Synchronous fires and fire danger challenge US capacity to respond

John Abatzoglou, Alison Cullen, and Susan Prichard

The 2021 fire season has coincided with yet another hot, dry summer. As of 17 September, 2021, a total of 2.5 million acres (1.01 million ha) had burned in the Northwest (defined here as Oregon, Washington, Idaho, and Montana), including the Bootleg fire in southern Oregon, which burned over 413,000 acres (167,000 ha). The relentless smoke from fires burning in the western United States and Canada has impacted human communities both near to and far from the fires. During 2021, cities such as Pullman, Washington, have already experienced more days with unhealthy air quality from regional smoke than in any of the past 15 years. Statewide fire restrictions in Washington have been in place since early July, and fire danger as of 20 July led the state to close public access to all state forest lands in eastern Washington.

The combination of widespread drought and record-setting heat increased the flammability of vegetation throughout much of the West this summer, with wildfires extending from central California into the Northwest and Northern Rockies (Figure 1). These climatic factors, and the high plant biomass due to the legacy of fire exclusion over the past century, have set the stage for fires that resist containment efforts and pose risks to both ecosystems and human communities. Local-to-regional fire suppression resources become limited due to some combination of widespread ignitions (e.g., from dry lightning), large fire incidents that threaten communities, and extensive high fire danger. When available, fire suppression resources from nearby regions can be mobilized to assist in large fire incidents. However, fire suppression resources may be unavailable to assist in ongoing incidents if firefighters are grappling with challenges in the location where they are stationed.

Fire activity across the West started early in 2021 and quickly overwhelmed fire suppression resources. As a result, the national preparedness level reached its maximum, 5, on 14 July – the earliest it has reached this stage in a decade. The preparedness levels provide a measure of strain on national fire suppression capacity. A preparedness level of 1 refers to limited commitment of national resources. At preparedness level 5, large fires are active in multiple regions and national suppression resources are fully committed. In recent years, countries such as Australia and New Zealand contributed additional personnel and equipment to aid in the US firefight (and we mobilize resources to these countries during their active fire seasons). Resources also may be shared between the United States and Canada, but this is not necessarily a viable option when Canada’s resources are also stretched thin due to widespread fire activity within its own borders, as was the case this year.

Not all fires garner the same fire suppression response. Rather, a complex set of factors and resource availability determine fire management approaches. Smaller lightning-caused fires burning in relatively remote areas may be assigned limited resources when fire suppression resources are depleted. In most of these cases, fire management strategies other than full suppression are employed, thereby reserving fire suppression resources for fires that pose direct threats to communities and infrastructure. Allowing such fires to burn also may have beneficial ecological impacts (e.g., reducing fuel loads, improving habitat for some species).

In 2021, the second driest March-July and the hottest June-July since 1895 in Washington, Idaho, Oregon, and Montana (Fig. 1) primed the landscape for widespread fire activity.

Plot of temperature in June and July, represented as departure from twentieth century average, against precipitation from March through July, represented as percentage of twentieth century average. Data from the 1890s through present.
Figure 1. March-July precipitation (percentage of twentieth century average) and June-July temperature (departure from twentieth century average) in the Northwest during 2021. Data source: NCEI climate at a glance.

Fire danger indices largely have reflected the anomalous combination of warm and dry conditions, and have tracked well above normal for much of summer. By early August, fire weather indices were high across much of the western United States, covering over 80% of forested lands on 5 August, after nearly three weeks at preparedness level 5 (Fig. 2). These index values coincided with over 100 ongoing large fire incidents and over 23,000 personnel on active fires in the West, primarily in the Northwest, Northern Rockies, and northern California.

Map of the extent of high fire weather conditions, defined as exceedance of the local 90th percentile value of the Canadian Forest Fire Weather Index, on 5 August 2021.
Figure 2. Extent of high fire weather conditions, as defined by exceedance of local 90th percentile values of the Canadian Forest Fire Weather Index (FWI), on 5 August 2021. Fire danger was high over 80% of forested areas in the western United States. Data source: gridMET surface meteorological data.

Recent studies highlight the fire management challenges caused by synchronous fire activity and fire danger. Abatzoglou et al. (2021) reported a strong link between the occurrence of widespread high fire danger days across forests in the West and national fire suppression resources. For example, on 6 September 2020, the day prior to the major expansion of the Labor Day fires in western Oregon, fire danger was high over 85% of forested lands in the western United States. Abatzoglou et al. (2021) found that since 1979, the number of days per year with widespread high danger has increased by 25, likely compounding fire management challenges. Furthermore, climate model projections suggest a doubling of such days by 2051-2080 under a moderate emissions scenario (RCP 4.5).

Recent research indicated that peak synchronicity, or co-occurrence of very large wildfires, coincides with low fuel moisture and high proportions of lightning-caused fires, which in turn hinders firefighting effectiveness via multiple mechanisms (Podschwit and Cullen 2020, Cullen et al. 2021). This work identified an increase in the synchronous occurrence of large wildfires (> 1000 acres [405 ha]) across the western and southern United States over the period 2000-2015 relative to 1985-2000. Additionally, researchers found a positive relation between wildfire synchronicity and preparedness-level designations of 4 and 5. These results provide the basis for the development of new forecasting tools to support proactive risk management, such as optimized positioning of resources in areas of greatest fire danger, recall of personnel from off-duty status, requests for back-up resources from other areas, and decisions about fuel treatment and risk reduction, such as prescribed burning.

Ongoing research suggests that without substantial changes in wildland fuels management and suppression preparedness, climate change will continue to overwhelm fire suppression capacity. As a result, proactive fire and land management approaches are gaining momentum (e.g., Prichard et al. 2021). Such management approaches include revitalization of Indigenous fire practices, more-extensive use of prescribed fire and forest thinning, and use of managed fire, acknowledging that potential solutions will vary among fire regimes, resource availability, and impacts to human communities and ecosystems.

Drs. Cullen, Prichard, and Abatzoglou are part of a research team working on “Managing Future Risk of Increasing Simultaneous Megafires,” a project funded by U.S. National Science Foundation grant #2019762.

Literature Cited

Abatzoglou, J.T., C.S. Juang, A.P. Williams, C.A. Kolden, and A.L. Westerling. 2021. Increasing synchronous fire danger in forests of the western United States. Geophysical Research Letters 48:e2020GL091377. doi: 10.1029/2020GL091377.

Cullen, A.C., T. Axe, and H. Podschwit. 2021. High-severity wildfire potential – associating meteorology, climate, resource demand and wildfire activity with preparedness levels. International Journal of Wildland Fire 30:30–41. doi: 10.1071/WF20066.

Podschwit, H., and A.C. Cullen. 2020. Patterns and trends in simultaneous wildfire activity in the United States from 1984 to 2015. International Journal of Wildland Fire 29:1057-1071. doi: 10.1071/WF19150. Prichard, S.J., et al. 2021. Adapting western North American forests to climate change and wildfires: ten common questions. Ecological Applications e02433. doi: 10.1002/eap.2433.

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