By Guest Author 12/11/2019

Grant Pearce

The effects of the current Australian bushfires in New South Wales and Queensland (and also again in California) are devastating and far-reaching.

To date, the fires have resulted in several lives being lost and many homes and properties destroyed. Here in New Zealand, the impacts have been only minor, with New Zealand firefighters gearing up for possible deployment to help out, and smoke from the Australian fires creating amazing sunsets, eerie daytime light conditions, and false smoke reports thought to have been local fire outbreaks.

The bushfires are the result of prolonged drought conditions, which in some areas extend back several years, that have contributed to extreme drying of forests and other vegetation fuels. Grass fuels are fully cured, leaving only dead, dry brown stalks. And under the exceptionally dry conditions, all of the forest vegetation biomass present has become available to burn. This includes the larger woody branches and logs and soil organic layers, in addition to the fine leaves and twigs in the forest canopy, surface litter on the ground, and any shrub understory present. This increases the overall fuel loadings available to burn, contributing to more intense fires with greater flame lengths and energy output that are harder to control.

On top of this, Eucalyptus species are the predominant forest trees species. These contain volatile oils which are released into the atmosphere in greater quantities as the trees become temperature and drought-stressed, raising the flammability of not only the foliage but also of the air around it. Many eucalypt species also produce flammable bark, which comes off in strips providing a perfect fuel source for embers that can burn for long periods. This can result in short-range mass ember storms and when carried to great heights by the fires’ towering convection columns, long-distance spotting that starts new fires many kilometres downwind.

When combined with the hot, dry conditions and strong, gusty winds, these fires burn very intensely and spread extremely rapidly, producing towering pyrocumulus smoke columns that extend many kilometres into the atmosphere. The fires interact with the upper atmosphere, creating their own weather and often over-riding the other factors of terrain and surface winds that would normally drive the behaviour of smaller fires. Rising rapidly due to high-intensity burning, these convection columns can even form thunderstorms (pyrocumulonimbus, or pyroCb), producing lightning which can start more fires.

Suppression of such fires is difficult if not impossible, due to the high fire intensities and erratic fire spread. It is too unsafe to put firefighters on the ground ahead of such fires, except in exceptional circumstances to protect lives or critical assets (where safe to do so). Any water or retardants, made even more scarce by the drought conditions, is quickly evaporated by the intense heat before it can wet the vegetation, whether applied through hoses or aircraft, even large water-bombers dropping tens of thousands of litres at a time. Firefighting becomes restricted to controlling the less intense back and flanks of the fire, getting control lines around the fire with heavy machinery, and undertaking essential property protection. In some cases, it may be possible to backburn from firebreaks or control lines out ahead to remove the fuels that feed the fire. This practice is more common in Australia, but only rarely used here in New Zealand.

Australian fire managers have in some instances called the current fires unprecedented, in terms of the fire weather conditions under which they are burning, in how early such severe fires are occurring in the fire season, and in the number of scale and ferocity of the multiple fires burning. This is despite both NSW and Queensland (and other parts of Australia) having a long history of extreme fire events. However, the frequency of such incidents, and of severe fire seasons generally, has increased in recent years.

Research has shown that fire risk in New Zealand will likely also increase with climate change, with a greater frequency of severe fire weather days in many parts of the country – in some cases by 2-3 times current levels. Fire seasons are also getting longer, both starting earlier and extending longer into autumn.

This means the potential for not only more fires, but more larger fires exhibiting the sorts of extreme fire behaviour seen in Australia and California, and locally in the 2019 Nelson/Tasman and 2017 Port Hills wildfires. We are already seeing an increasing trend in New Zealand in human impacts from wildfires, especially in the rural-urban interface where urban development meets rural vegetation fuels, with more homes being destroyed or damaged, and greater numbers of people being evacuated.

More fires also mean more wildfire smoke, with greater potential health impacts for vulnerable people, including people with respiratory conditions, and the very young and elderly. Smoke can also impact on air travel and tourism, due to particulate matter in the air, reduced visibility and air quality, as well as to the wine industry through wine taint.

Whilst not having as severe a fire climate as Australia, or the US, due for the most part to our maritime environment, complex terrain and associated microclimates, New Zealand does still have a wildfire problem in many parts of the country (such as Marlborough and the east coast of both islands). Significant wildfires are experienced in many areas every fire season. There is also the potential for our fire climate to worsen significantly in future, bringing an increase in the number of fires, including extreme fire events such as those currently being experienced in NSW and Queensland.

Grant Pearce is a fire scientist with Scion’s Rural Fire Research Group. The research programme is funded by MBIE and receives co-funding from Fire and Emergency New Zealand, NZ Forest Owners Association/Forest Growers Levy Trust, Department of Conservation and NZ Defence Force.

Featured image: 2010 Captain Creek fire, Wikimedia Commons.