Figure 1 (poster image above) Prescribed fire at Independence Lake near Truckee, California safely reduces surface fuels and risk of wildfire while improving the health of the forest. ©Edward Smith/The Nature Conservancy

The devastating August 2023 wildfire on Maui drew the world’s attention to a region that has little historical experience with catastrophic fires, but to many of us is a stark reminder of the ferocity and suddenness of wildfire activity in the western United States (WUS) and other parts of the globe. In the WUS (but also globally), escalating adverse societal impacts have raised public awareness about their dangers and the need for improved preparation and response plans. Observed increases in wildfire severity and impacts to communities have diverse anthropogenic causes—including the legacy of historical fire suppression policies, increased development in high-risk zones, and aridification by a warming climate. “Prescribed fire,” or fire intentionally set to achieve management objectives is increasingly viewed as an important component of ecological forest management designed to reduce risk to humans and their infrastructure and restore composition, structure and function to fire adapted grassland, shrubland and forested ecosystems around the world. Prescribed fire consumes accumulated dead, drying and some living vegetation, releasing nutrients back into the soil; the reduction in biomass can moderate subsequent wildfire behavior to a greater degree than mechanical removal of material alone.

In this work, we studied two critical meteorological determinants that influence managers’ ability to conduct prescribed burns safely and effectively now and into the future: the alignment of multiple weather variables that comprise “burn windows” of opportunity and the dryness of large woody debris (as well as finer fuels) and their contribution to fire behavior and safety. Separately, we also consider trends in atmospheric air stagnation and their potential to inhibit toxic smoke dispersal. The bad news? Climate change is likely to decrease the number of days on which conditions conducive to safe and effective prescribed fire occur across much of the WUS. The good news, however, is that we still have options available to us to adapt to changing conditions on our landscapes—but they will require increased financial investments, changes in management, and redoubled efforts to coordinate across sectors of society.

Historically, most prescribed burning in the western US has occurred during spring or autumn, when weather and vegetation conditions are most conducive to fire behavior that meets objectives but can still be controlled. These conditions collectively make up the “burn prescription” —or specified ranges of weather and live and dead vegetation moisture parameters that ensure fires burn completely enough to achieve objectives (e.g., consuming large fuels and woody debris) but that they don’t burn so hot as to present control problems or to have undesired ecological consequences (e.g., soil scorch and increased tree mortality). Periods of time where these prescribed conditions are likely to be met are called the “burn windows.”

Although considered a widely applicable solution, there are many impediments to prescribed fire implementation, including staffing and funding limitations, risk tolerance, and smoke impacts). For these and other reasons, prescribed fires are not implemented during all suitable burn windows—suggesting that, to date, climate has not been the primary inhibitor to implementation and that present-day burn windows are often underutilized. However, in recent years, the combined effects of severe short-term drought and long-term aridification have contributed to a reduction of adequate spring and autumn burn windows in some regions, raising concerns that climate change will add to the many existing challenges to prescribed fire implementation.

A specific example may help illustrate these challenges from an “on the ground” perspective. After two years of successfully burning hundreds of acres at The Nature Conservancy’s Independence Lake Preserve in the Sierra Nevada (Figure 1), in autumn 2019 we tried unsuccessfully to burn another site, but never met all prescription parameters of our burn plan due to delayed onset of wet weather. In discussing with our climatologist colleagues about the apparent lack of a “burn window,” initial data analysis revealed that we had achieved our prescription parameters, but only for approximately 22 hours, insufficient time to deploy crews and conduct a multi-day burn safely and effectively. This was an eye-opening moment and led to the current, larger study.

How many days are favorable for prescribed fire in the Western U.S.?

The occurrence of days favorable for prescribed fire from a meteorological and vegetation moisture perspective (henceforth, “RxDays”) varies widely across the WUS. Using climate data from 1981 to 2020 in the observed record (GridMET), we calculate the total annual occurrence of RxDays across the WUS (Fig. 2). We define RxDays as days with surface weather (temperature, relative humidity, and wind) and vegetation moisture (1, 10, 100, and 1000-h FM plus ignition component) conditions that are deemed suitable for prescribed fire based on prescriptions in existing burn plans. The number of annual RxDays ranges from fewer than 10 days per year in extremely moist regions of western Washington and Oregon to greater than 70 days per year in multiple subregions, mainly those characterized by non-forest vegetation types (Fig. 2a). These numbers depend on vegetation dryness for in extremely dry and hot areas, they are often too dry to burn safely. On the other hand, in very wet and cool regions, vegetation is usually too damp to burn properly, or it might not burn at all. So, it's all about finding the Goldilocks conditions: not too wet, not too dry, but just right.

Figure 2 Maps of observed RxDay climatology across the WUS. Maps depicting the observed number of RxDays across the western United States (WUS) on an annual (a) and seasonal (b–e) basis. ObservedRxDays are calculated using meteorological data from the gridMET dataset over the years 1981–2020.

How will the occurrence of prescribed fire days change in a warming climate?

We also looked at how climate change will affect the number of days suitable for planned fires (RxDays) in the western United States from 2021 to 2060. We used the “moderate emissions” RCP4.5 scenario that assumes stabilizing greenhouse gas concentrations with around 2C of global warming by 2060. Our findings show that due to climate change caused by human activities, there will be fewer RxDays in most of the western United States in the future—but with considerable regional and seasonal variability. In particular, projected RxDay losses are generally largest in the Southwest (including California, Arizona, and New Mexico) and smallest across the Northern Tier—and smaller losses are generally found in winter and autumn versus spring and summer (see Fig. 3).

Figure 3 Heat maps of projected change in RxDays across the WUS, 2021–2060 vs. 1981–2020. Maps depicting the projected change in the number of RxDays across the western United States (WUS) on an annual (a) and seasonal (b–e) basis. The change in projected RxDays is calculated as the difference between the RxDay counts during 2021–2060 vs. 1981–2020 on a “moderate warming” (RCP4.5) trajectory using meteorological data from the downscaled CMIP5 climate model ensemble dataset (MACA).

Importance of large-diameter fuel moisture and long-term drying

Given recent high-profile public discussions regarding the implications of escaped prescribed fires occurring under conditions characterized by unusually low vegetation and dead FM, including specific calls to use indices relevant to large-diameter FM (e.g., energy release component), we quantify the importance of considering large-diameter dead FM in WUS prescribed burn plans. We find, where such fuels occur, that large-diameter FM is a very strong constraint on RxDay occurrence across the WUS. In most forested regions, consideration of 100-/1000-h FM results in 40–80 fewer RxDays per year, yet 1000-h FMs are not consistently included in burn plans in the WUS. These widespread decreases in large-diameter dead FM and their contribution to undesirable fire effects (including difficult-to-control fire behavior and enhanced smoke production and carbon emissions) highlight the importance of explicitly including metrics such as 100/1000-h FM and/or drought indices in burn prescriptions to better reflect long-term aridification trends amid a warming climate.

Figure 4 Heat maps of projected change in 1000-h dead fuel moisture across the WUS, 2021–2060 vs. 1981–2020.

Maps depicting the projected change in 1000-h dead fuel moisture across the western United States (WUS) on an annual (a) and seasonal (b–e) basis. The change is calculated as an 18-model average difference between 2021–2060 and the 1981–2020 period on a “medium warming” (RCP4.5) trajectory using meteorological data from the downscaled CMIP5 climate model ensemble (MACA) dataset (1981–2060). Note: these values are only relevant in places where heavy dead and down fuels actually exist locally.

Trends in air stagnation relevant to prescribed fire smoke emissions

When using prescribed fires near populated areas, there are concerns about the smoke they produce and its impact on local and regional air quality. Although the smoke from planned fires is usually less harmful than smoke from wildfires that aren't controlled, there are rules from regional air quality authorities, and concerns of local communities about health risks, which can make it challenging to use prescribed burns.

We looked into how air conditions that can trap smoke close to the ground might change in the future. We call this "low-level stagnation" or "LLS." Our study shows that, on average across the western United States (WUS) and throughout the year, LLS is expected to broadly increase by about 5% on average (~5.7 more days per year) between 1981 and 2060 (see Figure 5).

All these changes in LLS due to climate shifts could lead to more restrictions on planned fires because of air quality regulations. That is why we suggest that the people in charge of planned fires and air quality regulators should have discussions about their goals, how much land they plan to use for fires in the upcoming season, and where and when they might face issues with local communities. In the Sierra Nevada region, recent experience suggests that regular meetings between land and air management groups have been helpful in reducing conflict.

Figure 5 Maps of projected change in low-level air stagnation days across the WUS, 1981–2020 vs. 2021–2060.

Maps depicting the projected change in the number of low-level air stagnation (LLS) days across the western United States (WUS) on an annual (a) and seasonal (b–e) basis. Here, positive values (blue regions) represent areas expected to see an increase in low-level air stagnation days. The change in projected stagnation days is calculated as the difference between the air stagnation day counts for 1981–2020vs. the 2021–2060 period on a “medium warming” (RCP4.5) trajectory using meteorological data from the downscaled CMIP5 climate model dataset (MACA).

Even in a warming climate, we can still find ways to increase prescribed burning

One thing that’s important to keep in mind is that conditions favorable for prescribed fire are not rare across most of the WUS: a majority of the non-forest covered areas (as well as some forested regions) experience at least one cumulative month (30 days) of RxDays per year, and many non-forested areas experience two or more cumulative months (≥60 days) of RxDays per year. This highlights the widespread climatic potential for prescribed fire to be utilized across a diverse range of regional sub-climates throughout the WUS (Fig. 2).

In some cases, however, the available opportunity windows are at times of year that traditionally have not been utilized to conduct burn operations. A recent literature review and synthesis that analyzed the ecological impacts of prescribed burning at different seasons concluded that seasonality had less negative ecological effects than burn intensity and concluded that a variable fire regime that includes use of prescribed burning at different times of year may improve outcomes for biodiversity (Knapp et al 2009). This implies that as long as the burn prescription is aligned with burn objectives, and the burn prescription parameters are followed, regardless of time of year, ecological impacts should be acceptable if fire mangers are operating within the burn prescription. This approach may also align with cultural burning practices that utilize burn windows when and where they occur.

Finally, we offer some specific recommendations surrounding prescribed fire policy and implementation:

• Prescribed fire management staff and line officers should explore using other seasons (including winter, spring and summer) for prescribed fire implementation windows.
• Increase dedicated funding, staffing & training for prescribed burners: an expanded workforce will be required to capture and utilize the available burn windows if they fall outside of traditional seasons and when crews are unavailable. An excellent example is The Nature Conservancy’s Prescribed Fire Training Exchange (TREX) network and USDA TREX.
• Expand partnerships with Native nations. For specific examples and to learn more about existing projects, see the Indigenous Peoples Burning Network.
• Use burn index and heavy fuel (100 and 1000-hr) moisture content in burn prescriptions to improve integration of antecedent dry periods into fire behavior and smoke emission predictions.
• Improve regular coordination among fire managers, air quality regulators, and public health agencies so they know what to expect in acres planned to be burned, and when and where emissions will occur.
• Increase outreach to public about objectives and tradeoffs – these efforts will help the public understand that prescribed fire produces less smoke, less damage, with fewer health impacts and is less expensive and disruptive to people.