Large bushfires under drought and extreme temperature conditions in southeastern Australia: a probabilistic assessment using copulas

The occurrence of large bushfires in southeastern Australia has been linked to the occurrence of extreme climate conditions, such as droughts and extreme temperatures. Several extreme bushfires have occurred following or during severe droughts and heatwaves, namely the Black Saturday bushfires in 2009 and the Black Summer of 2019-2020. Fire-prone weather conditions have become more severe in this region during the past years and are expected to worsen as the frequency of compound drought and extreme temperature events is expected to increase in the future, even under lower emission scenarios. This is particularly important as the impacts of compound climate events are usually larger when compared to the impacts resulting from an individual event. Therefore, compound drought and temperature extremes are likely to increase the probability of large bushfires, compared to the occurrence of an individual event.

In this work, the trivariate relationship between burned area in the months of December to February, drought conditions, and temperature extremes was analyzed using copulas for the period 2001-2020. Burned area across forests were computed using the GlobFire dataset. Drought conditions were assessed using the Standardized Precipitation Evapotranspiration Index (SPEI), computed with monthly precipitation and temperature data from the CRU TS4.05 dataset. The indices Number of Hot Days (NHD) and Number of Hot Nights (NHN) were used to identify conditions of extreme temperature and were computed using hourly temperature data from ERA5. The influence of concurrent and previous climate conditions on the burned area was assessed, for up to 3 months before the beginning of the fire event.

The results show a clear influence on the probability of occurrence of large fires under conditions of drought and extreme temperature. Drought conditions in the months before the fire event had a larger effect than temperature extremes. Moreover, the probability of occurrence of large fires is higher when compound drought and hot events are present than given only one individual extreme event.

 

Acknowledgements: This study was supported by the H2020 FirEUrisk project (EU H2020, Grant Agreement 101003890) and by FCT (Fundação para a Ciência e Tecnologia, Portugal) through national funds (PIDDAC) – UIDB/50019/2020 and project Floresta Limpa (PCIF/MOG/0161/2019).