Science - Mega bushfires

By: Ralph Cooney

The recent news from across the Tasman during the early summer months has been dominated by reports of exceptional bushfires in Queensland and New South Wales. The appearance of these destructive fires has led to loss of life, damage to homes and loss of habitat and death of native animals, including many koala. These events have been interpreted as a trend towards a new type of mega-fire. Australia is somewhat different from New Zealand, in that Australia exhibits long climatic cycles and bushfires caused by periodic drought. The predominant eucalyptus forests, rich in natural flammable tree oils (known as terpenes) are primary sources. Nevertheless, the extreme Nelson fires of last year and the similarity of some climatic zones in Aotearoa (such as Canterbury and the East Cape) to Australian fire zones at similar latitudes, provides a caution against complacency in this country. In other very different locations, especially California, mega-fires are now a regular feature of the warmer months. This confirms that the Australian fires may be frightening, but are certainly not unique.

Geoscience Australia gives details of the main factors causing bushfires. Fuel load is the quantity of combustible bark, leaf litter and small branches retaining natural oils that accumulate in forests. Low moisture levels are important as dry fuel loads during hot drought months are an obvious factor. Wind speed can cause fire-spotting ahead of the actual fire front by blowing burning embers into dry unburnt forest zones. High ambient temperatures are critical because the natural forest oils have ignition point temperatures above which ignition occurs spontaneously. Low humidity is important because forest leaves release their moisture more readily under dry conditions. Slope-angle plays a role as fires accelerate up slopes and decelerate down slopes, because mobile fires pre-condition the area in front of them via radiation and convection. Ignition sources include both natural (mainly lightning) and man-made (arson) sources. Therefore, it is important that the psychology of arsonists be examined, understood and public strategies developed to reduce this risk. All of the above factors would apply in Aotearoa, as well as in other countries.

The broadly accepted public view in Australia is that bushfires have been a natural long-term cyclic feature of the driest continent on earth. There is now a consensus among senior fire ecology experts and scientists that the recent mega-fires represent a climate change enhancement of past cyclic fire patterns. Diverse news sources including the ABC, The Conversation, The Guardian (Australia), the BBC and others have conveyed this consensus. The enhancing role of climate change in mega-fires is also endorsed by the Australian Climate Council, which is a national network of concerned scientists. In the USA, the Governors Fourth Climate Change Assessment Report and the respected National Geographic have drawn the same conclusion about the increasing role of climate change in wildfires. Aotearoa needs to be vigilant as the fire season approaches.

Professor Ralph Cooney


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