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

Domain: earth_science
Published: 2024-08-20
Editorial outcome: accept

Abstract
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.

Introduction
The 2023 wildfire season in Canada was one of superlatives and broken records. At approximately 15 Mha, the area burned was by far the highest since the start of comprehensive national reporting (c. 1972), shattering the previous record of 6.7 Mha in 19891. Four provinces and territories registered their highest recorded annual area burned, with one fire complex reaching over 1 Mha, larger than any fire since 1950. Over 200 communities had to be evacuated—some twice—under stressful or logistically complex circumstances. There were several fatalities, and hundreds of homes and important infrastructure were destroyed. The smoke produced by Canadian wildfires had widespread consequences, affecting not only nearby communities, but also major population centers >1000 km from the wildfires, notably in southern Canada and on the east coast of the USA2. An unprecedented contingent of wildland firefighters was deployed to respond to the wildfires and international help was obtained from 12 countries and the European Union. The aftermath of the 2023 fire season will likely affect communities and ecosystems for years, if not decades to come.

The 2023 fire season was enabled by record-setting climate. Canada’s climate has been warming at twice the global rate: the mean annual temperature has increased 1.7 °C since 1948, with larger increases at high latitudes and during winter and spring3. Nationally, the annual area burned in the country has been increasing since the 1950s4. While 2023 was unique in both magnitude and character, scientists and managers have long anticipated the growing potential for increased fire activity5,6. The length of the wildfire season—the window of opportunity for wildfire to ignite and grow—has been steadily increasing due to warming over the last 50 years4,7,8. Projections indicate more frequent and intense fire-conducive weather, especially in the central and western part of the country9,10. Moreover, recent wildfire seasons have been associated with extreme weather phenomena affecting an uncommonly large area. For example, the 2021 “heat dome” that covered much of northwestern North America brought record temperatures and enabled one of the worst wildfire seasons of the last century in the province of British Columbia11, contributing to the abrupt increase in fire activity in the province since ~200012. Undoubtedly, the increasing potential for wildfires in Canada is symptomatic of the changing climate; in fact, it was estimated that the heat dome would have been 150 times less likely to occur without the ongoing anthropogenic climate disruption13.

Despite this growing body of knowledge about changing fire patterns, the 2023 wildfire season has challenged our understanding of wildland fire in Canada. Canada is about the width of a Rossby wave, the planetary wave pattern14 associated with meridional flow of the Jet Stream15. Severe fire weather is more likely to develop on one side of a wave, leading typical active fire years to occur regionally, rather than throughout the country16. Nationwide fire activity, affecting nearly all forested areas in a single season as experienced in 2023 was not anticipated until later in the century based on climate change projections5. However, recent studies report an earlier-than-expected emergence of anthropogenic climate change in many parts of North America17,18. In fact, a formal climate attribution study demonstrated that anthropogenic climate change strongly contributed to the 2023 wildfire activity in Quebec19, and that the current levels of activity are enabled by global warming in western Canada20,21. Equally surprising in 2023 was the relentless fire activity: except for a handful of days, many large wildfires burned without respite from late April to early October, and several regions underwent two or more significant surges in fire activity. Uncommon fire behavior, including a pyro-tornado in British Columbia (https://www.uwo.ca/ntp/), and a record number of pyrocumulonimbus events were observed due to the frequency and intensity of extreme weather and fire intensity. Consequently, large areas of less-flammable vegetation, such as broadleaf-dominated forests and recently burned stands, were affected by wildfires.

Here, we provide an overview of this exceptional year, and examine the main drivers and impacts of the record-breaking 2023 wildfire season in Canada. Specifically, we: (i) describe the spatio-temporal patterns of fire activity, (ii) investigate the drivers of fire activity (i.e., weather conditions, ignition source, and fire duration), and (iii) examine the wildfires’ observed and potential impacts on people and ecosystems. To achieve this, we use several datasets quantifying wildfire activity, weather and climate, ignitions, fire management response, and societal impacts. We interpret these observations relative to past wildfire seasons, both nationally and regionally. Finally, we discuss the globally relevant impacts of this extraordinary event in the context of rapid climate change.

Results
Overview of the wildfire season
The 2023 wildfire season extended across seven months (Fig. 1), following early snowmelt that coincided with record-breaking heat. Many of these wildfires caused evacuations, loss of homes, and infrastructure; several continued to burn for the rest of the fire season, reaching very large sizes (e.g., ≥105 ha). The fire season abruptly began in mid-April with an evacuation in southern British Columbia. Shortly afterward, hundreds of mainly human-caused wildfires started in Alberta in early May. In late May, fast-spreading wildfires on the eastern coast—an area where large wildfires are relatively rare—led to the evacuation of thousands of people from several communities in Nova Scotia and the destruction of hundreds of buildings outside of Halifax and in southern Nova Scotia. In early June, multiple convective cells with associated lightning ignited a string of fires across south-central Quebec, followed by another series of wildfires three weeks later that ignited to the north of the initial ones. The Quebec wildfires burned for weeks and produced a colossal plume of smoke that caused severe smoke pollution in several major cities in Canada and the USA, eventually spreading throughout much of the northern hemisphere2. In midsummer, large wildfires burning in the Northwest Territories caused extensive structural losses and the evacuation of about 70% of the population, including the territorial capital city of Yellowknife. In mid-August, wildfires across British Columbia also burned close to communities, destroying hundreds of structures in south-central British Columbia. Embers transported over 3 km across Okanagan Lake started spot fires that threatened the city of Kelowna. Across Canada, there were many rapidly spreading crown fires over the fire season with extreme fire intensity and towering convection columns. Extreme fire behavior and instability generated 140 pyrocumulonimbus events in the country, 83% of the total observed globally in 2023 (D. A. Peterson, pers. comm.). As a testament to the widespread and sustained nature of the 2023 wildfire activity, September 22nd—usually a quiet time at the end of the season—was the largest single-day area burned in Canada (~440,000 ha) since satellite records began in 1972.

Record-breaking annual area burned
The 2023 Canadian fire season was remarkable due to coast-to-coast fire activity lasting from mid-April to late October, resulting in a record-breaking total area burned of approximately 15 Mha, corresponding to around 4% of Canada’s forest area and more than seven times the historical national average. At the level of individual provinces and territories (Table S1), Quebec, the Northwest Territories, British Columbia, and Alberta all registered a record year for total area burned. In comparison with the 1986–2022 period, the area burned in 2023 was more than double the previous record of 6.7 Mha burned in 1989 (Fig. 2). The widely reported preliminary figure of 18.5 Mha burned is based on active fire data from the fire management agencies for the National Fire Situation Report (CIFFC, https://ciffc.net/). The agencies provide invaluable real-time information during the fire season, but the data are considered to be preliminary estimates for daily operations. Final quality control to account for water bodies and unburned vegetation islands is performed postseason. Our revised estimate is based on a hybrid NBAC-M3 dataset, combining ~12 Mha mapped at high resolution with Landsat and Sentinel-2 satellite imagery and ~3 Mha of perimeters estimated from thermal anomalies.

There were about ~6700 reported wildfire ignitions in 2023, which represents—surprisingly—a lower number than the average of about 8000 wildfires per year4. Lightning ignited 59% of the wildfires and lightning-caused wildfires accounted for 93% of the total area burned (1959–2015 average = 91%)4. Lightning storms that caused multiple simultaneous ignitions that are beyond initial attack capacity led to several large fires, some of which eventually coalesced. Lightning fires that started on four days, May 13th, May 27th, June 1st and July 5th, were responsible for 30% of the total annual area burned. Interestingly, 2023 had the third lowest overall lightning detections since 1998. The lightning-ignition efficiency was high due to extensive areas of dry, receptive ground fuels coinciding with the specific timing and location of lightning events22, a phenomenon that is expected to increase as a function of the ongoing warming and drying23. Human-caused ignitions were responsible for a comparatively low proportion of the total area burned (7%) in 2023, though it is difficult to confidently assign a human cause to some fires due to ongoing investigations and the eventual intermingling of individual fires with multiple ignition sources in large wildfire complexes. These ignitions were numerous early in the season before the greening of vegetation and the prevalence of lightning storms. Due to their proximity to communities and infrastructure, human-caused ignitions are associated with a disproportionately large fraction of evacuations and loss of structures24,25.

Early season drought conditions
In 2023, much of Canada faced a significant moisture deficit at the onset of the fire season, primarily attributed to prolonged drought and early snowmelt. The country was snow free considerably earlier than the 2004–2022 average, particularly in northern and west-central Canada (British Columbia, Alberta, and Saskatchewan), and along the east coast (Fig. 3a, b). The maximum root zone soil moisture drying rate increased in western Canada in early May, followed by rapid, intense drying in eastern Ontario and southern Quebec in late May and early June (Fig. 3c, d). Standardized anomalies of the Canadian Fire Weather Index (FWI) System’s Drought Code (DC), show similar trends, with severe drought conditions in some northern and western areas in May, and new-onset drought conditions in Quebec and Nova Scotia in June (Fig. 3d, e). Early-season drought is a common occurrence in western Canada26, due to persistent drought carried over from the previous year and exacerbated by a low winter snowpack27 (Fig. S1). In contrast, the 2023 fire season started with near-average levels of soil moisture following snowmelt in the eastern provinces, but above-average temperatures and rapid drying caused what could be described as a ‘flash drought’, an emerging phenomenon that we are only beginning to understand28.

Persistent extreme fire weather
Although wildfire activity is influenced by many factors, the 2023 season was largely driven by extreme weather enabled by anthropogenic climate change. Canada experienced extreme fire weather conditions in 2023, recognized as the hottest year on record globally29. Mean fire season daily temperature, precipitation, and vapor pressure deficit (VPD) anomalies confirm that it was significantly hotter and drier in Canada than average (Fig. 4). The mean May to October fire season temperature was 2.2 °C warmer than the 1991–2020 average, with higher values in northern Canada and average-to-below average values in southern Ontario, Quebec, and New Brunswick. This anomaly is already comparable to what is expected in summertime temperature increases over Canada for ~2050, based on projections from CMIP6 climate models under the SSP5-8.5 scenario (https://climate-scenarios.canada.ca/?page=cmip6-scenarios). VPD is a good predictor of fuel moisture globally30 and area burned regionally31. Many daily mean temperature and VPD extremes were observed across the country in 2023; for example, on July 7 the temperature at Norman Wells, NWT (65°N), just south of the Arctic Circle, reached 38 °C. Precipitation deficits were widespread, particularly in regions of western and northern Canada and western Quebec, all areas that experienced unusually large wildfires during 2023.

Fire growth is largely determined by the coincidence of dry fuels, ignitions, and the occurrence of extreme weather conditions32,33. Here, we define extreme fire weather when the Fire Weather Index (FWI) exceeds the climatological 95th percentile of FWI calculated at each location (FWI95). The FWI is a meteorological-based measure of overall fire danger used extensively in fire management34. In 2023, there was a strong correlation between the forested area where FWI exceeded FWI95 and the daily burned area in each province and territory, particularly in western Canada (Fig. 5 and Table 1). Because conditions were extreme for many regions of the country for an extended period of time (e.g., 2–5 months), this enabled the synchronous burning observed in both western and eastern Canada. Synchronous extreme fire weather conditions across large areas has previously been identified as a proxy for constraints on fire suppression resources35,36. The highest number of extreme fire weather days occurred in northeastern British Columbia, northern Alberta, and southern Northwest Territories (Fig. 6a). Nationwide, fire weather was most extreme in May and June (Fig. 6b), as a result of significant spring drought conditions over much of the country (Figs. 3 and 4). When examining the mean proportion of daily forested area exceeding FWI95, 2023 was also the most extreme fire weather year since at least 1940 (Fig. 6d). Similar relationships were also found for two other FWI System components34, the Initial Spread Index (ISI) and the Buildup Index (BUI) (Fig. S3). Although drought conditions dominated much of the 2023 fire season, days of extreme fire growth were also related to the presence of high near-surface winds, as evidenced by the largest growth days typically coinciding with short-lived (1–3 days) surges in the FWI (Fig. S6).

Large-scale atmospheric circulation patterns have a strong influence on weather extremes. Atmospheric ridges and the associated positive 500-hPa geopotential height anomalies are associated with fire conducive weather, such as ‘blocking’ highs37,38. Blocking can be characterized by persistent positive anomalies in 500-hPa geopotential heights39. Sharma et al.40 found that persistent positive anomalies in North America were associated with positive fire weather anomalies and a seven-fold increase in the likelihood of wildfire ignitions. In 2023, the persistent and widespread extreme fire weather conditions were likely related to the presence of frequent blocking events. Whereas for the baseline period (1991–2020) the mean number of annual (April–October) blocking days in Canada is ~15, the number of blocking days during 2023 far exceeded this value in most parts of the country with a mean value of ~50 (Fig. S2).

Fire size and duration
Most wildfires in Canada’s managed forest are contained or self-extinguished before they reach 200 ha in size; however, fires that survive and exceed 200 ha (“large fires”) account for approximately 97% of the area burned4,41. In 2023, there were 834 large fires in Canada, more than 2.5 times the 1986–2022 average of 320 (Fig. S4), as well as a substantially higher frequency of very large fires >50,000 ha (Fig. 7a). The contribution of very large fires in 2023 is noteworthy: 60 of them were responsible for 73% of the total area burned, compared to an average of 7 very large fires with 41% of the total area burned in the historical period (Fig. 7a). An uncommon event during most years, wildfire complexes where two or more wildfires eventually burn together resulted in 6 of the 10 largest, long-burning wildfires on record (Figs. S6, S7).

How did so many fires become so large in 2023? The likelihood of fire growth in any time period depends on the probability of extreme weather favoring growth, as well as the absence of fire-ending events such as significant rain or the onset of winter, assuming no fuel limitations42. Because the number of possible spread days for a given fire is limited by the fire duration, fire size also covaries with the time from ignition to control or extinguishment43. This is illustrated by the growth of the 10 largest wildfires that burned in Canada in 2023 (Figs. S6, S7). Of the 43 very large fires (>50,000 ha) that occurred in western Canada (west of 85°W), the median duration was 82 days (90% CI: 32–156) in contrast to only 40 days (22–89) for the 17 very large fires that occurred in eastern Canada. The relationship between the number of extreme fire weather days (FWI > FWI95) and fire size is shown in Fig. 7b. In fact, a robust power-law relationship exists between extreme fire weather days (i.e., potential spread days) and fire size throughout Canada10. Interestingly, large fires in western Canada burned over a longer period than those in eastern Canada, but under similar numbers of extreme fire weather days, relative to the local FWI climatology. Although conditions were extreme in each regional context, typical fire weather conditions in western Canada are more severe overall due to climatologically higher aridity. The fires in eastern Canada that grew very large over a relatively short duration, compared to those in western Canada, may also reflect landscape factors such as fuel continuity and topography although further work is required to understand these regional differences32.

The seasonal timing of ignitions also influences fire size, and this was particularly evident in 2023, as environmental conditions favoring fire growth over extinguishment persisted for extended periods in many regions of Canada (Figs. S6 and S7), particularly in western Canada where some fires burned for five months. Wildfires that began in the spring and were not extinguished quickly grew larger (Fig. 7c) and accounted for an outsized proportion of the total burn area. The proportions of fires >1000 ha ignited in May, June, July, and August corresponded to 0.12, 0.24, 0.37, 0.08, respectively, but were responsible for substantially different proportions of the total area burned (0.35, 0.34, 0.24, 0.05, respectively; see Fig. 7d). A smaller number of large fires that began in April and September had a negligible contribution to the area burned nationally.

Fire management response
In 2023, approximately 79% of wildfires detected in Canada received full suppression response, with the remaining 21% receiving modified or monitored response (i.e., no direct intervention). However, modified and monitored fires contributed 61% percent of the total area burned. The number of active wildfires placed an overwhelming demand on resources throughout most of the 2023 fire season (Fig. S4). A national preparedness level of 5, signifying an extreme fire load, insufficient national resources, and limited exchange capabilities, was declared on May 11 and persisted for a record 120 consecutive days. Unprecedented personnel imports and exchanges occurred within Canada (over 1700 individuals), and over 5500 individuals from 12 countries and the European Union provided assistance. Additionally, the collective fire-fighting effort included contract firefighting crews, structural firefighters from municipal and local governments, individuals who volunteered, and the Canadian Armed Forces that were deployed to aid in firefighting efforts in multiple provinces. A significant number of firebreaks were created to halt or slow fire spread and protect communities and critical infrastructure. As a result of this record fire-suppression effort, approximately 85% of full-response fires were contained to under 200 hectares, in contrast to 62% of modified- and monitored-response fires that did not exceed this size.

Societal and ecosystem impacts
The human dimensions of wildfire also include the vulnerability of communities or individuals to wildfire smoke, evacuations, the loss of homes and structures, and in the worst-case scenarios, human lives44. Wildfires in Canada in 2023 resulted in profound societal impacts. Hundreds of thousands of people in communities were affected by evacuations, structure losses, power outages, and business interruptions, and millions of people in North America were exposed to wildfire smoke. Tragically, eight people working on wildfires in Canada were killed during the 2023 fire season. The approximately 15 Mha burned represents a vast area of fire-affected forest, with important implications for post-fire ecological recovery, habitat conservation, human use and enjoyment of forest resources, and the Canadian forestry industry.

Fine particulate matter with an aerodynamic diameter < 2.5 µm (PM2.5) is a known air pollutant; exposure to PM2.5 from wildfire smoke has been shown to significantly impact human health45. For communities near sources of wildfires, smoke causes direct visibility degradation and hazardous exposures to air pollutants. Smoke can be transported hundreds to thousands of kilometers downwind, thus causing population-level exposure to elevated levels of PM2.5 across large regions. Figure 8c shows the air quality warnings (bulletins) issued by forecasters at Environment and Climate Change Canada (ECCC) for the possible onset of poor air quality conditions46 (Air Quality Health Index (AQHI)) for years between 2017 and 2023. Within the current decade, most AQHI alerts are the result of wildfire events across the country, with peaks from June to September. Compared to the annual average of about ~1300 alerts from 2017–2022, 2023 stands out with ~5000 alerts issued (Jan–Nov), due to the extremely poor air quality conditions from summer wildfires across Canada (see also Table S3). Fires in the southern Northwest Territories and northwestern Quebec were particularly influential sources of smoke in 2023 (Fig. 8a). In some regions, almost half of the summer (>60 days) was spent in unsafe air quality conditions, sometimes in air containing more than 18 times the level of PM2.5 required to trigger an air quality warning (Fig. 8b). Smoke transport from fires as point sources (Fig. 8b) also affected many large communities along several eastern states of the USA. Furthermore, satellite imagery from early June 2023 also showed smoke from Quebec fires transported across the Atlantic, impacting countries in western Europe2. In 2023, Canadians experienced approximately eight days of poor air quality on average during the fire season; however, severely affected regions with lower populations, such as the Northwest Territories, endured up to 44 poor air quality days (Tables S5 & S6).

Most of Canada’s population of ~41 million is concentrated in the southern part of the country, where fire activity is less prevalent. However approximately 12.3% of Canadians live within or adjacent to forested areas, in the wildland-urban interface47, and these communities are more likely to experience an evacuation when threatened by fire. Indigenous communities, of which 32.1% of the on-reserve population is located within the wildland-urban interface, are especially vulnerable to evacuation24,48. Evacuations are extremely disruptive for community cohesion and economics, and have negative physical and mental health impacts49,50.

In 2023, wildfires triggered evacuations of communities in 12 of the country’s 13 provinces and territories51. Preliminary estimates indicate that approximately 232,000 people were evacuated in 282 events; the most evacuees of any fire season since records began in 198051, and nearly three times as many people as were evacuated due to the Horse River wildfire that burned through Fort McMurray in 2016. Five of the 10 largest evacuations ever recorded in Canada occurred this year (Table S4), with some communities such as Edson in Alberta and Lebel-sur-Quévillon in Quebec experiencing repeated evacuations. The 2023 fire season will undoubtedly have lasting social and health impacts throughout the country.

Most Canadian ecosystems are adapted to large, intense wildfires, but their resilience may be compromised if wildfires are too frequent. Although recent wildfire activity has remained within the historical range of variability52, the scale of this wildfire season (~4% of the total forest area burned) in Canada is well outside what was observed in recent decades. As such, the 2023 wildfire season will significantly alter forest landscapes. Nationally, wildfires predominantly burned forests (87.5%), except in ecotonal areas that transitioned to grassland ecosystems in the south (i.e., herbs vegetation type) and those that transitioned to tundra in the north (i.e., herbs and shrub types) (Table S2). Wildfires chiefly burned conifer and mixedwood forests, but a substantial area of broadleaf forests burned in Alberta due to this vegetation type regional dominance and the propensity of springtime, pre-greenup burning53.

The 2023 fire season notably caused widespread burning of young forests (e.g <30 years since fire or harvest), with over 1 Mha affected. This disturbance has the potential to cause extensive post-fire tree regeneration failures, because immature trees cannot provide enough seeds following a fire54,55. For instance, between 300 and 400 kha of forests might suffer post-fire regeneration failures in Quebec’s commercial forests alone56,57. These failures, compounded by logging legacies, drought, and insect outbreaks, could reduce forest productivity58 and carbon stocks59, and accelerate the transition from boreal forests to open taiga, prairies or parklands60–62. Forest landscape changes from the 2023 wildfire season will have profound effects on forest ecosystem processes and biodiversity, with species adapted to early-stage or open-canopy forests benefitting, whereas those reliant on mature or old-growth forests being most negatively affected in the near term63,64. The cumulative impacts of the area burned in 2023 coupled with the extensive anthropogenic disturbance legacies on the landscape will challenge the resilience of forest ecosystems, especially if fire activity continues to increase, as projected65,66.

Future outlook
The 2023 Canadian fire season shattered national and multiple regional records, causing profound societal and health impacts. The combination of extreme weather and an extended fire season led to longer-lasting, hence larger, wildfires, resulting in a burned area approximately seven times the national average. Unprecedented synchronous fire activity strained fire management and response capacities across the country. Anthropogenic climate change facilitated the 2023 fire season, which is consistent with expectations of global warming put forward over the past several decades6. Disentangling the relative influence of human landscape legacies and worsening fire weather8,18,67 on the current Canadian fire environment is ongoing, but it is inescapable that extreme heat and moisture deficits enabled the record-breaking 2023 fire season. The disproportionate effect a few days of extreme weather can have on the total area burned is also evident in this fire season68,69, leading to worrisome prospects given projected future conditions10. Fire-fuel feedbacks may offset some potential for future fire activity70,71; however, increasingly severe weather overwhelms this resistance to burning33. Although this trajectory cannot easily be changed, we are not without options. Increased focus on innovative and integrated fire management strategies that prioritize risk mitigation are gaining momentum. This will require all stakeholders to engage in fostering more fire resilient environments, as the 2023 fire season heralds the emergence of severe climate change impacts on fire activity decades earlier than previously anticipated.

Methods
Wildfires in Canada
Wildfires have been a longstanding ecological process in many ecosystems of Canada since the end of the last glaciation. Fire-prone regions of Canada have predominantly boreal and temperate continental climates, with the majority of wildfires occurring between April 1 and September 30. Even though about half are ignited by humans, lightning-caused fires are responsible for a greater proportion (~90%) of the area burned4, occurring in remote areas72. In some parts of the country, Indigenous cultural burning during spring or fall also drove local fire regimes, though these practices were suppressed beginning in the early 20th century73,74. From a meteorological standpoint, the Canadian wildfire season is typically characterized by a series of high-pressure atmospheric systems resulting in drying periods of several days to more than one week, interspersed with rain events75. Lightning storms occurring during or after these dry spells can cause several hundred fire starts within a few days in a given region. When followed by persistent warm and dry weather and strong winds, extensive forested areas can burn as crown fires76.

Fire data
In Canada, high-quality spatial wildfire perimeters are available from the National Burned Area Composite (NBAC)1. At the time of writing, the NBAC product had not yet been completed and consolidated with agency data, so we considered a variety of provisional fire data sources to form a complete picture of fire activity for 2023.

Agency fire data
During the fire season in Canada, fire information reported by fire management agencies are collated by the Canadian Interagency Forest Fire Centre (CIFFC). These data include reported date, fire ignition cause, and fire management response type. We downloaded reported data for 2023 (https://ciffc.net/, accessed December 29, 2023) to determine the number of incidents, management responses, and fire cause.

Subdaily fire detections
Near-real-time fire perimeter polygons were obtained from the M3 fire perimeters77 derived from thermal anomaly (hotspot) point detections using a two-step buffering process as follows. First, VIIRS (375m) hotspots (VIIRS and MODIS (1km) in Quebec) are buffered out rout meters to form circular polygons. Boundaries between intersecting circular polygons are dissolved to form larger polygons. Second, perimeters of the resulting polygons are buffered in (contracted) rin meters. The choice of rout and rin for each ecozone were determined by comparing buffered polygons from 2012–2021 with higher-resolution fire perimeters from the NBAC. Selected radii maximize the intersecting area, and minimize commission and omission errors.

National Burned Area Composite
The provisional 2023 NBAC fire perimeters were created using cloud and shadow-free satellite imagery from the Google Earth Engine Collection-2 catalog (Landsat-8 and -9 data combined at 30-meters resolution, and Sentinel-2 data at 20-meters). We produced post-fire mosaics using median compositing of images collected over a 30-day period starting after the last hotspot acquisition date of the corresponding M3 perimeter. A pre-fire mosaic was derived likewise using the dates of the post-fire mosaic one year earlier. Image compositing was performed independently for Landsat and Sentinel, and an analyst selected the best quality pre- and post-fire mosaics for image mapping. In this procedure, an adaptive threshold was applied based on the differenced Normalized Burn Ratio to map pixels corresponding to burned areas1. The largest fires contributing the greatest area burned were targeted for NBAC mapping.

NBAC-M3 2023 hybrid dataset
We produced a hybrid fire perimeter dataset (“NBAC-M3”) using the combination of all preliminary NBAC fire perimeters and, for the 18% of the 2023 area burned which was not yet mapped, M3 perimeters78. We removed water bodies using the CanVec hydrographic features polygons79. It was necessary to correct the apparent burned area from M3 buffered hotspots, since buffered points typically overestimate area burned due to the presence of unburned islands and water bodies that are not accounted for in the buffering process. We did this by dividing the NBAC area burned by the total area burned reported in historical M3 buffered hotspots for 2012–2022, stratified by ecozone and year, thereby producing ecozone-specific calibration factors. We multiplied the calculated area of the M3 polygons for 2023 by the correction factors, within ecozones. Finally, polygons less than 1 ha in size were removed from the final dataset to minimize any introduced artifacts from the above process. We then summed the calibrated M3 area burned, with the mapped area burned from NBAC polygons to estimate the total area burned in 2023.

We calculated the proportion of land cover type that burned by intersecting the final NBAC-M3 dataset with the SCANFI classified land cover raster dataset80, excluding nonfuel. This dataset uses National Forest Inventory plots and satellite data to classify land cover into Water, Rock, Bryoid, Herbs, Shrub, Treed broadleaf, Treed mixed and Treed conifer land cover classes. Proportion land cover type burned, by province or territory, is reported in Table S2.

Daily fire growth
To estimate daily fire growth, we interpolated the fire detection date from hotspots for all wildfires in the NBAC-M3 dataset with a final area over 500 ha. Detection date77 was corrected to local standard time and interpolated using ordinary kriging to a 180-m grid, following Parks81. A filter was applied to detections prior to April 1st to remove false detections outside of the main fire season. In total, 641 fires were interpolated, accounting for ~99.5% of the total area burned in 2023.

Snowmelt timing
Daily snow cover data were obtained from the Interactive Multisensor Snow and Ice Mapping System produced by the United States National Ice Center (USNIC)82 at 4-km resolution. We defined snowmelt timing at each location as the first day of the longest snow free period at that location for any given year. In addition, snowmelt timing departures for 2023 were determined by the anomaly (days) of the 2023 snowmelt timing from the mean timing of the period 2004–2022.

Drought data
The root zone soil moisture is an indicator of the moisture content of the top 1 m of soil, and as such, is a useful measure of drought. We obtained the root zone soil moisture from the Global Land Data Assimilation System (GLDAS-2.2)83, a land surface model constrained by Data Assimilation of the Gravity Recovery and Climate Experiment. The GLDAS-2.2. data are daily at 0.25° resolution, and were subset to North America. To determine rapid onset of drought, we calculated the maximum amount of drying (i.e., reduction in root zone soil moisture values, kg m−2) that occurred using a 14-day sliding window from May 1st to June 30th. The timing of rapid onset was defined as the midpoint of the window corresponding to the maximum 14-day drying.

Weather and fire weather
Weather data were obtained from the ERA5 reanalysis via the Copernicus Climate Change Service84. ERA5 includes an extensive set of surface and upper air atmospheric variables, available hourly and globally at 0.25° resolution. We downloaded 2-m temperature, 2-m dewpoint temperature, precipitation, and the 10-m U and V components of wind. Relative humidity and vapor pressure deficit (VPD) were estimated from temperature and dewpoint temperature, using the equation from Alduchov and Eskridge for VPD85.

We examined mean daily anomalies of temperature, precipitation and VPD averaged between May 1st and October 31st (fire season) of 2023 compared with the baseline period (1991–2020); we also plotted the rank of the fire season mean of each variable for 2023 compared with the period 1940–2023.

The Canadian Fire Weather Index System (CFWIS)—part of the Canadian Forest Fire Danger Rating System34—is an empirical model that combines surface temperature, relative humidity, 10-m open wind speed, and 24-hour accumulated precipitation collected at solar noon34. The System outputs represent proxies to fuel moisture in the forest floor, potential fire behavior and an overall fire danger rating (the Fire Weather Index, FWI). We calculated CFWIS outputs from ERA5 weather following a procedure to account for interseasonal drought conditions86. Fire season was determined using the startup condition that daily maximum temperature should exceed 12 °C for three consecutive days, and annual shutdown conditions when temperatures fall below 5 °C for three consecutive days, serving as a proxy for snow free periods7.

We calculated a climatology of extreme FWI values at each ERA5 grid cell, defined as the 95th percentile of FWI values (FWI95) between 1991–2020 (omitting values outside of the fire season). An extreme fire weather day is then any day for which FWI > FWI95. Using percentiles to define extreme fire weather accounts for the fact that the fire weather relationship (e.g., fire ignition and spread) are heterogeneous across the landscape. The extent of extreme fire weather conditions was further defined as the forested area exceeding the local 95th percentile of the FWI87,88 (FWI95, defined by the fire season period during 1991–2020). We defined forested area by applying a threshold of >20% canopy cover to the NASA Making Earth System Data Records for Use in Research Environments (MEaSUREs) Vegetation Continuous Fields (VCF) Version 1 data product89 (VCF5KYR).

To examine the influence of large-scale atmospheric patterns over Canada in 2023—and in particular, atmospheric blocking events—we applied the identification algorithm for persistent positive anomalies in 500-hPa geopotential heights, previously developed by Sharma et al.40. A persistent positive anomaly was identified for contiguous regions where the 500-hPa geopotential heights exceeded one standard deviation above the local seasonal climatological mean (for the baseline period 1991–2020) and persisted for at least 5 days, with the additional criterion that each event reached at least 100,000 km2 during its evolution. We applied the algorithm using a domain that covers the entirety of the forested area of Canada. We considered the sum of days with a persistent positive anomaly present as a proxy for the number of blocking days at each location.

Air quality
We compiled PM2.5 concentration maps from the first 24-hour model outputs of the ECCC operational Regional Air Quality Deterministic Prediction System-FireWork90. FireWork is a state-of-science numerical air quality forecast system with near-real-time wildfire emissions89,91. The core of the FireWork system is the GEM-MACH meteorology-chemical-coupled model that simulates the dynamics of chemical composition, accounting for detailed gas-, aqueous- and particle-phase chemical reactions and atmospheric physics. The model domain covers North America with a 10-km grid resolution. Hourly wildfire emissions were estimated using the Canadian Forest Fire Emissions Prediction System (CFFEPS)89 for fire grid locations representing hotspot areas (past-24 hrs satellite detection by VIIRS sensors). We estimated emissions with forecast meteorology, fuel type, and burn areas through the NRCan Canadian Wildland Fire Information System92. Output of the FireWork system consists of hourly concentrations of gas-phase and aerosol species. The total fine PM components represent simulated PM2.5 concentrations. We adopted thresholds for ambient levels of PM2.5 as regulated under the Canadian Ambient Air Quality Standards (CAAQS, https://ccme.ca/en/air-quality-report), with daily and annual standards determined as a 24-hour average concentration of 27 µg m−3 and an annual average of 8.8 µg m−3.

Air Quality (AQ) Alert Bulletins were issued by MSC-ECCC, 2017-2023. Data represents the number of all AQ Alert bulletins issued throughout the period. Each AQ alert has a minimum of 2 bulletins issued (issuance and termination), plus a number of continued bulletins based on the persistence of the event. Each AQ Alert’s area of coverage varies in size based on the geographical extent of the AQ event.

Evacuations
Evacuation data for Canada for 1980–2023 were obtained by request from the Canadian Wildland Fire Evacuation Database51. These data are compiled using an exhaustive search of media reports and quality control measures as described by Tepley et al.24. Evacuation data from 2023 are provisional and are subject to updates as evacuation numbers are confirmed.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "See attached PDF.\n[Editorial Note: this PDF is displayed after the comments of Reviewer #2]", "reviewer_2": "Comments for the Manuscript NCOMMS-24-16816 “Canada Under Fire – Drivers and Impacts of the\nRecord-Breaking 2023 1 Wildfire Season”\nSummary:\nThe authors present an overall assessment of the record breaking 2023 fire season for Canada,\nproviding information about the causes, the impacts, and the implications. Throughout the\nmanuscript, authors provide quantitative information to support their analysis, based on several\nlines of evidence, datasets and literature. Authors provide a comprehensive overview of the wildfire\nseason, with regional statistics referring to the burned area and the drivers that led to it. They\neffectively summarize the drought and fire weather conditions that led to the exceptional fire year,\nand present considerations referring to the fire management intensity and its implications to the\nburned area. Finally they consider socioeconomic impacts of the fire season. The manuscript is\nwell written and well organized, while the figures (except for some minor comments provided here)\nare of great quality.\nMain comments:\nWhile there is not much to criticize in this work on what is already in the manuscript, what seems to\nbe missing is a good insight into the vegetation that was burned, i.e. types of trees/forest per region,\nas well as the vegetation conditions prior the wildfire events compared to the climatology.\nInformation about the types of vegetation may also partly explain the regional differences between\nthe Eastern and Western Canada. Regarding the vegetation conditions, while the DC code of the\nFWI partly covers this aspect, the DC is a very simple soil moisture accounting scheme and hence\ncan provide an estimate on the “agricultural” drought status, but not the vegetation – despite the\nfact that the FWI was initially created for Canadian wildfires. A suggestion would be to expand their\nanalysis into a remotely sensed index (like NDVI if there is not any other better?) as a comparison\nbetween the climatological values and the 2023 values.\nSubchapter “Fire size and duration”: Authors define the potential spread days as the days where\nthe FWI exceeds the 95th percentile of May-October climatology. But is this true? To what I can\ninterpret from Fig S6, in many cases the fire sustains (while not flourishing) for long periods (in days)\nwith pyro-meteorological conditions below 95th percentile. So the question that arises is whether\nthere is a rough threshold in the FWI that can be considered as a \"fire-ending event\"?\nOther comments\nL224: Figure 5 does not seem to provide any correlation information between FWI and BA. There is\nobviously a covariation in both quantities. It would be interesting to also provide some type of\ncorrelation (Pearson or Spearman maybe?) . As also stated in the manuscript, Fire weather index\ncan correlate well with the logarithm of the BA.\nIn figure 3a, the snowmelt timing pattern in the central Quebec region, shows a rapid transition\nbetween May and ~Jun values. Is there any elevation/aspect/soil/climate reason for that? The same\nreason also causes the same pattern in Figure 3b, and for some reason partly in the Figure 3f (June\nDC) but interestingly not in the Figure 3e (May DC)? Authors should check for potential issues in the\ndata or their processing.\nFigure 4: Authors should add titles in the sub-figures of Figure 4 as they have done in Figure 3. Also\nFigure 4 caption does not make any sense, something is going wrong.\nFigure 5: What is the x2 and x10 values mentioned at the right of Figure 5a? This should be\nmentioned in the figure caption. Also, is there any way to include y-axis information?", "reviewer_3": "Review of Jain et al., “Canada Under Fire…”\nPhilip Higuera, Missoula, MT, 13 April, 2024\nThis is an excellent contribution that I was excited to review. Given the record-breaking nature\nof Canada’s 2023 fire season, there will no doubt be broad interest in this work, and it has the\npotential to be widely cited by scientists, managers, and policy makers. The data and analyses are\nsimple and sound, providing a critical context for how society makes sense of the 2023 fire\nseason in Canada. This is critical work.\nGiven the importance of the topic and the likely broad interest, the bulk of my feedback is\nintended to help clarify and simplify messages and make this easily digestible by non-specialists.\nIn my experiences with Nature Communications and other high-profile journals (as a reader,\nreviewer, and [co-]author), the reach of the work will be significantly improved with simple\nlanguage. This differs from if this paper were published in a fire-specific journals (e.g., IJWF).\nThis work could be taken up by researchers from multiple fields, journalists, and policy makers,\nso there is great opportunity to provide a clear message to those audiences.\n1. Use numbers whenever possible and be as precise as possible. For example, consider\nreplacing superlatives like “shattering” and “eclipsed” or subjective qualifiers like “many\nmore” and instead use numbers, e.g., “X-times larger than” or “more than X-time higher\nthan…” Use the hard-earned numbers produced by this research. Suggestions are below.\n2. I kept waiting for a clear statement on climate and climate change, and text that\nexplicitly helps the reader fit 2023 into the broader context of global warming and\nshifting fire regimes. This is where this work will end up, regardless of the authors’\nintentions. The statistics are impressive, and one might think the statistics could “speak for\nthemselves\", but the authors have an opportunity to be explicit with how the audience should\nintegrates 2023 in the context of topics like “wildfire crisis,” “climate change,” “human\ndevelopment”, and “forest management.” Connect the dots for the readers.\nIt was not until the last sentence of the paper that a critically important statement is made:\n“…the 2023 fire season may foreshadow the emergence of severe climate[-]change impacts\non fire activity decades earlier than previously anticipated.” This seems like a very relevant\nthesis for the paper itself, but it’s not a message that comes through clearly throughout the\npaper, particularly the “earlier than previously anticipated” point. That has huge implications.\nOverall, there are several paragraphs where “the lead is buried” a bit, noted below.\n3. The data presented clearly support the statement that “extreme climate enabled the record-\nsetting 2023 fire season.” Is that in fact a main point of this work? Full disclosure, I am a fan\nof the term “climate enabled” (e.g., Higuera and Abatzoglou 2021, GCB). The term clearly\nattributes climate as a critical contributor, but it does not hang everything on climate, since\n(as the authors know and communicate well) ignition and extreme fire weather are also\nneeded. All elements are well documents in the manuscript. I encourage the use of “enabled”\nin several parts of the paper, detailed below.\n1\n4. The paper lacks the important context that “fire is a longstanding component of” Canadian\nforest ecosystems, for millennia. This is simple, but without this, many readers could walk\naways thinking that 2023 was a crazy climate-change impact entirely. It was not; it was an\nexpression of a longstanding natural phenomenon, under climate change (or “on steroids”).\n5. Consider using the term “average” instead of “normal” – simply given the normative use of\nthe term “normal” outside of climatology, statistics, and research in general. This will\nimprove the way this is communicated to non-specialists, particularly journalists.\n6. Limit parenthetical statements. If something is important, keep it in the sentence. Limit the\nuse of acronyms, to keep things digestible for a broad audience. Suggestions below.\n7. “Data” is used as a singular. Convention is that “data” is plural…but conventions change.\nDetailed comments:\nL 54: Consider flipping this sentence to start with “The 2023 wildfire season in Canada was\nunprecedented in its scale and intensity” – and, to clearly justify “unprecedented” consider\nsimply adding “…scale, intensity, and human impacts”. It was 100% unprecedented in human\nimpacts – no arguing about that part.\nL 56: Consider clarifying “periods of dense smoke that caused public health concerns” – e.g.,\n“thousands exposed to hazardous air quality [from wildfire smoke]…”\nL 58: Take the actual statistic out of the parentheses here – this is a big number that will be cited.\nE.g., “The record-breaking fire season, with over 15 Mha burned, can be attributed to…”\n*Here is the first opportunity to make a clear statement on attribution: both items listed\nunder “can be attributed to” are climate-related: snowmelt and drought. In the authors’\nview, was the 2023 fire season largely a function of climate? State that; otherwise,\nreaders will wonder about fuels, forest management, human ignitions, etc.\nIt is true that the record-setting 2023 fire season was…enabled by extreme climate\nconditions, consistent with expectations of a warming planet? And, perhaps emerging\nearlier than expected under climate-change scenarios? If so, highlight that in the abstract.\nL 61: Example of replacing “normal” with “average” and removing parenthetical statements:\n“…was a staggering 2.2 C warmer than the 1991-2020 average, enabling…” And here, these\nconditions enabled dry fuels – not really extreme fire weather per se, no? When I read “extreme\nfire weather,” I’m thinking about high winds, although I know the FWI integrates fuel moisture.\nIt seems to me that drought (+) created extremely dry fuels, which then enabled successful\nignitions and rapid fire spread when winds were high.\nL 63: Suggestion: “…in the country and across North America” OR “…across North America.”\nAs noted, smoke impacts on the NE USA impacted health and prompted much discussion.\nL 80: The text would be improved with a transition between these first two paragraphs. The\nsecond paragraph is highlighting climate as an important contributor to the 2023 fire season, but\nthis is backed into over a number of sentences. The transition from the impacts on communities\nto climate statistics is sharp. Consider something like: “The 2023 fire season was enabled by\nrecord-setting climate. CLIMATE STATITICS…”\nL 92: “entrained” here seems odd. Was this intended to be “enabled”? That would make sense.\nL 94-95: This seems like the topic sentence (buried at the end of the paragraph), or at least an\nimportant topic: increasing wildfire activity in Canada as symptomatic of changing climates. Or,\nmore specifically, symptomatic of global (and regional) warming.\nL 103-105: Again, this seems like a clear topic sentence, hiding in the paragraph: “…2023…was\nunprecedented and…not anticipated until later in the century based on climate[-]change\nprojections.” This is a really critical point, worth highlighting and leading with.\nL 113: For brevity, consider: “Here we provide an overview...and examine selected drivers and\nimpacts…” Note, only selected driver, and esp. only selected impacts, are presented here.\nConsider stating the objective as completed contributions, vs. the “-ing” of each objective.\nFinally, it’s worth emphasizing the importance of this work in helping contextualize what was an\nextraordinary event, not only for Canada, but globally. Documenting and contextualizing these\nimpacts of global warming is a critical task more broadly.\nL 148: “…since satellite records began in 19XX” – let the reader know when the records began.\nL 150: The section title could be confusing, with “high” potentially read to refer to altitude.\nConsider: “Exceptionally large total area burned” or simply “Record-setting annual area burned.”\nL 153: Note that “total burned area” and “area burned” are both used. Either works, but being\nconsistent will help for a general audience.\nL 154: “…historical national average.”\nL 168: Consider: “…the total area burned (19XX-20XX average – 91%)” – to avoid the reader\ngetting distracted by asking the question “over what period?”.\nL 182: Is it known, with the statistics on hand, what % of wildfires that resulted in home or\nstructure loss originated from human ignitions? I asked from my own perspective and interests\n(e.g., Higuera et al. 2023, PNAS Nexus), but I know others would wonder this too. This is\ncritical in the western US, as it clearly points to the fact that most of the “wildfire crisis” (i.e.,\nhome and structure loss) is attributable to human ignitions, whereas 88% of wildfires don’t result in any\nhome or structure loss. If not possible with the stats. on hand, no need to dig.\nL 188: Consider “early snowmelt” vs. “premature”? i.e., objectively earlier than average.\nL 197: “near-average”\nL 209: “hotter and drier…than average”\nL 221: Consider: “Fire growth is largely determined by the coincidence of dry fuels, ignitions,\nand the occurrence of extreme weather conditions…” The text mixes “extreme weather” and\n“extreme fire weather.” I know the latter inherently includes dry fuels, but to the general reader,\n“extreme weather” is most easily linked to things like high winds.\nL 244: Consider spelling out PPA, since the acronym is only used once in this paragraph.\nL 249: For brevity, consider keeping figure references in parentheses at the end of the sentence.\nL 256: Consider: “In 2023, the 834 large fires in Canada was more than 2.5 times the 1987-2022\naverage of 230.”\nL 272-273: Isn’t the definition of potential spead days imply “days with FWI > FWI ”? FWI\n95 95\nwas previously defined in the text.\nL 280: Consider just deleting “bottom-up factors” and just state “may also reflect fuel continuity\nand topography” as “bottom-up factors” is a fairly fire-specific term.\nL 294: Subsection title here is a little vague, with “considerations” Consider “response\".\nL 306-307: Not sure if this holds, but in the US we emphasize that fire/fuel breaks are intended\nto change fire behavior (including slower rate of spread), but not necessarily halt fire spread.\nL 314: Strongly suggest deleting “Aside from fire management…” as this qualifier suggests a\nnarrow view of fire impacts. It is well acknowledged that fire impacts – and specifically human\nimpacts – go well beyond fire management personnel. Suggestions: “The human dimensions of\nwildfire include the vulnerability of communities or individuals to wildfire smoke, evaluations,\nand loss of homes and structures, and in the worst-case-scenarios human lives.” The “even lives”\nmakes it sounds like a rare possibility – it’s happening more and more each year. In fact, it is\nremarkable – literally worth noting in the text – that the death toll form the 2023 fire season in\nCanada (not in the US) was kept so low.\nConsider citing, in the first sentence of this paragraph, a paper on human dimensions of wildfires\nand vulerability: “Social drivers of vulnerability to wildfire disasters: A review of the literature:\nhttps://www.sciencedirect.com/science/article/pii/S0169204623001160\nL 332: Over 3x more?\nL 381: For the ecological impacts of the 2023 wildfire season, consider citing Coop et al. (2020),\nfor a broad, recent, North American overview:\nhttps://academic.oup.com/bioscience/article/70/8/659/5859066\nL 387-391: This is all true, but generally a weak way to link the 2023 fire season to climate\nchange, global warming. Aren’t we past the stage where we wonder if we’re seeing the impacts\nof global warming manifest through wildfire activity? Consider something like “Although future\nresearch is necessary to quantify the degree to which anthropogenic climate change facilitated\nthe 2023 fire season, it is clear that extreme climate conditions, all consistent with expectations\nof global warming,…” This is my opinion, but I think it's a disservice if this text comes off as\nwondering if climate change was a factor here; details and specifics, yes, but overall, this is\nconsistent with what we’ve expected and predicted, since Flannigan and Van Wagner 1991 (!).\nL 396: “…moisture deficits enabled the record-breaking 2023 fire season.” This fits under the\nframework of “climate enabled, weather driven” as an explanation for extensive fire activity.\nL 402: This suggestion of prioritizing fire prevention is just doubling down of fire suppression?\nThis seems odd, not to mention very dangerous.\nL 404-405: As note above…this is a HUGE and very important point. The paper would benefit is\nthis was highlighted throughout, and expended upon a bit more.\nL 409: “Wildfire in Canada” – this is at least one place where it should be noted that wildfire in\nCanada is a longstanding process, for millennia.\nL 485: This page of method has a lot of acronyms. If the acronym is only used once or twice in a\nparagraph, consider eliminating it and just spelling out the term, for clarity.\nL 519: I’m familiar with the CFFDRS…but I don’t know what “screen-level temperature” are.\nL 534: What size is the “grid-level” here?\nFigures\nFigure 1: Strongly suggest changing the text (in the upper-right corner) to “…largest extent of\nland impacted by fire in Canada’s history” less you perpetuate the misconception that wildfires\ndestroy all land they burn across. The land is not “consumed” – it’s still there (thankfully)!\n*For Fig. 1 and 2 – are these statistics for “total area” or just “forested area”? I think the\nformer…but specifying would be helpful.\n*Fig. 3: Suggest using “average” here too instead of “norm”.\n*Fig. 4: Including titles after the panel labels (“a),” etc.) would be helpful here, as in Fig. 3.\n*Fig. 5: Great figure! Is it possible to have a y-axis for each panel, at least for one of the\nsubpanels? There’s no ways gauge the value of area burned (a) or number of days (b) currently.\nFig. 6: Having panel (a) as an anomaly (relative to average) as well would be useful. It would\narguably be more useful than panel (c) in this figure (which could go to the appendix). That\nwould keep this as a four-panel figures.\nFig. 7: In my opinion, this entire figure could go to an appendix.\nFig. 8: Consider having panel (b) as an anomaly, instead of the PM2.5 values. That would help\nhighlight what’s unusual about 2023."}, "author_response": "REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\nPhilip Higuera, Missoula, MT, 13 April, 2024\nThis is an excellent contribution that I was excited to review. Given the record-breaking nature\nof Canada’s 2023 fire season, there will no doubt be broad interest in this work, and it has the\npotential to be widely cited by scientists, managers, and policy makers. The data and analyses\nare simple and sound, providing a critical context for how society makes sense of the 2023 fire\nseason in Canada. This is critical work.\nResponse: We thank the reviewer for their positive assessment of this paper. Their feedback\nhas greatly improved the manuscript. Please see detailed responses to each of the comments\nbelow.\nGiven the importance of the topic and the likely broad interest, the bulk of my feedback is\nintended to help clarify and simplify messages and make this easily digestible by non-\nspecialists. In my experiences with Nature Communications and other high-profile journals (as a\nreader, reviewer, and [co-]author), the reach of the work will be significantly improved with\nsimple language. This differs from if this paper were published in a fire-specific journals (e.g.,\nIJWF). This work could be taken up by researchers from multiple fields, journalists, and policy\nmakers, so there is great opportunity to provide a clear message to those audiences.\n1. Use numbers whenever possible and be as precise as possible. For example, consider\nreplacing superlatives like “shattering” and “eclipsed” or subjective qualifiers like “many more”\nand instead use numbers, e.g., “X-times larger than” or “more than X-time higher than...” Use\nthe hard-earned numbers produced by this research. Suggestions are below.\nResponse: We agree that some of the subjective language could be changed. However, where\nnumbers are also given to back up these statements, we do not believe the superlative requires\nremoval as it relates to a quantitative statement. For example, this is the case on Lines 69-71:\n“At approximately 15 Mha, the area burned was by far the highest since the start of\ncomprehensive national reporting (c. 1972), shattering the previous record of 6.7 Mha in 1989”\nwe have decided to keep the word “shattering” as the relevant numbers are also provided.\nAs suggested by Reviewer #1, we have made the following changes:\nLine 60 - (abstract): removed ‘shattering’\nLine 72 - replaced “a massive” with “one”\nLine 171 - replaced “eclipsed” with “was more than double”.\nLine 281 - we removed the expression “many more” and used the number of large fires directly.\n2. I kept waiting for a clear statement on climate and climate change, and text that\nexplicitly helps the reader fit 2023 into the broader context of global warming and shifting\nfire regimes. This is where this work will end up, regardless of the authors’ intentions. The\nstatistics are impressive, and one might think the statistics could “speak for themselves\", but the\nauthors have an opportunity to be explicit with how the audience should integrates 2023 in the\ncontext of topics like “wildfire crisis,” “climate change,” “human development”, and “forest\nmanagement.” Connect the dots for the readers.\nIt was not until the last sentence of the paper that a critically important statement is made:\n“...the 2023 fire season may foreshadow the emergence of severe climate[-]change impacts on\nfire activity decades earlier than previously anticipated.” This seems like a very relevant\nthesis for the paper itself, but it’s not a message that comes through clearly throughout the\npaper, particularly the “earlier than previously anticipated” point. That has huge implications.\nOverall, there are several paragraphs where “the lead is buried” a bit, noted below.\nResponse: We appreciate the reviewer’s feedback with respect to potential improvements to the\nflow and the overall messaging. We made several changes that we feel have addressed this\ncomment, in particular in Paragraphs 2 and 3 of the Introduction (specific changes listed in\ncomments below). We feel like the manuscript is now a lot more forthcoming in terms of linking\nthe 2023 fire season to the anthropogenic climate disruption.\n3. The data presented clearly support the statement that “extreme climate enabled the record-\nsetting 2023 fire season.” Is that in fact a main point of this work? Full disclosure, I am a fan of\nthe term “climate enabled” (e.g., Higuera and Abatzoglou 2021, GCB). The term clearly\nattributes climate as a critical contributor, but it does not hang everything on climate, since (as\nthe authors know and communicate well) ignition and extreme fire weather are also needed. All\nelements are well documents in the manuscript. I encourage the use of “enabled” in several\nparts of the paper, detailed below.\nResponse: We accept this suggestion and have added this language, including the expression\n‘climate enabled’ in several places. We were cautious, however, to not be too heavy handed,\nbecause a formal climate attribution has so far only been performed for part of the country.\nSee lines 82, 96, 114, 229 and 437.\n4. The paper lacks the important context that “fire is a longstanding component of” Canadian\nforest ecosystems, for millennia. This is simple, but without this, many readers could walk\naways thinking that 2023 was a crazy climate-change impact entirely. It was not; it was an\nexpression of a longstanding natural phenomenon, under climate change (or “on steroids”).\nResponse: This statement was added to the Material and Methods section, as follows (Lines\n453-454): “Wildfires have been a longstanding ecological process in many ecosystems of\nCanada since the end of the last glaciation.”\n5. Consider using the term “average” instead of “normal” – simply given the normative use of the\nterm “normal” outside of climatology, statistics, and research in general. This will improve the\nway this is communicated to non-specialists, particularly journalists.\nResponse: We agree with this suggestion and made the change in the five places where\n‘normal’ or ‘norm’ was used (Lines 60, 220, 221, 232 and the Fig. 3 caption).\n6. Limit parenthetical statements. If something is important, keep it in the sentence. Limit the\nuse of acronyms, to keep things digestible for a broad audience. Suggestions below.\nResponse: As suggested, we limited the use of parenthetical statements and acronyms\nthroughout. We have removed the acronym for Ice Mapping System (IMS), persistent positive\nanomalies (PPAs), and Root Zone Soil Moisture (RZSM), and wildland urban interface (WUI)\nexcept where needed for figure appearance.\n7. “Data” is used as a singular. Convention is that “data” is plural...but conventions change.\nResponse: thank you for pointing this out. We have modified this in multiple places (Lines 175,\n478, 538, 550, 557, 628, 643, 647)\nDetailed comments:\nL 54: Consider flipping this sentence to start with “The 2023 wildfire season in Canada was\nunprecedented in its scale and intensity” – and, to clearly justify “unprecedented” consider\nsimply adding “...scale, intensity, and human impacts”. It was 100% unprecedented in human\nimpacts – no arguing about that part.\nResponse: We have made the suggested change.\nL 56: Consider clarifying “periods of dense smoke that caused public health concerns” – e.g.,\n“thousands exposed to hazardous air quality [from wildfire smoke]...”\nResponse: We modified as suggested, although we used the specific wording “millions exposed\nto hazardous air quality from wildfire smoke” as the data supports this.\nL 58: Take the actual statistic out of the parentheses here – this is a big number that will be\ncited. E.g., “The record-breaking fire season, with over 15 Mha burned, can be attributed to...”\n*Here is the first opportunity to make a clear statement on attribution: both items listed\nunder “can be attributed to” are climate-related: snowmelt and drought. In the authors’\nview, was the 2023 fire season largely a function of climate? State that; otherwise,\nreaders will wonder about fuels, forest management, human ignitions, etc.\nIt is true that the record-setting 2023 fire season was...enabled by extreme climate\nconditions, consistent with expectations of a warming planet? And, perhaps emerging\nearlier than expected under climate-change scenarios? If so, highlight that in the\nabstract.\nResponse: Although we tried as much as possible to accommodate the reviewer’s concerns, we\nwere limited by the abstract’s strict 150-word limit. We did manage to emphasize the role of\nclimate, albeit with some caution, given that a formal attribution analysis was not undertaken.\nWe made reference to the climate crisis in the following sentence (Lines 58-60): “Anthropogenic\nclimate change enabled sustained extreme fire weather conditions throughout the fire season,\nas the mean May–October temperature over Canada in 2023 was 2.2°C warmer than the 1991–\n2020 average.”\nL 61: Example of replacing “normal” with “average” and removing parenthetical statements:\n“...was a staggering 2.2 C warmer than the 1991-2020 average, enabling...” And here, these\nconditions enabled dry fuels – not really extreme fire weather per se, no? When I read “extreme\nfire weather,” I’m thinking about high winds, although I know the FWI integrates fuel moisture. It\nseems to me that drought (+) created extremely dry fuels, which then enabled successful\nignitions and rapid fire spread when winds were high.\nResponse: We have replaced “normal” with “average” and modified the sentence as suggested.\nWhile it is true that strong winds are often associated with extreme fire weather (specifically in\nthe Fine Fuel Moisture Code, Initial Spread Index, and Fire Weather Index outputs of the\nCanadian FWI System), extreme fire weather can also refer to extremes in fuel aridity and not\nonly fire behavior indices. For example, during the 2021 heat dome, regions in western North\nAmerica experienced record high temperatures and low humidity, which led to extremes in\nvapor pressure deficit (VPD) and the Fine Fuel Moisture Code (See Jain et al. Communications\nEarth & Environment volume 5, Article number: 202, 2024). Moreover, pyrocumulonimbus\nevents - many that occurred in both 2021 and 2023 - are a function of fuel amount and\nespecially fuel aridity, but tend to occur under lighter winds. In general, the FWI System uses an\nempirical model to combine several variables (surface temperature, relative humidity, wind\nspeed and precipitation) in a nonlinear way, and extremes in the outputs may occur due to the\ncombination of extreme values in one or more (but not necessarily all) of the inputs.\nL 63: Suggestion: “...in the country and across North America” OR “...across North America.” As\nnoted, smoke impacts on the NE USA impacted health and prompted much discussion.\nResponse: We prefer to not generalize to the continent in the abstract, given the focus of the\nwildfire season in Canada. Also, we are running into the word limit and feel that some of the\nother concerns raised by the reviewer should take precedence over this suggestion addition.\nNote that we made sure that the impacts beyond the borders of Canada were properly\nacknowledged throughout the manuscript.\nL 70: “...(c. 1975) over two times the previous record of 6.7 Mha...”\nResponse: We feel like the current formulation essentially holds the same information and opted\nto keep it as is. While we made an effort to remove superlatives, we do want to convey the\nexceptional nature of the 2023 wildfire season.\nL 80: The text would be improved with a transition between these first two paragraphs. The\nsecond paragraph is highlighting climate as an important contributor to the 2023 fire season, but\nthis is backed into over a number of sentences. The transition from the impacts on communities\nto climate statistics is sharp. Consider something like: “The 2023 fire season was enabled by\nrecord-setting climate. CLIMATE STATITICS...”\nResponse: We adopted the reviewer’s suggestion as a topic sentence. Note that an effort was\nmade to improve the transitions among ideas throughout the Introduction. In particular, the link\nto climate change was strengthened in Paragraphs 2 and 3 to address the reviewer’s main\nconcern.\nL 92: “entrained” here seems odd. Was this intended to be “enabled”? That would make sense.\nResponse: thank you for pointing this out. I believe we were another victim of autocorrect! (it\nhas been fixed).\nL 94-95: This seems like the topic sentence (buried at the end of the paragraph), or at least an\nimportant topic: increasing wildfire activity in Canada as symptomatic of changing climates. Or,\nmore specifically, symptomatic of global (and regional) warming.\nResponse: It is true that this general statement could have made an acceptable topic sentence.\nHowever, it would have not allowed us to start the paragraph by mentioning the record-breaking\nclimate, which we feel needs to be emphasized early on in the paragraph. In light of this, we\nrather chose to use the simple, yet impactful, topic sentence suggested by the reviewer (see\nprevious comment).\nL 103-105: Again, this seems like a clear topic sentence, hiding in the paragraph: “...2023...was\nunprecedented and...not anticipated until later in the century based on climate[-]change\nprojections.” This is a really critical point, worth highlighting and leading with.\nResponse: We respectfully disagree with the reviewer regarding the topic sentence. Although\nthis sentence indeed conveys an important idea, it remains one of the many outcomes that\n“challenged our understanding”, as stated in the current topic sentences. This said, some\nunpacking of this broad idea is justified; the text was added to address the reviewer’s concern\n(Lines 110-111): “However, recent studies report an earlier-than-expected emergence of\nanthropogenic climate change in many parts of North America (Abatzoglou et al. 2018; Jain et\nal. 2022).”\nL 113: For brevity, consider: “Here we provide an overview...and examine selected drivers and\nimpacts...” Note, only selected driver, and esp. only selected impacts, are presented here.\nConsider stating the objective as completed contributions, vs. the “-ing” of each objective.\nFinally, it’s worth emphasizing the importance of this work in helping contextualize what was an\nextraordinary event, not only for Canada, but globally. Documenting and contextualizing these\nimpacts of global warming is a critical task more broadly.\nResponse: We accepted the reviewers suggestions, except that we used ‘main’ instead of\n‘selected’ to describe the drivers and impacts. We reframed the latter part of the paragraph to\naddress the reviewer’s concern as follows (Lines 129-132): “We interpret these observations\nrelative to past wildfire seasons, both nationally and regionally. Finally, we discuss the globally\nrelevant impacts of this extraordinary event in the context of rapid climate change.”\nL 148: “...since satellite records began in 19XX” – let the reader know when the records began.\nResponse: We have added “in 1972” to reflect the fact that the first Landsat mission\n(Multispectral Scanner, MSS) provided data from that year (Line 162).\nL 150: The section title could be confusing, with “high” potentially read to refer to altitude.\nConsider: “Exceptionally large total area burned” or simply “Record-setting annual area burned.”\nResponse: We have replaced the section heading with “Record-breaking annual area burned”.\nL 153: Note that “total burned area” and “area burned” are both used. Either works, but being\nconsistent will help for a general audience.\nResponse: We agree it would be better to be consistent and have changed two instances of\n“total burned area” to “total area burned” to be consistent with the majority of cases (see lines\n167, 324).\nL 154: “...historical national average.”\nResponse: we have made the suggested change.\nL 168: Consider: “...the total area burned (19XX-20XX average – 91%)” – to avoid the reader\ngetting distracted by asking the question “over what period?”.\nResponse: We added the time period, as suggested (Line 183).\nL 182: Is it known, with the statistics on hand, what % of wildfires that resulted in home or\nstructure loss originated from human ignitions? I asked from my own perspective and interests\n(e.g., Higuera et al. 2023, PNAS Nexus), but I know others would wonder this too. This is critical\nin the western US, as it clearly points to the fact that most of the “wildfire crisis” (i.e., home and\nstructure loss) is attributable to human ignitions, whereas 88% of wildfires don’t result in any\nhome or structure loss. If not possible with the stats. on hand, no need to dig.\nResponse: Unfortunately, we do not have these statistics in Canada at this point in time. We\nused the reviewer’s comment to strengthen the argument that human-caused wildfires are\nresponsible for a disproportionate share of the evacuations and structure loss more generally\n(Lines 190-197): “Human-caused ignitions were responsible for a comparatively low proportion\nof the total area burned (7%) in 2023, though it is difficult to confidently assign a human cause\nto some fires due to ongoing investigations and the eventual intermingling of individual fires with\nmultiple ignition sources in large wildfire complexes. These ignitions were numerous early in the\nseason before the greening of vegetation and the prevalence of lightning storms (Parisien et al.\n2023) and many became wildfires that burned for months and caused evacuations of\ncommunities. Due to their proximity to communities and infrastructure, human-caused ignitions\nare associated with a disproportionate fraction of evacuations and loss of structures (Tepley et\nal. 2022; Higuera et al. 2023).”\nL 188: Consider “early snowmelt” vs. “premature”? i.e., objectively earlier than average. L 197:\n“near-average”\nResponse: We have replaced “premature” with “early” and “near normal” with “near-average” as\nsuggested (Lines 210, 220)\nL 209: “hotter and drier...than average”\nResponse: Modified as suggested (Line 232).\nL 210: “...warmer than the 1991-2020 average,...”\nResponse: Modified as suggested (Line 233)\nL 221: Consider: “Fire growth is largely determined by the coincidence of dry fuels, ignitions,\nand the occurrence of extreme weather conditions...” The text mixes “extreme weather” and\n“extreme fire weather.” I know the latter inherently includes dry fuels, but to the general reader,\n“extreme weather” is most easily linked to things like high winds.\nResponse: Modified as suggested. We also added ‘fire’ to ‘extreme weather’ in all cases that\ndidn’t strictly consider weather, independent of fire occurrence or fire behavior (Line 244).\nL 244: Consider spelling out PPA, since the acronym is only used once in this paragraph.\nResponse: Done (Line 269)\nL 249: For brevity, consider keeping figure references in parentheses at the end of the\nsentence.\nResponse: We have moved the figure reference to parentheses at the end of the sentence. This\nis consistent with other figure callouts in the results section. (Line 275)\nL 256: Consider: “In 2023, the 834 large fires in Canada was more than 2.5 times the 1987-\n2022 average of 230.”\nResponse: We have changed the text to be more in line with this suggestion (Lines 281-282).\nL 272-273: Isn’t the definition of potential spead days imply “days with FWI > FWI ”? FWI was\n95 95\npreviously defined in the text.\nResponse: Yes, FWI was previously defined. For consistency with the earlier definition, we\n95\nnow refer to potential spread days here as “extreme fire weather days” and use the term\n“potential spread days” in connection with the citation to Wang et al. (2020). The definition in\nparentheses was replaced with “FWI > FWI ” to avoid the redundancy (Lines 298, 299).\nand topography” as “bottom-up factors” is a fairly fire-specific term.\nResponse: We replaced the word “bottom-up” with “landscape” (line 307) because we believe\nthere may be other relevant landscape factors other than fuel continuity and topography that are\nimportant. In fact, Reviewer #2 has requested an analysis (which we have performed, see\nbelow) to determine what types of vegetation burned and if there are any differences that may\naccount for the different observed fire behavior between western and eastern Canada in 2023.\nL 294: Subsection title here is a little vague, with “considerations” Consider “response\".\nResponse: Modified as suggested (Line 320).\nL 306-307: Not sure if this holds, but in the US we emphasize that fire/fuel breaks are intended\nto change fire behavior (including slower rate of spread), but not necessarily halt fire spread.\nResponse: This is a good point and we have added the words “or slow” to address this (Line\n333).\nL 314: Strongly suggest deleting “Aside from fire management...” as this qualifier suggests a\nnarrow view of fire impacts. It is well acknowledged that fire impacts – and specifically human\nimpacts – go well beyond fire management personnel. Suggestions: “The human dimensions of\nwildfire include the vulnerability of communities or individuals to wildfire smoke, evaluations, and\nloss of homes and structures, and in the worst-case-scenarios human lives.” The “even lives”\nmakes it sounds like a rare possibility – it’s happening more and more each year. In fact, it is\nremarkable – literally worth noting in the text – that the death toll form the 2023 fire season in\nCanada (not in the US) was kept so low.\nResponse: We have removed the qualifier “Aside from fire management” and accepted the\nreviewer’s suggested change (Line 339). In the modern era, the death toll from wildfires has\nbeen relatively low in Canada when compared with other countries with active fire seasons; this\nis due to a combination of factors, not least being the relatively low population density in\nCanada, as well as the management of risk through the early and proactive issuance of\nevacuation orders. While we agree with the reviewer that the low death toll is noteworthy, we\nprefer to not mention it because it is an extremely sensitive topic in Canada and may be\nperceived by some as diminishing the gravity of the loss or lives that did occur.\nConsider citing, in the first sentence of this paragraph, a paper on human dimensions of\nwildfires and vulerability: “Social drivers of vulnerability to wildfire disasters: A review of the\nliterature: https://www.sciencedirect.com/science/article/pii/S0169204623001160\nResponse: As suggested, we have added this citation to the end of the first sentence of that\nparagraph (Line 341).\nL 332: Over 3x more?\nResponse: This is true, but in this case we do not believe it is necessary to further qualify the\nstatement.\nL 381: For the ecological impacts of the 2023 wildfire season, consider citing Coop et al. (2020),\nfor a broad, recent, North American overview:\nhttps://academic.oup.com/bioscience/article/70/8/659/5859066\nResponse: This reference was added to the last sentence of the paragraph (Line 420).\nL 387-391: This is all true, but generally a weak way to link the 2023 fire season to climate\nchange, global warming. Aren’t we past the stage where we wonder if we’re seeing the impacts\nof global warming manifest through wildfire activity? Consider something like “Although future\nresearch is necessary to quantify the degree to which anthropogenic climate change facilitated\nthe 2023 fire season, it is clear that extreme climate conditions, all consistent with expectations\nof global warming,...” This is my opinion, but I think it's a disservice if this text comes off as\nwondering if climate change was a factor here; details and specifics, yes, but overall, this is\nconsistent with what we’ve expected and predicted, since Flannigan and Van Wagner 1991 (!).\nResponse: With a step back, we fully agree with the reviewer. We removed the somewhat tepid\ntext underlined by the reviewer and replaced it with the following sentence (Lines 428-430):\n“Anthropogenic climate change facilitated the 2023 fire season, which is consistent with\nexpectations of global warming put forward over the past several decades (e.g., Flannigan and\nVan Wagner 1991).” In addition, we strengthened the text related to the effect of global warming\nand included it higher up in the manuscript (see earlier comment).\nL 396: “...moisture deficits enabled the record-breaking 2023 fire season.” This fits under the\nframework of “climate enabled, weather driven” as an explanation for extensive fire activity.\nResponse: We replaced ‘drove’ by ‘enabled’, as suggested (Line 437).\nL 402: This suggestion of prioritizing fire prevention is just doubling down of fire suppression?\nThis seems odd, not to mention very dangerous.\nResponse: We replaced `prevention and mitigation’ by ‘risk mitigation’. In this context,\nprevention refers to the avoidance of human-caused ignition, which, as pointed out in the text,\ncauses significant problems every year in Canada (as in most fire-prone areas). However, given\nthat the term can mean different things to different people, we opted to remove it.\nL 404-405: As note above...this is a HUGE and very important point. The paper would benefit is\nthis was highlighted throughout, and expended upon a bit more.\nResponse: This comment is related to an earlier comment. We made an effort to emphasize this\npoint throughout the manuscript. To strengthen the statement, we modified the end of sentence\nas follows (Lines 445-456): “ [...] as the 2023 fire heralds the emergence of severe climate\nchange impacts on fire activity decades earlier than previously anticipated.”\nL 409: “Wildfire in Canada” – this is at least one place where it should be noted that wildfire in\nCanada is a longstanding process, for millennia.\nResponse: We have added the following sentence (Lines 453-454): “Wildfires have been a\nlongstanding ecological process in many ecosystems of Canada since the end of the last\nglaciation.”\nL 485: This page of method has a lot of acronyms. If the acronym is only used once or twice in a\nparagraph, consider eliminating it and just spelling out the term, for clarity.\nResponse: We have removed acronyms that were used only once or twice, as suggested.\nL 519: I’m familiar with the CFFDRS...but I don’t know what “screen-level temperature” are.\nResponse: Screen-level meteorological measurements are usually performed at 1.3-2 m in the\npresence of a shelter (eg. Stevenson screen) to shield measuring apparatus from local sources\nof precipitation or heat. However, the point is taken that perhaps this is unnecessary detail,\nparticularly because this term is not relevant when using reanalysis data to compute fire\nweather indices. We have therefore replaced the words “screen-level” with “surface” (Line 570).\nL 534: What size is the “grid-level” here?\nResponse: As mentioned earlier in the text, each grid cell is at 0.25 degree resolution for the\nERA5 reanalysis data, which corresponds to approximately 31 km at the equator. To extract the\nexact physical size of each grid cell for a specific location would require a coordinate\ntransformation with a latitudinal adjustment to account for the fact that grid cells are smaller for\nincreasing latitude. Here, the correct transformation with area adjustment was used when\ncomputing the extent of extreme fire weather. However, we do not believe it is necessary to add\nthis rather technical detail here as this is a standard procedure that is performed with built-in\ntools available in most GIS software. In fact, the words “grid-level” are not required here so we\nhave now omitted them.\nFigures\nFigure 1: Strongly suggest changing the text (in the upper-right corner) to “...largest extent of\nland impacted by fire in Canada’s history” less you perpetuate the misconception that wildfires\ndestroy all land they burn across. The land is not “consumed” – it’s still there (thankfully)!\nResponse: Modified as suggested.\n*For Fig. 1 and 2 – are these statistics for “total area” or just “forested area”? I think the\nformer...but specifying would be helpful.\nResponse: This refers to the total area, which, in Canada is quite close to the forested area\n(See the new Supplemental Table S2). Fig. 1 has the text “Total area burned by province” to\nmake this clear.\n*Fig. 3: Suggest using “average” here too instead of “norm”.\nResponse: Changed as suggested.\n*Fig. 4: Including titles after the panel labels (“a),” etc.) would be helpful here, as in Fig. 3.\nResponse: Added as requested. We have also slightly modified the figure caption to clarify that\nthe rankings are for 2023 values relative to the period 1940-2023.\n*Fig. 5: Great figure! Is it possible to have a y-axis for each panel, at least for one of the\nsubpanels? There’s no ways gauge the value of area burned (a) or number of days (b)\ncurrently.\nResponse: We have added a second axis to both panels a and b as requested. We have only\nadded this axis to the top (Quebec) curve as to not crowd the plot although the scaling is the\nsame for other curves except where the multiplicative factor has been applied (as indicated).\nNote that panel b represents the forested area in which FWI is exceeding the 95th percentile of\nFWI, and not the number of days.\nFig. 6: Having panel (a) as an anomaly (relative to average) as well would be useful. It would\narguably be more useful than panel (c) in this figure (which could go to the appendix). That\nwould keep this as a four-panel figures.\nResponse: We have replaced panel (c) with the suggested figure (now panel b) and modified\nthe caption accordingly. The previous blocking figure has been moved to the supplementary as\na new figure (S2) with subsequent supplementary figure numbers incremented and figure\ncallouts appropriately changed. We also added the text to methods to clarify that days with\npositive persistent anomalies (in 500-hPa geopotential heights) are being used as a proxy for\nblocking days (blocking occurring when large-scale pressure patterns are nearly stationary for\nseveral days) . On revisiting the data, we also decided to add the mean number of blocking\ndays across Canada, which was an extraordinary ~50 compared with the mean value ~15; this\nwas explicitly stated (see Line 274).\nFig. 7: In my opinion, this entire figure could go to an appendix.\nResponse: We respectfully disagree with the reviewer. One of the main features of the 2023 fire\nseason was the large size of fires—many wildfires were immense (>500,000 ha). As a result,\nthe 2023 fire size distribution diverted wildly from the average distribution. One of the reasons\nfor so many large wildfires is that many of them started early in the season and burned for\nseveral months, a phenomenon that is not often described in the literature and it, in our opinion,\nwas worth highlighting.\nFig. 8: Consider having panel (b) as an anomaly, instead of the PM2.5 values. That would help\nhighlight what’s unusual about 2023.\nResponse: Although this would help emphasize how exceptional 2023 was, we feel that the\nabsolute value of PM2.5 is a relevant metric to display. Extreme values of PM2.5 correspond\nwith elevated health impacts, and an anomaly map would unfortunately downplay the impacts\nfelt by northern communities, which are more regularly exposed to smoke events.\nIt is worth noting that since PM2.5 is a known pollutant, countries (including WMO) have\nestablished absolute concentration/exposure limits:\n- Canadian's CAAQS is 27 ug/m3 (24hr avg) https://ccme.ca/en/air-quality-report#slide-7\n- USA's NAAQS is 35 ug/m3 (24hr avg) https://www.epa.gov/criteria-air-pollutants/naaqs-table\n- WMO is 15 ug/m3 (24-hr short term AQG level) https://iris.who.int/handle/10665/345329\nAnalysis of the PM2.5 anomaly is nonetheless a good suggestion and we hope to consider it in\na future study.\nReviewer #2 (Remarks to the Author):\nComments for the Manuscript NCOMMS-24-16816 “Canada Under Fire – Drivers and Impacts\nof the Record-Breaking 2023 1 Wildfire Season”\nSummary:\nThe authors present an overall assessment of the record breaking 2023 fire season for Canada,\nproviding information about the causes, the impacts, and the implications. Throughout the\nmanuscript, authors provide quantitative information to support their analysis, based on several\nlines of evidence, datasets and literature. Authors provide a comprehensive overview of the\nwildfire season, with regional statistics referring to the burned area and the drivers that led to it.\nThey effectively summarize the drought and fire weather conditions that led to the exceptional\nfire year, and present considerations referring to the fire management intensity and its\nimplications to the burned area. Finally they consider socioeconomic impacts of the fire season.\nThe manuscript is well written and well organized, while the figures (except for some minor\ncomments provided here) are of great quality.\nResponse: We are grateful for these positive comments.\nMain comments:\nWhile there is not much to criticize in this work on what is already in the manuscript, what\nseems to be missing is a good insight into the vegetation that was burned, i.e. types of\ntrees/forest per region, as well as the vegetation conditions prior the wildfire events compared to\nthe climatology. Information about the types of vegetation may also partly explain the regional\ndifferences between the Eastern and Western Canada. Regarding the vegetation conditions,\nwhile the DC code of the FWI partly covers this aspect, the DC is a very simple soil moisture\naccounting scheme and hence can provide an estimate on the “agricultural” drought status, but\nnot the vegetation – despite the fact that the FWI was initially created for Canadian wildfires. A\nsuggestion would be to expand their analysis into a remotely sensed index (like NDVI if there is\nnot any other better?) as a comparison between the climatological values and the 2023 values.\nResponse: We agree that more information pertaining to the vegetation type that burned and a\nmore in-depth characterization of vegetation condition prior to burning would add a useful\ndimension to this overview of the 2023 fire season. Although we added an analysis to partially\naddress the reviewer’s concern, it was beyond the scope of this study to undertake a detailed\nanalysis of the fine-scaled factors that drive fire spread.\nWe computed the vegetation type that was burned by provinces and territories to make a simple\ncomparison among areas (see new Table S2). The data shows that the vast majority of the area\nburned occurred in forest systems (87.5%). A larger proportion of non-forests burned in areas\nwhere other fuel types are common or dominant, such as in the tundra (eg, Nunavut) or\npeatland-rich areas (eg, Newfoundland and Labrador); however, in relative terms there was little\nfire activity in these areas. We added the following text to support these new results (Lines 397-\n401): “Nationally, wildfires predominantly burned forests (87.5%), except in ecotonal areas that\ntransitioned to grassland ecosystems in the south (i.e., herbs vegetation type) and those that\ntransitioned to tundra in the north (i.e., herbs and shrub types) (Table S2). Wildfires chiefly\nburned conifer and mixedwood forests, but a substantial area of broadleaf forests burned in\nAlberta due to this vegetation type regional dominance and the propensity of springtime, pre-\ngreenup burning (Parisien et al. 2023b).”.\nAs for the vegetation conditions, what the reviewer suggests represents a massive undertaking\nthat would be best addressed in one or more follow-up studies. Although the data exists to\nexamine the daily condition before or during a given wildfire, doing a proper analysis would\nrequire an examination of all the main factors that control wildfire ignition and spread (including\ntopography, land cover, and the spatial configuration of these factors on the landscape). This\nsaid, some important general inferences can be made from the analysis of drought and prior\nweather conditions (e.g., Fig. 4 and 6) and the timing and seasonality of ignition (Fig. 7).\nSimilarly, the suggested NDVI analysis is not without challenges. In many northern landscapes,\nNDVI varies substantially among vegetation types; therefore, the correspondence between on-\nthe-ground vegetation and the spectral indices must be established. For instance, compared to\nforests, a rich wetland (e.g., an open fen) can have a very high NDVI without having a\nparticularly impressive biomass load. An even greater challenge is depicting the intra-annual\nvariability observed in these landscapes. In most Canadian landscapes, there are strong\nphenological signals, but also important fluctuations related to drying/wetting in wetlands. This is\nnot to say that these indices are not useful, but the work that would be required to meaningfully\ninterpret them in different fuel types, seasons, and topo-edaphic settings is beyond the scope of\nthis study.\nSubchapter “Fire size and duration”: Authors define the potential spread days as the days where\nthe FWI exceeds the 95th percentile of May-October climatology. But is this true? To what I can\ninterpret from Fig S6, in many cases the fire sustains (while not flourishing) for long periods (in\ndays) with pyro-meteorological conditions below 95th percentile. So the question that arises is\nwhether there is a rough threshold in the FWI that can be considered as a \"fire-ending event\"?\nResponse: This apparent discrepancy can be understood in the context of “spread days”;\nextreme fire weather days (whether defined by FWI exceeding the 95th percentile or some other\nthreshold, see for example Wang et al. 2023) are related to daily linear fire spread and not total\ndaily area burned. As a fire grows large, relatively small linear growth can lead to large area\nburned due simply to geometric considerations (specifically, wildfire growth through time follows\na power function).\nIn terms of a FWI threshold for a fire-ending event, this needs to be considered probabilistic,\nbecause no single set of conditions can be confidently considered ‘fire ending’. For example, 20\nmm or rain could easily decrease the potential for ignition or spread of a wildfire, but may not be\nenough to extinguish a fire smoldering in deep organic soils (i.e., peatlands) that may rekindle\n(eg. holdover or overwintering fires). This said, a combination of factors may be preferable in\ndefining a “fire-ending event”; however, this is outside the scope of the present work, which was\nto provide an overview of the drivers and impacts of the 2023 fire season.\nOther comments\nL224: Figure 5 does not seem to provide any correlation information between FWI and BA.\nThere is obviously a covariation in both quantities. It would be interesting to also provide some\ntype of correlation (Pearson or Spearman maybe?) . As also stated in the manuscript, Fire\nweather index can correlate well with the logarithm of the BA.\nResponse: We have calculated the spearman correlation coefficient, which being a rank based\nquantity gives the same correlation coefficient between area FWI > FWI and BA or the\n95\nlogarithm of BA. We have added this as panel c of Fig. 5 and made reference to the fact that the\ncorrelations are larger in western Canada than in the central and eastern parts of the country in\nthe main text (Line 253).\nIn figure 3a, the snowmelt timing pattern in the central Quebec region, shows a rapid transition\nbetween May and ~Jun values. Is there any elevation/aspect/soil/climate reason for that? The\nsame reason also causes the same pattern in Figure 3b, and for some reason partly in the\nFigure 3f (June DC) but interestingly not in the Figure 3e (May DC)? Authors should check for\npotential issues in the data or their processing.\nResponse: The data has been double checked, and in so far as the data source is accurate,\nthere are no problems with the data. Although it appears as a discontinuity, it should be noted\nthat the reason for this is simply that this corresponds to the pattern of snow on the ground in\nlate May (see for example, the snow map for May 24 below). It is also not surprising that this\npattern is therefore evident in the May DC as this is the month in which the snow melted for this\nregion (the presence of snow would affect several inputs to the DC calculation including\ntemperature and overwintering adjustment to precipitation). The discontinuity disappeared by\nJune after the snow had fully melted. Indeed, the fact that this feature is shown in two\nindependent datasets lends further confidence to the results shown.\nFigure 4: Authors should add titles in the sub-figures of Figure 4 as they have done in Figure 3.\nAlso Figure 4 caption does not make any sense, something is going wrong.\nResponse: We have added caption titles as per Fig. 3. Similar captions have also been added\nto Fig. 6 for consistency (but not Fig. 7 which does not display maps). Also, thank you for\nalerting us to the problem with the caption for Fig. 4; there was an issue with the references to\nthe individual panel figures that is now fixed.\nFigure 5: What is the x2 and x10 values mentioned at the right of Figure 5a? This should be\nmentioned in the figure caption. Also, is there any way to include y-axis information?\nResponse: We explained the x2 and x10 in the caption. As also suggested by Reviewer #1, we\nhave added a second axis to both panels a and b as requested. We have only added this axis to\nthe top (Quebec) curve as to not crowd the plot although the scaling is the same for other\ncurves except where the multiplicative factor has been applied (as indicated).\nREVIEWERS' COMMENTS\nReviewer #2 (Remarks to the Author):\nComments for the Revised Manuscript R1-NCOMMS-24-16816 “Canada Under Fire – Drivers and\nImpacts of the Record-Breaking 2023 1 Wildfire Season”\nDear Authors\nThank you for your response and the additional analysis provided. I appreciate the effort made to\naddress my comments, especially the inclusion of the Table S2 and the supplementary text which\nenhance the manuscript's insights into the vegetation types burned. I understand and acknowledge\nthe constraints mentioned regarding a more detailed analysis of vegetation conditions prior to\nburning and the challenges associated with implementing a remotely sensed index like NDVI. While\nI still believe that the amount of additional work doesn’t justify one or more follow-up studies, I am\ncontent with the revisions and the overall response. Therefore, I agree to proceed with the\npublication as it stands. Thank you for your attention to my feedback and the diligent work in\nrevising the manuscript.\nAll the best"}]

Decision letter