Cyclone Dylan bears down on the Queensland coast. Picture: Supplied Source: News Limited
THE frequency of tropical cyclones is at its lowest level in 1,500 years in Western Australia and the lowest in 550 years in Queensland, a team of researchers has found.
The new study also shows the number of cyclones lashing Australia has dropped greatly since the industrial revolution, suggesting that global warming might be causing a decline in the extreme events sooner than expected.
A team of scientists led by Jordahna Haig, of James Cook University in Queensland, will publish their results in the British journal Nature today.
Cyclone frequency in both states has been oscillating for centuries but it dropped significantly in Queensland from 1743, around the start of the industrial revolution. In Western Australia, overall cyclone frequency fell from 1650, declining abruptly from 1960.
"The dramatic reductions in activity since the industrial revolution suggest that climate change cannot be ruled out as a causative factor," said team member Jonathan Nott, of James Cook University's School of Earth and Environmental Sciences.
He stressed that the results should be treated with caution.
"It remains to be seen whether this downward trend in activity will continue," he said.
Scientists had long predicted that the frequency of tropical cyclones in the Australian region would fall as the planet warmed, Professor Nott said.
The outlook, which also included projections of an increase in the intensity of the most severe tropical cyclones, was based on climate modelling simulating mechanisms of cyclone formation.
This latest research provides the first empirical evidence supporting the model predictions of a decline in cyclone frequency but it suggests that the fall is happening much sooner than expected.
Scientists had not expected the decline to happen until mid to late this century, Professor Nott said.
The James Cook University team used environmental archives written in the calcium carbonate of stalagmites from caves in Chillagoe, in north-eastern Queensland, and Cape Range, near Exmouth, Western Australia.
Stalagmites are cylindrical structures that grow from the floors of limestone caves from drops of water when calcium carbonate in solution precipitates out. Some have annual growth bands like tree rings. The isotopic composition of oxygen in the calcium carbonate records rainfall information
Rain dumped by cyclones is depleted in the heavy isotope, oxygen-18, compared with normal monsoonal rain.
The team calibrated their dataset against Bureau of Meteorology's instrumental records of the past 50 years.
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