Ex-tropical Cyclone Alfred has curiously unleashed online claims downplaying the way climate change affects the frequency and severity of major storms in Australia.
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While it's true tropical cyclones have become less frequent in Australian waters, experts say claims that they're not intensifying over time miss important context.
One particular viral post on X shared by thousands of users in the wake of Alfred, including coalition Senator Matt Canavan, recently caught the eye of AAP FactCheck researchers.
It features a Bureau of Meteorology graph detailing Australian region tropical cyclones and severe cyclones over the past half century.
"There is no evidence that tropical cyclones are becoming more frequent or more intense in the waters surrounding Australia," the post caption claimed.
Senator Canavan's office did not respond to AAP's request for comment.
Tropical cyclones affect the tropical coasts of Western Australia, the Northern Territory and Queensland from late November to April and are characterised by extreme wind speeds, heavy rain, flooding and storm surges.
Experts consider three key aspects when examining cyclone behaviour: frequency, severity and intensity.
Frequency refers to the number of cyclones identified via satellite in the Australian region each season, with Bureau of Meteorology saying climate data confirms they have become slightly less so each season since reliable satellite observations began in 1982.
However part of that decline is due to the change in the definition of a tropical cyclone in 1978, which resulted in storm systems that would previously have been categorised as such, being instead designated as sub-tropical systems.
The bureau's graph also appears to support the claim that tropical cyclones are not intensifying, as the proportion of systems categorised as 'severe' has fluctuated since 1970.
Cyclones are categorised as 'severe' on the graph based on "current intensity value", which is estimated using infrared and satellite imagery and atmospheric pressure data.
However the graph does not capture the full suite of conditions that indicate the intensity of tropical cyclones, such as rainfall, flooding and storm surges, where the sea rises well above the highest tide of the year.
The bureau says the intensity trend is harder to quantify than frequency due to uncertainties in estimating the force of individual cyclones and the small number of intense cyclones.
"Analysis of historical tropical cyclone data has limitations due to changes in observing practices and technology that have occurred over time," according to a bureau spokesperson.
Liz Ritchie, a tropical cyclone expert at the ARC Centre of Excellence for Weather of the 21st Century, says the relatively small number of cyclones in Australia means making definitive statements about intensity trends is difficult.
"Changes in frequency don't necessarily mean changes in intensity," Prof Ritchie says.
She says a 2019 review by two ARC researchers shows that in the 30 years to 2010, the proportion of category 4 and 5 cyclones in the Australian region has increased.
That paper also notes that the number of tropical cyclone days has risen slightly over that period.
"You can have a decrease in # TCs but if the TC days increases then the total impact has still increased," Prof Ritchie says.
A 2022 data review suggested a possible five per cent increase in cyclone intensity worldwide for a two-degree rise in global temperature, she notes, while the most recent IPCC report suggests there would be little change in the South Pacific and South Indian oceans.
"All indications are that globally averaged potential intensity will increase under increased greenhouse gas emissions," she says.
The bureau says current projections indicate the intensity of rainfall associated with cyclones is expected to increase and higher sea levels caused by climate change will worsen the impact of storm surges.
Changes in the technology used to measure cyclones, such as aircraft and satellite monitoring, along with the difficulty of measuring a cyclone hundreds of kilometres out to sea, have all made predicting future cyclone behaviour harder, Prof Ritchie says.
Australian Associated Press
