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Bushfires, Fuels and Climate Change

Friday, 1 November 2013  | Philip Zylstra

Recent house losses and political activities have once again brought bushfires into general discussion. It is becoming increasingly important that this discussion is informed, because our opinions will influence whether we provide adequate protection to homes and lives and may even determine the survival of entire species. The following article is intended to inform some of the most relevant points in the discussion.


Fire and fuels

Bushfire fuel is everything in the forest that burns in a fire. If a fire just burns the dead leaves on the ground then we get a flame a metre or two high, but if trees are alight then the energy released by the fire is so great that it can change the weather, producing its own pyrocumulonimbus cloud complete with lightning and even tornados.[1] The difference between the two scenarios is decided mostly by the size of the gaps between plants. If the trees are too high above the flames then they won’t ignite—which is why on Black Saturday the oldest Ash forests were much less flammable than the more recently burnt or logged forests.[2] We cannot meaningfully talk about the fuel of a forest without knowing first whether the spaces between grasses, shrubs and trees will be crossed by the fire, and this is a complex question.[3]

In the 1960s Alan McArthur attempted to describe the flammability of a forest without addressing this complexity and instead measuring only the fine dead material on or near the forest floor, which he called the fuel load. To prove his argument, he conducted nine experimental fires in West Australian Jarrah forests and concluded that if the fuel load was doubled, then fires spread at twice the speed. None of his work was published transparently for others to examine as per general scientific practice, so in order to adequately test his proposition Neil Burrows revisited the location and conducted hundreds of experimental fires under rigorous conditions.[4] What he found was that fuel load had no effect whatsoever on rate of spread.

McArthur also developed a Grassland fire behaviour model, but although it worked well it didn’t have fuel load as an input so he developed another model to fit his expectations. The models were tested shortly after when a record grass fire was measured to spread at 22km/h. The model without fuel load performed admirably, but the newer model estimated only 8km/h. A second record fire spread at 23km/h and the model without fuels predicted 20km/h, but in even lighter fuels, the newer model predicted only 3.6km/h. McArthur’s successor Phil Cheney spent years after this lighting fires in grasslands to find the link to fuels, but in the end the overwhelming evidence forced him to produce another model that once again did not use fuel load. The reality is that where the science has gone through the process of transparent peer-review, studies around the world since the 1940s have shown little to no effect of fuel load on the speed at which a fire spreads.

Prescribed or hazard reduction burning is the attempt to reduce fuel load in order to reduce the bushfire risk. Not only is there a problem with the fuel load idea though, there is the reality that fire has other much larger effects on the bush. It may increase gaps by killing plants but it may also reduce gaps by germinating others; bushfires for example are more than twice as hard to control if re-burning Snowgum forests that have been burnt within the previous 14 years.[5]

Prescribed burning certainly does help in some instances, but if we examine all of the cases we find that in Southwest WA for example, effectively only one in four burns helped.[6] The stats vary between areas from slightly more effective to very much less effective but the message is the same: on average, prescribed burning helps, but only a bit. It’s not ‘PC’ but it is important: history is emphatic across most of Australia that more burning will not save us from fires. When a bad day comes, the fire will sweep across the previous burns as it did in the Blue Mountains and on Black Saturday in Victoria. One thing we do know though—frequent fire is what we call a Key Threatening Process: when a fire re-burns a recently burnt area, there is a genuine risk that some species will be lost. Consider that in context with the statement by the Australian Bureau of statistics that “Australia has experienced the largest documented decline in biodiversity of any continent over the past 200 years”.[7] Forcing fire into the bush does have a cost and it should be weighed against its benefits.


Climate change and fire

We know that smoking increases the risk of cancer, but we can’t always say that someone would not have developed cancer if they had not smoked; all we might say is that it was a factor. Global Warming is giving south-eastern Australia a hotter, drier climate. There is less rain and what we have evaporates more quickly, plants get heat-stressed and ignite more easily[8] so that the spaces are crossed and more fuel burns[9], and sheltered green areas that used to break up fire paths are now more often dry enough to create the “landscape connectivity” that gives us massive fires.[10] There is even evidence that we may get more lightning;[11] potentially starting enough fires simultaneously to burn millions of hectares at once as happened in the Alps in 2003 and 2006 without any known precedents.[12] This is a recipe for a lot of fire, and while it’s difficult to point to a single fire and blame climate change in the same way that we can’t blame someone’s cancer on a particular cigarette, we can say with confidence that climate change is giving us more fire.

In that context, nothing that Environment Minister Greg Hunt saw in Wikipedia was wrong; the problem was what he chose not to say. Figure 1 (below) shows the Wikipedia list of major fires[13] in Australia counted against the decade in which they occurred. The message in the data he saw is clear: as the global temperature increases, so does the number of major fires. Our global addiction to fossil fuels has changed the climate in a way that was paid for over the last weeks by the residents of Winmalee and surrounding towns.

Figure 1. The number of major fires per decade recorded on the Wikipedia page used by Environment Minister Greg Hunt to argue that there was no connection between climate change and fire. The line shows the global temperature anomaly, and the arrow marks the point when scientists first began warning that excessive fossil fuel use would warm the globe.

We can’t engineer a solution by forcing more and more fire into the bush; the only way to burn more than we do is by burning in dangerous weather and unwittingly creating the bushfires we are trying to avoid—like the one that destroyed 34 homes in Margaret River two years ago.  The peer-reviewed evidence is clear and growing that prescribed burning may help, but it will not be enough.

But don’t think for a moment that we are powerless.

We know what is causing the increase in hot dry fire days, and we know that to limit it we need to pollute less, to waste less. The best evidence so far says that to pull our own weight, Australia needs to forget the politically favoured 5% reduction and instead reduce our emissions by 27% to 34% by 2020[14]. The hard reality is that this will not happen unless it is driven by a community that cares enough about vulnerable people and about the beauty in our care to say no to our Western gospel of excess. The only question is whether we want to be that community or not.


Dr. Philip Zylstra
Centre for Environmental Risk Management of Bushfires, University of Wollongong

[1] McRae RHD, Sharples JJ, Wilkes SR, Walker A. 2012. An Australian pyro-tornadogenesis event. Natural Hazards 65: 1801–1811.

[2] Lindenmayer DB, Hobbs RJ, Likens GE, Krebs CJ, Banks SC. 2011. Newly discovered landscape traps produce regime shifts in wet forests. PNAS 108: 15887–15891.

[3] Zylstra P. 2011. Forest Flammability: Modelling and Managing a Complex System. PhD Thesis, UNSW: Canberra

[4] Burrows ND. 1999. Fire behaviour in Jarrah forest fuels. CALMScience 3: 31–84.

[5] Zylstra P. 2013. The historical influence of fire on the flammability of subalpine Snowgum forest and woodland. Victorian Naturalist (In Press)

[6] This is expressed as fire leverage, or area protected per area treated. The study (Boer MM, Sadler RJ, Wittkuhn RS, McCaw WL, Grierson PF. 2009. Long-term impacts of prescribed burning on regional extent and incidence of wildfires—Evidence from 50 years of active fire management in SW Australian forests. Forest Ecology and Management 259: 132–142) measured a leverage of 1ha protected for every 4 treated.

[8] Yebra M, Dennison PE, Chuvieco E, Riaño D, Zylstra P, Hunt ER, Danson FM, Qi Y, Jurdao S. 2013. A global review of remote sensing of live fuel moisture content for fire danger assessment: Moving towards operational products. Remote Sensing of Environment 136: 455–468.

[9] Zylstra P. 2011. Rethinking the fuel - fire relationship. In: Thornton RP, ed. Proceedings of Bushfire CRC & AFAC 2011 Conference Science Day, September, 2011 Sydney Convention Centre. Darling Harbour: Bushfire Cooperative Research Centre, 87–99.

[10] Caccamo G, Chisholm LA, Bradstock RA, Puotinen MLA. 2012. Using remotely-sensed fuel connectivity patterns as a tool for fire danger monitoring. Geophysical Research Letters 39.

[11] Price C, Rind D. 1994. Possible implications of global climate change on global lightning distributions and frequencies. Journal of Geophysical Research 99: 10823–10831.

[12] Zylstra P. 2006. Fire History of the Australian Alps. Jindabyne: Australian Alps Liaison Committee.

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