Martin Jucker , ARC Centre of Excellence for Climate Extremes

Satellite image from 1 January 2020 , shows a brown plume of smoke from bushfires in Australia . The smoke was transported around globe and caused serious health issues across Australia . Credit : Image from Himawari 8 , 2020 – 01 – 01 12:00 EAST , https :// himawari8 . nict . go . jp

The end of 2019 saw eastern Australia swallowed by bushfire smoke . One contributor to the bushfire season was a rare and extreme weather event . We found out exactly how rare it was and how rare it will be into the future .

The spring of 2019 was exceptional for eastern Australia : Scorching heat , the culmination of a three-year drought and bushfires raging much earlier in the season than any other time in recorded history . By summer , the burning and related loss of flora , fauna and human lives , as well as respiratory health impacts , were at a disastrous scale . While the reasons for this devastating natural catastrophe were many — and still subject to intensive research — a recent paper shows how rare one of the main contributors to that extreme fire weather was , and how in a future warmer world it will become even rarer to the point of being almost impossible .

Interestingly , the usual suspect for fire weather , the El Niño Southern Oscillation ( or ENSO ) wasn ’ t doing much that year . But there were other obvious factors like a multi-year drought and a strong Indian Ocean Dipole ( IOD ) that played a role . And , in 2019 , there was a third factor : a strongly negative Southern Annular Mode ( SAM ), which is known to be related to hot and dry conditions over eastern Australia . The SAM can be driven by many different things , but in 2019 the obvious driver was a very rare one indeed .

In the spring of 2019 , the usually very strong winds in the stratosphere ( about 300km / h at a height of 30km ) almost completely collapsed within a matter of days — a so-called Sudden Stratospheric Warming was underway . These events high in the atmosphere are known to influence the wind structure at the surface and cause fire-prone conditions at the ground . That ’ s why in early September , the stratospheric research community argued that something very rare was happening and might make things much worse .

Unfortunately , the researchers were right .

A Sudden Stratospheric Warming has only been recorded once before in the Southern Hemisphere — in the spring of 2002 ( which was also followed by fires that summer ). What we know from climate models is that events like these are usually followed by hot and dry conditions in Australia and that these conditions prevail for many months ( eg . into the summer ).

But because this had only happened for the second time on record , it was impossible to say how rare this event was and , crucially , we didn ’ t know if this frequency would alter with climate change . Was this second event after 2002 a result of climate change ? Does this mean we should expect more of those to happen given we are still happily pumping greenhouse gases into the atmosphere ? The only reliable measurements of the stratosphere come from satellites —