Scientists have long known that fires release substantial amounts of greenhouse gases and pollutants into the atmosphere.
However, estimating the total climate impact of fires is challenging.
Now, new satellite data has shed fresh light on the complex interplay between the climate and fires in different landscapes around the world.
It suggests that global emissions from fires are much higher than previously assumed.
In this article, we unpack the latest update to the Global Fire Emissions Database (GFED) – a resource that combines satellite information on fire activity and vegetation to estimate how fires impact the land and atmosphere.
The latest update to the database – explored in new research published in journal Scientific Data – includes data up to and including the year 2024.
It reveals that, once the data from smaller fires is included, fire emissions sit at roughly 3.4bn tonnes of carbon (GtC) annually – significantly higher than previous estimates.
It also shows that carbon emissions from fires have remained stable over the past two to three decades, as rising emissions from forest fires have been offset by a decline in grassland fire emissions.
The database update also illustrates how the amount of area burned around the world each year is falling as expanding agriculture has created a fragmented landscape and new restrictions on crop residue burning have come into force.
Landscape fires
Fire events vary widely in cause, size and intensity. They take place across the globe in many types of landscapes – deserts and ice sheets are the only biomes that are immune to fire.
When vegetation burns, it releases greenhouse gas emissions, which contribute to global warming. It also releases pollutants that cause local air pollution and, on a global scale, have a cooling effect on the climate.
Forest fires often generate considerable media attention, especially when they threaten places where people live.
However, the forest fires that make the news represent just a small fraction of all fires globally.
More than 95% of the world’s burned area occurs in landscapes with few trees, such as savannahs and grasslands.
Fires have helped maintain tropical savannah ecosystems for millions of years. Savannahs have the perfect conditions for fire: a wet season which allows grasses and other “fuels” to grow, followed by an extended dry season where these fuels become flammable.
Historically, these fires were ignited by lightning. Today, they are mostly caused – intentionally or accidentally – by humans.
And yet, despite their prevalence, these fires receive relatively little media attention. This is not surprising, as they have been part of the landscape for so long and rarely threaten humans, except for their impact on air quality.
Fires also occur in croplands. For example, farmers may use fire to clear agricultural residues after harvest, or during deforestation to clear land for cultivation.
The term “landscape fires” is increasingly used to describe all fires that burn on land – both planned and unplanned.
(The term “wildfire”, on the other hand, covers a subset of landscape fires which are unplanned and typically burn in underdeveloped and underinhabited land.)
Calculating the carbon emissions of landscape fires is important to better understand their impact on local air quality and the global climate.
New data
In principle, calculating carbon emissions from fires is straightforward. The amount of vegetation consumed by fire – or “fuel consumption” – in one representative “unit” of burned area has to be multiplied by the total area burned.
Fuel consumption can be determined through field measurements and satellite analysis.
For example, the burned area of a relatively small fire can be measured by walking around the perimeter with a GPS device. Fuel consumption, meanwhile, can be derived by measuring the difference in amount of vegetation before and after a fire, something that is usually only feasible with planned fires.
In practice, however, fires are unpredictable and highly variable, making accurate measurement difficult.
To track where and when fires occur, researchers rely on satellite observations.
For two decades, NASA’s MODIS satellite sensors have provided a continuous, global record of fire activity. To avoid too many false alarms, the algorithms these satellites use are built in a way so fires are flagged only when they burn an entire 500-metre grid cell.
However, this approach misses many smaller fires – resulting in conservative estimates of total burned area.
The latest update to the GFED includes, for the first time, finer-resolution satellite data, including from the European Space Agency’s “sentinel missions”.
This data shows that fires too small to be picked up by a satellite with a 500-metre spatial resolution are extremely common. So common, in fact, that they nearly double previous estimates of global burned area.
The data shows that, on average, 800 hectares of land – an area roughly the size of Australia – has burned annually over the past two decades.
The map below shows the frequency of fires around the world. Regions shaded in dark red burn, on average, 50-100% each year. In other words, fires occur annually or biannually. Regions in dark blue, on the other hand, are those where fires occur, but are very infrequent. Most regions fall in between these extremes.
The map shows that the areas most prone to fire are largely found in the world’s savannah and agricultural regions.
Falling burned area
Over recent decades, the total burned area globally each year has been declining.
This is largely due to land-use change in regions which used to have frequent fires.
For example, savannah is being converted to croplands in Africa. This transforms a frequently burning land-use type to one that does not burn – and creates a more fragmented landscape with new firebreaks which limit the spread of fire.
The decline in burned area is also due to the introduction of more stringent air quality regulations limiting crop residue burning in much of the world, including the European Union.
The amount of “fuel” – or biomass – in a unit area of land varies greatly. Arid grasslands are biomass-poor and, therefore, produce less carbon emissions when burned, whereas fuel consumption in tropical forests with peat soils is extremely high.
Maps of carbon emissions from fires closely resemble maps of burned area. However, they typically highlight biomass-rich areas, such as dense forests.
This is illustrated in the map below, which shows how fires in regions coloured dark red on the map produce, on average, 1,000-5,000 grams of carbon per square metre. In these places, much more carbon is lost during fires than gained through photosynthesis.
Meanwhile, much of the world’s savannah regions are coloured in yellow and orange on the map, indicating that fires here produce between 100-500 grams of carbon per square metre.
Rising forest fire carbon emissions
The boost in fire emissions captured by the latest version of the GFED is most pronounced in open landscapes, including savannahs, grasslands and shrublands.
Forest fire emissions, on the other hand, have barely changed in the updated version of the database. This is because most forest fires are relatively large and were already well captured by the coarse resolution satellite data used previously.
However, the trend in forest fire emissions is sloping upwards over the study period.
Overall, current estimates – which take into account the new data from smaller fires – suggest that, over 2002-22, global fire emissions averaged 3.4GtC per year.
This is roughly 65% higher than estimates set out in the previous update to the GFED, which was published in 2017.
For comparison, today’s fossil fuel emissions are around 10GtC per year.
Comparisons between fire and fossil fuel carbon emissions are somewhat flawed, as much of the carbon released by fires is eventually reabsorbed when vegetation regrows.
However, this is not the case for fires linked to deforestation or the burning of tropical peatlands, where regrowth is either much slower – or non-existent, if forests are converted to agriculture. These fires account for roughly 0.4GtC each year – just less than 12% of total fire emissions – and contribute directly to the long-term rise in atmospheric carbon dioxide (CO2).
The traditional view of forest fires as “carbon-neutral” is increasingly uncertain as the climate changes due to human activity. Longer fire seasons, drier vegetation and more lightning-induced ignitions are increasing fire frequency in many forested regions.
This is most apparent in the rapidly-warming boreal forests of the far-northern latitudes. The year 2023 saw the highest emissions ever recorded by satellites in boreal forests, breaking a record set just two years before.
Moreover, the fires in boreal forests are becoming more intense – meaning they burn hotter and consume a larger fraction of vegetation. This, in turn, jeopardises the recovery of forests.
In cold areas, fires also cause permafrost to break down faster. This happens because fires remove an organic soil layer that has an insulating effect which prevents permafrost thaw.
The map below shows the dominant fire type in different regions of the world, including boreal forest fires (dark green), cropland fires (red), open savannah (darker yellow) and woody savannah (brown).
Changing ‘pyrogeography’
Thanks to more precise satellite data we now know that fire emissions are higher than we thought previously, with the new version of GFED having 65% higher overall fire emissions than its predecessor.
However, all evidence suggests that emissions from fires have been stable over the past two to three decades. This is because an increase in forest fire emissions is being offset by a decline in grassland fire emissions.
The world’s changing “pyrogeography” is illustrated in the bar chart below, which breaks down annual fire emissions across different types of biome.
It shows how low-intensity grassland fires with modest fuel consumption – represented in yellow and brown – have declined over time, while high-intensity forest fires – illustrated in green colours – are becoming more prominent, albeit with substantial variability in emissions year-on-year.
The post Guest post: Why carbon emissions from fires are significantly higher than thought appeared first on Carbon Brief.
Guest post: Why carbon emissions from fires are significantly higher than thought
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Drought and heatwaves occurring together – known as “compound” events – have “surged” across the world since the early 2000s, a new study shows.
Compound drought and heat events (CDHEs) can have devastating effects, creating the ideal conditions for intense wildfires, such as Australia’s “Black Summer” of 2019-20 where bushfires burned 24m hectares and killed 33 people.
The research, published in Science Advances, finds that the increase in CDHEs is predominantly being driven by events that start with a heatwave.
The global area affected by such “heatwave-led” compound events has more than doubled between 1980-2001 and 2002-23, the study says.
The rapid increase in these events over the last 23 years cannot be explained solely by global warming, the authors note.
Since the late 1990s, feedbacks between the land and the atmosphere have become stronger, making heatwaves more likely to trigger drought conditions, they explain.
One of the study authors tells Carbon Brief that societies must pay greater attention to compound events, which can “cause severe impacts on ecosystems, agriculture and society”.
Compound events
CDHEs are extreme weather events where drought and heatwave conditions occur simultaneously – or shortly after each other – in the same region.
These events are often triggered by large-scale weather patterns, such as “blocking” highs, which can produce “prolonged” hot and dry conditions, according to the study.
Prof Sang-Wook Yeh is one of the study authors and a professor at the Ewha Womans University in South Korea. He tells Carbon Brief:
“When heatwaves and droughts occur together, the two hazards reinforce each other through land-atmosphere interactions. This amplifies surface heating and soil moisture deficits, making compound events more intense and damaging than single hazards.”
CDHEs can begin with either a heatwave or a drought.
The sequence of these extremes is important, the study says, as they have different drivers and impacts.
For example, in a CDHE where the heatwave was the precursor, increased direct sunshine causes more moisture loss from soils and plants, leading to a drought.
Conversely, in an event where the drought was the precursor, the lack of soil moisture means that less of the sun’s energy goes into evaporation and more goes into warming the Earth’s surface. This produces favourable conditions for heatwaves.
The study shows that the majority of CDHEs globally start out as a drought.
In recent years, there has been increasing focus on these events due to the devastating impact they have on agriculture, ecosystems and public health.
In Russia in the summer of 2010, a compound drought-heatwave event – and the associated wildfires – caused the death of nearly 55,000 people, the study notes.
The record-breaking Pacific north-west “heat dome” in 2021 triggered extreme drought conditions that caused “significant declines” in wheat yields, as well as in barley, canola and fruit production in British Columbia and Alberta, Canada, says the study.
Increasing events
To assess how CDHEs are changing, the researchers use daily reanalysis data to identify droughts and heatwaves events. (Reanalysis data combines past observations with climate models to create a historical climate record.) Then, using an algorithm, they analyse how these events overlap in both time and space.
The study covers the period from 1980 to 2023 and the world’s land surface, excluding polar regions where CDHEs are rare.
The research finds that the area of land affected by CDHEs has “increased substantially” since the early 2000s.
Heatwave-led events have been the main contributor to this increase, the study says, with their spatial extent rising 110% between 1980-2001 and 2002-23, compared to a 59% increase for drought-led events.
The map below shows the global distribution of CDHEs over 1980-2023. The charts show the percentage of the land surface affected by a heatwave-led CDHE (red) or a drought-led CDHE (yellow) in a given year (left) and relative increase in each CDHE type (right).
The study finds that CDHEs have occurred most frequently in northern South America, the southern US, eastern Europe, central Africa and south Asia.
Threshold passed
The authors explain that the increase in heatwave-led CDHEs is related to rising global temperatures, but that this does not tell the whole story.
In the earlier 22-year period of 1980-2001, the study finds that the spatial extent of heatwave-led CDHEs rises by 1.6% per 1C of global temperature rise. For the more-recent period of 2022-23, this increases “nearly eightfold” to 13.1%.
The change suggests that the rapid increase in the heatwave-led CDHEs occurred after the global average temperature “surpasse[d] a certain temperature threshold”, the paper says.
This threshold is an absolute global average temperature of 14.3C, the authors estimate (based on an 11-year average), which the world passed around the year 2000.
Investigating the recent surge in heatwave-leading CDHEs further, the researchers find a “regime shift” in land-atmosphere dynamics “toward a persistently intensified state after the late 1990s”.
In other words, the way that drier soils drive higher surface temperatures, and vice versa, is becoming stronger, resulting in more heatwave-led compound events.
Daily data
The research has some advantages over other previous studies, Yeh says. For instance, the new work uses daily estimations of CDHEs, compared to monthly data used in past research. This is “important for capturing the detailed occurrence” of these events, says Yeh.
He adds that another advantage of their study is that it distinguishes the sequence of droughts and heatwaves, which allows them to “better understand the differences” in the characteristics of CDHEs.
Dr Meryem Tanarhte is a climate scientist at the University Hassan II in Morocco, and Dr Ruth Cerezo Mota is a climatologist and a researcher at the National Autonomous University of Mexico. Both scientists, who were not involved in the study, agree that the daily estimations give a clearer picture of how CDHEs are changing.
Cerezo-Mota adds that another major contribution of the study is its global focus. She tells Carbon Brief that in some regions, such as Mexico and Africa, there is a lack of studies on CDHEs:
“Not because the events do not occur, but perhaps because [these regions] do not have all the data or the expertise to do so.”
However, she notes that the reanalysis data used by the study does have limitations with how it represents rainfall in some parts of the world.
Compound impacts
The study notes that if CDHEs continue to intensify – particularly events where heatwaves are the precursors – they could drive declining crop productivity, increased wildfire frequency and severe public health crises.
These impacts could be “much more rapid and severe as global warming continues”, Yeh tells Carbon Brief.
Tanarhte notes that these events can be forecasted up to 10 days ahead in many regions. Furthermore, she says, the strongest impacts can be prevented “through preparedness and adaptation”, including through “water management for agriculture, heatwave mitigation measures and wildfire mitigation”.
The study recommends reassessing current risk management strategies for these compound events. It also suggests incorporating the sequences of drought and heatwaves into compound event analysis frameworks “to enhance climate risk management”.
Cerezo-Mota says that it is clear that the world needs to be prepared for the increased occurrence of these events. She tells Carbon Brief:
“These [risk assessments and strategies] need to be carried out at the local level to understand the complexities of each region.”
The post Heatwaves driving recent ‘surge’ in compound drought and heat extremes appeared first on Carbon Brief.
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