Storm activity runs in centuries-long cycles (Image: KeystoneUSA-ZUMA/Rex Features)
We know from historical records that the numbers of tropical storms in the North Atlantic and Pacific rise and fall over multi-decadal cycles, but those data only span 100 to 150 years. For evidence of longer-term cycles, researchers must turn to the geological record, where ancient storms can be detected by the destruction they caused – washing sand into lagoons and leaving a series of ridges along low-lying coasts.
Jonathan Nott and Anthony Forsyth at James Cook University in Cairns, Australia, have now completed the first global analysis of ancient storms to hunt for patterns. Their data stretches back about 6000 years, to the time when global oceans reached present levels. Nott and Forsyth correlated records that they and other researchers have gathered from a dozen sites in Australia, North America, Japan and Puerto Rico. The results revealed that severe storm activity runs in cycles lasting several hundred years.
"Every single one of the records shows periods of greater activity and periods of lesser activity," says Nott. "We still don't understand why."
El Niño effect
Circumstantial evidence suggests a link with the El Niño Southern Oscillation (ENSO) of the Pacific, because areas that seem to have experienced similar prehistoric patterns of storm activity are affected by ENSO in similar ways. An increase in El Niño activity appears to be associated with an increase in storm activity in western Australia and southern Japan, for instance, but a decrease in storm activity in North America and eastern Australia. Good climate data does not go back far enough to make a firm link, though.
The long-term variations identified by Nott and Forsyth raise doubts about regional risk assessments based on historical data, according to Nott.
"Last year a very severe cyclone hit the area where I live [northeast Queensland], and it put sand ridges along the coast," he says. It was the first severe cyclone northeast Queensland had suffered since records begin in 1870 – but the geological record revealed prehistoric intense storms in the region, suggesting storms may be more likely there than previously thought.
Kam-biu Liu of Louisiana State University, who was not involved in the work, has previously studied the ancient record of storms in the southeastern US. "I applaud their efforts and contributions to the emerging field of palaeotempestology," he says.