Off the chart: cyclone activity in Australia has been lower over the last 40 years than at any time in the past 1500 years. But the seemingly good news comes with a sting in the tail for people living on the coast.
Radar and satellite records of tropical cyclones – rotating storm systems – stretch back only about four decades. For an idea of trends on the longer term, researchers must go underground.
Compared with typical monsoonal rains, the severe rains associated with tropical cyclones are unusually low in the heavier oxygen isotope – oxygen-18. Stalagmites forming in caves record this difference, so by analysing their growth bands – which form each year in the wet season – geologists can establish whether or not a given year was characterised by cyclone activity.
The future, today
Jordahna Haig and Jonathan Nott of James Cook University in Cairns, Queensland, Australia, and their colleagues examined stalagmites from the coast of Queensland and Western Australia. Their results show that cyclone activity in Australia since 1900 has been dramatically lower than at any time since about AD 500. There was an even steeper drop from about 1960, corresponding closely with the sharp increase in global temperatures.
Climate models predict that cyclone frequency will decrease – and individual cyclones will become more intense – as the world warms. However, these effects were only expected to be significant by about 2050. "Models are saying this is going to occur later on. What we're saying is that we're seeing this now," says Nott.
Nott says anthropogenic climate change cannot be ruled out as a factor in the cyclone activity trend, although the link will be difficult to prove. But he notes that a cyclone record going back 1500 years provides an opportunity for long-term comparisons with records of other things that might affect cyclone activity, like solar minima and El Niño. It seems that these have not caused such a change in cyclone activity in the past.
Grossly underestimated risk
If cyclones are to become less frequent but more intense, that is bad news for people living on Australia's coast. It suggests they may have to deal with stronger cyclone-induced storm surges and flooding in future. "We've grossly underestimated the risks with building close to sea level," says Nott.
Scott Power from the Bureau of Meteorology in Canberra, Australia, says the issue is particularly important today because there are more people living along Australia's coast than ever before.
Whether a drop in cyclone activity around Australia means an increase in other regions of the planet is not clear, says Kevin Walsh at the University of Melbourne, Australia. For that, similar studies would need to be done elsewhere.
Nott and his colleagues are already on the case. They are now analysing geological records from Madagascar, to the Cook Islands and Vanuatu. "We're working on the whole southern hemisphere," he says.
Journal reference: Nature, DOI: 10.1038/nature12882
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