Bushfires and grassfires are common headlines on today’s news. Grassfires are medium intensity but fast-moving events, lasting usually from five to ten seconds and smoldering within minutes, whereas bushfires are slower moving, with a higher intensity. They may last longer (from two to five minutes) but they can smolder for days. Both fire types are considered part of Australia’s natural environment and Australian natural ecosystems have evolved with them so much that many of their native plants became highly combustible and fire prone.
According to Tim Flannery, a researcher at the University of Melbourne, fire is the most important ecological force at work in the Australian environment: some native plant species have evolved a variety of mechanisms to survive fires and others require bushfires to regenerate. For example, plants with epicormic shoots or lignotubers sprout after a fire, and plants with fire-triggered seeds or oil-containing leaves, like the eucalyptus, encourage fires .
Aboriginal people used to set fires to facilitate hunting and produce lusher leaves which would be used to fatten kangaroos. This may be the reason why James Cook called Australia “This continent of smoke”. When European settlers moved to Australia, population growth and urban sprawling in bushlands increased the risk of bushfires, as proven by the “Black Sunday” episode of 1925-1926. Over a two-month period, 60 people were killed in Walbourton, near Melbourne. Similarly, the “Black Tuesday” episode of 1967 killed 62 people in Tasmania. Nowadays, fires have not gone extinct but have actually exacerbated due to the climate crisis effects. The September 2019 – March 2020 bushfires season was indeed the worst fire season of Australia’s history.
The basic factors that determine whether a bushfire will occur are the presence of fuel, oxygen and the source of ignition. The fire intensity and speed at which a bushfire spreads depends on the ambient temperature, the fuel load, fuel moisture, wind speed and the slope angle. Fuel load refers to the amount of vegetation in the landscape and their quality (twigs, leaf litter and branches burn quickly). Dry fuel burns quickly so the time passed since the last rainfall is taken as significant weather indicator for bushfire ocurrence prediction. Other considerable factors are also related to weather: strong winds promote the rapid spread of fires by lifting burning embers into the air. Among all this, the climate crisis does not help.
The Australian climate has warmed by more than one degree Celsius in the past century: heatwaves and droughts have dried out the underground and created conditions that increased the risk of bushfires . Stakes are high: in 2020, an international team of scientists discovered that the probability of another 2019-2020 bushfires crisis could increase by eight times if global air temperature warmed by 2 °C. Furthermore, bushfires themselves contribute to the climate crisis. It is believed that the 2019-2020 bushfires have released approximately 350 million tons of carbon dioxide. David Bowman, professor of Pyrogeography and Fire science at the University of Tasmania, warned that Australian forests have been so damaged that they may need more than 100 years to re-absorb the carbon that has been released so far .