Beware the Black Swan

The Black Summer fire season was extraordinary (Canadell et al 2021); it occurred due to the culmination of multiple factors, including protracted drought and a favourable configuration of tropical climate drivers. Global warming also played a role, with an Oldenborgh et al (2020) attribution study showing that warming already experienced increased the likelihood of the 2019/2020 bushfire season by ~30%. Attribution studies typically try to understand the role of anthropogenic warming in driving extreme events, but one major driver of the Black Summer fires was completely unexpected. The difficulty with seasonal forecasts and climate projections is accounting for the unexpected: either because a phenomenon is understood and not predictable, or because it has not previously been observed.

The phrase “black swan” is a metaphor that has been around since the 2nd century and is used to describe unexpected events of large magnitude and consequence. It arose at a time when black swans were presumed not to exist, but the term took on an even more nuanced meaning in the 16th century when Dutch explorers discovered black swans in Western Australia. The term is now used to describe an extreme event, with high consequence, which was not predicted, but perhaps in hindsight should have been. The catastrophic weather of the past few years occurred in conjunction with climate forcing events that were not predicted by any weather or climate forecast models.

Starting in September 2019 a sudden warming of the stratosphere occurred (Figure 3) which perturbed westerly winds over the Southern Ocean resulting in increased frontal activity over southeast Australia. Fronts in spring and summer are one of the key drivers of dangerous bushfire weather and dust storms. They are usually proceeded by hot dry north-westerly winds which desiccate the landscape meaning bushfires are easier to start and can quickly grow to become out of control. Sudden stratospheric warmings (SSW) are commonly observed in the northern hemisphere, but were almost unprecedented in the southern hemisphere, with only one previously observed in 2002. Although the SSW was unexpected, the Bureau of Meteorology quickly realised the implications and issued warnings that it could lead to drought and severe bushfire weather (Hendon et al 2019). By spring 2019 the landscape was already extremely dry, and the climate drivers were aligned for a dangerous bushfire season. The SSW could be considered the “black swan” that amplified it from dangerous to catastrophic.

But that’s not all: one of the main complexities of the climate system is that nothing happens in isolation, and this is one of the factors which makes attribution studies so difficult. Recent research shows the Black Summer bushfires put such large quantities of ash and smoke into the stratosphere that it strengthened the polar vortex, prolonged the ozone hole and led to unusually cool southern hemisphere temperatures in spring 2020 (Damany-Pearce et al 2022). More importantly, Fasullo et al (2023) found that the bushfire emissions increased cloud albedo across the Pacific, leading to sea surface cooling and that this was likely a major contributor to the strength and persistence of the 2020–2022 strong La Niña events.

Connecting the dots, the September 2019 SSW was the “black swan” that predicated 3 years of climate turmoil, including some of the most disruptive and costly natural catastrophes in Australia’s history. So what is the chance of another SSW any time soon? A modelling study by Jucker et al (2021) showed that SSW could be expected to occur about once every 22-years. With accurate measurements of the stratosphere only available since the late 1970s it is understandable that this event came as a surprise. However, Jucker et al (2021) did have some good news; their climate model simulations showed that under a warmer climate SSW should become less frequent—about once every 300 years.