"There were fires around us on all sides, burnt leaves were falling in our backyard, and I was on ember watch for days."

The 'Black Summer' fire season in 2019-20 was one of the most intense and catastrophic on record in Australia with 34 direct deaths, nearly 10,000 properties destroyed, and a damage bill estimated at $3 billion.

The experience of living through those hellish bushfires prompted University of Wollongong (UOW) student Rebecca Ryan to start researching the frequency and intensity of bushfires in Australia over the past 200 years.

Rebecca was on a property just outside Mittagong in the Southern Highlands when the fires came so close, nearly five years ago.

"It was all very real, trying to prepare our property as best we could; and now, seeing how many people were impacted by those fires and are still impacted and recovering," Rebecca said. 

Her research project, published in the International Journal of Wildland Fire, has expanded our historical understanding of how fires impact, and interact with, the Australian landscape. It also sheds a light on how fire characteristics have evolved, not just in the past two centuries, but over more than 3000 years, with the most rapid change taking place in just the past 200 years.

Exciting and daunting

"It was both exciting and daunting to know the trajectory we've been on for the past 3000 years," said Rebecca, from UOW's School of Earth, Atmospheric and Life Sciences.

She said previously most existing records of fire characteristics only stretched back 50 years, when satellites came into use.

"But we need at least a 100-year record to understand how fire characteristics may change in the future," Rebecca said.

As the lead author for the research, which included contributions from the United Kingdom and Slovakia, Rebecca collected sediment samples in the NSW Blue Mountains and Namadgi National Park in the ACT. Using Fourier Transform Infra-Red (FTIR) spectroscopy, she analysed changes in the chemical bonds of the sediments to unravel the changes in fire intensity over thousands of years. She then compared these with pollen and drought records to understand paleoenvironmental drivers for fire behaviour.

The researchers found that the past 200 years have seen a significant increase in the frequency and intensity of fires in south-eastern Australia, a result, Rebecca said, of the complex interactions between climate, people and vegetation.

No sole driver for increased fire events

"These three factors are so interconnected  and entangled that it is near impossible to attribute just one of these factors as the sole driver for these more frequent and intense fire events in the past 200 years."

She said over the past two centuries, since colonisation, there had been a shift in vegetation species, particularly the expansion of eucalypts, "which has increased the ability for the fire to burn into the canopy".

"The volatile oils present within the eucalypt species also increases the energy output of the fire and promote fire spread," Rebecca said.

"The increased frequency of extreme and prolonged droughts means that there is an increased incidence of human-ignited fires coinciding with severe fire weather and higher fuel loads."

This research forms the basis of Rebecca's PhD thesis, which she recently submitted, and the results are part of the Australian Research Council's Discovery Project "Shaping a Sunburnt Country'.

Helping us prepare for future fires

"Understanding how fire behaviour has changed is vital to inform how we prepare for and fight these fires in the future," Rebecca said.

"We saw the damage of the Black Summer fires to the environment and communities. There are still many areas and people who haven't recovered from that. This new record expands our knowledge of the history of fire characteristics in SE Australia. It's important to inform how organisations such as the Rural Fire Service and National Parks and Wildlife Service approach land management practices and fight more frequent fires that burn at higher intensities.

"There is also a moral obligation to understand these changes and ensure we all do all we can to mitigate the risk and prepare for these events."