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As Baby Boomers hit retirement, about is now over the age of 65. The number of Americans living with dementia is — but the proportion of older Americans who develop dementia has actually decreased. The exact reason why is uncertain, but various lifestyle and environmental factors can of cognitive decline.����

One recently discovered risk is air pollution. exposure to a type of air pollution called fine particulate matter, or PM2.5, with an increased risk of developing dementia, and researchers suspect that some sources of PM2.5 may pose a greater risk than others.��

New research led by the ����Ӱ�Ӵ�ý found that wildfire smoke is especially hazardous. An analysis of the health care records of 1.2 million Southern California residents found that higher long-term smoke exposure was associated with a significant increase in the odds that a person would be diagnosed with dementia.������

The researchers at the Alzheimer’s Association International Conference in July and

“There have been studies that have found total PM2.5 is related to people developing dementia, but no one had looked specifically at wildfire PM2.5,” said lead author , a UW associate professor of environmental & occupational health sciences. “Wildfire smoke is a different animal, in that it’s much spikier. There are many days where there’s no wildfire smoke, and there are some days where exposure is really, really extreme.”��

Researchers analyzed the health records of 1.2 million members aged 60 and older of Kaiser Permanente Southern California between 2008 and 2019, all of whom were free from dementia at the start of the study period. They estimated each person’s long-term exposure to both wildfire and non-wildfire PM2.5 as a three-year rolling average, and then identified people who received a dementia diagnosis.��

Researchers found that for every 1 microgram per cubic meter (µg/m3) increase in three-year average wildfire PM2.5 concentration, the odds of a dementia diagnosis increased by 18%. Exposure to non-wildfire PM2.5 also increased a person’s risk of dementia, but to a much lesser degree.����

“One microgram per meter cubed might sound fairly small, but we have to think about how people are exposed to wildfire smoke,” Casey said. “Most days they aren’t exposed at all, so this might represent a few days of exposure at a concentration of something like 300 µg/m3, where the AQI is over 200 in someone’s community. When you think about it, it’s actually a few really severe wildfire smoke days that might translate into increased risk.”��

That risk further increased among racialized people and those living in high-poverty census tracts, following long-term trends in which vulnerable populations often experience disproportionate effects of environmental hazards. The authors suggested that disparities might be related to lower-quality housing, which can increase the amount of smoke that enters people’s homes, or lower-income families’ inability to afford air filtration systems.����

The study period does not include the summers of 2020 and 2021, which produced the most recorded in California. The climate crisis has the frequency and severity of wildfires across the American West, introducing “smoke season” in many West Coast regions The influx of smoke has at air quality improvements made over the last century.��

“The main culprit here is climate change,” Casey said. ���t’s a global problem. While individuals can protect themselves with air filters and masks, we need a global solution to climate change. It’s going to have to be many-pronged��— many people have to be involved to solve this highly complex problem.”��

Co-authors on this study are Holly Elser of the University of Pennsylvania; Timothy Frankland of the Kaiser Permanente Hawaii Center for Integrated Health Research; Chen Chen and Tarik Benmarhnia of the Scripps Institution of Oceanography at UC San Diego; Sara Tartof and Gina Lee of Kaiser Permanente Southern California; Elizabeth Rose Mayeda of UCLA; Dr. Alexander Northrop of Columbia University; and Jacqueline Torres of UC San Francisco. This research was funded by the National Institute on Aging and the National Institute for Environmental Health Sciences.��

Across the American West, managers of fire-prone landscapes are increasingly using a practice that seems counterintuitive: setting small fires to prevent larger, more destructive ones. Commonly called “prescribed burns,” these targeted, controlled fires keep forests healthy by reducing the buildup of grasses, leaves, branches, and other debris that can fuel larger wildfires and smoke out nearby communities.����

But smoke from prescribed burns also presents health risks. Today’s forest managers must ask themselves — how much prescribed burning is too much? When do the long-term benefits of fuel reduction no longer outweigh the short-term smoke costs? And how can nearby communities better prepare for a fire season?����

An international team led by researchers at the ����Ӱ�Ӵ�ý built a framework to help land managers assess the air quality implications of land management scenarios with different levels of prescribed burning. To apply the framework, researchers��linked together a series of models that estimate the smoke effects of various levels of prescribed burning on ecosystems and nearby communities.����

After using those models to estimate the smoke produced under six different levels of prescribed burning across California’s Central Sierra range, the researchers found that moderate amounts of burning would reduce overall smoke levels. All tested levels of prescribed fires led to less wildfire smoke overall. But greater amounts of prescribed fires could present notable health hazards of their own.����

The researchers reported their findings — specific to the Central Sierra landscape — in a pair of recently published papers. The first, , estimated how different levels of prescribed burning affected the total amount of smoke produced during an average wildfire season. The second, , analyzed the impacts on the region’s outdoor agricultural workers.��

“We haven’t had a good way to put numbers to that smoke exposure trade-off previously because of challenges in integrating data and methods across sectors,” said , a UW doctoral alum of the Department of Environmental & Occupational Health Sciences and lead author of both papers. She is now a postdoctoral scholar at UCLA. “We know that if we can reduce fuel density, then wildfires may be less severe when they do come through. Emissions may also be lower, and thus subsequent smoke exposure and health impacts will be less. We also must consider that the location and timing of prescribed burns are planned, which is not the case for wildfires. That’s the concept. But I think communicating that has previously been difficult.����

“What’s cool about this work is we were finally able to quantify the trade-off between reducing wildfire risks and its impacts on human health through prescribed burning at a local scale.”��

Researchers focused on the Tahoe Central Sierra Initiative, a 2.4 million acre expanse covering public, private and commercial land. A consortium of land managers in the area developed six forest management scenarios with increasing levels of prescribed burning. They ranged from Minimal Management, with no prescribed burns and limited efforts to trim back excess fuels, to a scenario dubbed Fire++, with an estimated 30,000 acres of prescribed burning each year.����

Those scenarios were fed into a series of models that estimated the amount of smoke generated by wildfires and prescribed burns in each scenario, and the health impacts on nearby communities.����

Every scenario that included prescribed burning in the Tahoe Central Sierra Initiative resulted in a shorter wildfire smoke season, with less overall smoke, than those without prescribed burns. As a result, nearby communities and outdoor agricultural workers could be exposed to less smoke.����

The model predicted that overall smoke levels as measured by concentrations of fine particles (PM 2.5) were lowest with a moderate amount of prescribed burning — a scenario researchers called, simply, Fire. Scenarios that involved greater amounts of burning — Fire+ and Fire++ — produced slightly more total smoke than the moderate scenario. ��

Schollaert hopes forest managers across the country will replicate the methods, so they can better incorporate public health considerations into management planning on their specific landscapes.��

“The exact placement of that sweet spot of prescribed burning is going to vary. But when mitigating extreme wildfire risk, the more you can lower severity of fire, the lower your emissions are going to be, generally,” Schollaert said. “And baked into that sweet spot is also coordination with health agencies, because you can theoretically plan for smoke from prescribed burns. That’s the kind of planning I’m hoping can come from this.”��

Other authors on both papers include and of the UW Department of Environmental & Occupational Health Sciences; of the UW School of Environmental and Forest Sciences and of the UW Department of Civil and Environmental Engineering, among others.����

Research for the Nature Sustainability paper was funded by Science for Nature and People Partnerships. Research for the Environmental Research Letters paper was funded by NASA and the U.S. Department of Energy.��

For more information, contact Schollaert at cschollaert@ucla.edu.��

doesn’t want her work to scare people. It’s already unsettling when wildfire smoke descends upon a community, when eyes burn and throats scratch and people trickle into emergency rooms. She’d rather people see her research, which ties wildfire smoke to an increased risk of emergency department visits, as a step toward protecting themselves. ��

��� think it’s useful to see it as more information, and use that to help us figure out what we can do to protect ourselves,” said Doubleday, who completed the research while working toward her doctorate in environmental health at the UW and now works on air quality for the Washington State Department of Health. “For me the takeaway is we’re all at risk of health impacts. Obviously some more than others, such as those with pre-existing respiratory or cardiovascular conditions, but we all should be taking steps to reduce exposure and watching for any symptoms.”��

That’s the crux of two papers recently published in Environmental Research: Health by researchers at the ����Ӱ�Ӵ�ý, which found an increased risk of hospital service encounters in the days following wildfire smoke events. Taken together, their findings suggest that wildfire smoke poses a risk to people of all ages, not just young children and older adults. ��

The researchers found that the risk of respiratory-related emergency department encounters increased most sharply for those between the ages of 19 and 64. The findings suggest that public health messaging should also target younger and middle-aged adults, who may not see themselves as vulnerable to wildfire smoke. ��

“We do have this younger age group in there who may think they’re invincible, or that the risk messaging doesn’t apply to them because they’re not very young or elderly,” said , teaching professor of environmental and occupational health sciences at the UW and co-author of both papers. Isaksen is also co-director of the , which has produced a string of papers on the risks of wildfire smoke. ��

“Knowing that essentially all age groups are at risk of negative health outcomes during wildfire smoke events is an important finding and a shift in how we think of who is vulnerable in our population during these events,” Busch Isaksen said. ��� expect these results will be informative to public health risk communication strategies aimed at reducing wildfire smoke exposure in all age groups through behavior change such as limiting time outdoors, actively cleaning your indoor air, etc. “��

The first study, led by Doubleday and , analyzed emergency department (ED) data from hospitals across Washington state. It found an increased risk of respiratory-related ED visits, including visits for asthma, in the five days following a smoke event. Researchers also observed a delayed increase in the odds of cardiovascular-related ED visits. ��

The analysis also found a correlation between the amount of smoke in the air and the risk of ED encounters. For every 10 µg��−3 increase in the concentration of fine particle pollution — PM 2.5 or particulate matter 2.5 micrometers or smaller — the odds of ED visits rose accordingly. ��

The second study, led by recent UW graduate , is among the first to document the health effects of wildfire smoke on children in Washington state. , it analyzed 15 years of data from Seattle Children’s Hospital’s emergency department and in-patient hospital admissions, comparing rates of visits on days with and without smoke. ��

Researchers linked wildfire smoke events to a 7% increase in the odds of all-cause hospital admissions. Notably, the odds of hospitalization remained elevated in the week after smoke events, highlighting the need to monitor children’s symptoms well after exposure.��

“We definitely want to be more cognizant of exposure when it comes to children during wildfire smoke season,” said Iyaz, who earned a master’s in environmental health from the UW and now works in extreme heat mitigation for King County. “After children are exposed to wildfire smoke, keep monitoring symptoms for a couple of days, because they can lag, especially if there are underlying health conditions that might contribute.”��

The study did not find any change in visits to the emergency department, which researchers attributed to the unique population served by Seattle Children’s. As a Level I trauma center, the hospital draws medically complicated cases from across the region, so its patients may be at greater risk of hospitalization than the general population. Parents may also be more likely to bring a sick child to the nearest emergency room, where their visit wouldn’t be captured by this specific dataset.��

Even before these papers were published, the findings began to show real-world impacts on public health.��Iyaz designed an easy-to-read summary of how smoke can affect children’s health, so patients’ families can better prepare for future events. ��

“Wildfire smoke days are relatively new, and not all people may understand them,” Iyaz said. ���f people aren’t aware of what wildfire smoke is and the impacts it can have, that makes it more important to meet communities where they are and talk about what the health effects can be.”��

For more information, contact Busch Isaksen at tania@uw.edu, or visit the

In 2006, the Tripod Complex Fire burned more than 175,000 acres in north-central Washington. The fire, which was within the Okanogan-Wenatchee National Forest, was more than three times the size of Seattle. Yet while considered severe at the time, even larger wildfires in 2014, 2015 and 2021 have since dwarfed Tripod.

Past research shows that large and severe wildfires like these were much rarer in the western U.S. and Canada prior to the late 20th century.

“Fire exclusion policies for much of the 20th century yielded many dense forests with largely uniform composition,” said , a research scientist with the UW School of Environmental and Forest Sciences. “By the turn of this century, we had mature and densely treed, multi-layered forests with high fuel content — and as a result, large, destructive wildfires can ignite and spread more easily. There’s simply more to burn across large landscapes.”

Prichard, along with colleagues from the U.S. Forest Service’s Pacific Northwest Research Station — , Nicholas Povak and Brion Salter — and consulting fire ecologist Robert Gray, have created a modeling tool that will allow managers and policymakers to imagine and realize a different future: one where large, severe wildfires like Tripod are once again rare events, even under climate change.

The tool, known as REBURN, can simulate large forest landscapes and wildfire dynamics over decades or centuries under different wildfire management strategies. The model can simulate the consequences of extinguishing all wildfires regardless of size, which was done for much of the 20th century, or of allowing certain fires to return to uninhabited areas. REBURN can also simulate conditions where more benign forest landscape dynamics have fully recovered in an area.

In a pair of papers published and in the journal Fire Ecology, the team applied REBURN to the region in north-central Washington where the 2006 Tripod Complex Fire burned. Simulations showed that setting and allowing smaller wildfires to burn can yield more varied and resilient forests over time. Such forests are made up of forest condition “patches” of different sizes and shapes, and all at different stages of recovery from their most recent fire. Patches that recently burned acted as “fences” to the flow of fire for at least the next 5 to 15 years, preventing wildfires from spreading widely. REBURN simulations showed that a forest landscape comprised of 35 to 50% “fence” areas had far fewer large-scale and damaging wildfires.

“Landscapes had tipped to more ‘benign’ burning conditions,” said Hessburg.

REBURN simulations showed that, when fence areas were less abundant across a region, larger and more severe wildfires tended to dominate how the landscape developed over time.

“The model allows us to simulate what can happen when different management scenarios are applied before the fact, including how small or medium-sized fires in uninhabited areas can reshape forest vulnerability to fires,” said Prichard. “We found that having a more complex forest environment — in terms of tree age, composition, density, fuel content — makes it harder for large fires to spread and become severe.”

“We also found that non-forest areas comprised of grasslands, shrublands, wet and dry meadows, and sparsely treed woodlands were key ingredients of wildfire-resilient forests,” said Hessburg. “REBURN showed us that our policy of extinguishing all wildfires created forests like those that exist today, with large, severe wildfires growing more prevalent. In addition to destroying homes and blanketing cities and towns with smoke, conflagrations like these displace wildlife, destroy habitats, and can burn large areas severely, sometimes making it difficult for forests to return.”

Short intervals between forest reburns can be especially harmful for long-term recovery by destroying young trees that have not yet produced cones, they added.

From 1940 to 2005 in Washington’s North Cascades, fire crews extinguished more than 300 fires in their early stages in the Tripod area — most triggered by lightning strikes. By the 1980s and 1990s, forests in the region had become high-density tinderboxes, loaded with older, dying trees and lots of dead wood and other fuel on the ground.

Research has shown that before large-scale European colonization of the area, smaller wildfires shaped forests in north-central Washington and elsewhere in the Pacific Northwest. The Methow people and other tribes in the region actively set fires through cultural burning practices. Aerial photos show that, as recently as the 1930s, forests in north-central Washington had a “patchwork quilt” structure that kept large wildfires from forming easily.

“Forests with more complex structure — including densely and lightly treed areas like meadows and grasslands, shrublands, and spare woodlands — also create a wider variety of habitats for wildlife,” Hessburg said. “Recently burned areas can develop into wet or dry meadows that can host deer or moose. Other, younger tree-dense areas can host lynx and snowshoe hares.”

REBURN can be adapted to other regions in the western U.S. and Canada. Prichard, Hessburg and their colleagues are currently adapting it to simulate forest development in the vast forests of southern British Columbia and northern California, including regions recently hit by wildfires and those culturally burned by Indigenous people.

But knowing when — or even whether — to allow a small fire to burn in an uninhabited region is no easy task, since fire managers must protect people, their homes and livelihoods. The team hopes ongoing research will help refine the model and the insight it can provide to modified forest management strategies.

“This is a new type of tool that couples forest and non-forest development models over time, fuel fall-down after fires, and a fire growth model,” said Hessburg.

“We hope that it will help people who make major decisions about our forests understand the long-term consequences of different practices and policies when it comes to wildfires,” said Prichard. “We hope it will make these conversations easier to have by grounding our predictions in sound forest science.”

The research was funded by the Joint Fire Science Program and the U.S. Forest Service Pacific Northwest Research Station.

For more information, contact Prichard at 509-341-4493 and sprich@uw.edu and Hessburg at 509-423-6738 and paul.hessburg@usda.gov.

With wildfire smoke ��for next week in Seattle and the Occupational Safety and Health Administration in Oregon for keeping workers safe during increasingly smoky conditions and heat in that state, we caught up with a ����Ӱ�Ӵ�ý expert on worker safety for advice.

, an industrial hygiene program director and assistant professor in the UW Department of Environmental & Occupational Health Sciences, provided these insights to UW News:

“We are still living through the COVID-19 pandemic and often the advice that we are giving to keep people safe from COVID is at odds with the advice we give to keep people safe from wildfire smoke. With COVID we want as much outside air as possible inside, and with wildfire smoke we want as much outside air kept outside as possible. But the one thing that is protective against both is an N95 mask. HEPA filters and upgraded filters on ventilation systems are also protective against both,” Baker said.

For more information from Professor Baker, check out the video at left.

For more information on the hazards of wildfire smoke and how individuals can protect themselves, please check out this informational website published by UW’s Department of Environmental & Occupational Health Sciences:

Firefighters have reported that Western wildfires are starting earlier in the morning and dying down later at night, hampering their ability to recover and regroup before the next day’s flareup.

A study by ����Ӱ�Ӵ�ý and U.S. Forest Service scientists shows why: The drying power of nighttime air over much of the Western U.S. has increased dramatically in the past 40 years. The was published online in July in Geophysical Research Letters, a journal of the American Geophysical Union.

“Nighttime is an important time in fire management. When fires die down at night it gives firefighters a chance to rest, move equipment and strategize. The problem firefighters are reporting is an unexpected increase in nighttime fire activity,” said lead author , a UW research scientist at the Cooperative Institute for Climate, Ocean & Ecosystem Studies, a joint center with the National Oceanic and Atmospheric Administration. “Our findings support that this has been going on over the last 40 years over much, but not all, of the Western U.S.”

Earth’s atmosphere is warming due to climate change, and warming in many places has been greater at night. Warmer night air had been suspected as the culprit , with burns continuing later into the night.

The new study, however, shows it’s not just that the night air is warmer, but also a dramatic shift from 1980 to 2019 in its drying power — how much moisture the nighttime air can carry away from the fuels — over much of the Western U.S. This shift is not captured in climate models, and the authors say it could be related to natural long-term cycles rather than to climate change.

“We paid special attention to the change in recent years compared to the conditions seen in the ’80s and ’90s, which is when many of the current firefighters started their careers, and presumably formed their ideas about what normal fire behavior should look like,” Chiodi said. “We tried to quantify the changes that we were hearing about from firefighters.”

The study looks at the “vapor pressure deficit,” or the difference between the moisture in the air and the saturation moisture level at that air temperature. This difference is a measure of the air’s drying power.

���n the southern Sierra Nevada, the average summer nighttime vapor pressure deficit for the recent decade was 50% higher than the average in the ’80s and ’90s,” Chiodi said. ��� was surprised — it’s unusual to see geophysical data change that dramatically.”

Some of this shift in vapor pressure deficit is happening because warmer nighttime air, caused by climate change, produce higher saturation values. But part of the drying power is happening because the nighttime air in some regions has less moisture, and that effect is not predicted by climate change models, at least this much or in this pattern. The authors find a possible connection to the Pacific Decadal Oscillation, a long-term cycle that can influence inland weather.

The increased drying power of nighttime air is especially pronounced in California’s San Joaquin Valley and in the Bitterroot-Blue Mountain Region — including parts of the Idaho Panhandle, southeast Washington, northeast Oregon and western Montana.

“Firefighters had been saying for several years that they feel some fires burn later into the evening than they used to,” said co-author at the U.S. Forest Service’s Pacific Wildland Fire Sciences Laboratory. “We found that in some areas, the amount of water in the air is decreasing, sort of doubling up on the warmer nights. These areas, including where the Snake River Complex and Lick Creek fires are burning right now, are much more likely to have fires burn late into the night.”

The analysis used hourly weather outputs from the European Centre for Medium-Range Weather Forecasts. The recently released hourly reconstructions of historical weather allowed investigation of daily cycles.

The next step, Chiodi said, is to further explore the causes of these changes in nighttime vapor pressure deficit. After that, he hopes to connect the atmospheric conditions more directly to fuel moisture and fire behavior.

The other co-author is at the U.S. Forest Service’s Pacific Wildland Fire Sciences Laboratory in Seattle. The research was funded by the U.S. Forest Service through its research team and by NOAA.

###

For more information contact Chiodi at chiodi@uw.edu or Potter at brian.potter@usda.gov.

Exceptionally hot and dry weather this summer has fueled dozens of wildfires across the western U.S., spewing smoke across the country and threatening to register yet another record-breaking year. More than a century of fire exclusion has created dense forests packed with excess trees and brush that ignite and spread fires quickly under increasingly warm and dry conditions.

Scientists largely agree that reducing these fuels is needed to make our forests and surrounding communities more resilient to wildfires and climate change. But policy and action have not kept pace with the problem and suppressing fires is still the norm, even as megafires become more common and destructive.

Seeing the urgent need for change, a team of scientists from leading research universities, conservation organizations and government laboratories across the West has produced a that clearly lays out the established science and strength of evidence on climate change, wildfire and forest management for seasonally dry forests. The goal is to give land managers and others across the West access to a unified resource that summarizes the best-available science so they can make decisions about how to manage their landscapes.

“Based on our extensive review of the literature and the weight of the evidence, the science of adaptive management is strong and justifies a range of time- and research-tested approaches to adapt forests to climate change and wildfires,” said co-lead author Susan Prichard, a research scientist in the ����Ӱ�Ӵ�ý’s School of Environmental and Forest Sciences.

These approaches include some thinning of dense forests in fire-excluded areas, prescribed burning, reducing fuels on the ground, allowing some wildfires to burn in backcountry settings under favorable fuel and weather conditions, and revitalizing Indigenous fire stewardship practices. The were published Aug. 2 as an invited three-paper feature in the journal Ecological Applications.

The authors studied and reviewed over 1,000 published papers to synthesize more than a century of research and observations across a wide geographic range of western North American forests. The analysis didn’t include rainforests in the Pacific Northwest or other wet forests where thinning and prescribed burning wouldn’t be advised.

“The substantial changes associated with more than a century of fire exclusion jeopardize forest diversity and keystone processes as well as numerous other social and ecological values including quantity and quality of water, stability of carbon stores, air quality, and culturally important resources and food security,” said co-lead author and UW researcher .

This ambitious set of articles was inspired by the reality that under current forest and wildfire management, massive wildfires and drought are now by far the dominant change agents of western North American forests. There is an urgent need to apply ecologically and scientifically credible approaches to forest and fire management at a pace and scale that matches the scope of the problem, the authors say.

Part of the solution involves addressing ongoing confusion over how to rectify the effects of more than a century of fire exclusion as the climate continues to warm. Land managers and policymakers recognize that the number and size of severe fires are rapidly increasing with climate change, but agreement and funding to support climate and wildfire adaptation are lagging.

To that end, these papers review the strength of the science on the benefits of adapting fire-excluded forests to a rapidly warming climate. The authors address 10 common questions, including whether management is needed after a wildfire, or whether fuel treatments (thinning, prescribed burning) work under extreme fire weather. They also discuss the need to integrate western fire science with traditional ecological knowledge and Indigenous fire uses that managed western landscapes for thousands of years.

Although climate change brings with it many uncertainties, the evidence supporting intentional forest adaptation is strong and broad based. The authors clearly demonstrate that lingering uncertainties about the future should no longer paralyze actions that can be taken today to adapt forests and communities to a warming climate and more fire.

“This collection represents a blending of scientific voices across the entire disciplinary domain,” said co-lead author , a research ecologist with the U.S. Forest Service and affiliate professor at the UW. “After reviewing the evidence, it is clear that the changes to forest conditions and fire regimes across the West are significant. The opportunity ahead is to adapt forests to rapidly changing climatic and wildfire regimes using a wide range of available, time-tested management tools.”

Co-authors on this special report are from University of Arizona, University of British Columbia, University of California, Berkeley, University of California, Merced, University of Idaho, University of Montana, University of New Mexico, Northern Arizona University, Oregon State University, The Pennsylvania State University, Utah State University, U.S. Forest Service research stations (Pacific Northwest, Pacific Southwest, Rocky Mountain), U.S. Forest Service, Pacific Southwest Region, Washington State Department of Natural Resources, California Department of Forestry and Fire Protection, U.S. Fish and Wildlife Service, U.S. Geological Survey, The Nature Conservancy, R.W. Gray Consulting, Rocky Mountain Tree-Ring Research and Spatial Informatics Group.

This research was funded by U.S. Fish and Wildlife Service, The Wilderness Society, The Nature Conservancy of Oregon, Conservation Northwest, The Ecological Restoration Institute, Washington State Department of Natural Resources, U.S. Forest Service (Pacific Northwest and Pacific Southwest Research Stations), and the California Department of Forestry and Fire Protection.

For more information, contact Prichard at sprich@uw.edu, Hessburg at paul.hessburg@usda.gov��and Hagmann at hokulea@uw.edu

In recent years, wildfires on the West Coast have become larger and more damaging. A combination of almost a century of fire suppression and hotter and drier conditions has created a tinderbox ready to ignite, destroying homes and polluting the air over large areas.

New research led by the ����Ӱ�Ӵ�ý and the University of California, Santa Barbara, looks at the longer-term future of wildfires under scenarios of increased temperature and drought, using a model that focuses on the eastern California forests of the Sierra Nevada. The , published July 26 in the journal Ecosphere, finds that there will be an initial roughly decade-long burst of wildfire activity, followed by recurring fires of decreasing area.

“That first burst of wildfire is consistent with what we’re seeing right now in the West. The buildup of fuels, in conjunction with the increasingly hot and dry conditions, leads to these very large, catastrophic fire events,” said lead author , assistant professor at the ����Ӱ�Ӵ�ý Tacoma. “But our simulations show that if you allow fire to continue in an area, then the fire could become self-limiting, where each subsequent fire is smaller than the previous one.”

How climate change, tree growth and wildfires will interact over coming decades is only beginning to be explored, Kennedy said, through experiments and simulations. Existing models of vegetation often assume wildfires will strike at set intervals, like every 10 years, or based on past patterns of wildfire risk for that ecosystem. But those previous patterns may not be the best guide to the future.

“The big question is: What’s going to happen with climate change? The relationships that we’ve seen between climate and wildfire over the past 30 years, is that going to continue? Or is there going to be a feedback? Because if we keep burning up these fuels, and with extreme drought that limits new growth, there will eventually be less fuel for wildfires,” Kennedy said.

The new study used a model that includes those feedbacks among climate, vegetation growth, water flows and wildfire risk to simulate the Big Creek watershed outside Fresno, California, near the site of the September 2020 . Climate models suggest that here, as in other parts of the West, conditions will likely continue to get hotter and drier.

Results of the 60-year simulations show that under increased drought and rising temperatures, the large wildfires will continue for about a decade, followed by recurring wildfires that occur in warm and dry conditions, but are smaller over time. Even without wildfire the trees in the forest declined in number and size over time because they were less productive and more stressed in the hot and dry conditions. These findings would likely apply to other forests that experience drought, said Kennedy, who’s now using the model on other regions.

What happens with wildfires over the longer term matters now for planning. Current understanding is that communities will have to coexist with wildfire rather than exclude it entirely, Kennedy said. A combination of prescribed burns and forest thinning will likely be the future of managing forests as they contend with both wildfires and climate change.

“With such high density in the forest, the trees are pulling a lot of water out of the soil,” Kennedy said. “There is growing evidence that you can relieve drought stress and make more drought-resilient forests if you thin the forests, which should also help with, for example, reducing the impact of that initial pulse of wildfire.”

After thinning out smaller trees, managers could then do controlled burns to remove kindling and smaller material on the forest floor. But knowing how to manage forests in this way requires understanding how local weather conditions, plant growth and wildfire risk will play out in future decades.

���t’s important to include climate change so we have an idea of the range of variability of potential outcomes in the future,” Kennedy said. “For example, how often do you need to repeat the fuels treatment? Is that going to be different under climate change?”

Kennedy was also a co-author of another that uses the same model to tease apart how much climate change and fire suppression increase wildfire risk in different parts of Idaho.

“Our ‘new normal’ is not static,” said , a professor at UC Santa Barbara who is a co-author on both studies and developed the RHESSys-FIRE model that was used in the research. “Not only is our climate continuing to change, but vegetation — the fuel of fire — is responding to changing conditions. Our work helps understand what these trajectories of fire, forest productivity and growth may look like.”

This research was funded by the National Science Foundation and the U.S. Forest Service. Other co-authors are Ryan Bart at the University of California, Merced, and Janet Choate at UC Santa Barbara.

For more information, contact Kennedy at mkenn@uw.edu or Tague at ctague@bren.ucsb.edu. ��

NSF grant: EAR-1520847, USFS

Wildfires burning in the West affect not only the areas burned, but the wider regions covered by smoke. Recent years have seen hazy skies and hazardous air quality become regular features of the late summer weather.

Many factors are causing Western wildfires to grow bigger and to generate larger, longer-lasting smoke plumes that can stretch across the continent. An analysis led by the ����Ӱ�Ӵ�ý looks at the most detailed observations to date from the interiors of West Coast wildfire smoke plumes.

The multi-institutional team tracked and flew through wildfire plumes from the source to collect data on how the chemical composition of smoke changed over time. A resulting , published Nov. 2 in the Proceedings of the National Academy of the Sciences, shows that smoke forecasts may incorrectly predict the amount of particles in staler smoke.

The new results could significantly change the estimate for particles in staler smoke, which could be the difference between “moderate” and “unhealthy” air quality in regions downwind of the fire.

“Wildfires are getting larger and more frequent, and smoke is becoming a more important contributor to overall air pollution,” said lead author , a UW professor of atmospheric sciences. “We really targeted the smoke plumes close to the source to try better understand what’s emitted and then how it can transform as it goes downwind.”

Knowing how newly generated wildfire smoke transitions to stale, dissipated smoke could lead to better forecasts for air quality. Communities can use those forecasts to prepare by moving outdoor activities inside or rescheduling in cases where the air will be unsafe to be outdoors, as well as limiting other polluting activities such as wood-burning fires.

“There are two aspects that go into smoke forecasts,” said first author , a UW postdoctoral researcher in atmospheric sciences. “One is just where is the smoke plume going to go, based on dynamics of how air moves in the atmosphere. But the other question is: How much smoke gets transported — how far downwind is air quality going to be bad? That’s the question our work helps to address.”

When trees, grass and foliage burns at high temperatures they generate soot, or black carbon, as well as organic particles and vapors, called organic aerosols, that are more reactive than soot. Fires can also produce “” aerosol, a less-well-understood form of organic aerosol that gives skies a brownish haze.

Once in the air, the organic aerosols can react with oxygen or other molecules already in the atmosphere to form new chemical compounds. Air temperature, sunlight and concentration of smoke affect these reactions and thus alter the properties of the older smoke plume.

The multi-institutional team measured these reactions by flying through wildfire plumes in July and August 2018 as part of WE-CAN, or the field campaign led by Colorado State University.

Research flights from Boise, Idaho, used a C-130 aircraft to observe the smoke. The study flew through levels of 2,000 micrograms per cubic meter, or about seven times the worst air experienced in Seattle this summer. Seals on the aircraft kept the air inside the craft much cleaner, though researchers said it was like flying through campfire smoke.

“We tried to find a nice, organized plume where we could start as close to the fire as possible,” Palm said. “Then using the wind speed we would try to sample the same air on subsequent transects as it was traveling downwind.”

The analysis in the new paper focused on nine well-defined smoke plumes generated by the in southwestern Oregon, the Bear Trap fire in Utah, the Goldstone fire in Montana, the South Sugarloaf fire in Nevada, and the Sharps, Kiwah, Beaver Creek and Rabbit Foot wildfires in Idaho.

“You can’t really reproduce large wildfires in a laboratory,” Palm said. ���n general, we tried to sample the smoke as it was aging to investigate the chemistry, the physical transformations that are happening.”

The researchers found that one class of wildfire emissions, phenols, make up only 4% of the burned material but about one-third of the light-absorbing “brown carbon” molecules in fresh smoke. They found evidence of complex transformations within the plume: Vapors are condensing into particles, but at the same time and almost the same rate, particulate components are evaporating back into gases. The balance determines how much particulate matter survives, and thus the air quality, as the plume travels downwind.

“One of the interesting aspects was illustrating just how dynamic the smoke is,” Palm said. “With competing processes, previous measurements made it look like nothing was changing. But with our measurements we could really illustrate the dynamic nature of the smoke.”

The researchers found that these changes to chemical composition happen faster than expected. As soon as the smoke is in the air, even as it’s moving and dissipating, it starts to evaporate and react with the surrounding gases in the atmosphere.

“When smoke plumes are fresh, they’re almost like a low-grade extension of a fire, because there’s so much chemical activity going on in those first few hours,” Thornton said.

The authors also performed a set of inside a research chamber in Boulder, Colorado, that looked at how the ingredients in smoke react in daytime and nighttime conditions. Wildfires tend to grow in the afternoon winds when sunlight speeds up chemical reactions, then die down and smolder at night. But very large wildfires can continue to blaze overnight when darker skies change the chemistry.

Understanding the composition of the smoke could also improve weather forecasts, because smoke cools the air underneath and can even change wind patterns.

���n Seattle, there are some thoughts that the smoke changed the weather,” Thornton said. “Those kinds of feedbacks with the smoke interacting with the sunlight are really interesting going forward.”

The research was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration. Other co-authors are graduate students and and research scientist in the UW Department of Atmospheric Sciences; Lauren Garofalo, Matson Pothier, Delphine Farmer, Sonia Kreidenweis and Emily Fischer at Colorado State University; Rudra Pokhrel, Yingjie Shen and Shane Murphy at the University of Wyoming; Wade Permar and Lu Hu at the University of Montana; and Teresa Campos, Samuel Hall, Kirk Ullmann, Xuan Zhang and Frank Flocke at the National Center for Atmospheric Research in Boulder, Colorado.

For more information, contact Thornton at joelt@uw.edu or Palm at bbpalm@uw.edu.

With most of the Northwest blanketed by wildfire smoke, public officials and health experts suggest staying inside as much as possible to reduce exposure to the significant health risks of wildfire smoke.

However, inequity in our communities means not every home provides great protection and many workers in disadvantaged populations can’t afford to stay home, says , assistant professor of epidemiology in the UW School of Public Health.

�Ჹ�Ჹ��’s expertise covers the impact air pollution — including from wildfires — has on disadvantaged populations.

Related quotes from Hajat:

��� think it’s a really important issue. We know that disadvantaged communities tend to live in older housing and more crowded housing. The messaging we get from public health says we need to stay indoors, but if you’re living in a home that’s pretty leaky, then you’re getting minimal protection from staying indoors. So poorer and minority communities who tend to live in older homes will be less protected from wildfire smoke.”

“Also, lower income and minority populations also tend to work more essential jobs. So with the dual issues of COVID and wildfire smoke, they still have to go to work. So their smoke exposure potentially will be higher than for people who are able to work from home and stay indoors.”

“There’s lots of different parties that could play a role in terms of remedying these exposures. If we’re talking about older housing, we could focus a on landlords and try to incentivize them to improve the conditions of their properties. We know that can be expensive, so some involvement from the city could help incentivize landlords to do that.”

���ndividual residents can also create low cost filtration systems or air purifiers.”

For more information about this video, go here.

���t’s also important that the state think about these vulnerable workers, specifically people that have to work outdoors. It can institute occupational standards that could potentially alleviate the risks to those workers who are the hardest hit such as farm and construction workers.”

For more information and to speak with Professor Hajat, please contact Jake Ellison at jbe3@uw.edu

More information on smoke and wildfires:

• More UW experts on smoke impacts and wildfires