Michael G. L. Baillie is Professor Emeritus of Palaeoecology at Queen's University of Belfast, in Northern Ireland. Baillie is a leading expert in dendrochronology, or dating by means of tree-rings. In the 1980s, he was instrumental in building a year-by-year chronology of tree-ring growth reaching 7,400 years into the past. Comet theories 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 claims 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 Baillie explains the general absence of mainstream historical references to this event by the fact it was described in terms of biblical metaphors since at that time "Christian beliefs included the dogma that nothing that happens in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 has devoted much of his attention to uncovering the causes of these global environmental downturns. He believes 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 has searched the written record and traditions embodied in myths. There he has 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