Australian bushfires are getting worse and it’s being driven by climate change, according to a massive analysis of 90 years’ worth of fire data.
Not only has the number of megafires in Australia spiked since 2000, but a greater expanse of land is being burnt, and they’re happening more and more in autumn and winter.
Even the cooler La Niña seasons don’t offer much respite – in fact, the data shows that fires tend to be worse directly after La Niña years.
The researchers, led by the CSIRO’s Pep Canadell, warn that we should prepare for another severe fire season in the summer of 2022/23.
“Our results really underscore the overwhelming role of climate change and associated changes of fire weather in driving the observed increased trends in burnt area,” says Canadell.
“Understanding these trends will help to inform emergency management, health, infrastructure, natural resource management and conservation.”
The major findings
“We have always seen large fires in the history of Australia,” says Canadell. “But clearly since 2000, what we call megafire seasons – of more than one million hectares [burnt in forests] – have increased quite significantly.”
A major finding of the study, published in Nature Communications, is that of the four forest megafires in Australia since 1930, three have happened since 2000.
Nine of the 11 biggest fire years, each with more than 50 million hectares burned, have also occurred since 2000.
The study specifically focused on fires in Australian forest ecosystems.
“Fire dynamics are very different in woody-dominated ecosystems versus grass-dominated ecosystems, including the rangelands that dominate the whole of Australia,” Canadell explains.
“We have focused on 324,000 square kilometres of forests, rich in biodiversity, mostly on the on the eastern board of Australia and then some in the southwest.”
The team found that from 1988 to 2018, Australian forests saw a 350% increase in burned areas – if the Black Summer fires of 2019/20 are included, that number jumps to 800%.
The cooler months don’t even provide a break – since 1988, autumn fires have burnt three times more forest than previously, and winter fires have burnt five times more forest.
This is in stark contrast to decades past, when fires were concentrated in spring and summer months.
“All these things have completely changed over the last 15 years, where fundamentally fire is spreading throughout the entire 12 months,” says Canadell.
The study shows that forests are burning more frequently, too. There are now fewer years between them – fire is coming back time and again. In the 1980s, there was an average of 70 years between fires in any given area. In the 2010s, the number was down to 39 years.
And many areas are burning every 20 years or less.
“There are now regions in the southeast and south with fire intervals shorter than 20 years, preventing certain types of vegetation to reach maturity and therefore posing a risk to local ecosystem collapse,” says Canadell.
These forests are dominated by mountain ash or alpine ash, which don’t resprout after a fire, as other species do.
“They need forests to reach a certain level of maturity to be able to build what we call a seed bank in the soil – so when there is a fire, they can then regenerate out of those seeds,” says Canadell.
“Forests need at least 20 years to produce that seed bank.
“The data shows that we are increasing the risk of – at least to begin with – local collapse of some of these ecosystems if we continue with these very high fire frequencies.”
Bob Hill, director of the Environment Institute at the University of Adelaide, says that the process of local ecosystem collapse is complex.
“Nevertheless, this is an obvious potential source of local extinction and one that will be exacerbated by climate change,” says Hill, who was not involved in the study.
“This should be an important focus for those charged with vegetation management and especially controlled burning, to help to ensure that such species are given the best possible chance of surviving into the long term.”
The whole continent is at risk
The reach of Australia’s fire is not limited to forests. The team also looked at the area of land burnt across the continent as a whole, and found that it’s increasing.
And wetter weather cycles such as La Niña, Canadell says, will not reduce the risk of fires across the country.
“The highest years of burned areas are actually right after La Niña years, because the wetness across the continent really brings up the fuel loads – the biomass – mostly of the herbaceous components of the rangelands and woodlands across the entire continent,” he says.
Australia is dominated by rangelands, which are covered with plants such as spinifex that react quickly to above-average rainfall. They grow fast and increase in biomass, before drying out in the following season. While forests will likely see less fire after La Niña years, the study says, these rangeland ecosystems will be primed to burn.
“After the big 2010/11 La Niña, we saw what was probably the first 100-million-hectare fire season in Australia – which is about four times the area of the UK,” says Canadell.
A lot of this fire burned through central Australia and went on for months, according to co-author and rangeland ecologist Garry Cook from CSIRO.
He says that the same thing is happening to our drier ecosystems now.
“Sometime in the next year or two or three, a lot of vegetation production will burn, it’s fairly inevitable,” he says. “We’ve got a vast, connected landscape, so once a fire starts, it can keep burning for months on end.”
Next year, the researchers say, we need to be prepared.
“We expect that most likely next year – unless there is another La Niña, which is highly unlikely – that there will be…a very large fire season across Australia,” says Canadell.
The climate cause
The cause of the concerning fire trend in forests is clear: climate.
The study examined eight main drivers of fire, and found that days where conditions are ripe for dry lightning have soared, and so have super-hot days. There is also less rainfall, especially in winter, and soils have dried out. All combine to prime a forest to burn.
All these factors can be linked to climate change, with Australia warming by 1.4°C since 1910.
These drivers feed into the Forest Fire Danger Index (FFDI), a measure of the potential danger of a bushfire on a given day and location.
Many previous studies have found that we are now experiencing more and more very high fire danger days, as measured by the FFDI.
But this study found that even a small change in the number of these days leads to a huge increase in burned areas.
“We find this incredibly tight relationship, which is an exponential relationship,” says Canadell. For every additional very high fire danger day over a baseline of 25 per year, there was a 21% increase in area burnt.
The relationship between FFDI and fire days was so tight, he says, that it could have “predicted that a really massive, unprecedented burned area event was going to happen in 2019”.
Would prescribed burning reduce the size of fires?
In the wake of the Black Summer, many argued that fires could be reduced by management practices such as prescribed burning.
But the study did not find evidence for this on a broad scale.
Though there was variability, there was no clear relationship found between total area burnt and management.
“We are burning 1% per year of the forest, which is a really a small amount if that prescribed burning were to try to control and keep the fuel loads low across the entire forested landscapes,” says Canadell.
Whatever effect reducing fuel load had on the total burned area is completely swallowed by the effect of climate change.
Data on fuel loads does not exist far into the past, Canadell adds, so it’s hard to tell, but “it’s very difficult to imagine that fuel loads would have a large, important component in driving what we’ve seen – it’s really climate and weather”.
Jason Sharples, bushfire expert from UNSW Canberra, who was not involved in the study, says that the lack of clear influence of fuel load is concerning because it “is one of the few factors influencing fire behaviour that we have the ability to control.
“This suggests that bushfire risk-reduction measures such as prescribed burning may be of only limited effectiveness in the absence of broader action to mitigate climate change.”
Undertaking more prescribed burning would be difficult anyway, especially since emergency-services resources are stretched thin.
The CSIRO’s Mick Meyer, co-author of the study, says prescribed burning is mainly geared towards protecting assets such as houses, and it does its job in preventing flames from spreading in those situations.
“[Burning] 1% of the area is actually quite sensible – it’s directing the resources to where they’re going to have an effect,” he says. “If you tried to burn the whole of the country, you’d be changing the ecosystems.”
What does this mean for people, animals and the land?
“The fires are a direct risk for infrastructure, burning houses downs, whole towns – a direct threat to lives and livelihoods,” says Cook. “The smoke from the fires is a health hazard. It causes people to have asthma attacks and greatly increases hospitalisation from asthma and heart-related issues.”
The more frequent interval of fires – where they happen in the same area more often – is also a direct threat to ecosystems, and not just to native animals.
“The rainforest areas have very, very low fire tolerance – frequent fires will wipe them out,” says Cook.
As the length of the fire season increases, he warns that people working to manage fires won’t be getting a break: “There won’t be an off season.
“The autumn and winter are going to very quickly become more and more significant as a fire season in the southeast corner of Australia.
“We’re going to face an increasing difficulty managing this and it’s going to make it increasingly hard to live and work in the area of these forests – and that means in capital cities next to forests.
“I’m thinking of those scenes of Sydney bathed in orange light from the smoke. This is going to be increasingly our future.
“We’re going to have to adapt to this unless we as a society can bring climate change under control.”
Where did this fire data come from?
This study is the result of a massive effort to collect fire data from across the country.
This data is collected on a state and territory level, but a centralised database doesn’t exist.
“The multiple inquiries that we’ve seen on fire recently, they all say exactly the same [thing]: we don’t have an international data repository,” says Canadell.
“So when we need to look at exactly what has happened, and the trends, and how they can inform us into the future, this thing does not exist.”
An impressive part of this study was therefore simply bringing together multiple lines of data – both ground- and aerial-based – from agencies across the country, spanning the last 90 years.
The team also took data from two further sources – satellites from NOAA and NASA in the US, covering the last 32 years – to make sure their findings were robust.
None of these three data sources had been well investigated in the past.
“I think that this will probably change in the near future,” says Canadell. “We must build a national-level data repository on all things concerning fire.”