Temperature memory of the trees is no longer correct
Previously, the average summer temperatures of past years and decades could be estimated from the annual rings of trees. However, as a large-scale study shows, this temperature memory of trees got confused in the second half of this century. "The higher the summer temperatures, the denser the wood in the respective annual ring," was the rule of thumb. To check the matter, the English scientists at the University of East Anglia in Norwich (UK) collected data from hundreds of locations in the northern hemisphere. In fact, there was great agreement between the wood densities in the annual rings and the temperatures, at least in the first half of this century (Nature).
For a few decades, however, the anomalies have increased. This means that temperature curves and measured wood densities in the annual rings of the trees are increasingly inconsistent. The researchers have not yet found any conclusive explanations for the causes of this phenomenon. They point out, however, that studies using trees' temperature memory would need to be reconsidered. In part, statements about the climatic conditions of the past were made on the basis of the annual ring analysis.
The investigations also have an impact on forecasts of carbon dioxide concentrations in the atmosphere. Because trees and forests are considered as so-called sinks for carbon dioxide, which means that the greenhouse gas is stored in the wood, so to speak. If the wood densities are lower, less carbon dioxide is also stored, so the forecasts for future concentrations in the air would have to be corrected upwards.