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July 30, 2020

Under climate change, winter will be the best time for bush burn-offs – and that could be bad news for public health

If climate change continues at its current rate, how will this change the days suitable for prescribed burning?

At the height of last summer鈥檚 fires, some commentators claimed 鈥済reenies鈥� hazard reduction burns 鈥� also known as prescribed burns 鈥� in cooler months. They argued that such burns would have reduced the bushfire intensity.

repeatedly dismissed these claims. As then NSW Rural Fire Service Commissioner Shane Fitzsimmons noted in January this year, the number of available days to carry out prescribed burns had reduced because climate change was altering the weather and causing longer fire seasons.

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This public conversation led our research team to ask: if climate change continues at its current rate, how will this change the days suitable for prescribed burning?

, published today, were unexpected. Climate change may actually increase the number of burn days in some places, but the windows of opportunity will shift towards winter months. The bad news is that burning during these months potentially increases the public health impacts of smoke.

A hot debate

Hazard reduction involves removing vegetation that could otherwise fuel a fire, including burning under controlled conditions. But its effectiveness to subdue or prevent fires in the scientific community.

Commissioner Fitzsimmons weighs in on a national debate about hazard-reduction burns.

 

Those with experience on fire grounds, Fitzsimmons, say it鈥檚 an important factor in fire management, but 鈥渘ot a pancea鈥�.

Despite the debate, it鈥檚 clear hazard reduction burning will continue to be an important part of bushfire risk management in coming decades.

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Modelling future weather

Before conducting prescribed burns, firefighting agencies consider factors such as vegetation type, proximity to property, desired rate of spread and possible . But we wanted to distil our investigation down to daily weather factors.

We reduced those factors to five key components. These were maximum temperature, relative humidity, wind speed, fuel moisture and the (the index used to forecast fire danger in southeast Australia).

We looked at these elements on prescribed burning days between 2004-2015. We then used climate models to simulate how the conditions would change with global warming over southeast Australia, relative to a baseline historical 20-year period for 1990-2009.

To make a valid 20-year comparison, we compared the historical period to a modelled period from 2060-2079, assuming emissions continue to rise at their current pace.

 

Surprisingly, we found, with one regional exception, the number of days suitable for prescribed burning did not change. And in many places, the number increased.

As the fire season lengthened under a warming climate, the number of days suitable for burning just shifted from autumn to winter.

Shifting seasons

Our research indicated that by 2060 there鈥檒l be fewer prescribed burning days during March, April and May. These are the months when most burning happens now.

But there will be significantly more opportunities for burning days from June to October. This is because the conditions that make for a good day for prescribed burning 鈥� such as mild and still days 鈥� start to shift to winter. Today, weather in these months is unsuitable for conducting burns.

Interestingly, these results aren鈥檛 uniform across southeast Australia. For example, much of the Australian east coast and South Australia would see seasonal shifts in burning windows, with around 50% fewer burning days in March to May.

Much of Victoria and in particular the southern regions saw an increase in burning windows during April to May and, in some parts of the state, through September and October as well.

Only the east Queensland coast would see a total reduction in prescribed burn days from April to October.

The smoke trap

This may be good news for firefighters and those agencies who depend on prescribed burning as a key tool in bushfire prevention. But, as so often is the case with climate change, it鈥檚 not that simple.

A is smoke, and it鈥檚 a very significant health issue.

Last year, showed global warming will strengthen an atmospheric layer that traps pollution close to the land surface, known as the 鈥渋nversion layer鈥�. This will happen in the years 2060-79, relative to 1990-2009 鈥� especially during winter.

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Unfortunately, the conditions that create inversion layers 鈥� including cool, still air 鈥� correspond with conditions suitable for prescribed burning.

For asthmatics and those sensitive to air pollution, smokier burn days could make winter months more difficult and add further stress to the health system.

It also creates an additional challenge for firefighting agencies, which must already consider whether smoke will linger close to the surface and potentially drift into populated regions during prescribed burns.

This is just one factor our firefighting agencies will need to face in the future as bushfire risk management becomes more complex and challenging under climate change.

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, Research associate, ; , Post Doctoral Research Fellow, ; , Research Fellow, ; , Professor, ; , Professor of Bushfire Dynamics, School of Science, UNSW Canberra, , and , Graduate Director, ARC Centre of Excellence for Climate Extremes,

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