Between June 2019 and March 2020, thousands of wildfires spread devastation across Australia at the tragic cost of many lives, vast areas of burnt forest, and estimated economic losses upward of AU$100 billion. Exceptionally hot and dry weather conditions, and preceding years of severe drought across Australia, contributed to the severity of the wildfires. Here we present analysis of a very large ensemble of initialized climate simulations to assess the likelihood of the concurrent drought and fire-weather conditions experienced at that time. We focus on a large region in southeast Australia where these fires were most widespread and define two indices to quantify the susceptibility to fire from drought and fire weather. Both indices were unprecedented in the observed record in 2019. We find that the likelihood of experiencing such extreme susceptibility to fire in the current climate was 0.5%, equivalent to a 200 year return period. The conditional probability is many times higher than this when we account for the states of key climate modes that impact Australian weather and climate. Drought and fire-weather conditions more extreme than those experienced in 2019 are also possible in the current climate.

2019 年 6 月至 2020 年 3 月期间，数千场野火在澳大利亚各地蔓延，造成许多人丧生、大片森林被烧毁，经济损失估计高达 1000 亿澳元。异常炎热和干燥的天气条件，以及澳大利亚前几年的严重干旱，导致了野火的严重性。在这里，我们对大量初始化气候模拟进行了分析，以评估当时同时发生干旱和火灾天气条件的可能性。我们关注澳大利亚东南部这些火灾最普遍的一个大区域，并定义了两个指数来量化干旱和火灾天气对火灾的敏感性。这两个指数在 2019 年的观察记录中都是前所未有的。我们发现在当前气候下经历这种极端火灾敏感性的可能性为 0.5%，相当于 200 年的重现期。当我们考虑影响澳大利亚天气和气候的关键气候模式的状态时，条件概率比这高很多倍。在当前气候下，也可能出现比 2019 年更极端的干旱和火灾天气条件。