While we have all watched in horror the fires and devastation in Australia, it is instructive to better understand the causes of the bushfire season. Despite the political smokescreen, scientists are in no doubt that global heating has contributed to Australia’s fire emergency. Graham Readfearn provides a good review of the causes in an article in The Guardian.
Explainer: what are the underlying causes of Australia’s shocking bushfire season?
As Australia’s unprecedented bushfire season continues to unfold, competing arguments have been made about the principal causes of the human and environmental tragedy – particularly around the role of climate change.
The prime minister, Scott Morrison, has acknowledged that climate change has had an influence on the fires and has defended his government’s climate record.
But Morrison has also said that “job-destroying, economy-destroying, economy-wrecking targets and goals” on climate change “won’t change the fact that there have been bushfires or anything like that in Australia”.
Backbench MP Craig Kelly denied any link between climate change and bushfires in a combative interview on British TV.
Conservative media have concentrated on other factors, such as the amount of hazard reduction burning carried out, or the activities of arsonists – a claim shown to have been inflated and misrepresented.
Bushfire experts say that in normal years hazard reduction is a way to control the behaviour of fires, but the changing climate is making it harder to carry out prescribed burns and, according to fire chiefs, it is not a “panacea” for extreme bushfires.
Here is what we know about the long-term influences on the bushfire catastrophe.
Why has this bushfire season been so devastating?
Extreme heat and dryness are two important influencers of fire and, on both measures, 2019 was remarkable for Australia.
Australia experienced its hottest year on record in 2019, with average temperatures 1.52C above the 1961-1990 average. Our second hottest year was 2013, followed by 2005, 2018 and 2017.
New South Wales – one state hard hit by the bushfires – broke its record by a greater margin, with temperatures 1.95C above average, beating the previous record year, 2018, by 0.27C.
At a very basic level, rising levels of greenhouse gases in the atmosphere change the Earth’s radiation balance, allowing less heat to escape.
Australia also had its driest ever year in 2019, with rainfall 40% lower than average, based on records going back to 1900. NSW also had its driest year.
A visualisation from Prof Nerilie Abram, a climate scientist at the Australian National University, examines hot and dry years in Australia since 1910 and how they correlate with major bushfires.
Fire authorities and the Bureau of Meteorology look at the risk of bushfires using the forest fire danger index, a combined measure of temperature, humidity, wind speed and the dryness, but not the amount, of fuel on the ground.
Australia’s 2019 spring months of September, October and November were the worst on a record going back to 1950 for bushfire risk.
What about ‘natural’ weather patterns?
There have been two other meteorological patterns that helped generate the extreme conditions Australia has been experiencing, and both these “modes of variability” were in “phases” that made conditions worse.
The Indian Ocean dipole was in a “positive phase”, meaning the Indian Ocean off Australia’s north-west was cooler than normal and the west of the ocean was warmer.
Positive dipole events draw moisture away from Australia and tend to deliver less rainfall.
But there is evidence that the extra greenhouse gases in the atmosphere are also impacting the dipole and another phenomenon, known as the southern annular mode (SAM).
A 2009 study found that positive dipole events “precondition” the south of the country for dangerous bushfire seasons and that these events were becoming more common.
A 2018 study in the journal Nature Communications found the number of extreme positive dipole events goes up as climate heating continues.
At 1.5C of global warming, the frequency of extreme positive dipole events doubles compared with the pre-industrial period.
The southern annular mode was in a “negative phase” as the bushfires took hold in November and December. This phase was generated by a sudden warming event in the stratosphere above Antarctica.
This caused westerly winds to track further north, blowing hot air across the continent into fire-prone areas, further fanning flames.
Abram’s own research has found that the SAM is being pushed towards more positive phases which, when they occur in Australia’s winter, tend to dry the continent.
Prof Matt England, of the University of New South Wales Climate Change Research Centre, said: “These modes of variability are not changing in a way that’s good for south-east Australia.
“We know with certainty that we are stacking the dice for the chances of these extreme drought years because of the changes in the modes.”
What has happened to Australia’s fire weather?
Scientists have already detected a trend towards more dangerous fire weather in Australia.
A 2017 study of 67 years of FFDI data found a “clear trend toward more dangerous conditions during spring and summer in southern Australia, including increased frequency and magnitude of extremes, as well as indicating an earlier start to the fire season”.
That trend continued in 2019, which was the riskiest year for bushfires on a record going back to 1950.
What role is climate change playing in the risk of fire?
A study of Queensland’s historic 2018 bushfire season found the extreme temperatures that coincided with the fires were four times more likely because of human-caused climate change.
In advice issued in November 2019, Australia’s National Environmental Science Program was unambiguous.
“Human-caused climate change has resulted in more dangerous weather conditions for bushfires in recent decades for many regions of Australia.
“Observations show a trend towards more dangerous conditions during summer and an earlier start to the fire season, particularly in parts of southern and eastern Australia.
“These trends are very likely to increase into the future, with climate models showing more dangerous weather conditions for bushfires throughout Australia due to increasing greenhouse gas emissions.”
Despite such unequivocal statements, Scott Morrison has been irritated that interviewers have asked about his government’s record on climate change, saying it was “just ridiculous” to link “any one emissions reduction policy to any of these fires”.
Morrison’s argument that no emissions reduction policy can be tied to individual events is spurious, as the same argument could be put for any and all efforts to reduce emissions anywhere in the world, at any time.
Scientists also believe that 2019 was a “standout” year in Australia for the formation of extreme bushfires that became “coupled” with the atmosphere, generating their own lightning and gusty, violent and unpredictable winds. Rainfall is replaced with blackened hail and embers that can be shot out over distances of 30km.
Another study has found that global heating will create more favourable conditions for these “pyroCB” storms to form in Australia.
What about the future?
Climate studies show that conditions in Australia for extreme bushfires will only get worse as more greenhouse gases are added to the atmosphere.
On Friday afternoon the president of the Australian Academy of Science, Prof John Shine, said Australia would need to further improve its climate modelling ability and understanding of fire behaviour to mitigate against the extreme events that would become more frequent and intense because of climate change.
“Australia must take stronger action as part of the worldwide commitment to limit global warming to 1.5° C above the long-term average to reduce the worst impacts of climate change,” he said.
England said: “We are loading the dice for more and more of these summers. But we have had knowledge of this for some time.
“What we have seen in Australia this year will just be a normal summer if we warmed the planet by 3C. And an extreme summer would be even worse than we’ve seen now.”
Abram said: “Even from my perspective, I am surprised by just how bad 1C of warming is looking.
“It’s worrying that we are talking about this as a new normal, because we are actually on an upward trajectory. Currently the pledges in the Paris agreement are not enough to limit us to 1.5C – we are looking more like 3C.”