The science of dendrochronology might be traced back 500 years to when Leonardo Da Vinci observed what seemed to be a direct relationship between the width of tree-rings and precipitation. As time went by, archaeologists began to use this concept to unlock a potential treasure-house of information: they used tree-rings to fix the age of wood.
The technique is based on the principle that if you slice through the trunk of a tree, the resulting disc is embossed with concentric circles, known as tree-rings. The width of a treering varies with the weather of the year in which it formed, low temperature and low rainfall both producing narrower rings than usual; each tree, therefore displays a pattern which reflects the weather during its lifetime.
By analysing old timbers, all from the same locality and known to have overlapped somewhat in their periods of growth, it is possible to identify a unique tree-ring pattern corresponding to the weather over the centuries in a particular region. From this standard pattern, any piece of wood from the same area can be dated accurately - by noting to which period its tree-rings correspond. And this technique is used in reverse by climatologists; they deduce the weather pattern during the lifetime of an ancient piece of wood whose age is known.
But as the song says, "if you go down to the woods today, you're in for a big surprise". Recent studies at hundreds of sites across Asia, Europe and North America suggest that trees in the northern hemisphere are becoming less responsive to the weather of their time. It seems that during the second half of the 20th century, the relationship between the texture of tree-rings and the prevailing temperature has become much less reliable than it used to be; the wood formed by trees at a given temperature is not as dense as heretofore.
Now trees accrete their wood through photosynthesis, which involves absorbing carbon dioxide from the atmosphere. The compelling explanation for this latest phenomenon is that forests must be absorbing less carbon dioxide than they were wont to do, and this, if true, means that there is more CO 2 left in the atmosphere to enhance the greenhouse effect and contribute further to global warming.
Nobody knows yet why the trees of the world are behaving in this way. Maybe it is because more untraviolet radiation is reaching the ground following depletion of the ozone layer; or maybe it has something to do with the effects of acid rain. Neither do we know if this is an ephemeral phenomenon, or the beginning of a trend.