“Our results are in agreement with model forecasts, which predict that the frequency of tropical cyclones will decrease in the Australian region due to climate change,” Jordahna Haig of James Cook University, Australia told environmentalresearchweb. “The expectation is that while the total frequency may decrease, the intensity of tropical cyclones will increase.”

While models suggest that these changes will occur mid to late this century, Haig and colleagues’ study suggests the decline in cyclone frequency has already begun. “We cannot be sure that the decrease in tropical cyclone activity highlighted in our study is due to climate change, but nonetheless our results are mirroring these model forecasts,” said Haig.

To create the cyclone activity index, the team examined two cylindrical stalagmites, from caves in Queensland and Western Australia. Rainfall from cyclones contains relatively little heavy oxygen so analysing the oxygen isotope ratio of calcite in stalagmites can reveal the extent of cyclone activity.

The team then compared the oxygen isotope ratio for each year’s wet season from 1990 to 2010 with a cyclone activity index of the average accumulated energy expended, based on factors such as number of cyclones, cyclone strength, size and time on storm track. The two correlated well, enabling the team to calculate the cyclone activity index from stalagmite data for the past 700 or 1500 years, depending on the stalagmite.

“Recent works have shown that stalagmites from tropical regions do in fact record individual tropical cyclone events within the chemistry of their annually deposited layers, but until now it was not possible to quantify this information,” said Haig. “By calibrating these modern deposits with instrumental records of actual events using this index we can provide a meaningful measure of activity at an annual resolution over thousands of years.”

The index revealed that the current low levels of storm activity on the mid-west and north-east coasts of Australia are unprecedented over the past 550 to 1500 years. Cyclones in the region appear to have a multi-century cycle, with the most recent one starting around 1700 AD.

“Our results indicate that the underlying assumption that the short instrumental record is a true reflection of longer-term tropical cyclone behaviour is incorrect,” said Haig.

The analysis provides “one seamless index which combines both instrumental and prehistorical tropical cyclone records”, according to Haig. “This brings with it the ability to more accurately analyse long-term trends in tropical cyclone behaviour.”

The trends in Australia are in stark contrast to those in the Northern Hemisphere, where tropical cyclones have increased in frequency and destructiveness since 1970 in the Atlantic Ocean and western North Pacific.

“As new methods provide new opportunities we plan to examine further linkages between tropical cyclone activity and other climate indices and ways in which this new index could be incorporated into climate or forecasting models,” said Haig.

The team reported the results in Nature.

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