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(I favor the comet/asteroid theory. Of the five catastrophic events recorded in the tree ring data (supported by the ice core records), in the last 10k years, the 540 AD event is the only one that does not have an acid layer (volcanos) in the ice core data. The five catastrophic events were: 3195 BC(Stonehenge built), 2354BC(start of Bronze Age), 1628BC(Exodus, Stonehenge abandoned), 1159BC(David plague), 540AD(Dark Ages,Justinian plague) and two minor events at 207BC and 44BC.)

Friday, 8 September, 2000, 14:55 GMT 15:55 UK

Tree rings challenge history

By BBC News Online's Jonathan Amos

Could a comet hitting the Earth 1,500 years ago have triggered a global disaster in which millions of people lost their lives?

It is an old claim that historians say has little evidence in written records to support it, but now a tree ring expert has said the idea must be re-examined.

Mike Baillie, professor of palaeoecology at Queen's University in Belfast, UK, said it was very clear from the narrowness of growth rings in bog oaks and archaeological timbers that a great catastrophe struck the Earth in AD 540.

"The trees are unequivocal that something quite terrible happened," he told the British Association's Festival of Science. "Not only in Northern Ireland and Britain, but right across northern Siberia, North and South America - it is a global event of some kind."

Dark Ages

Professor Baillie favours the idea that cometary fragments smashed into the atmosphere throwing up dust and gas that blocked out the Sun. This, in turn, led to crop failures, famine and even plague among the weakened peoples of the world.

Professor Baillie said astronomers from Armagh Observatory in Northern Ireland had published research 10 years ago in which they said the Earth would have been at risk from cometary bombardment between the years AD 400 and AD 600.

"This event is in AD 540, so it fits very nicely into the window," he said.

"We know from the tree rings to the year exactly when this event happened. And some archaeologists and historians are beginning to come round to the opinion that this was the date when the Dark Ages began in Northern Europe. It wasn't just when the Romans left."

Oral tradition

However, there are many more historians who believe that if such a major event had occurred there would be much clearer references to the disaster in written texts. But Professor Baillie urged them to go back and look again - "to read between the lines".

He said mythical stories certainly seemed to point to a comet striking the Earth at about the right time. He said King Arthur died in this period and some stories talk about long arms in the sky delivering mighty blows.

"Mythology tells you and history doesn't and that raises some very interesting questions because the implication is that you could suppress the written word but you couldn't suppress the oral tradition."

Professor Baillie said chemical analysis would be carried out on the tree rings to investigate the comet idea further. He hopes also to get access to ice cores to see if they record any interesting data that might support the comet theory.

(For more, read Mike Baillie's book, Exodus To Arthur)