Fire Weather: SE Australia
5.6 Fire weather
A substantial increase in fire weather risk is likely at most sites in south-eastern Australia. Such a risk may exist elsewhere in Australia, but this has yet to be examined.
Bushfires are an integral part of Australia’s environment. Its natural ecosystems have evolved with fire, and its landscapes and their biological diversity have been shaped by both historical and recent patterns of fire (Cary 2002). South-eastern Australia has the highest bushfire risk in spring, summer and autumn. This region has the reputation of being one of the three most fire-prone areas in the world, along with southern California and Mediterranean Europe.
Fire risk is influenced by a number of factors, including fuels, terrain, land management, fire suppression and weather. The Forest Fire Danger Index (FFDI) is used operationally to provide an indication of fire risk based on near-surface daily maximum temperature, daily total precipitation, 3 pm relative humidity and 3 pm wind speed. The FFDI has five intensity categories: low (less than 5), moderate (5-12), high (13-25), very high (25-49) and extreme (at least 50).
When the FFDI is extreme, a Total Fire Ban Day is usually declared. During the Canberra fires on 19 January 2003, the FFDI exceeded 100 (Figure 5.43).
Fire danger indices were calculated using daily weather records from 1974-2003 for 17 sites in south- eastern Australia (Hennessy et al. 2006). It was not possible to calculate changes in fire danger based on any of the CMIP3 models in Table 4.1. The results presented here are based on the study by Hennessy et al. (2006) using two climate change simulations with CSIRO’s Cubic Conformal Atmospheric Model (CCAM), which has 50 km resolution over Australia. One simulation (denoted CCAM Mark2) was driven by boundary conditions from the CSIRO Mark 2 coupled ocean-atmosphere model, while the other simulation (denoted CCAM Mark3) was driven by boundary conditions from the CSIRO Mark 3.0 model. Data from these simulations were then used to generate climate change scenarios per degree of global warming, including changes in daily weather variability. These changes were scaled by the IPCC (2001) global warming ranges for 2020 and 2050, then applied to the daily weather records from 1974-2003 at 17 sites in south-eastern Australia. Fire danger indices were then calculated for the modified 30 year datasets centred on 2020 and 2050 (Hennessy et al. 2006).
An increase in fire weather risk is simulated at most sites, including the average number of days when the FFDI rating is very high or extreme. The combined frequencies of days with very high and extreme FFDI ratings increase 4-25% by 2020 and 15-70% by 2050 (Table 5.7). For example, Canberra has an annual average of 25.6-28.6 very high or extreme fire danger days by 2020 and 27.9-38.3 days by 2050, compared with a present average of 23.1 days. The increase in fire weather risk is generally largest inland. Tasmania is relatively unaffected. It is likely that the higher fire weather risk in spring, summer and autumn will increasingly shift periods suitable for prescribed burning toward winter.
This study of CSIRO/CCAM results does predict increased fire risk in SE Australia, but does not appear to extend much of that risk to Tasmania.