Extreme Fire Weather Research Articles

Enhanced prediction of extreme fire weather conditions in spring using the Hot-Dry-Windy Index in Alberta, Canada

Background Fire weather indices forecast fire behaviour and provide valuable information for wildland fire prevention, preparedness, and suppression. However, these indices do not directly account for atmospheric conditions aloft. The province of Alberta, Canada has experienced extreme fire weather conditions during spring for decades, leading to the continued occurrence of disastrous wildland fires. Aims We examined the Hot-Dry-Windy Index (HDWI) and spread days over the first 4 days of 80 large wildland fires that started in May 1990–2019 in Alberta. Methods HDWI values were calculated using ERA5 reanalysis data from the 1000, 975 and 950 hPa levels. Differences between HDWI distributions on spread days and non-spread days were examined using permutation tests. Initial Spread Index was also examined as it is considered an important Fire Weather Index System value for wildland fire spread during spring in Alberta. Key results Higher median HDWI values were observed on spread days than non-spread days, where median Initial Spread Index values showed little to no difference. Conclusions This analysis suggests that HDWI can contribute to the prediction of significant spring wildland fire spread in Alberta. Implications Forecasted HDWI and HDWI climatologies may provide additional decision support for wildland fire management agencies.

Impact of elevated fine particulate matter (PM2.5) during landscape fire events on cardiorespiratory hospital admissions in Perth, Western Australia

BackgroundAustralia has experienced extreme fire weather in recent years. Information on the impact of fine particulate matter (PM2.5) from landscape fires (LFs) on cardiorespiratory hospital admissions is limited.MethodsWe conducted a...

South America is becoming warmer, drier, and more flammable

South America is experiencing severe impacts from climate change. Although the warming of the subcontinent closely follows the global path, the rise of temperatures has been more pronounced in some regions, which have also seen a parallel increment in the occurrence of droughts and weather conditions associated with enhanced fire risk. Here, we use reanalysis datasets to analyze the progression of the concurring warm, dry, and high fire risk conditions (i.e., dry compounds) since 1971. We show that the frequency of these compound extremes has surged in key South American regions including the northern Amazon, which have seen a 3-fold increase in the number of days per year with extreme fire weather conditions (including high temperatures, dryness, and low humidity). Our results also suggest that the surface temperature of the tropical Pacific Ocean modulates the interannual variability of dry compounds in South America. While El Niño enhances the fire risk in the northern Amazon, dry extremes in the Gran Chaco region appear to be more responsive to La Niña.

Forest fire size amplifies postfire land surface warming

Climate warming has caused a widespread increase in extreme fire weather, making forest fires longer-lived and larger1–3. The average forest fire size in Canada, the USA and Australia has doubled or even tripled in recent decades4,5. In return, forest fires feed back to climate by modulating land–atmospheric carbon, nitrogen, aerosol, energy and water fluxes6–8. However, the surface climate impacts of increasingly large fires and their implications for land management remain to be established. Here we use satellite observations to show that in temperate and boreal forests in the Northern Hemisphere, fire size persistently amplified decade-long postfire land surface warming in summer per unit burnt area. Both warming and its amplification with fire size were found to diminish with an increasing abundance of broadleaf trees, consistent with their lower fire vulnerability compared with coniferous species9,10. Fire-size-enhanced warming may affect the success and composition of postfire stand regeneration11,12 as well as permafrost degradation13, presenting previously overlooked, additional feedback effects to future climate and fire dynamics. Given the projected increase in fire size in northern forests14,15, climate-smart forestry should aim to mitigate the climate risks of large fires, possibly by increasing the share of broadleaf trees, where appropriate, and avoiding active pyrophytes.

Changes in snow‐dominated streamflow quantity and timing following an extensive wildfire in British Columbia

AbstractThe length and frequency of extreme fire weather has increased across the globe in recent decades, with potential deleterious consequences to streamflow quantity, timing and quality. Changes in the hydrologic regime following wildfire can have substantial downstream consequences, affecting communities and ecosystems through flooding, erosion, loss of habitat and degraded water quality. While there are many studies that address post‐wildfire hydrology across the globe, there are few studies in the snow‐dominated regions. The 2017 Elephant Hill wildfire in south‐central BC burned across or adjacent to four watersheds with long‐term streamflow gauges providing a rare opportunity to evaluate hydrologic change. Several approaches were used to identify patterns of change following the wildfire, all of which suggest increased post‐fire flows. The before‐after‐control‐impact design showed significant increases in annual, spring and summer water yield from the small (49 km2) Arrowstone Creek watershed (30%, 21% and 86%, respectively). Significant increases in spring water yield were observed in the larger (5318 km2) Bonaparte River watershed (48%). Annual and summer water yield increased in the Bonaparte River (31% and 58%, respectively) but these changes were not statistically significant. In both the Bonaparte River and Arrowstone Creek, the onset of spring freshet (26 days earlier in both) was significantly advanced, however, the timing of maximum snowmelt discharge was significantly advanced (27 days earlier) only in Arrowstone Creek. Smaller changes were also observed in the reference watersheds; however, these were not statistically significant. The difference in results between the small and large watershed, as well as the effects of weather and watershed attributes, highlight the need for continued research into the relationships between wildfire and hydrologic regime across diverse landscapes.

Drivers and Impacts of the Record-Breaking 2023 Wildfire Season in Canada.

The 2023 wildfire season in Canada was unprecedented in its scale and intensity, spanning from mid-April to late October and across much of the forested regions of Canada. Here, we summarize the main causes and impacts of this exceptional season. The record-breaking total area burned (~15 Mha) can be attributed to several environmental factors that converged early in the season: early snowmelt, multiannual drought conditions in western Canada, and the rapid transition to drought in eastern Canada. Anthropogenic climate change enabled sustained extreme fire weather conditions, as the mean May-October temperature over Canada in 2023 was 2.2 °C warmer than the 1991-2020 average. The impacts were profound with more than 200 communities evacuated, millions exposed to hazardous air quality from smoke, and unmatched demands on fire-fighting resources. The 2023 wildfire season in Canada not only set new records, but highlights the increasing challenges posed by wildfires in Canada.

Synoptic-scale drivers of fire weather in Greece

The identification of the large-scale atmospheric circulation patterns which are associated with extreme fire weather is of great importance for developing early warning systems, management strategies, and for increasing awareness and preparedness of all the involved entities, including both the public and practitioners. Such a forecasting approach is currently missing in Greece and many other countries. Furthermore, considering climate projections over the Mediterranean, which indicate an environment more conducive to wildfire activity, the need for timely forecasting of extreme fire weather becomes increasingly urgent. Here, we present an alternative fire weather forecasting framework using ERA5 reanalysis data of atmospheric variables and fire weather indices of the Canadian Forest Fire Weather Index System (CFFWIS) during the period June–October from 1979 to 2019. Within this framework, we define the critical fire weather patterns (CFWPs) of Greece associated with different levels of fire weather severity by applying Self-Organizing-Maps (SOMs) on mid-tropospheric geopotential height. We quantify the fire weather conditions associated with each CFWP. Using a set of CFFWIS indices and key fire weather variables, our SOM-based analysis reveals five distinct CFWPs linked to different levels and characteristics of fire weather severity. The lowest fire weather severity is associated with lower than average geopotential heights, and anomalous cold and moist weather. The highest fire weather severity is associated with higher than average geopotential heights, and anomalous hot, dry, and windy conditions, suggesting the potential for wind-driven wildfires. Our analysis yields elevated fire weather severity linked to a CFWP, when hot and dry conditions are accompanied by atmospheric instability, suggesting the potential for plume-driven wildfires and the potential for pyroconvection. The main advantage of this forecasting framework is that it could be used for providing valuable information regarding the upcoming fire weather conditions even up to 7–12 days in advance depending on the atmospheric predictability.

Forest thinning and prescribed burning treatments reduce wildfire severity and buffer the impacts of severe fire weather

BackgroundThe capacity of forest fuel treatments to moderate the behavior and severity of subsequent wildfires depends on weather and fuel conditions at the time of burning. However, in-depth evaluations of how treatments perform are limited because encounters between wildfires and areas with extensive pre-fire data are rare. Here, we took advantage of a 1200-ha randomized and replicated experiment that burned almost entirely in a subsequent wildfire under a wide range of weather conditions. We compared the impacts of four fuel treatments on fire severity, including two thin-only, a thin-burn, a burn-only, and an untreated control. We evaluated four fire severity metrics—tree mortality, average bole char height, percent crown volume consumed (PCVC), and percent crown volume affected (PCVA)—and leveraged data from pre-fire surface and canopy fuels to better understand the mechanisms driving differences in wildfire severity among treatments and how they changed with fire weather.ResultsWe found strong mitigating effects of treatments on fire behavior and tree mortality, despite 20 years having elapsed since mechanical thinning and 10 years since the second entry of prescribed fire. The thin-burn treatment resulted in the lowest fire severity across all four metrics and the untreated control the highest. All four fire severity metrics were positively associated with pre-fire canopy and surface fuel loads, with the exception that PCVC (a fire severity metric related to crown fire behavior) was not associated with surface fuel load. The fire weather conditions under which fuel treatment was most effective varied among fire severity metrics. Fuel treatment benefit was maximized at intermediate burning index values for tree mortality, intermediate to high burning index values for PCVA, and high burning index for bole char height and PCVC.ConclusionsWe conclude that reducing canopy bulk density via mechanical thinning treatments can help to limit crown fire behavior for 20 years or more. However, reducing surface fuels is necessary to limit scorching and the total crown impacts associated with tree mortality. Further, while fuel treatment effectiveness may decline under the most severe fire weather conditions for fire severity metrics associated with tree mortality, it is maximized under severe fire weather conditions for fire severity metrics associated with crown fire behavior (bole charring and torching). Our results provide strong evidence for the use of fuel treatments to mitigate fire behavior and resulting fire severity even under extreme fire weather conditions.

Extreme fire weather in Chile driven by climate change and El Niño–Southern Oscillation (ENSO)

A string of fierce fires broke out in Chile in the austral summer 2023, just six years after the record-breaking 2017 fire season. Favored by extreme weather conditions, fire activity has dramatically risen in recent years in this Andean country. A total of 1.7 million ha. burned during the last decade, tripling figures of the prior decade. Six of the seven most destructive fire seasons on record occurred since 2014. Here, we analyze the progression during the last two decades of the weather conditions associated with increased fire risk in Central Chile (30°–39° S). Fire weather conditions (including high temperatures, low humidity, dryness, and strong winds) increase the potential for wildfires, once ignited, to rapidly spread. We show that the concurrence of El Niño and climate-fueled droughts and heatwaves boost the local fire risk and have decisively contributed to the intense fire activity recently seen in Central Chile. Our results also suggest that the tropical eastern Pacific Ocean variability modulates the seasonal fire weather in the country, driving in turn the interannual fire activity. The signature of the warm anomalies in the Niño 1 + 2 region (0°–10° S, 90° W–80° W) is apparent on the burned area records seen in Central Chile in 2017 and 2023.

Prescribed burning mitigates the severity of subsequent wildfires in Mediterranean shrublands

BackgroundPrescribed burning (PB) is becoming relevant in fuel reduction and thus fire hazard abatement in fire-prone ecosystems of southern Europe. Yet, empirical evidence on the effectiveness of this practice to mitigate wildfire severity in Mediterranean shrublands is non-existent, despite being the focus of PB efforts in this region. Here, we intended to quantify the protective effect of PB treatment units (2005–2021) to subsequent wildfire severity in shrublands across mainland Portugal, as well as the relative contribution and complex interactions between drivers of wildfire severity in PB-treated areas and untreated neighboring counterparts through Random Forest regression. We leveraged cloud-computing remote sensing data processing in Google Earth Engine to estimate fire severity (PB and wildfire) as the Relativized Burn Ratio (RBR) using Landsat data catalog.ResultsPB treatment was particularly effective at mitigating wildfire severity at the first PB-wildfire encounter in shrublands, with a mean reduction of around 24% in RBR units. Fuel age (i.e., time since prescribed burning) in PB-wildfire intersection areas overwhelmed to a large extent the effect of fire weather, burning probability, and PB severity. The mitigating effect of PB on wildfire severity persisted for a fuel age of around 5 years. However, this effect decreased with increasingly adverse fire weather conditions, such that variation in wildfire severity was somewhat insensitive to fuel age under extreme fire weather. Similarly, the lowest wildfire severity experienced in sites with high burning probability, along with the interaction effect observed between burning probability and fuel age, suggest that repeated PB treatments may be useful in controlling fuel accumulation and mitigating wildfire severity. The relative contribution of fire weather in explaining wildfire severity was exceedingly high in untreated areas, doubling that of the other variables in the model in the absence of PB treatment variables.ConclusionsOur results suggest that the implementation of PB treatments at intervals of less than 5 years is of paramount importance to control fuel build-up and fire hazard under extreme fire weather in productive Mediterranean shrublands. Further research on this topic is warranted in other shrublands worldwide, namely in Mediterranean-type climate regions.

Extending MESMER-X: a spatially resolved Earth system model emulator for fire weather and soil moisture

Abstract. Climate emulators are models calibrated on Earth system models (ESMs) to replicate their behavior. Thanks to their low computational cost, these tools are becoming increasingly important to accelerate the exploration of emission scenarios and the coupling of climate information to other models. However, the emulation of regional climate extremes and water cycle variables has remained challenging. The MESMER emulator was recently expanded to represent regional temperature extremes in the new “MESMER-X” version, which is targeted at impact-related variables, including extremes. This paper presents a further expansion of MESMER-X to represent indices related to fire weather and soil moisture. Given a trajectory of global mean temperature, the extended emulator generates spatially resolved realizations for the seasonal average of the Canadian Fire Weather Index (FWI), the number of days with extreme fire weather, the annual average of the soil moisture, and the annual minimum of the monthly average soil moisture. For each ESM, the emulations mimic the statistical distributions and the spatial patterns of these indicators. For each of the four variables considered, we evaluate the performances of the emulations by calculating how much their quantiles deviate from those of the ESMs. Given how it performs over a large range of annual indicators, we argue that this framework can be expanded to further variables. Overall, the now expanded MESMER-X emulator can emulate several climate variables, including climate extremes and soil moisture availability, and is a useful tool for the exploration of regional climate changes and their impacts.

Resource objective wildfire leveraged to restore old growth forest structure while stabilizing carbon stocks in the southwestern United States

Wildfire futures and aboveground carbon (C) dynamics associated with forest restoration programs that integrate resource objective wildfire as part of a larger treatment strategy are not well understood. Using simulation modeling, we examined alternative forest and fuel management strategies on a 237,218-ha study area within a 778,000-ha landscape that is a high priority target for federal restoration programs. We simulated two wildfire management scenarios combined with three levels of conventional forest restoration treatments over 64 years using a detailed landscape disturbance and succession model developed in prior work. We found accelerated forest restoration used in concert with resource objective wildfire was the most effective at returning old growth forest structure, while stabilizing aboveground C stocks and restoring the fire return interval to its historic range of variation. In scenarios without forest restoration, the continued practice of resource objective wildfires during shoulder fire seasons reduced summer emissions in a negative feedback loop. In the short term, scenarios without forest restoration increased live tree C, but also increased the likelihood of C loss during wildfire activity driven by extreme fire weather. We found scenarios most effective at restoring fire-excluded pine forests to their historical old growth conditions came at a short-term cost of lost C, but with the long-term benefit of substantially increasing fire-resistant live tree C. Our results inform how local decision making can best balance competing goals of sequestering C, and stabilizing C stocks in frequent-fire pine forests using the principles of local fire ecology to restore and maintain old growth forest structure.

Fuel dynamics and rarity of fire weather reinforce coexistence of rainforest and wet sclerophyll forest

Rainforest, wet sclerophyll forest and eucalypt forest exist as Alternative Stable States moderated by fire. Eucalypt forest is flammable and subject to regular fire. Insights into the flammability of rainforest and wet sclerophyll forest are difficult to obtain because of the rarity of fire, particularly in sub-tropical and tropical environments. This study aimed to determine the relative flammability of rainforest and wet sclerophyll forest with a canopy of Lophostemon confertus in subtropical Australia. Fires in 2019, coinciding with extreme fire weather and following a long period of drought, burnt rainforest and wet sclerophyll forest in south-east Queensland. The weather record indicates these conditions recur about three to four times a century. Remapping of the vegetation overlaid with the fire-scar mapping, indicated that the 2019 fires burnt 23 % of rainforest, much of which was disturbed, but 60 % of wet sclerophyll forest suggesting greater flammability of the latter. This study revealed that above-ground biomass and litter fuel loads are more than twice as high in wet sclerophyll forest than rainforest. The litter of Lophostemon confertus, a Myrtaceae tree dominating the upper canopy, on average decomposes at less than half the rate of rainforest litter, contributing to the higher fuel loads and thus greater flammability of wet sclerophyll forest compared to rainforest. The Alternative Stable States of wet sclerophyll and rainforest are partly stabilised by the fire retardance of rainforest, the rarity of weather conditions suitable for fire in the subtropical environment and the higher fuel loads in wet sclerophyll forest that allow occasional fire. Wet sclerophyll forest requires rare fire events and the legacy of these fires over hundreds of years is evident in the multiple cohorts of ‘giant’ canopy trees.

Changes in European fire weather extremes and related atmospheric drivers

Recent destructive fire seasons around the world have increased concern that climate change is escalating the frequency, intensity, and extent of wildfires. While the contributions of individual factors can be debated, the scientific literature concludes that fire weather is one prominent driver of fire activity. Our analysis of the relationship between fire weather extremes and burned area in Europe revealed that the number of extreme fire weather days per year correlates positively with the annual burned area over most of the study domain. The evidence presented in our study strongly suggests that fire weather extremes in Europe have become more frequent in recent decades while affecting increasingly larger areas and occurring earlier and later in the fire season. Increases in fire weather extremes occurred abruptly in several parts of Europe, as evidenced by the detection of statistically significant change-points in the time series of the annual extreme fire weather days. Our analysis revealed that, at the regional scale, significant change-points occurred around the late 1990s and mid-2000s in the Mediterranean, marking an abrupt rise in the median of extreme fire weather days per year. We found that high-latitude blocking and sub-tropical ridging drive fire weather extremes in northern and southern Europe, respectively. The results of our study suggest that fire weather extremes are today more likely to occur than in the past, and the increasing occurrence of extreme fire weather could lead to rapidly reaching adaptation limits, thereby reversing currently stagnating or even declining trends reported for fire activity in Europe. Corresponding with extreme fire weather years reported here, recent destructive fire seasons exemplify the limits of fire suppression and the need to adapt to the reality of more frequent extreme fire weather.

Refuge-yeah or refuge-nah? Predicting locations of forest resistance and recruitment in a fiery world.

Climate warming, land use change, and altered fire regimes are driving ecological transformations that can have critical effects on Earth's biota. Fire refugia-locations that are burned less frequently or severely than their surroundings-may act as sites of relative stability during this period of rapid change by being resistant to fire and supporting post-fire recovery in adjacent areas. Because of their value to forest ecosystem persistence, there is an urgent need to anticipate where refugia are most likely to be found and where they align with environmental conditions that support post-fire tree recruitment. Using biophysical predictors and patterns of burn severity from 1180 recent fire events, we mapped the locations of potential fire refugia across upland conifer forests in the southwestern United States (US) (99,428 km2 of forest area), a region that is highly vulnerable to fire-driven transformation. We found that low pre-fire forest cover, flat slopes or topographic concavities, moderate weather conditions, spring-season burning, and areas affected by low- to moderate-severity fire within the previous 15 years were most commonly associated with refugia. Based on current (i.e., 2021) conditions, we predicted that 67.6% and 18.1% of conifer forests in our study area would contain refugia under moderate and extreme fire weather, respectively. However, potential refugia were 36.4% (moderate weather) and 31.2% (extreme weather) more common across forests that experienced recent fires, supporting the increased use of prescribed and resource objective fires during moderate weather conditions to promote fire-resistant landscapes. When overlaid with models of tree recruitment, 23.2% (moderate weather) and 6.4% (extreme weather) of forests were classified as refugia with a high potential to support post-fire recruitment in the surrounding landscape. These locations may be disproportionately valuable for ecosystem sustainability, providing habitat for fire-sensitive species and maintaining forest persistence in an increasingly fire-prone world.

The effect of broadleaf forests in wildfire mitigation in the WUI – A simulation study

The increasing occurrence of large wildfires in Southern European countries calls for the adoption of more effective measures of fire prevention, to protect people and infrastructures, namely in the Wildland-Urban Interface (WUI). Previous research suggests that broadleaf forests could mitigate the effects of these wildfires due to their lower flammability. However, few attempts have been made to investigate the possibility of using broadleaf-based green firebreaks to protect infrastructures, as an alternative to current less ecologically and economically sustainable fuel reduction techniques. Here, we assess the relevance of a broadleaves fuel model in reducing fire hazard in the WUI, by analyzing a set of six Industrial Zones (IZs) in Central Portugal, severely affected by wildfires during the catastrophic fire season of 2017. We developed alternative scenarios for the spatial simulation of fire behavior, using a factorial combination of weather conditions (standard and extreme), buffer around each IZ (distances of 100 m and 500 m) and land cover (current and broadleaves fuel model). The simulations were grounded on real-world data obtained from reconstructed fire front isochrones and fieldwork. Our results suggest that replacing the flammable vegetation present in the WUI with broadleaf forests could reduce fireline intensity by up to five times, even under extreme weather conditions. This reduction occurs subtly as the broadleaf cover interface is expanded from 100 m to 500 m. Our results show that fires that exceed the suppression capacity in pine and eucalypt forests (>4 m flame length) can be effectively suppressed in broadleaf forests under extreme fire weather (1.4 m flame length) and easily suppressed in broadleaf forests in standard weather (0.8 m flame length). Due to significant changes in land use and extreme weather events, future large wildfires could occur again in Portugal. In this regard, our results corroborate the urgent need to discuss forest management in the country, which has already proven to be insufficient to prevent fire disasters in the WUI.

The Impact of Stratospheric Aerosol Injection on Extreme Fire Weather Risk

AbstractStratospheric aerosol injection (SAI) would potentially be effective in limiting global warming and preserving large‐scale temperature patterns; however, there are still gaps in understanding the impact of SAI on wildfire risk. In this study, extreme fire weather is assessed in an Earth system model experiment that deploys SAI beginning in 2035, targeting a global temperature increase of 1.5°C above pre‐industrial levels under a moderate warming scenario. After SAI deployment, increases in extreme fire weather event frequency from climate change are dampened over much of the globe, including the Mediterranean, northeast Brazil, and eastern Europe. However, SAI has little impact over the western Amazon and northern Australia and causes larger increases in extreme fire weather frequency in west central Africa relative to the moderate emissions scenario. Variations in the impacts of warming and SAI on moisture conditions on different time scales determine the spatiotemporal differences in extreme fire weather frequency changes, and are plausibly linked to changes in synoptic‐scale circulation. This study highlights that regional and spatial heterogeneities of SAI climate effects simulated in a model are amplified when assessing wildfire risk, and that these differences must be accounted for when quantifying the possible benefit of SAI.

Fire weather index data under historical and shared socioeconomic pathway projections in the 6th phase of the Coupled Model Intercomparison Project from 1850 to 2100

Abstract. Human-induced climate change is increasing the incidence of fire events and associated impacts on livelihood, biodiversity, and nature across the world. Understanding current and projected fire activity together with its impacts on ecosystems is crucial for evaluating future risks and taking actions to prevent such devastating events. Here we focus on fire weather as a key driver of fire activity. Fire weather products that have a global homogenous distribution in time and space provide many advantages to advance fire science and evaluate future risks. Therefore, in this study we calculate and provide for the first time the Canadian Fire Weather Index (FWI) with all available simulations of the 6th phase of the Coupled Model Intercomparison Project (CMIP6). Furthermore, we expand its regional applicability by combining improvements to the original algorithm for the FWI from several packages. A sensitivity analysis of the default version versus our improved version shows significant differences in the final FWI. With the improved version, we calculate the FWI using average relative humidity in one case and minimum relative humidity in another case. We provide the data for both cases while recommending the one with minimum relative humidity for studies focused on actual FWI values and the one with average relative humidity for studies requiring larger ensembles. The following four annual indicators, (i) maximum value of the FWI (fwixx), (ii) number of days with extreme fire weather (fwixd), (iii) length of the fire season (fwils), and (iv) seasonal average of the FWI (fwisa), are made available and are illustrated here. We find that, at a global warming level of 3 ∘C, the mean fire weather would increase on average by at least 66 % in duration and frequency, while associated 1-in-10-year events would approximately triple in duration and increase by at least 31 % in intensity. Ultimately, this new fire weather dataset provides a large ensemble of simulations to understand the potential impacts of climate change spanning a range of shared socioeconomic narratives with their radiative forcing trajectories over 1850–2100 at annual and 2.5∘ × 2.5∘ resolutions. The produced full global dataset is a freely available resource at https://doi.org/10.3929/ethz-b-000583391 (Quilcaille and Batibeniz, 2022) for fire danger studies and beyond, which highlights the need to reduce greenhouse gas emissions for reducing fire impacts.

Critical climate thresholds for fire in wet, temperate forests

Climate models predict more frequent droughts and more severe fire weather. Wildfires in wet forests, while historically uncommon, can have catastrophic impacts on forest values and services. Therefore, it is important to ask whether climate change is increasing the frequency of fires in wet forests. Long-term fire histories and weather records were compared in wet Eucalyptus forests supplying water to Melbourne, Australia, to identify the combination of dryness and fire weather under which stand-replacing fires occur. The effect of climate change on stand replacement frequency was predicted using down-scaled regional climate change projections for RCP4.5 and RCP8.5.An index of surface soil dryness (SSD), which is a proxy for live and dead fuel moisture, and daily maximum vapor pressure deficit (Dmax), which is a proxy for daily fire weather, were used to rank each day of each fire season from 1900/01 to 2019/20, within 898 km2 of water supply catchments. The two indices were compared for days with and without stand-replacing fire activity within the study area. Threshold values for the combination of both indices were identified. Damaging fires occurred on about 10 % of days falling above this threshold. The two worst fires, between them burning 86 % of the wet forest in the study area, occurred on days with moderate rather than severe SSD (within the top 2 % of days but only about half of the maximum SSD) but with the most extreme Dmax (highest and second highest ranked out of 43,920 days).The frequency of long fire seasons (SSD > 65 mm for 30 or more days) increased from 1 in ∼30 years during the 20th century to 1 in 4 years in the past 15 years, due to a doubling in the number of warm dry days (Dmax > 2.0 kPa) per fire season. The frequency of extreme fire weather days (Dmax > 5.5 kPa) was also higher in the past 15 years than for most of the previous century. These observations suggest that fire frequency and severity are likely to increase in these wet forests, potentially threatening a suite of ecosystem services. Based on regional climate change projections, increases in the frequency of stand replacing fire will be driven more by increases in maximum temperature than by reductions in rainfall. Under RCP4.5, stand replacing fire frequency in the study area could increase from 1 in ∼140 years historically to 1 in ∼40 years by 2050 and 1 in ∼22 years by 2090 (1 in ∼6 years under RCP8.5), which would result in widespread loss of these iconic forests, including Eucalyptus regnans, the world’s tallest Angiosperm.

Characterization of biophysical contexts leading to severe wildfires in Portugal and their environmental controls

Characterizing the fire regime in regions prone to extreme wildfire behavior is essential for providing comprehensive insights on potential ecosystem response to fire disturbance in the context of global change. We aimed to disentangle the linkage between contemporary damage-related attributes of wildfires as shaped by the environmental controls of fire behavior across mainland Portugal. We selected large wildfires (≥100 ha, n = 292) that occurred during the 2015–2018 period, covering the full spectrum of large fire-size variation. Ward's hierarchical clustering on principal components was used to identify homogeneous wildfire contexts at landscape scale on the basis of fire size, proportion of high fire severity, and fire severity variability, and their bottom-up (pre-fire fuel type fraction, topography) and top-down (fire weather) controls. Piecewise Structural Equation Modeling was used to disentangle the direct and indirect relationships between fire characteristics and fire behavior drivers. Cluster analysis evidenced severe and large wildfires in the central region of Portugal displaying consistent fire severity patterns. Thus, we found a positive relationship between fire size and proportion of high fire severity, which was mediated by distinct fire behavior drivers involving direct and indirect pathways. A high fraction of conifer forest within wildfire perimeters and extreme fire weather were primarily responsible for those interactions. In the context of global change, our results suggest that pre-fire fuel management should be targeted at expanding the fire weather settings in which fire control is feasible and promote less flammable and more resilient forest types.