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. The evidence suggests 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 Baillie explains the general absence of mainstream historical references to this event by the fact it was described in terms of biblical metaphors since at that time "Christian beliefs included the dogma that nothing that happens in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."