23 Feb 2010
Scientists are trying to explain why tree-rings that successfully track northern hemisphere temperature changes for about a century, fail to do so from the 1960s.
In an article in the NZ Herald, environment reporter Eloise Gibson says 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. Generally trees grow more in warmer years and this is reflected in tree-rings - the wider the ring, the more the tree grew that year.
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.
Tree-ring records are often combined with other 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.
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 and logs buried in swamps.
Source: NZ Herald, 22 February 2010