Upon examining the tree-ring record, Baillie noticed indications of severe environmental downturns around 2354 BC, 1628 BC, 1159 BC, 208 BC, and AD 540. He claimed that these environmental downturns were wide-ranging catastrophic events; the AD 540 event in particular is attested in tree-ring chronologies from
Siberia through
Europe and
North and
South America. This event coincides with the second largest ammonium signal in the Greenland ice in the last two millennia, the largest being in AD 1014, and both these epochs were accompanied by cometary apparitions. Baillie explained the general absence of mainstream historical references to this event by the fact it was described in terms of biblical metaphors since at that time "Christian beliefs included the dogma that nothing that happens in the heavens could have any conceivable effect on the Earth." Since then, he devoted much of his attention to uncovering the causes of these global environmental downturns. He believed that impacts from
cometary debris may account for most of the downturns, especially the AD 540 event. This hypothesis is supported in work by British cometary
astrophysicists, who find that
earth was at increased risk of bombardment by cometary debris in the AD 400-600 timeframe, based on the frequency of fireball activity in the
Taurid meteor streams recorded in Chinese archives. To provide further support to his cometary debris theory, Baillie searched the written record and traditions embodied in myths. There he found evidence that the dates of the environmental downturns listed above are often associated with collapses of civilizations or turning points in history. The AD 540 event, for example, may have been associated with a catastrophe that ushered in the
Dark Ages of
Europe. His book,
Exodus to Arthur: Catastrophic Encounters with Comets (Batsford, 1999), relates the findings of his tree-ring studies to a series of global environmental traumas over the past 4400 years that may mark events such as the biblical
Exodus, the disasters which befell
Egypt, collapses of
Chinese dynasties, and the onset of the European Dark Ages.
The Celtic Gods: Comets in Irish Mythology (Tempus, 2005), co-authored with Patrick McCafferty, focuses on the AD 540 event as recorded in the historical records and
myths of
Ireland and shows that the imagery in the myths and the times between events are consistent with a comet with an earth-crossing orbit similar to P/Encke, as described by the British astronomers
Victor Clube and
Bill Napier. His latest book,
New Light on the Black Death: The Cosmic Connection (Tempus, 2006), shows how the tree-ring and Greenland ice core evidence and descriptions in annals, myths and metaphors adduced in support of the global environmental downturn at AD 540, which included the
Justinian plague, also applies to conditions extant at the time of the
Black Death in AD 1348. ==Controversy over releasing data==