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Scientists who took samples from New Zealand kauri say they show five of the 10 strongest El Nino events in the past 400 years have been since 1982.
A team led by Dr Anthony Fowler at the University of Auckland has built up the world's longest record of
El Nino activity from tree rings - a continuous picture of the past 3722 years.
The El Nino Southern Oscillation (Enso) - changes in sea surface temperatures and atmospheric pressure across the Pacific Ocean linked to droughts, flood, and cyclones - has been compared to the heartbeat of climate systems.
The Auckland-based team is about to publish a detailed tree-ring study of El Nino events, extracted from the larger study, covering the past 423 years.
The team used 426 cores from 191 trees at 17 sites to build the record.
The recent cluster of five big El Nino events was reflected in the tree rings: 1982, 1986, 1991, 1992 and 2002.
Kauri rings are sensitive to the cycle, with wide growth rings in their trunks often associated with the cool dry El Ninos and narrow rings with rainy La Nina patterns.
Initial analysis of the most recent four centuries of kauri rings showed a 50 to 80-year Enso cycle, which has repeated seven times since 1580.
If the pattern persisted, the active El Nino period of the past few decades would be the latest such peak and a subsequent easing of activity was plausible, Dr Fowler said.
As well as kauri growth rings, a wider tree-ring study of native species including kaikawaka was being used to built a climatic record for northern New Zealand over several millennia, and for central and southern regions for the last millennia.
By feeding information into climate models, scientists will be able to better predict whether global warming will increase the strength or frequency of El Nino events.
New Zealand's kauri evolved about 190 million years ago, before the split from Gondwana.
These remnants of ancient forest contain some of the longest-living trees in Australasia. Some are more than 2000 years old, with girths of more than 10m.
Scientists bored three small samples by hand, each the width of a piece of straw. The rings were then measured under a microscope.
Maori elders had banned scientific research for 20 years in the 5000ha Warawara native forest, in the Far North, as a result of culturally insensitive behaviour by previous researchers.
A member of Dr Fowler's team, Australian Dr Joelle Gergis, of the University of NSW, convinced the elders of the worth of the research.
Samples from giant trees that had fallen centuries ago and been preserved in swamps were studied, together with logging relics kept in museums.
The team also went into the forest and found stumps of old trees that had been cut down.
- NZPA
