Tropical cyclones are occurring less frequently around the world due to climate change, Australian scientists have found.
The team led by Savin Chand from Federation University discovered tropical cyclones were happening about 13 per cent less frequently than in the pre-industrial period.
"We have consistently found that cyclone numbers are going down around the world," Dr Chand said.
However, the authors said their study only looked at the frequency of tropical cyclones, not their intensity, which they said was increasing due to climate change.
"As the atmosphere warms, tropical cyclones forming have more fuel for their severity," Dr Chand said.
"Even though cyclones will get fewer, they will get more intense."
Reconstructing hurricane history
The authors had to find a way to go back in time to reconstruct a history of tropical cyclones around world back to the 1850s.
Before the satellite era, the only way to know a cyclone had occurred out to sea was if a ship or an aeroplane had the misfortune to run into it.
"How frequently these things have been happening historically has been very controversial because of the short period of data," Dr Chand said.
However, new weather data sets and sophisticated computer reconstructions of past weather have allowed scientists to look back in time with greater accuracy than before.
"There has been quite huge progress in what we call reanalysis," Dr Chand said.
"It uses a model which assimilates available observations and then goes even further. And then we use all this data to detect past tropical cyclones."
Tropical cyclones decline in Australia
The scientists found tropical cyclone numbers in the Australian region had declined by about 11 per cent since 1900.
Study co-author Kevin Walsh, from the University of Melbourne, said tropical cyclone numbers had been decreasing "quite a bit" in the South Pacific region in the past few decades.
"This decrease agrees with the likely impact of anthropogenic warming," Professor Walsh said.
The researchers said the decline in Australia had increased since the 1950s as global warming accelerated.
They said it was a trend mirrored in ocean basins around the world.
But there was one exception.
Tropical cyclones have actually become more frequent in the North Atlantic since the 1960s, according to the study.
"This may be because the basin is recovering from a decline in tropical cyclone numbers due to human-related aerosol emissions in the late 20th century," Dr Chand said.
"The number of annual storms is still, however, lower than in pre-industrial times."
Less frequent but more intense
The finding has not been a total surprise for climate scientists.
As the climate has warmed over the 20th century, scientists have suggested that changes in underlying atmospheric conditions like humidity and wind shear have created an environment that is more hostile for tropical cyclone formation globally.
Professor Walsh said the results needed to be interpreted in perspective as the study counted all tropical cyclones, severe or otherwise.
"The really intense ones are the ones that cause most of the damage," he said.
"So what that means is you can have a situation where the total numbers are decreasing, but the total damage actually increases, because there are some pretty good theoretical reasons to believe that in the future, the numbers of really intense storms will increase rather than decrease."
