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.