EXODUS TO ARTHUR
Mike Baillie
Batsford, £19.99
ISBN 0-7134-8352-0 hb

As tree-ring sequences accumulate from different parts of the world, it should become possible to discern `signal' from `noise': to identify patterns which are consistent across large areas, rather than just describing local ups and downs. Dendrochronology could thus move towards dendroclimatology.

One sort of signal which it is tempting to search for in such a record are traces of regionally or globally synchronous climatic events resulting from the impact of extraterrestrial bodies such as meteorites or planetesimals. The importance of such phenomena has been increasingly recognised since Luis Alvarez's hypothesis that the Cretaceous Period came to a dramatic close some 65 million years ago as the result of such a major impact. It is now known that an asteroid some six miles across landed on the Yucatan peninsula, where an explosion equivalent to 10,000 times the energy of detonating the world's entire nuclear arsenal left a crater 180 miles across and wiped out a major portion of the life of the planet in an `impact winter' and subsequent greenhouse effect.

If this seems a long time ago, then remember that in 1908 an impact equivalent to a ten-megaton explosion left a 1.2km crater in Tunguska (Siberia), and that events of this magnitude probably occur with a frequency of between once a century and once a millennium. Worth searching for in the tree-rings.

Mike Baillie is a leading dendrocatastrophist who believes he had found them. There are certainly some dramatic and widespread anomalies in tree-ring thicknesses, though many may be associated with volcanic eruptions rather than impacts. Sadly, the hard evidence does not yet make a story. Undeterred, Baillie recruits an amazing mixture of historical records for various kinds of unusual phenomena and their associated mythologies, in a way which goes far beyond any credibility as serious science.

`Mercy on us,' wrote Horace Walpole, on surveying the second volume of Archaeologia in 1771, `what a cartload of bricks and ruins and Roman rubbish they have piled together!' It is tempting to echo the judgement; but a serious idea still awaits investigation.

Dr Andrew Sherratt works at the Ashmolean Museum in Oxford   
source http://www.britarch.ac.uk/ba/ba46/ba46book.html

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