Increasing frequency and intensity of the most extreme wildfires on Earth

Abstract

Climate change is exacerbating wildfire conditions, but evidence is lacking for global trends in extreme fire activity itself. Here we identify energetically extreme wildfire events by calculating daily clusters of summed fire radiative power using 21 years of satellite data, revealing that the frequency of extreme events (≥99.99th percentile) increased by 2.2-fold from 2003 to 2023, with the last 7 years including the 6 most extreme. Although the total area burned on Earth may be declining, our study highlights that fire behaviour is worsening in several regions—particularly the boreal and temperate conifer biomes—with substantial implications for carbon storage and human exposure to wildfire disasters.

Full text

Nature Ecology & Evolution | Volume 8 | August 2024 | 1420–1425 1420
nature ecology & evolution
Brief Communication
https://doi.org/10.1038/s41559-024-02452-2
Increasing frequency and intensity of the
most extreme wildfires on Earth
Calum X. Cunningham , Grant J. Williamson & David M. J. S. Bowman
Climate change is exacerbating wildre conditions, but evidence is lacking
for global trends in extreme re activity itself. Here we identify energetically
extreme wildre events by calculating daily clusters of summed re
radiative power using 21 years of satellite data, revealing that the frequency
of extreme events (≥99.99th percentile) increased by 2.2-fold from 2003
to 2023, with the last 7 years including the 6 most extreme. Although the
total area burned on Earth may be declining, our study highlights that re
behaviour is worsening in several regions—particularly the boreal and
temperate conifer biomes—with substantial implications for carbon storage
and human exposure to wildre disasters.
Extreme wildfires are sporadic features of fire-prone landscapes
globally
1
, and they carry major ecological, economic and social con-
sequences. Australia’s ‘Black Summer Bushfires’ of 2019 and 2020, for
example, were unprecedented in their scale and intensity (measured as
radiative power)2. These energetically extreme fires released extraordi-
nary quantities of carbon emissions and smoke3,4, killed an estimated
~2.8 billion vertebrates
5
and burned the entire geographic ranges of
116 plant species6. The 2015 wildfires in Indonesia likewise had major
social and economic effects: densely populated cities of southeast Asia
were blanketed with smog, leading to an estimated 100,000 additional
deaths from smoke-related respiratory problems
7
and causing an esti-
mated US$16 billion in direct and indirect economic losses8.
Most fires on Earth are small9, ignited by humans10, and not remark-
ably damaging. Indeed, fire plays a crucial role in the health of most
fire-adapted ecosystems
11
. It has been widely reported that the area
burned globally has decreased this century12–16, but this trend is mostly
driven by declines in low-intensity fires in African grasslands and savan-
nas13,16. Globally, average fire intensity has also been decreasing this
century (with some regional increases)
15
, but burn severity, an ecologi-
cal measure of a fire’s immediate effects (for example, biomass loss and
mortality), is increasing in more regions than it is decreasing14.
In contrast to the majority of fires, energetically extreme wildfires
are associated with extreme ecological, social and economic conse-
quences
1
, including emitting vast quantities of smoke and greenhouse
gases that threaten to further accelerate warming3,4. Despite their
importance, there remains no systematic evidence of temporal trends
in extreme wildfires. In a study of energetically extreme wildfires1, no
temporal trend was revealed, potentially because of the relatively short
satellite record used in the study (12 years, 2002–2013). Likewise, global
trends have not yet been observed for pyrocumulonimbus events
(storms triggered by extremely intense wildfires)17. A lack of trends is
unexpected because warming temperatures and increasing vapour
pressure deficit are drying fuels and worsening fire weather across
most of the Earth18–23. Climate change has already caused fire weather
to depart from its historical variability across ~20% of burnable land
area globally
24
, and recent extremely destructive wildfire seasons have
occurred in the Amazon25, Australia2, Canada26, Chile27, Portugal28,
Indonesia8, Siberia and the western United States19,29. While the notion
of increasingly dangerous wildfires pervades the media, such trends
have not been systematically shown30.
Here we use 21 years of satellite observations of the radiative power
released by wildfires to evaluate whether energetically extreme wildfire
events are increasing in frequency and/or magnitude. We use a similar
approach to ref. 1 by calculating the daily summed fire radiative power
(FRP; megawatts) of clusters of active fire hotspots observed by the
Moderate Resolution Imaging Spectroradiometer (MODIS) and Aqua
satellites
31
. To do this, we summed the FRP (ΣFRP) of hotspots in a 0.2°
equal-area lattice across the Earth for each of the 7,639 days between
1 January 2003 and 30 November 2023. As distinct from analysing the
FRP of individual hotspots, which measure the intensity of a fire at a
single time and location, our daily ΣFRP approach characterizes the
integrated radiant energy released by broader fire ‘events’ that may
burn concurrently at multiple nearby locations and at multiple time
points during the day, thus distinguishing energetically extreme fires
from energetically extreme hotspots. This process reduced 88.4 mil-
lion satellite observations to 30.7 million daily ΣFRP events. We then
evaluated temporal trends and biogeographic associations in the most
extreme of these events, defined here as those exceeding the 99.99th
Received: 22 January 2024
Accepted: 26 May 2024
Published online: 24 June 2024
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Fire Centre, School of Natural Sciences, University of Tasmania, Hobart, Tasmania, Australia. e-mail: calum.cunningham@utas.edu.au
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