The threat of catastrophic bushfires in Australia has been “severely underestimated”, climate scientists have warned. 

By analysing sea salt concentrations in an Antarctic ice core drilled decades ago, scientists have reconstructed bushfire weather patterns in south-east Australia dating back to 2,000 years ago.

The new study, published in the journal Communications Earth and Environment, confirmed how devastating fire events could be with just natural climate variability, meaning without the added impact of human-induced climate change.

Scientists have been studying the ice at Law Dome, in Antarctica, since the 1980s.(Supplied: Tessa Vance)

“We know climate change is ramping up the frequency and severity of fire weather,” lead author and climatologist Danielle Udy said.

“Climate variability alone can can toss up more severe bushfire weather than what we have seen, including the 2019–20 bushfires.

“Given human-caused climate change is loading that dice even more for worse bushfire weather, we are most likely underestimating how bad bushfires can be in Australia.”

Dr Udy said equal or more severe bushfires to the 2019 Black Summer fires — until now thought to be unprecedented — have occurred at least seven times over the past 2,000 years.

The study found they occurred in the summers of 485, 683, 709, 760, 862, 885 and 1108 AD.

“We need to plan for that in our bushfire seasons now and into the future,” she said.

The Black Summer bushfires of 2019–20 raged through more than 24 million hectares, destroyed more than 3,000 homes, displaced tens of thousands of people, and was estimated to have killed billions of animals.

Thirty-three people died directly and nearly 417 more people lost their lives from smoke inhalation.

During the Black Summer bushfires in 2019–20, decreased wind speeds resulted in less sea salt spray near Antarctica.

Paleoclimate weather data evident in ice core 

The report explained how the high and low pressure weather systems south of Australia were so large they connected the two continents, even though they were more than 3,000 kilometres apart.

As Australian bushfire weather records extend back only to the 1950s, insight into bushfire observations has been limited. 

To counter this, paleoclimate data collected from ice core extracted from Law Dome, an icy cap on the east coast of Antarctica, roughly due south of Perth, has provided a record of two millennia of weather from the Southern Ocean.

By drilling into an ice core at Law Dome and measuring levels of sea salt found in different sections, Dr Udy said scientists could “disentangle the knots of what is climate change and what is climate variability”.

When south-east Australia experienced extreme bushfire weather over summer, there was less wind around Antarctica, which meant less sea-salt spray laid down at the ice core site.

In short, the ice core’s sea salt concentrations indicated how much sea was sprayed onto ice by wind across the ocean, providing a “direct weather link”.

Cores from deep in the ice provide a record of Earth’s climate history available nowhere else. (Supplied: Australian Antarctic Division)

“We used an ice core in Antarctica to reconstruct the past 2,000 years of south-east Australia’s fire weather, and then compared the 2019-20 severity and frequency of that extreme fire weather to the past 2,000 years,” Dr Udy said.

“As it gets windier across the ocean, you get more sea spray. A small portion of that goes up into the atmosphere and water molecules join onto it, it forms snowflakes, and it falls out as snow over Antarctica.”

“And then when we drill the ice core, we’re able to measure the sea salt concentration … back through time.”

Tessa Vance with an ice core sample.(ABC News: Cameron Gooley)

Report co-author Tessa Vance, a paleoclimatologist from the Australian Antarctic Program Partnership at the University of Tasmania, said as Antarctic water was so clean, its ice was a good indicator of how atmospheric circulation changed over time.

“There are still very tiny quantities of things like volcanic ash or sea salt aerosols that are bound up in the snowfall,” she said.

“In Antarctica, these impurities are at very, very low concentrations. So we’re measuring things in parts per billion range.

“One of the things that we can measure is sea salts … and that can tell us about wind across the Southern Ocean, and even across the sea ice.”

Warning of future risk ‘buried in the ice’

While the study found catastrophic fires occurred more frequently than previously thought, Dr Udy said they were “extremely rare”.

However, she urged that continued global warming heightened the risk of these extreme events.

“The key thing is, the range of climate variability that is possible when everything collides is larger than what we used to know about,” Dr Udy said.

“Climate change and climate variability are an additive together that increases our potential of having a Black Summer fire next summer.”

Dr Udy said it “was only a matter of time” before the same type of weather system that brought the bushfire devastation of 2019 occurred again.

“That aligning with the four or five extra years of emissions increases that risk of another devastating fire season.

“We need legislation and we need more policy in place to meet our targets. And we need to actually start seeing our emissions come down, not continue growing as they have been doing.”