Abstract
Abstract. Disastrous bushfires during the last months of 2019 and January 2020 affected Australia, raising the question to what extent the risk of these fires was exacerbated by anthropogenic climate change. To answer the question for southeastern Australia, where fires were particularly severe, affecting people and ecosystems, we use a physically based index of fire weather, the Fire Weather Index; long-term observations of heat and drought; and 11 large ensembles of state-of-the-art climate models. We find large trends in the Fire Weather Index in the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) Atmospheric Reanalysis (ERA5) since 1979 and a smaller but significant increase by at least 30 % in the models. Therefore, we find that climate change has induced a higher weather-induced risk of such an extreme fire season. This trend is mainly driven by the increase of temperature extremes. In agreement with previous analyses we find that heat extremes have become more likely by at least a factor of 2 due to the long-term warming trend. However, current climate models overestimate variability and tend to underestimate the long-term trend in these extremes, so the true change in the likelihood of extreme heat could be larger, suggesting that the attribution of the increased fire weather risk is a conservative estimate. We do not find an attributable trend in either extreme annual drought or the driest month of the fire season, September–February. The observations, however, show a weak drying trend in the annual mean. For the 2019/20 season more than half of the July–December drought was driven by record excursions of the Indian Ocean Dipole and Southern Annular Mode, factors which are included in the analysis here. The study reveals the complexity of the 2019/20 bushfire event, with some but not all drivers showing an imprint of anthropogenic climate change. Finally, the study concludes with a qualitative review of various vulnerability and exposure factors that each play a role, along with the hazard in increasing or decreasing the overall impact of the bushfires.
Highlights
The year 2019 was the warmest and driest in Australia since standardized temperature and rainfall observations began, following 2 already dry years in large parts of the country
The four climate models investigated show that the probability of a Fire Weather Index this high has increased by at least 30 % since 1900 as a result of anthropogenic climate change
As the trend in extreme temperature is a driving factor behind this increase and the climate models underestimate the observed trend in extreme temperature, the attributable increase in fire risk could be much higher. This is reflected by a larger trend in the FWI7x-SM in the reanalysis compared to models
Summary
The year 2019 was the warmest and driest in Australia since standardized temperature and rainfall observations began (in 1910 and 1900), following 2 already dry years in large parts of the country. Interruptions of the supply of power, fuel and food supplies have been reported, and road closures have been common This has resulted in total isolation of some communities, or they have been only accessible by air or sea when smoke conditions allow (https://reliefweb.int/sites/ reliefweb.int/files/resources/IBAUbf050220.pdf, last access: 6 March 2021). While the FWI was originally developed for the Canadian forests, the physical basis of the models allows it to be used for many different climatic regions of the world (e.g. Camia and Amatulli, 2009; Dimitrakopoulos et al, 2011) and has been shown to provide a good indication of the occurrence of previous extreme fire events in the southeastern Australian climate (Dowdy et al, 2009). Previous attribution studies on Australian extreme heat at regional scales have generally indicated an influence from anthropogenic climate change. We briefly discuss non-climate factors, such as exposure and vulnerability, that have contributed to the impacts of the extreme fire season of 2019/20
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