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The changing risk and burden of wildfire in the United States
Edited by B. L. Turner, Arizona State University, Tempe, AZ, and approved November 24, 2020 (received for review June 30, 2020)

Abstract
Recent dramatic and deadly increases in global wildfire activity have increased attention on the causes of wildfires, their consequences, and how risk from wildfire might be mitigated. Here we bring together data on the changing risk and societal burden of wildfire in the United States. We estimate that nearly 50 million homes are currently in the wildland–urban interface in the United States, a number increasing by 1 million houses every 3 y. To illustrate how changes in wildfire activity might affect air pollution and related health outcomes, and how these linkages might guide future science and policy, we develop a statistical model that relates satellite-based fire and smoke data to information from pollution monitoring stations. Using the model, we estimate that wildfires have accounted for up to 25% of PM2.5 (particulate matter with diameter <2.5 μm) in recent years across the United States, and up to half in some Western regions, with spatial patterns in ambient smoke exposure that do not follow traditional socioeconomic pollution exposure gradients. We combine the model with stylized scenarios to show that fuel management interventions could have large health benefits and that future health impacts from climate-change–induced wildfire smoke could approach projected overall increases in temperature-related mortality from climate change—but that both estimates remain uncertain. We use model results to highlight important areas for future research and to draw lessons for policy.
Over the past four decades, burned area from wildfires has roughly quadrupled in the United States (Fig. 1A) (1). This rapid growth has been driven by a number of factors, including the accumulation of fuels due to a legacy of fire suppression over the last century (2) and a more recent increase in fuel aridity (Fig. 1B, shown for the western United States), a trend which is expected to continue as the climate warms (3, 4). These increases have happened parallel to a substantial rise in the number of houses in the wildland–urban interface (WUI). Using data on the universe of home locations across the United States and updated national land cover maps, we update earlier studies (5, 6) and estimate that there are now ∼49 million residential homes in the WUI, a number that has been increasing by roughly 350,000 houses per year over the last two decades (Fig. 1C and SI Appendix). As firefighting effort focuses substantially on the protection of private homes (7), these factors have contributed to a steady rise in spending on wildfire suppression by the US government (Fig. 1D), which in recent years has totaled ∼$3 billion/y in federal expenditure (1). Total prescribed burn acreage has increased in the southeastern United States but has remained largely flat elsewhere (Fig. 1E), suggesting to many that there is underinvestment in this risk-mitigation strategy, given the massive overall growth in wildfire risk (8).
Trends in the drivers and consequences of wildfire. (A and B) Increases in burned area in public and private US lands (A) (1) have been driven in part by rising fuel aridity, shown here over the western United States (4) (B). (C and D) The number of homes in the WUI has also risen quickly (C, our calculations; SI Appendix), which has contributed to rising suppression costs (D) incurred by the federal government. (E) Prescribed burn area has increased substantially in the South but is flat in all other regions (1). (F and G) Smoke days have increased throughout the United States (F), perhaps undermining decadal improvements in air quality across the United States (G). (H) We calculate an increasing proportion of overall
What are the consequences of this change in fire activity for overall air quality and for health outcomes, and how should policy respond? Large increases in wildfire activity have been accompanied by substantial increases in the number of days with any smoke in the air across the United States (Fig. 1F), as estimated from satellite data (9). Such increases have been observed throughout the continental United States, not just in the West, and threaten to undo the substantial improvements in air quality observed across the United States over the last two decades (Fig. 1G). The fingerprints of wildfire are already visible in upward-trending spring- and summertime organic carbon concentrations observed in rural areas in the US South and West (SI Appendix, Fig. S1), respectively, and studies find that having any smoke in the air can increase morbidity and mortality among exposed populations (10, 11).
A challenge in understanding the broader contribution of changing wildfire activity to air quality is the difficulty in accurately linking fire activity to related pollutant exposures in often-distant population centers (12). Satellite-based measures of smoke exposure are increasingly available and are appealing because plume monitoring intuitively links source and receptor regions. Such data, however, cannot yet be used to precisely measure smoke density or to separate surface-level smoke from smoke higher in the atmospheric column, and thus they are difficult to link to existing exposure–health response relationships (13, 14). Chemical transport models (CTMs), which can directly model the movement and evolution of wildfire emissions, offer an alternate approach for linking local pollution concentrations to specific fire activity. However, generating accurate exposure estimates from CTMs requires surmounting several major uncertainties in the pathway between source and receptor. First, large uncertainties in wildfire emissions inventories have been shown to lead to many-fold differences in wildfire-attributed
To further understand the changing contribution of wildfire to particulate matter exposure across the United States and to illustrate key remaining scientific and policy questions at the intersection of wildfire, pollution, and climate, we train and validate a statistical model that relates changes in satellite-estimated smoke plume exposure and fire activity to ground-measured
Our results show that the contribution of wildfire smoke to PM2.5 concentrations in the US has grown substantially since the mid 2000s, and in recent years has accounted for up to half of the overall PM2.5 exposure in western regions as compared to <20% a decade ago (Fig. 1H). While increases in contribution of smoke to PM2.5 are concentrated in the western US, they can also be seen in other regions (Fig. 2 A and B), a result of long-distance transport of smoke from large fires. Indeed, in midwestern and eastern regions of the United States, a growing share of smoke is estimated to originate from fires in the western United States or from outside the United States (13) (Fig. 2 C and D), mirroring recent findings on the substantial transboundary movement of overall
The quantity, source, and incidence of wildfire smoke. (A and B) Average predicted micrograms per cubic meter of
These trends and patterns highlight important points of tension between existing air quality regulation and the growing threat from wildfire smoke and raise important unanswered research questions that will be critical to informing policy choice. Current approaches to regulation in the United States treat air quality primarily as a local problem, wherein counties are penalized if pollutant concentrations exceed designated short- or long-term thresholds. Current regulation under the Clean Air Act also potentially exempts wildfire smoke—but not smoke from prescribed burns—from attainment designation. These approaches appear at odds with the transboundary nature and growing contribution of wildfire smoke to air quality.
To better guide policy, a first key scientific contribution will be a better quantification of smoke exposures and agreed-upon methods for validating these exposures. Both statistical and transport-based approaches to exposure assessment have their strengths and shortcomings, and the performance of both should be evaluated based on metrics relevant to the measurement of downstream health responses. In particular, to isolate smoke exposure from potential confounds, most statistical approaches in recent health impact studies use variation over time in pollution exposure to estimate health effects. This implies that smoke models used for estimating health impacts should be evaluated in their ability to predict temporal variation in
A second key scientific question is the nature of health responses to wildfire smoke. Growing evidence indicates a range of negative health consequence associated with wildfire smoke exposure (10, 28), consistent with a vast literature on the broader health consequences of polluted air. Most recent evidence suggests that there is no “safe” level of exposure to key pollutants such as
To illustrate this sensitivity, we combine pollution changes predicted from our statistical model with three recently published mortality response functions (29, 31, 32) to simulate changes in older-adult mortality predicted by various changes in
Health consequences of changes in smoke exposure depend on the assumed dose–response function and on the magnitude of management- or climate-driven changes in smoke. (A) Distributions of
The large potential health benefits of smoke mitigation also raise key questions about wildfire management strategies. For instance, existing evidence does not provide a comprehensive understanding of how a given prescribed burning intervention will change the timing, amount, and spatial distribution of smoke, and we find that alternate estimates of the efficacy of prescribed burning in reducing the subsequent size of wildfires (33) can lead to more than twofold differences in estimated health benefits of prescribed burns (Fig. 3). Similarly, current fire suppression efforts understandably focus on protecting homes and structures, but the overall population health impact of a heavily polluting wildfire that does not threaten structures could be much worse than that of a smaller fire that does threaten structures. In addition, fuels management activities are targeted at local community protection and ecosystem benefits and do not consider likely downstream impacts of wildfire on large populations. Additional quantitative work is needed to help navigate these difficult trade-offs.
A third key question is whether source-agnostic
Fourth, how might the interaction of climate change and wildfire risk shape policy priorities? A warming climate is responsible for roughly half of the increase in burned area in the United States (4), and future climate change could lead to up to an additional doubling of wildfire-related particulate emissions in fire-prone areas (36) or a many-fold increase in burned area (37, 38). Costs from these increases include both the downstream economic and health costs of smoke exposure, as well as the cost of suppression activities, direct loss of life and property, and other adaptive measure (e.g., power shutoffs) that have widespread economic consequences. It is currently unknown whether accounting for these wildfire-related costs meaningfully increases the estimated overall economic damages from climate change.
To begin to quantify the possible cost of climate-induced wildfire increases, we use our statistical model and stylized scenarios to calculate the change in smoke exposure and resulting mortality associated with projected increases in wildfire risk. Using projected increases in future smoke broadly consistent with existing literature (36⇓–38), we calculate that increased mortality from climate-change–induced wildfire smoke could approach projected overall increases in temperature-related mortality—itself the largest estimated contributor to economic damages in the United States (39) (SI Appendix). More detailed studies are needed to refine these estimates in terms of their magnitude, their geographic specificity, and the particular subpopulations that might be the most affected. A key related policy question will be whether and to what degree to modify current exceptions to the Clean Air Act granted to states for pollution impacts from wildfire smoke, as these erode gains from efforts aimed at reducing
Finally, wildfires have strongly interacted with the COVID-19 pandemic in ways that require further study. COVID-19 has to some degree impeded the ability of government and the private sector to respond to wildfire risk, before, while, and after fires occur. The scale of the 2020 wildfire season in many parts of the West, where drought in the 2019 to 2020 rainy season followed an accumulation of fuels during a relatively wet 2018 to 2019 season has presented particularly acute challenges. Wildland firefighter trainings were delayed or sometimes canceled, convict firefighter crews were unavailable due to early release from state prisons to avoid COVID outbreaks, many fuels management treatments did not occur in winter and spring, utilities faced at least some delays in wildfire risk reduction activities, and traditional approaches to wildfire evacuation have proved more challenging due to lowered capacity at evacuation centers resulting from social distancing requirements. It is currently unknown but probable that the historic fire season, and consequent smoke impacts, has also worsened COVID-related health outcomes, as early evidence suggests that exposure to air pollution increases both COVID cases and deaths in the United States (40, 41) (a finding consistent with the relationship between pollution and other viral respiratory illness) (42, 43). A better causal understanding of the impact of air pollution on COVID outcomes, including that from wildfires, is a critically urgent research priority, and scholars have provided guidelines on how air pollution/COVID relationships might be best studied (44). Findings from this research could be important in guiding labor- and finance-constrained firefighting effort and fuels management strategies as the pandemic continues.
Data Availability.
Data and code have been deposited in GitHub (github.com/burke-lab/wildfire-map-public) (45).
Acknowledgments
We thank the Robert Wood Johnson Foundation, William and Flora Hewlett Foundation, and NSF (CNH 1715557) for generous funding.
Footnotes
- ↵1To whom correspondence may be addressed. Email: mburke{at}stanford.edu.
Author contributions: M.B., A.D., S.H.-N., and M.W. designed research; M.B., A.D., J.X., and J.B. performed research; A.D., S.H.-N., J.X., and J.B. analyzed data; and M.B., S.H.-N., J.X., J.B., and M.W. wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2011048118/-/DCSupplemental.
- Copyright © 2021 the Author(s). Published by PNAS.
This open access article is distributed under Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND).
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