The extraordinary 2019/20 Australian bushfire season: contributing processes and environmental impacts

The recent Australian summer witnessed bushfire at a scale that is without historical precedent. We analysed the scale and severity of the fires, the main processes contributing to their scale, and environmental consequences that have already become apparent.  We did this by combining satellite-derived information of vegetation cover, biomass and history, of soil and vegetation moisture content, and of fire extent and severity. More than 80,000 km2 was burnt, much of it native forest. Fire severity varied, but was overall greater than in preceding years. A critical factor contributing to fire conditions was a multi-year drought in Eastern Australia, which culminated in 2019 with the hottest and driest year in more than a century. During the fire season, fire danger conditions were further exacerbated by oceanic modes in the Indian and Southern Oceans, which limited circulation and caused excessive heating of the Australian land mass. Fuel availability in forests was unusually high. Reasons for this were several, including afforestation and regrowth as well as effective fire suppression in preceding years, while a contributing role for CO₂ fertilisation is also plausible. Combined with the drought and associated vegetation mortality, this created a high and flammable fuel load. The fires strongly affected Australia’s total living carbon pool, which was already depleted by several years of below-average rainfall. Greenhouse gas releases associated with drought and bushfires are not considered in official emission accounts, but are of comparable magnitude. The smoke emissions also caused direct health impacts, affecting cities like Sydney, Melbourne and Canberra for prolonged periods. Most of the burnt forests are resilient to fire and will regenerate, assuming rainfall conditions improve. The severity, scale and connectedness of some of the fire complexes suggest ecological recolonization may be very slow, while a number of threatened species may not recover. Perhaps most concerning, some of the forests affected had burnt only years before, whereas other areas contained vegetation communities not experiencing fire for centuries, raising questions about their ability to regenerate and possibly permanent ecological regime shifts.