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Anthropogenic climate warming as key driver of extreme wildfire events

by Caden Chamberlain | Caden’s Google Scholar 

Wildfires have drastically impacted much of western North America over the past decade. Using machine and deep learning models, a recent study finds that extreme fire events will only become more common as the climate continues to warm.

Fig. 1|Caden Chamberlain conducting fieldwork in Washington, USA.

In western North America, wildfire has been used by Indigenous communities for thousands of years to steward forest landscapes1. Fire is an essential ecological process that engenders ecosystem resilience and enhances biodiversity. However, fire can also have negative impacts on ecological and social systems. Settler-colonial fire suppression through much of the 20th century increased fuel loads in forests2, contributing to increasingly extreme fire behavior over the last decade3-4. Writing in Nature, Brown et al.5 found that by 2100, anthropogenic-driven climate warming in the western US will substantially increase the frequency of extreme fire events in California, even under low greenhouse gas emission scenarios.

Wildfire trends can be quantified in many ways, including fire size distributions, cumulative area burned, severity of fire effects, shifts in fire behavior, and impacts on human communities. Trends in fire behavior and effects are governed by numerous, interacting environmental factors which can broadly be categorized as fuels, weather, and topography. Brown et al. (2023) modeled the relationship between extreme fire spread events and temperature, holding other influences on fire behavior constant. Extreme fire spread events are often associated with the greatest ecological and social impacts, and these events pose the greatest challenges to fire managers2. Brown et al. (2023) argue that understanding the influence of temperature on extreme fire spread is particularly important since most global climate change models predict increasing temperatures with relatively high confidence.

The authors identified a 25% increase in extreme spread days between the pre-industrial era and the modern era (2003-2020). Frequencies continued to increase until mid-century under both low and high emissions scenarios. By late-century, the low emissions scenario stabilized near mid-century frequencies, but under a high emissions scenario, they found that frequency of extreme spread days increased by 107% relative to the 2003-2020 period. As such, anthropogenic warming is likely to have a strong influence on fire activity over the next few decades, and potentially until the end of the century if emissions are not reduced. Importantly, the authors found that it was not necessarily the influence of warming on fire activity, rather that warming led to faster drying of fine- and coarse-fuel loads, which in turn influenced fire behavior.

Other recent studies have demonstrated how climate warming can affect fire activity, primarily using landscape or ecosystem simulation models6–9. These models are highly effective for capturing and projecting complex ecological relationships, however, in their current state, they do not operate at spatial resolutions suitable for quantifying fire behavior, especially as it relates to fire management. Brown et al. (2023) present a novel analytical approach in which machine and deep learning models were used to learn the relationship between temperature and extreme fire weather during the modern era, and these relationships were then applied under future warming scenarios derived from global climate models. In this way, they were able to quantify warming effects on fire behavior at a much finer spatial scale – the scale of individual spread days – compared to previous studies.

Recognizing that temperature itself typically is not the primary driver of fire behavior, the authors also propagated temperature metrics into other metrics related to fuel moisture, including vapor pressure deficit and 100-/1000-hour fuel moisture. The authors demonstrate that fuel moisture is an important mechanism that helps to predict fire behavior. Lastly, the authors show that increases in the frequency of extreme fire spread events were attributed primarily to a subset of fires during the contemporary period – those that burned under conditions close to distinct temperature and fuel moisture thresholds. On one hand, these results suggest that crossing of certain warming thresholds will lead to rapid increases in extreme fire activity; however, these results also point to the limited interpretability when using a binary response variable which defined extreme spread days as > 10,000 ha.

This article provides a robust evaluation of how climate change will influence extreme fire spread as a result of both warmer temperatures and drier conditions in western North America. These findings have widespread implications for the state of California, and other forested regions globally, as they demonstrate that fires are likely to have heightened environmental and social impacts in coming decades. These results call for more urgent measures to reduce greenhouse gas emissions, and they also signal a warning to fire managers and policymakers to prepare for a future characterized by increasingly extreme fire behavior.

Having developed an essential methodology for assessing climate change impacts on wildfire, this work will serve as a springboard for future studies investigating other factors that may exacerbate or buffer fire activity in the future. For example, the authors state that many factors will exacerbate future fire activity, suggesting that their projections of extreme fire spread frequencies may be conservative. However, other confounding factors may actually diminish fire activity in the future. For example, a recent study demonstrates that fires may begin to develop a negative feedback with vegetation, in which increased fire activity in the short-term will reduce fuel loading and thus stifle fire activity in the long-term9. As such, while Brown et al. (2023) show that warmer temperatures may prime fuels for more extreme fire behavior under climate change, there is also a chance that the fuels themselves will not be available to support widespread burning due to negative fire-vegetation feedbacks.

Another intriguing uncertainty is the role that management will play on future fire activity. In states like California, and many other regions globally, managers are rapidly increasing the pace and scale of fuel reduction treatments10. These efforts will undoubtedly influence the relationship between warming temperatures and fire behavior, likely reducing the frequency of extreme spread events, even under the warmest and driest futures. While Brown et al. (2023) do not address this question directly, the methodology posed in this research will be critical in further progressing research in the field to better understand the role of climate, feedbacks, and management in fire behavior in an increasingly uncertain future.

References Cited

  1. Falk, D. A. et al. Multi-scale controls of historical forest-fire regimes: new insights from fire-scar networks. Frontiers in Ecology and the Environment 9, 446–454 (2011).
  2. Hagmann, R. K. et al. Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests. Ecological Applications. 31(8): 24-. 31, 1–34 (2021).
  3. Coop, J. D., Parks, S. A., Stevens-Rumann, C. S., Ritter, S. M. & Hoffman, C. M. Extreme fire spread events and area burned under recent and future climate in the western USA. Global Ecology and Biogeography 31, 1949–1959 (2022).
  4. Wasserman, T. N. & Mueller, S. E. Climate influences on future fire severity: a synthesis of climate-fire interactions and impacts on fire regimes, high-severity fire, and forests in the western United States. Fire Ecology 19, 43 (2023).
  5. Brown, P. T. et al. Climate warming increases extreme daily wildfire growth risk in California. Nature 621, 760–766 (2023).
  6. Kitzberger, T., Falk, D. A., Westerling, A. L. & Swetnam, T. W. Direct and indirect climate controls predict heterogeneous early-mid 21st century wildfire burned area across western and boreal North America. PLOS ONE 12, e0188486 (2017).
  7. Liu, Z. & Wimberly, M. C. Direct and indirect effects of climate change on projected future fire regimes in the western United States. Science of The Total Environment 542, 65–75 (2016).
  8. Rabin, S. S. et al. The Fire Modeling Intercomparison Project (FireMIP), phase 1: experimental and analytical protocols with detailed model descriptions. Geoscientific Model Development 10, 1175–1197 (2017).
  9. Kennedy, M. C., Bart, R. R., Tague, C. L. & Choate, J. S. Does hot and dry equal more wildfire? Contrasting short- and long-term climate effects on fire in the Sierra Nevada, CA. Ecosphere 12, e03657 (2021).
  10. Forest Management Task Force. https://wildfiretaskforce.org/action-plan/.
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