Attribution of the Australian bushfire risk to anthropogenic climate change

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.

Pathways and pitfalls in extreme event attribution

“…As an example, moderate rainfall and moderate wind causing extreme water levels in the North of the Netherlands (van den Hurk et al 2015) can be analysed using a model of water level. Similarly, drought and heat can be combined onto a forest fire risk index, which lends itself to the class-based event event definition (Kirchmeier-Young et al 2017;Krikken et al 2019;van Oldenborgh et al 2020) The definition next includes the spatial and temporal properties of the variable: the average or maximum over a region and timescale, possibly restricted to a season. As an example, an event definition would be the April-June maximum of 3-day averaged rainfall over the Seine basin, with the threshold given by the 2016 flooding event (Philip et al 2018a).…”

“…These results are then synthesised into an overview of key factors which may have enhanced or diminished the impacts of the extreme natural hazard. As the methodology associated with this analysis has advanced, the integration of this information into attribution studies has progressed from an introductory framing to incorporating findings in synthesis or conclusions sections to most recently including a stand-alone vulnerability and exposure section (Otto et al 2015b;van Oldenborgh et al 2020).…”

Cascading Hazards in the Aftermath of Australia's 2019/2020 Black Summer Wildfires

“…2019 was the driest year on record in Australia (van Oldenborgh et al., 2021), with the lowest rainfall on record from July to December in many parts of southeastern Australia (Nolan et al., 2020; data accessible from http://www.bom.gov.au/climate/history/rainfall/). Neutral El Niño‐Southern Oscillation conditions and a positive Indian Ocean dipole were the main causes for the drought (King et al., 2020; van Oldenborgh et al., 2021). In summer 2019, this event was accompanied by the highest mean maximum temperatures since recording began in 1910, with the highest anomalies in December 2019 surpassing those of the “Angry Summer” of 2012/2013 (van Oldenborgh et al., 2021).…”

“…Neutral El Niño‐Southern Oscillation conditions and a positive Indian Ocean dipole were the main causes for the drought (King et al., 2020; van Oldenborgh et al., 2021). In summer 2019, this event was accompanied by the highest mean maximum temperatures since recording began in 1910, with the highest anomalies in December 2019 surpassing those of the “Angry Summer” of 2012/2013 (van Oldenborgh et al., 2021). This extraordinary drought was a key driver of the wildfires, whereas the role of fuel accumulation due to fire suppression is still disputed (Bradstock et al., 2020).…”

Drought, Flood, Fire

“…In 2019-2020, a staggering and globally unprecedented 21 percent of Australia's forested area burned, 13 as climate change contributed to extreme temperatures and dry conditions. 14 Ecologists estimate that a billion or more animals (mammals, birds, and reptiles) may have perished in these conflagrations.…”