff et al., 2018), fire suppression strategies evolved (Stephens and Ruth, 2005), and frequency and type of human-ignited wildfires changed (Balch et al., 2017; Keeley and Syphard, 2018). Changes in these non-climatic factors may also promote non-stationarity in fire-climate relationships, confounding efforts to isolate the influence of climate change on fire activity (Marlon et al., 2012; Higuera et al., 2015; Mann et al., 2016; Taylor et al., 2016; Littell, 2018; Hurteau et al., 2019).”

Methods section
“We did not control for non-climate effects on wildfire such as human effects on ignitions, fire suppression, or vegetation cover. Therefore, the climate-fire relationships that we identified have human impacts embedded within them (e.g., human effects on land cover may influence how wildfire responds to drought). When relevant, we evaluated the stability of fire-climate relationships during 1972–2018 by comparing regression statistics based on only data from the first half versus second half of the record. Similar statistical relationships for both periods would strongly suggest that, while non-climate factors may influence the fire-climate relationship in general, these factors (e.g., fuel accumulation due to wildfire suppression) did not cause a change in the fire-climate relationship during the study period.”

Results section
“The above results strongly suggest that the observed increase in California summer burned area during 1972–2018 (which mainly occurred in northern California forests) was mainly due to increased VPD and not concurrent changes in non-climate factors such as forest management, fire suppression practices, or human ignitions. This is not to say that non-climate factors are negligible in dictating modern annual burned areas. To the contrary, human ignitions greatly enhance the number of wildfires relative to that expected in their absence (Balch et al., 2017) and increased fuel density due to fire suppression (and warming/wetting trends in the high Sierra) may have enhanced the mean state of modern-day forest-fire extent, severity, and sensitivity to aridity (Minnich et al., 1995; Swetnam and Baisan, 1996; Dolanc et al., 2013; Harris and Taylor, 2015). However, while effects of human activities are evident in multi-century assessments of fire activity (e.g., Marlon et al., 2012; Taylor et al., 2016; Klimaszewski-Patterson et al., 2018), changes in background conditions such as fuel abundance during our short study period do not appear responsible for the observed increase in summer forest-fire extent during 1972–2018.”

“Importantly the sensitivity of burned area to aridity is modulated by background conditions such as fuel abundance and connectivity, ignition frequency, and resources dedicated toward suppression, all of which changed over the past century. However, the statistical relationship between vapor-pressure deficit and forest fire area remained stable during 1972–2018, supporting the interpretation that increased aridity was the primary driver of the increase in summer forest-fire area during this time.”

Rich Lentz
Rich Lentz
4 years ago

Why is there no discussion about the absurd “Smokey-the-Bear” program and lack of any, in fact, mandated regulations against removal of the overgrowth and underbrush? One only need look at the early 1900 1800 photos of the Model T driving through a giant Redwood and the complete lack of any underbrush and overgrowth around that tree and the surrounding forest and the present condition of that same area. A simple assessment of forest preserves where there has been no increase in overgrowth and underbrush over this same period may provide more validity to the theory.

Randolph D
Randolph D
4 years ago

Don’t buy this one bit. I mean we should take better care of the environment. But not EVERYTHING can be blamed on climate change, including ground fires.

Frederick Wooding
Frederick Wooding
4 years ago

Didnt even need to read this to know its b.s.. 1st off im a native to California and if you even know the history dating back over a hundred years ago the climate hasnt changed. What has change is the lack of the logging industry that was reduced greatly in the 80s. And coupled with a lack of maintenance. Carr fire was started by a flat tire on a travel trailer which might never have happened if the state had kept the roads mowed along the road edges. And most importantly the majority of the fires are also caused by homelessness. Something the geniuses fail to add in their reports.