Tree-ring anomaly mystifies scientists
The last part of Eloise Gibson's series talking to climate experts about what they don't know, what they wish they knew and how they can find out more.
Tree-ring records from the Southern Hemisphere are limited. Photo / Northern Advocate
Scientists are chipping away at a glitch in the climate records, hoping to explain why tree-rings that track temperature changes successfully until the 1950s suddenly veer off.
Researchers believe global warming or other man-made changes may be to blame for an unexplained slowdown in growth of some of the ancient trees used to track temperatures back more than 1000 years.
Although tree-rings records appear to be a good measure of temperature changes most of the time, nobody knows why the usual techniques used with them fail from the 1960s.
The divergence between tree-rings and real, thermometer measurements in the past 50 years has been used to attack temperature reconstructions used by the International Panel on Climate Change and others - particularly since emails from the University of East Anglia's climatic research unit revealed unit head Phil Jones discussing "hiding the decline" in tree-ring temperatures.
It turned out the email referred to a common technique of replacing tree-ring records with direct thermometer measurements from 1961, when tree-rings show temperature declining while real measurements from thermometers do not.
But although researchers of ancient climate have several theories, so far no single theory can explain why some Northern Hemisphere trees behaved differently in the past few decades.
Dr Andy Reisinger, a climate researcher at Victoria University who has followed the progress of proxy temperature reconstructions, said it could be that a lack of rain in recent decades had stunted tree growth in some high-altitude spots - or that when temperatures reached a certain point, trees began to react differently.
Whatever the cause, "the relationships [between tree-rings and temperature] that we've developed for the last 500-100 years may not apply in the last 50," he said.
So far, Dr Reisinger said tree-ring records from the Southern Hemisphere were limited.
"Tree-rings always only tell you something about a specific location and therefore tree-rings are always selective," he said.
Tree-ring records study the width of tree-rings - the wider the ring, the more the tree grew that year.
Generally trees grow more in warmer years.
Tree-ring records are often combined with other ancient reconstructions to form a "hockey stick" pattern, which shows late 20th century temperatures rising sharply from the long-term average.
Those reconstructions helped the IPCC conclude that the last 50 years of the 20th century were probably the warmest in the Northern Hemisphere in more than 1000 years.
Dr Reisinger said that for most of the record tree rings match other physical evidence from ice cores, sediment records, stalagmite and coral fairly closely.
There is no way of confirming the picture with real thermometer temperatures until 1850, although tree-rings are fairly accurate for most of the period when thermometers overlap.
Meanwhile, New Zealand researchers at Auckland University and the National Institute of Water and Atmospheric Research, including tree-ring specialist Andrew Lorrey, may be able to add to the picture by building a long-term climate record from Kauri stumps many thousands of years old.
Massive kauri trees found buried in Northland bogs have yielded some of the world's oldest tree-ring records - some about 30,000 to 60,000 years old.
Some kauri rings are already being used to build a partial record of droughts.
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Say goodbye to the classic Kiwi summer
More floods, droughts and extreme weather events associated with the El Nino and La Nina cycles can be expected in New Zealand in a warming world, Auckland researchers predict.
A group of University of Auckland scientists looking at 700 years of climate records found that El Nino and La Nina cycles "ramped up", or became more common as air and sea temperatures rose.
Their findings are published today in the journal Nature Climate Change.
Lead researcher Anthony Fowler said many large El Ninos and La Ninas had occurred with greater frequency in the past 30 years.
His team's study suggested that this trend might not be in isolation, but a sign of things to come.
"As the world continues to warm, New Zealand is likely to experience the impacts of El Nino and La Nina events with comparable intensity and frequency to what we have seen over the last three decades, and possibly more so," Dr Fowler said.
"This means that we should anticipate more extreme events, such as flooding and droughts, in the regions affected by these weather patterns."
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His team created a climate record for every year since AD1300 by studying kauri tree rings.
Kauri are highly sensitive to the ebb and flow of the cycles, with wide growth rings in their trunks often associated with the cool, dry El Ninos and narrow rings with rainy La Nina patterns.
The scientists previously created a 300-year-old record by studying the rings in living trees. To go further back in time, the group collected ring samples from logged wood, and colonial-era buildings - a burned-down church in the Bay of Islands, weatherboard homes and display pieces in museums.
By looking at how the rings widened and thinned as the world warmed over several centuries, the researchers learned how La Ninas and El Ninos behaved over this time.
These results allowed them to anticipate what might happen to our climate in future, Dr Fowler said.
"If I was talking to water resource planners, I would be suggesting that they might want to start to think about the possibility of more extreme or more frequent El Ninos.
"I can't say I can predict it, but it's a plausible possibility given what has been observed in the past."
Dr Fowler acknowledged it was ironic that the studies benefited from environmental destruction - the piecing together of the climate record was mainly possible because of devastating logging of kauri in the 19th and early 20th centuries.
STORM CHILDREN
The weather cycles known as El Nino and La Nina - Spanish for the boy and the girl - have the greatest influence on our climate after the seasons and monsoons, and have been responsible for devastating floods in the past three years in Pakistan and Australia.
The weather cycles were first noticed by Peruvian fishermen - hence their Spanish names. When anchovy stocks fell in the 1700s, that was blamed on a strange, warm current in the eastern Pacific. The event was named El Nino, "the boy child", because it arrived at Christmas.
El Nino (The boy child)
In New Zealand, El Nino generates cool southwesterly winds and is associated with droughts on the eastern sides of both islands.
La Nina (The little girl)
La Nina - which New Zealand has experienced this summer and last - brings wetter, warmer conditions, greater rainfall and sometimes floods.
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