Picture: Tatong Bushfire. Credit: Andrew Wallace (Flickr CC BY-NC-ND 2.0).

Catastrophic wildfires like the Black Saturday wildfires in 2009 and Canberra Wildfires of 2003, which were so large and dangerous that they generated their own weather systems – including the world’s first filmed fire tornado – are likely to be more frequent in the future as a result of climate change across southeast Australia.

New research in Geophysical Research Letters has found that the coupling between the fire and atmosphere, that helps generate these catastrophic pyrocumulonimbus wildfires in Australia will occur more frequently in the future as global warming takes hold. Importantly, these conditions will also occur earlier – in late spring, not just in summer.

“Some wildfires burn with such intense pyroconvection that they link with dry, unstable air 1500m above the ground and generate winds and internal weather conditions that transform them into unpredictable and powerful pyrocumulonimbus wildfires. These fires can send embers tens of kilometres ahead of the fire-front to start new blazes,” said lead author Dr Giovanni Di Virgilio from the University of New South Wales’ Climate Change Research Centre (CCRC).

“Our research reveals the perfect conditions for these most dangerous wildfires will start to appear in late spring rather than just summer. This will have clear implications for fire resource availability and policy.”

The key to the research was the understanding that it’s not just conditions on the ground, represented by the McArthur Forest Fire Danger Index, which leads to catastrophic pyrocumulonimbus wildfires. Historical observations showed the conditions in the lower troposphere were much better indicators of the development of these extreme fire events.

When the air 1.5km above the land surface is dry and unstable it can link with a large, very intense fire and get drawn down by convective mixing to fill the space where the plume of heat has pushed up air from closer to the ground. The process generates its own hot and dry wind completely separate from conditions across the rest of the landscape around the fire. This is when fire behavior is at its most erratic and dangerous.

Rugged landscapes again amplified the odds of the formation of these extreme fires. When these future changes to the atmosphere with climate change were modelled, it enabled the researchers to identify particular areas in Australia that were most at risk of pyrocumulonimbus events.

“The area that most stood out in terms of increased risk from pyrocumulonimbus wildfires was the bush-urban interface near Melbourne – a location where there is likely to be significant increases in population,” said fellow author Prof Jason Evans from the ARC Centre of Excellence for Climate Extremes.

“But Melbourne was not the only region at increased risk, this concordance of changes to upper atmospheric conditions, seasonal shifts in fire danger, rugged landscape and low humidity also puts major cities like Canberra, and key agricultural regions such as Western Victoria, in the firing line.”

“This adds a key piece of information to Australia’s future fire preparedness that will help agencies involved in climate impact adaption and the planning for and management of wildfires in Australia.”