Located at the border with Bolivia and Paraguay, the Brazilian Pantanal comprises more than 15  million hectares. The wetland floods seasonally, from November to April, then drains in the dry season from May to October. It holds a huge range of unique species, is home to many indigenous groups, provides important ecosystem services for the surrounding area, supports the livelihoods of tens of thousands of ranchers, farmers and fishers, and is a vast carbon store. 

Indigenous and traditional communities are among the worst affected by the wildfires, as traditional lands are destroyed, cultural practices disrupted and people displaced. Economic activities such as tourism and agriculture are also threatened, with crop losses and livestock deaths. The fires have also killed innumerable wild animals and birds, destroyed vital habitat and made life much more difficult for the animals that were able to escape, as food and water has become increasingly scarce.

Human-induced climate change is increasing wildfires in many regions of the world, as hot, dry and windy weather conditions increase the risk of fires both starting and spreading. Researchers from Brazil, the Netherlands, Sweden and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the weather conditions that fuelled the Pantanal wildfires, and how the conditions will be affected with further warming. Due to the difficulty of accounting for human activity in both starting and suppressing wildfires, we attribute the fire weather conditions, not the burned area itself.

To illustrate the extent and duration of extreme fire weather in the region, we use the cumulative Daily Severity Rating (DSR) for June, averaged over the Brazilian Pantanal (indicated by the solid black outline in Figure 1a). The DSR indicates how difficult it is to control a fire once it starts and it is commonly used to assess fire weather over monthly or longer periods. The DSR is derived from the Fire Weather Index (FWI), which uses meteorological information (temperature, humidity, wind speed and precipitation over the preceding weeks and days) to predict the expected energy release per length of the fire-front if a wildfire occurs. We focus on the Brazilian Pantanal where nearly all active fires in June occurred; however, including the wider region, which extends into Bolivia and Paraguay, would likely yield similar results. 

Main findings

• Fire weather is a critical driver of wildfires, although changes in vegetation (wildfire fuel) and fire management strategies also contribute to future wildfire risk. In the Pantanal, land use and land cover changes, such as clearing natural vegetation for pasture or agriculture, contribute to drier conditions and increase the availability of flammable vegetation. 
• In today’s climate with 1.2°C of global warming, intense fire weather conditions like the ones that drove the wildfires in the Brazilian Pantanal during June 2024 are a relatively rare event, expected to occur once every 35 years. This means there is about a 3% chance similar June fire weather conditions will occur in any given year.
• Observations show that similar June fire weather conditions, as defined by DSR, are about 3 times more impactful than they would have been in a 1.2°C cooler climate. They would have been about a factor 100 rarer had the climate not been warmed by humans. 
• To determine the role of climate change, we combine fire weather observations with climate models. Human-induced warming from burning fossil fuels made the June 2024 DSR about 40% more impactful and 4-5 times more likely. 
• These trends will continue with future warming. If warming reaches 2°C, similar June fire weather conditions will become around twice as likely, expected to occur on average about once every 17 years, and will become 17% more impactful. 
• To understand how the June fire-weather conditions are affected by human-induced climate change, we also investigate the weather variables comprising the DSR: maximum temperature, relative humidity, wind speed and rainfall. Most of these variables broke records in June 2024: it was the driest, hottest, and windiest June since observations began. Only relative humidity was the second lowest on record. 
• Next, we analyse how climate change alters likelihood and intensity of these four main weather variables. In the observations there is a strong drying trend and, as expected, increasingly high temperatures (figure 1b) accompanied by a reduction in relative humidity, while there is no clear trend in wind speeds. Thus, the increase in DSR can be explained by increasing temperatures – driven by climate change – and decreasing rainfall.
• Yearly rainfall in the Pantanal has been decreasing for over forty years. While natural decadal variability and deforestation in large ecosystems are known to affect rainfall patterns across South America, climate change may also be influencing the drying trend.
• The June 2024 fires spawned multi-ministry response actions to try to contain fires and save wildlife and livelihoods, such as the establishment of 13 new bases to accelerate the deployment of firefighters to remote areas. However, while significant steps have been taken to address the Pantanal wildfires, there are still substantial challenges to containment and extinguishment efforts. It is imperative that government agencies at all levels act swiftly and prepare for increasingly critical situations, as projections indicate a rise in such events.  

It leads to clear skies and high temperatures as well as strong winds due to an inverted pressure gradient between an intense subtropical high extending well into midlatitudes and the coastal low farther north. These conditions are highly conducive to wildfires i.e favour spread, especially during summer when conditions are warmer and drier. On February 2, 2024, wildfires ignited in the mountainous forested areas east of, forested areas east of the city Viña del Mar and around Lake Peñuelas.

The flames rapidly advanced into densely populated city outskirts despite authorities’ efforts to curb their spread. The humanitarian impact continues to worsen, with over 29,000 hectares burnt since February 4, resulting in 132 deaths, 300 missing, 7,200 houses destroyed and 40,000 people affected.

Researchers from Chile, Brazil, Colombia, the Netherlands, Germany, Sweden and the United Kingdom collaborated to assess to what extent human-induced climate change altered the likelihood and intensity of the weather conditions at the time of the fires, and how the conditions will be affected with further warming. 

Fire weather is defined by a combination of elevated temperatures, reduced humidity, minimal rainfall, and heightened winds persisting over a period. To capture the characteristics of this event, we use a fire weather index (HDWI) based on high wind speeds, high temperatures and low humidity. While not taking the build-up of fuel into account like other, more complex indices, it is an effective hazard metric for estimating threat to communities and difficulty of containment. We focussed on a coastal region affected by the wildfires that include Valparaíso and Viña del Mar.

Figure 2 shows the index values for the four days of highest fire intensity, when most impacts occurred.

Main findings

• There is significant interaction between extreme climate conditions and land management policies, leading to particularly devastating impacts in informal settlements.
• Fire weather is one important factor that drives wildfires, although changes in vegetation (wildfire fuel), ignition factors, and fire management strategies also contribute to future wildfire risk (‘risk’ refers to the combination of hazard, exposure and vulnerability, following common usage in climate science).
• To combine the different variables leading to high fire danger we compute the hot dry windy index (HDWI) that combines high temperatures, high wind speeds and low humidity. In this index, we find that the hot, dry and windy conditions that drove the wildfires of February 2024 are characterised as a 1 in 30-year event in today’s climate.
• To assess the role of climate change, we combine observation-based products and climate models and assess changes in the likelihood and intensity of a 1-in-30-year 4-day event over the region of the most devastating fires (fig. 2).
• We find that overall, there is a small increase in the HDWI in the observations and some models, but it is not significant. This is also true for the individual components of the index: maximum temperature, relative humidity and wind speed, none of which show a significant trend.
• These results are not surprising, given the coastal location of the region which has a well established wind-driven sea surface temperature cooling.
• The fires occurred in the coastal range of central Chile, right in the transition of coastal cooling and inland warming, rendering it difficult for most climate models to represent the event well. Only 5 models were able to simulate the event, and only 2 of these perform well in the model evaluation, thus results are very uncertain. Furthermore, the observation-based data are also comparably short and highly uncertain, particularly with respect to the wind component.
• Nevertheless, unless the world rapidly stops burning fossil fuels, fire danger due to high HDWI will increase. Using the same limited models as above, in a world 2°C warmer than preindustrial, the trend towards higher HDWI becomes significant.
• We conclude that despite the trend in HDWI not yet being significant, the risk of an increase in dangerous fire weather conditions attributable to human-induced climate change needs to be taken very seriously.
• We then assess to what extent El Niño is related to dangerous fire weather conditions. We test two different indices to characterise the influence of El Niño and find that El Niño has no significant influence on the HDWI.
• Across the studied area, fire risk is increasing notably due to current land management practices, such as the expansion of Wildland-Urban Interface areas (including the growth of informal settlements in forest zones) and widespread conversion from native to foreign and monoculture plantations.
• The existing investments in fire prevention and adaptation measures, coupled with low-risk perception among residents in fire-prone areas, have shown limited effectiveness in adequately mitigating the fire risk.
• Spared by the deadly flames, the pilot fireproofing program of Villa Botania
  showcases the life-saving potential of preparedness, including measures such as community-led vegetation control, embedding water points across the land, and robust emergency training.
• Measures to address the significant fire risk should encompass improved spatial planning; enhanced coordination, resource allocation, and community engagement in fire prevention and adaptation; and awareness raising campaigns

There are at least 17 direct fatalities linked to the fires, more than 150,000 people have been evacuated, and at least 200 structures, including homes, were damaged in the fires (AP News, 2023). The Canadian wildfires have severely impacted air quality locally in Canada, and in the neighbouring United States with Air Quality Index (AQI) values frequently exceeding safe levels in the midwest and northeast USA, and in some cases approaching record levels (e.g. on June 7th AQI reached 341 in New York City, considered hazardous for all residents) (CNBC, 2023). Similarly, in southern Ontario, including the cities of Ottawa and Toronto, air quality reached the “very high risk” level forcing officials to cancel public events and reduce hours for outdoor public services. Schools remained closed for several days in many states, including Nova Scotia, New York, New Jersey, and Connecticut. 

In this study we focus on the fires in eastern Canada, which experienced a particularly unusually active fire season in 2023 and are most directly linked with the very large-scale impacts on air quality. In order to identify the role of human-induced climate change we focus on fire-weather indices rather than on fire regime variables such as area burned, in order to identify whether and to what extent climate change altered fire-prone weather conditions.

To capture the extent and duration of the extreme fire weather across the region, we will use the cumulative daily severity rating (Figure 1). The DSR is a scaled power transformation of the Canadian Fire Weather Index (FWI), and reflects how difficult a fire is to suppress once ignition has occurred; it is commonly used for assessing fire weather on monthly or longer timescales (Van Wagner,, 1987). To capture the peak intensity of the fire season, we also take the annual maximum of the 7-day moving average of the FWI. This index has been used in previous attribution studies (eg. van Oldenborgh et al., 2021), where it has been found to have a good correlation with the area burned. 

Main messages

• Fire weather is one important condition driving wildfires, although changes in vegetation (wildfire fuel), ignition factors, and fire management strategies also contribute to future wildfire risk. 
• In today’s climate, intense fire weather like that observed in May-July 2023 is a moderately extreme event, expected to occur once every 20-25 years. This means in any given year such an event is expected with 4-5% probability. 
• Climate change made the cumulative severity of Québec’s 2023 fire season to the end of July around 50% more intense, and seasons of this severity at least seven times more likely to occur. Peak fire weather (FWI7x) like that experienced this year is at least twice as likely, and the intensity has increased by about 20% due to human-induced climate change. 
• Observed changes are typically larger than in the models. 
• As expected, likelihood and intensity are projected to increase further in a 2°C warmer world. 
• Changes in fire weather are associated with an increase in temperature and decrease in humidity, both of which are driven by human-induced warming; the effect was compounded in 2023 by unusually low precipitation
• The extent, magnitude, and location of concomitant wildfires posed significant challenges for wildfire management which largely focused on disaster response and wildfire containment to limit the impact on lives and infrastructure. 
• The wildfires had disproportionate impacts on indigenous, fly-in, and other remote communities who were particularly vulnerable due to lack of services and barriers to response interventions. 
• The consequences from the wildfires reached far beyond the burned areas with displaced impacts due to air pollution threatening health, mobility, and economic activities of people across North America.
• As fire weather risks increase, changes in fire management strategies and increased resources will be required to meet the increased challenges. 

Key findings

The key findings of the analysis of the 2019/20 fire season in southeastern Australia are:

Fire Weather Index

• We consider the highest weekly mean Fire Weather Index (FWI) of the fire season for each grid point as a measure of the most intense fire risk, and the Monthly Severity Rating as a measure of the overall seasonal fire risk. These are averaged over the area of most intense bushfires, between the mountains (the Great Dividing Range) and sea in New South Wales (including the Australian Capital Territory) and Victoria.
• Four climate models for which FWI could be calculated show that the probability of a Fire Weather Index this high has increased by at least 30% since 1900 as a result of anthropogenic climate change. As the trend in extreme heat is one of the main factors behind this increase and the models underestimate the observed trend in heat, the real increase could be much higher. This is also reflected by a larger trend in the Fire Weather Index in the observations.
• The observed index in 2019 was exceptional with a probability of about 3% in any given year in the current climate to occur. FWI shows a significant trend towards higher fire weather risk since 1979. Compared with the climate of 1900, the probability of Fire Weather Index as high as in 2019/20 has increased by more than a factor of four. For the Monthly Severity Rating the probability has increased by more than a factor of nine. We can attribute part of this trend to climate change.
• The Monthly Severity Index increased by a factor of two in the models, compared with 1900, but this is not significantly different from no change. Again, the real increase could be higher.
• Projected into the future, the models simulate that a Fire Weather Index at the 2019/20 level would be at least four times more likely with a 2 ºC temperature rise, compared with 1900. Due to the model limitations described above this is likely an underestimate.

Heat

• We analyse the highest 7-day mean maximum temperatures of the year averaged over the same region as the Fire Weather Index.
• Observations show that a heatwave as extreme as observed in 2019/20 would have been 1 to 2 ºC cooler at the beginning of the 20^th century. Similarly, a heatwave of this intensity is about 10 times more likely now than it would have been around 1900.
• While all eight climate models that were investigated simulate increasing temperature trends, they all have some limitations for simulating heat extremes: the variability is in general too high and the trend in these heat extremes is only 1 ºC, substantially lower than observed. We can therefore only conclude that anthropogenic climate change has made a hot week like the one in December 2019 more likely by at least a factor of two. Given the larger trend in observations, we suspect that climate models underestimate the trend in extreme heat due to climate change. Coupled with the models’ tendency to overestimate variability, the increase in the likelihood of such an event to occur is likely much higher than the models suggest.

Drought

• We consider annual mean low precipitation and the driest month of each year’s fire season September-February, both averaged over the same region.
• Observations show non-significant trends towards more dry periods like the record 2019 annual mean and a non-significant trend towards fewer fire season dry months like December 2019.
• All ten climate models we considered simulate the statistical properties of the observations well. Collectively they show no trend in dry extremes of annual mean precipitation nor in the driest month of the fire season (September–February). We conclude that there is no attributable trend in dry extremes like the ones observed in 2019.

2019 conditions

• These attributions are for events as defined above at the threshold, or above, set by the 2019/20 fire season and thus assessing the overall effect of human-induced climate change. In individual years, natural variability in large scale climate system drivers can have a higher influence on fire risk than the trend.
• Besides the influence of anthropogenic climate change, the particular 2019 event was made much more severe by a record excursion of the Indian Ocean Dipole and a very strong anomaly of the Southern Annular Mode, which together explain more than half of the amplitude of the drought in the second half of 2019.

Full study

• See the full study at NHESS: Attribution of the Australian bushfire risk to anthropogenic climate change