“It was really scary to see such extreme conditions in such a country as well prepared as Australia,” said Maarten van Aalst, a researcher with the Red Cross Red Crescent Climate Centre in The Hague, Netherlands, and one of the researchers.
Previous studies of Australian heat had identified a clear influence of climate change, including the 2012-13 summer.
The recent heat again offered a clear climate change signal, with temperatures driven higher in heatwaves that are 1-2 degrees hotter than 1900, the researchers found. The record-breaking temperatures during one week in December – which smashed previous Australia-wide daily maximums – were 10 times more likely than a century earlier, they said.
Identifying climate change’s role for the drought was more difficult. The scientists did not find an attributable climate change trend in either extreme annual drought or the driest month of the September-February fire season.
However, the climate influence was clear for the broader fire weather index, arguably before summer had begun.
The Bureau of Meteorology identified record high values during spring for almost 60 per cent of the country using its accumulated forest fire danger index.
Using a modified Canadian fire weather index that better accommodated wind speeds, the researchers found a trend towards higher risks since at least 1979.
They estimate the probability of an index as high as recorded during the recent summer had increased by at least 30 per cent – and as much as 80 per cent – since 1900 as a result of anthropogenic climate change.
“We think it could be much higher” than 30 per cent, said Geert Jan van Oldenborgh from the Royal Netherlands Meteorological Institute, and another of the researchers.
Climate models “are bad at capturing small-scale extremes, or extremes that depend on small-scale processes,” Professor van Oldenborgh said.
“This is partly resolution, but in the case of heat extremes these depend strongly on factors like land use, irrigation, urbanisation, air pollution and other non-climate factors that are often not represented in climate models, and if they are have large uncertainties.”
Australia has warmed about 1.4 degrees over the past century, or faster than the global rise of about 1 degree. With a global temperature rise of 2 degrees, the weather conditions behind the recent fires would be at least four times more common than in 1900, research found.
Taking the higher end of the model range estimate that the recent fires had been made 80 per cent more likely than 1900, the projection for conditions in a 2-degree world was for an eight-fold increase in similar fire weather than without the background climate change, Friederike Otto, from Oxford University’s Environmental Change Institute, said.
Dr Otto cautioned, though, the higher estimates were “very uncertain due to the model deficiencies”.
However, the world’s weak emissions reduction efforts leave temperatures on course to rise 4.1-4.8 degrees by 2100 compared with pre-industrial levels, Climate Action Tracker estimates.
While the attribution study did not examine fire weather risks associated with a 4-degree warmer world, there is “no reason to assume” fire threats won’t continue to climb, Dr Otto said.
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.