Researchers evaluate connection between California wildfires and human-caused climate change

A group of seven scientists published a paper last month that looks at the connection between California wildfires and human-caused climate change, titled Observed Impacts of Anthropogenic Climate Change on Wildfire in California. One of their main conclusions was that climate warming dries the atmosphere which in turn dries fuels, promoting forest fires in the summer.

Here is the abstract:

“Recent fire seasons have fueled intense speculation regarding the effect of anthropogenic climate change on wildfire in western North America and especially in California. During 1972–2018, California experienced a fivefold increase in annual burned area, mainly due to more than an eightfold increase in summer forest‐fire extent. Increased summer forest‐fire area very likely occurred due to increased atmospheric aridity caused by warming. Since the early 1970s, warm‐season days warmed by approximately 1.4 °C as part of a centennial warming trend, significantly increasing the atmospheric vapor pressure deficit (VPD). These trends are consistent with anthropogenic trends simulated by climate models. The response of summer forest‐fire area to VPD is exponential, meaning that warming has grown increasingly impactful.

“Robust interannual relationships between VPD and summer forest‐fire area strongly suggest that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased VPD. Climate change effects on summer wildfire were less evident in nonforested lands. In fall, wind events and delayed onset of winter precipitation are the dominant promoters of wildfire. While these variables did not change much over the past century, background warming and consequent fuel drying is increasingly enhancing the potential for large fall wildfires. Among the many processes important to California’s diverse fire regimes, warming‐driven fuel drying is the clearest link between anthropogenic climate change and increased California wildfire activity to date.”

An illustration from the paper:

Climate change California wildfires
Seasonal and annual burned areas in California for 1972–2018. (a) Total burned area in the four regions of focus: (b) North Coast, (c) Sierra Nevada, (d) Central Coast, and (e) South Coast. Annual burned area is decomposed into that which occurred in January–April (green), May–September (red), and October–December (orange). Significant (p < 0.05) trends are shown as bold black curves.

Below is an excerpt from the paper:

“In this study we evaluated the various possible links between anthropogenic climate change and observed changes in California wildfire activity across seasons, regions, and land cover types since the early 1970s. The clearest link between California wildfire and anthropogenic climate change thus far has been via warming‐driven increases in atmospheric aridity, which works to dry fuels and promote summer forest fire, particularly in the North Coast and Sierra Nevada regions. Warming has been far less influential on summer wildfire in nonforest areas. In fall, the drivers of wildfire are particularly complex, but warming does appear to enhance the probability of large fall wildfires such as those in 2017 and 2018, and this effect is likely to grow in the coming decades.

“Importantly, 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%, 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.”

Citation:
Observed Impacts of Anthropogenic Climate Change on Wildfire in California.
A. Park Williams John T. Abatzoglou Alexander Gershunov Janin Guzman‐Morales Daniel A. Bishop Jennifer K. Balch Dennis P. Lettenmaier
First published: 15 July 2019 https://doi.org/10.1029/2019EF001210

Wildfire risk in California no longer coupled to winter precipitation

Recent large fires may be harbingers of things to come, researchers say

Eiler Fire, Northern California, August 6, 2014. Photo by Bill Gabbert.

By Mari N. Jensen, University of Arizona College of Science

Wet winters no longer predict possible relief from severe wildfires for California, according to a new study from an international team that includes a University of Arizona scientist.

From 1600 to 1903, the position of the North Pacific jet stream over California was linked to the amount of winter precipitation and the severity of the subsequent wildfire season, the team found. Wet winters brought by the jet stream were followed by low wildfire activity, and dry winters were generally followed by higher wildfire activity.

After 1904, the connection between winter moisture brought by the jet stream from December through February and the severity of the wildfire season weakened. The weakened connection between precipitation and wildfires corresponds to the onset of a fire suppression policy on U.S. federal lands, the team reports in the March 4 issue of the Proceedings of the National Academy of Sciences.

The connection disappeared altogether after 1977.

Now, fuel buildup from decades of fire suppression in the 20th century plus rising temperatures from climate change means any year may have large fires, no matter how wet the previous winter, the team writes.

“The moisture availability over California is still strongly linked to the position of the jet stream, but fire no longer is,” said co-author Valerie Trouet, an associate professor of dendrochronology at the UA Laboratory of Tree-Ring Research.

The finding surprised Trouet.

“I didn’t expect there to be no relationship between jet stream dynamics and fire in the 20th century. I expected it to be maybe weaker than before, but not to completely disappear,” Trouet said.

California’s wet winter of 2016-2017 is a good example, she said. That winter was followed by many large fires in 2017, including the Tubbs fire in October and the Thomas fire in December. Twenty-four people died and 6,699 structures burned in those two fires, according to the California Department of Forestry and Fire.

“It’s not either climate change or historical fire management–it’s really a combination of the two that’s creating a perfect storm for catastrophic fires in California,” Trouet said.

To reconstruct California’s fire and moisture patterns and the position of the North Pacific jet stream for the past 400 years, the researchers combined instrumental and historical records of temperature, precipitation and fires with the natural archives of climate and fires stored in tree rings that go back in time for centuries.

Lead author Eugene R. Wahl of the National Oceanic and Atmospheric Administration said, “The method we used to determine the average winter jet stream conditions is a real advance. Coupled with independent precipitation and fire records, this is a state-of-the-art coupling of paleoclimate and paleoecology.”

The study is the first to show the close connection between winter precipitation in California and the position of the jet stream back to the year 1571, Trouet said. The study is also the first to examine the relationship of past winter precipitation, the position of the jet stream and past fire activity stretching back to 1600, she said.

The paper by Wahl, Trouet and two co-authors is, “Jet Stream Dynamics, Hydroclimate, and Fire in California from 1600 CE to Present.”

Initially, Wahl and co-author Eduardo Zorita of the Helmholz-Zentrum Geesthacht in Germany were working independently of Trouet and co-author Alan Taylor.

As part of a larger project to extend global reconstructions of temperature, precipitation and atmospheric circulation further into the past, Wahl and Zorita were figuring out how the North Pacific jet stream affected precipitation in California for centuries. Wahl, a paleoclimatologist at NOAA’s National Centers for Environmental Information in Boulder, Colorado, was a co-leader for the North America part of the larger project.

Trouet and Taylor of Penn State in University Park, Pennsylvania, had already reconstructed California’s fire history back to 1600, and Trouet had reconstructed the behavior of the North Atlantic jet stream back to 1725.

After Wahl heard Trouet give a presentation about her North Atlantic jet stream research, the four scientists joined forces to see whether there were links between the past behavior of the North Pacific jet stream and California’s fire and precipitation history.

“When the jet stream is positioned over California, it’s like a fire hose–it brings storms and moisture straight over California,” Trouet said. “What we see post-1900 is that the position of the jet stream is still an important driver of moisture to California–it brings moisture to California when it’s in the right position–but there’s a disconnect with fire.”

The likelihood that every year may be a high-fire year will be a significant societal challenge, Taylor said.

“Fire not being influenced by moisture anymore? That is surprising. It’s going to be a problem for people, for firefighters, for society,” he said. “The only thing we can control is fuels, so what it suggests is that we take that very seriously.

“The last three years may be a harbinger of things to come,” he said. “Between 1600 and 1903 there was not a single case of a high-precipitation year coupled with a high-fire year as occurred in 2017.”

The research team’s next step is to expand this research to see how the jet stream patterns correspond with fire in other types of forested ecosystems farther north.

The research was funded by the U.S. National Science Foundation, the U.S. Geological Survey’s Southwest Climate Science Center, the U.S. Department of Agriculture Forest Service, the German Science Foundation Cluster of Excellence Clisap, a George H. Deike, Jr. Research Grant, and the Swiss National Science Foundation.


Note from Bill. Even though the research was funded by universities and government agencies, you would still have to pay $10 to a private organization to read the report. It does not comply with Open Access policies.

Researchers develop model to predict wildfire occurrence

Their model uses temperature and precipitation to determine probability

A team of researchers from the University of Missouri and the U.S. Forest Service are continuing an effort to research how climate influences wildfire frequency. The model focuses on two variables – temperature and precipitation – to understand how climate drives wildfire across the world.

After acquiring historic fire occurrence data from tree ring and other studies they developed a mathematical model using temperature and precipitation as the two variables. In validation runs, the predictions the model generated were close to actual fire patterns.  As they continued to collect additional historic data from locations around the world during the last several years, they refined the model making it more accurate.

research wildfire probability temperature precipitation
A Combustion-Climate diagram (CCd) of climate influences on fire probability. Climate simulated fire probabilities for ‘natural’ ecosystems using mean maximum temperature and annual precipitation in the PC2FM. This rate diagram explains two temporal differences related to the combustion of ecosystems. Temperature and precipitation affect the reaction rate at the time the reaction occurs while the rate of fuel production determines the fuel concentration and its combustion rate. These two timing conditions differentially determine the rates of the two components of the PC2FM model: ARterm and the PTrc3. (From the team’s research)

“You can see patterns in global wildfire frequency that are obviously predictable,” Michael Stambaugh, an associate research professor in forestry, said. “For example, ¹Greenland doesn’t burn. It’s too icy and wet. It’s on one end of the spectrum. The other end of the spectrum is a place like the Sahara Desert, which doesn’t burn either. It’s too dry and there’s not enough fuel. Between those two extremes, we were confident that there was a way to describe the transition.”

The work is being done by Richard Guyette, Michael Stambaugh, Daniel Dey, and Rose-Marie Muzika who developed what they call the “Physical Chemical Fire Frequency Model (PC2FM)”.

More information about their research.

 

¹Note from Bill: To be clear, Greenland RARELY burns

Report highlights links between wildfires, climate change and health

Pollution from fossil burel-burning sources is decreasing but the air is getting dirtier during the wildland fire season.

As a long and brutal fire season in California starts to wind down, Climate Central issued a report that lays out links between climate change, wildfire, and health effects.  The report, titled Western Wildfires Undermining Progress on Air Pollution, analyzes air quality trends from 2000 through 2016 in two large California air basins — the Sacramento Valley and the San Joaquin Valley — that are heavily affected by pollution. 

The report finds that while the air quality continues to improve as pollution from power plants, trucks and other fossil fuel-burning sources declines, it is getting dirtier during the fire season. Studies have shown that fire seasons in the West are getting longer and that more large wildfires are breaking out as temperatures rise.

“We focused on an especially bad actor called “fine particulate matter”, or PM2.5 — particles that can reach deep into the lungs and exacerbate a wide array of health problems such as asthma, heart disease, and premature birth.  A lot of hard work has been done to decrease PM2.5 from other sources, so it’s troubling to see progress getting undercut by wildfires — and to know that a warming climate will likely have wildfires becoming more frequent and burning more area in California and the West,” said Todd Sanford, Ph.D. scientist with Climate Central.

Research: Wildfires in Sierra Nevada driven by past land use

Changes in human uses of the land have had a large impact on fire activity in California’s Sierra Nevada since 1600, according to research by a University of Arizona researcher and her colleagues.

Above: Indian Canyon Fire near Edgemont, SD, 2016. Photo by Bill Gabbert.

By Mari N. Jensen, University of Arizona College of Science

Forest fire activity in California’s Sierra Nevada since 1600 has been influenced more by how humans used the land than by climate, according to new research led by University of Arizona and Penn State scientists.

For the years 1600 to 2015, the team found four periods, each lasting at least 55 years, where the frequency and extent of forest fires clearly differed from the time period before or after.

However, the shifts from one fire regime to another did not correspond to changes in temperature or moisture or other climate patterns until temperatures started rising in the 1980s.

“We were expecting to find climatic drivers,” said lead co-author Valerie Trouet, a UA associate professor of dendrochronology. “We didn’t find them.”

Instead, the team found the fire regimes corresponded to different types of human occupation and use of the land: the pre-settlement period to the Spanish colonial period; the colonial period to the California Gold Rush; the Gold Rush to the Smokey Bear/fire suppression period; and the Smokey Bear/fire suppression era to present.

“The fire regime shifts we see are linked to the land-use changes that took place at the same time,” Trouet said.

“We knew about the Smokey Bear effect — there had been a dramatic shift in the fire regime all over the Western U.S. with fire suppression. We didn’t know about these other earlier regimes,” she said. “It turns out humans — through land-use change — have been influencing and modulating fire for much longer than we anticipated.”

Continue reading “Research: Wildfires in Sierra Nevada driven by past land use”

Was the 2014 wildfire season in California affected by climate change?

Happy Camp Complex, 2014
Happy Camp Complex in northern California in 2014. Photo by Kari Greer.

2014 was a busy year in California for wildland firefighters. Battles were fought over 555,044 acres of blackened ground in the state, which was the eighth largest number of acres burned in the last 28 years. So far in 2015, fires have covered 838,465 acres in California, which puts it fifth highest in 28 years.  (Stats from Cal FIRE and the NIFC National Situation Report.)

We have always been dubious of linkages between one weather event and long term climate change. When a senator brings a snowball onto the Senate floor or a governor talks about this summer’s fires to prove their cases that climate change does or does not exist, both may be over stating their “evidence”.

However, I’m not a meteorologist or climate scientist. But some of them who are, took a stab at investigating the possible attribution of extreme weather-related events in 2014 to global climate change. In their report, Explaining Extreme Events of 2014 from a Climate Perspective33 different research groups explored the causes of 29 different events that occurred that year.

The first event in the report is titled, Extreme Fire Season in California: A Glimpse Into the Future. It is debatable if the 2014 fire season in California was “extreme”, since like we wrote earlier, it had the eighth largest number of acres burned in the last 28 years according to data from the land management agencies. The authors, Jin-Ho Yoon, S.-Y. Simon Wang, Robert R. Gillies, Lawrence Hipps, Ben Kravitz, and Philip J. Rasch, reported “thousands more fires than the five-year average” between January 1 and September 20.

We don’t put very much stock in numbers of fires, since a small spot that can be stomped out by a couple of firefighters counts just as much as a 300,000-acre conflagration. Total burned acres is much more meaningful. The area burned data that the scientists studied was derived from satellite observations, which can underestimate wildfire extent due to its limit in the minimum detectable burned area, timing of the satellite overflights, light fuels cooling before being detected, and obscuration by cloud cover.

The report also examined the Keetch-Byram Drought index, and determined that “in terms of the KBDI and the extreme fire risk, 2014 ranks first in the entire state”, but it was not clear what time period they were referring to (it may have been since 1979).

The authors fall short of attributing the “extreme” 2014 fire season in California to global climate change:

Our result, based on the CESM1 outputs, indicates that man-made global warming is likely one of the causes that will exacerbate the areal extent and frequency of extreme fire risk, though the influence of internal climate variability on the 2014 and the future fire season is difficult to ascertain.

2014 climate events
Location and types of events analyzed in the publication. The image is from the study.