5.1. Bushfire risk
Global heating has substantially transformed global wildfire, increasing fire seasons across 25% of the Earth’s vegetated surface and doubling the global burnable area affected by long fire weather seasons [paper]. Global heating increases fire risk in several ways; most directly through an increase in the number of extremely hot days, but also through a decrease in the number of cool days and nights. The best predictor for the five-fold increase in bushfires in California between 1972 and 2018 is the number of hot dry days (as measured by the “vapor pressure deficit”, a combination of high temp and low humidity). Increased number of high VPD days increases the dry fuel load which increases with the area burned [paper],
“The response of summer forest‐fire area to VPD is exponential, meaning that warming has grown increasingly impactful. Robust interannual relationships between VPD and summer forest‐fire area strongly suggest that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased VPD.”
The Australian Bureau of Meteorology concludes that, “Climate change is influencing the frequency and severity of dangerous bushfire conditions in Australia and other regions of the world, including through influencing temperature, environmental moisture, weather patterns and fuel conditions. There have been significant changes observed in recent decades towards more dangerous bushfire weather conditions for various regions of Australia….” [report]. Similarly, the CSIRO concludes, “There has been an associated increase in the length of the fire weather season. Climate change, including increasing temperatures, is contributing to these changes” [report].
Historical data conform with this position. For example, a comprehensive database in the United States shows that large wildfire activity increased markedly in the mid-1980s, with higher frequency of large-wildfires, longer wildfire durations, and longer wildfire seasons [paper]. Similarly, anthropogenic climate change added an estimated 4.2 million hectares of forest fire in the USA between 1984–2015, nearly doubling the area that would have otherwise burned [paper]. In 2017 a record fire season occurred in British Columbia, Canada, burning over 1.2 million hectares. Over 95% of the observed maximum temperature anomalies across that season could be attributed to global heating, more than doubling the event’s high fire weather metrics and increasing the area burned by a factor of 7–11 [paper].
Fire models driven by global climate models (GCMs) consistently predict a long-term increase in fire probabilities, with an increased risk of greater than 60% at mid‐ to high‐latitudes, slightly offset by a decreased risk in tropical areas [papers]. Recent modelling has been particularly prescient given Australia’s current fires: “Pyrocumulonimbus” fires are extreme wildfires which involve a coupling between the fire and the atmosphere, driving dangerous fire conditions – even fire thunderstorms – that result in fatalities and considerable damage. Two papers appearing in July this year predicted increases in extreme weather and pyroconvection risk factors for Australian wildfires [paper, paper]. Pyrocumulonimbus wildfires have since occurred repeatedly in Australia’s current fire season, at times associated with internally generated thunder and lightning, in addition to fatal, catastrophic fire conditions at ground level (Fig. 9).
Pyrocumulonimbus wildfires magnify the impact of global heating on extinction events. Fires play an important role in ecological systems and as such, flora and fauna have not only adapted to survive fire, but many components of biodiversity depend on recurrent burning. Unfortunately, intense wildfires under the influence of global heating can impact negatively on remnant species populations in fragmented bushland. In particular, a sudden loss of numbers can decrease genetic diversity in small remnant populations leading to a “near‐extinction bottleneck” [paper]. That is, even if numbers recover, there remains increasing inbreeding and less fitness in the population. Recurrent events can lead to a species becoming “functionally extinct”; a population does not encompass sufficient numbers and genetic diversity for recovery and future viability. The 2019-2020 bushfires of Australia have killed substantial numbers (possibly hundreds of millions) of animals and will have substantially increased extinction pressures on a number of native species [paper].
Recent bushfires in south east Australia have caused sudden deaths of up to 35% of local, biodiverse koala populations [media]. While koala numbers have declined by approximately 99.5% in the last 200 years, as a whole they are not considered functionally extinct, but vulnerable or critically endangered (although some of the 20 different species are likely functionally extinct) [report]. Future projections predict further declines in this iconic marsupial species [paper] so that widespread regional extinction seems likely.
5.2 Bushfires and global burned area
An important distinction exists between global bushfire risk and global burned area. Bushfire risk represents the likelihood and severity of uncontrolled fires. The transition from risk to actual bushfires depends on additional factors such as ignition and fire management. The global burned area is a composite of the area burned due to uncontrolled bushfires plus deliberate, controlled burning. Controlled burning has historically made a larger contribution to global burned area than bushfires [paper], particularly the widespread controlled burning of grasslands and savannas associated with nomadic subsistence and agricultural practices in under-developed economies in Asia, Africa and South America [paper]. As these agricultural practices have changed, the area of controlled burned area has decreased and as a result, the total burned area (controlled plus uncontrolled) has declined by approximately 25% over the past 18 years [paper]. Analyses of historical data indicate that purposeful human activities have been the dominant factor in the decline of burned area in tropics and extra-tropics, and climate variation has been the primary factor influencing the variation of burned area at high latitudes [paper].
The decrease in global burned area has been used to argue that wildfire risk is not increasing, but this is clearly not true: Research in this area has conclusively found that the primary drivers of declining fire activity have been “agricultural expansion and intensification” [paper] despite the fact that “rising temperature and frequent droughts are becoming increasingly important and expected to increase wildfire activity in many regions of the world.”[paper].
5.3 Fuel load, prescribed burning and weather
It is generally accepted that fire load (the density of flammable material) plays an important role in bushfire intensity, size and damage. Prescribed, off-season burning is the accepted way reducing fire load (particularly smaller forest litter) and legislated at a state level [report]. For this reason, Australian states legislate an annual quote of prescribed burning that is coordinated across state forests, National Parks, rural fire services and private land-owners. However, the impact of prescribed burning on fire load and biodiversity are complex [paper]. One reason for this is that fires can “cure” (dry-out) forest material, increasing rather than decreasing fuel. For example, post-burn assessments of prescribed burns in the Blue Mountains (from 1990-97) found that 30 per cent of the burns had a negative result, 40 per cent were sub-optimal, and 30 per cent could be rated as effective burns. When they are effective, fuel re-accumulation may limit their effectiveness to a short post-treatment period (2–4 years) [paper], meaning that burning would be required every 5 years. Given the enormous expanse of Australia, this limits the effectiveness of prescribed burning as a large-scale fire mitigation strategy. It is therefore targeted to decrease fire intensity near populated areas, improving subsequent fire suppression efforts at the time of fire.
While fire plays an important role in ecosystem life cycles, repeated prescribed burning in a limited area can actually decrease local biodiversity. Moreover, quantitative analysis of previous fires across SE Australia suggest that the influence of extreme weather events are up to 50 times more important than the influence of the time since the last fire. For this reason, the best results of prescribed fire are thought to be in climates where the likelihood of extreme weather conditions is low [paper]. Expert opinion suggests that controlled burning modifies bushfire behaviour, making them easier to control [media].
Naturally occurring wildfires increase carbon cycling but are thought to be close to carbon neutral when factoring in regrowth: However, extreme fire under the influence of global heating may liberate carbon from soil and turn forests into carbon emitters [paper].
Also of note is that in NSW, only four per cent of fires start in National Parks (although they cover 7% of the state). Less than 10 per cent of fires that started in the national parks escaped the park while 20 per cent of the fires in national parks start in private property or other lands [report].